Tandem axle for a motor vehicle, in particular for a commercial vehicle, motor vehicle and method for operating such a motor vehicle

Abstract

Tandem axle (10) for a motor vehicle, comprising: - a drivable first axis (12) which has: ◯ a first differential gear (30), via which first vehicle wheels (14, 16) of the first axle (12) can be driven; and ◯ a first drive shaft (32) by means of which the first differential gear (30) and, via the first differential gear (30), the first vehicle wheels (14, 16) can be driven; - a drivable second axis (20) which has: ◯ a second differential gear (34) via which second vehicle wheels (22, 24) of the second axle (20) can be driven; ◯ a drive gear (40) by means of which the second differential gear (34) and, via the second differential gear (34), the second vehicle wheels (22, 24) can be driven; and ◯ a second drive shaft (36) by means of which the drive gear (40) can be driven; and - a clutch (70) which can be switched between: ◯ a coupling state in which the second drive shaft (36) is coupled to the first drive shaft (32) by means of the coupling (70) in a torque-transmitting manner, whereby the second drive shaft (36) can be driven by the first drive shaft (32) via the coupling (70); and ◯ a decoupling state in which the second drive shaft (36) is decoupled from the first drive shaft (32); characterized in that: - the second axle (20) has an axle housing (80) in which the drive gear (40) rotatable relative to the axle housing (80) is accommodated; - a speed sensor (88) designed to detect a speed of the drive gear (40) is provided, which has: ◯ a receiving element (92) attached to the axle housing (80); and ◯ a sensor element (94) connected to the drive gear (40) in a rotationally fixed manner, the rotational speed of which can be detected by means of the receiving element (92), whereby the rotational speed of the drive gear (40) can be detected by means of the rotational speed sensor (88); and - the drive gear (40) has a toothing (102) and a shaft stub (104) formed integrally with the toothing (102), on which the sensor element (94) is provided.

Description

The invention relates to a tandem axle for a motor vehicle, in particular for a commercial vehicle, according to the preamble of patent claim 1. The invention also relates to a method for operating a motor vehicle having such a tandem axle according to the preamble of claim 9.DE 10 2006 045 007 A1 and WO 2008 / 019759 A1 disclose a tandem axle having two drivable axles, each axle comprising a differential gear having a drive gearwheel drivable via a shaft as differential gear input and two drive half shafts driving the wheels of this axle via axleshaft wheels. Furthermore, EP 3 608 144 A1 discloses a method for controlling a drive axle system. US 4 724 935 A discloses an anti-lock brake system. JP H08-337 125 A discloses a drive system for vehicles having two rear axles.It is an object of the present invention to provide a tandem axle for a motor vehicle, in particular for a commercial vehicle, a motor vehicle having such a tandem axle and a method for operating a motor vehicle having such a tandem axle, so that particularly advantageous operation can be realized.This object is achieved by a tandem axle having the features of claim 1, by a motor vehicle having the features of claim 8 and by a method having the features of claim 9. Advantageous embodiments with expedient developments of the invention are specified in the other claims.A first aspect of the invention relates to a tandem axle for a motor vehicle, also referred to simply as a vehicle. This means that the motor vehicle, which is preferably designed as a commercial vehicle, in particular as a truck, has the tandem axle in its completely produced state. The tandem axle has a drivable, first axle, which is also referred to as a first vehicle axle. The first axle has a first differential gear, which is also referred to as a first differential or first differential gear or first final drive. First vehicle wheels of the first axle can be driven via the first differential gear. The first vehicle wheels are also referred to as first wheels. Furthermore, the first axle has a first drive shaft, by means of which the first differential gear and the first vehicle wheels can be driven via the first differential gear, as a result of which, for example, the motor vehicle as a whole can be driven. The feature that the first axle is a drivable first axle is understood to mean that the first vehicle wheels can be driven by the first drive shaft via the first differential gear.The tandem axle also has a drivable second axle which is provided in addition to the first axle and is also referred to as a second vehicle axle. In particular, for example, the axles are arranged consecutively in the vehicle longitudinal direction of the motor vehicle and thus one behind the other, in particular in such a way that the second axle connects to the first axle toward the rear in the vehicle longitudinal direction of the motor vehicle. In particular, it is conceivable that the motor vehicle in its completely manufactured state has at least or exactly three axles, also referred to as vehicle axles, namely the first axle, the second axle and a third axle, which is also referred to as the third vehicle axle. It is in particular conceivable for the third axle to be arranged in front of the first axle and in front of the second axle in the longitudinal direction of the vehicle, so that, for example, the third axle is a front axle. Since, for example, the first axle is arranged between the third axle and the second axle in the longitudinal direction of the vehicle, the first axle is, for example, a first rear axle, wherein, for example, the second axle is a second rear axle in the longitudinal direction of the vehicle because it is arranged behind the first axle and the third axle in the longitudinal direction of the vehicle. In particular, the first axle is thus, for example, a front rear axle, and the second axle is, for example, a rear rear axle.The second axle has a second differential gear provided in addition to the first differential gear, which is also referred to as a second differential, second differential gear or second final drive. Second vehicle wheels of the second axle can be driven via the second differential, wherein the second vehicle wheels are also referred to as second vehicle wheels as second wheels. When reference is made above to the vehicle wheels, unless otherwise stated, this is to be understood as meaning the first vehicle wheels and the second vehicle wheels. When the term axles or the vehicle axles is used above in the following, this is to be understood as meaning the first axle and the second axle unless otherwise indicated. The vehicle wheels of the axles are ground contact elements, by means of which the motor vehicle can be or is supported on a ground in the vertical direction of the motor vehicle downwards. If the motor vehicle is driven along the ground while the motor vehicle is supported on the ground via the vehicle wheels in the vertical direction of the motor vehicle downwards, the vehicle wheels roll off on the ground, in particular directly. The feature that the second axle is a drivable second axle is to be understood as meaning that the second vehicle wheels of the second axle are drivable, as a result of which, for example, the motor vehicle as a whole can be driven.The second axle has a drive gearwheel, which is also referred to as a pinion or drive pinion, for example. For example, the drive gear is a bevel gear, also referred to as a bevel gear. The drive gear can be driven into the second differential gear and the second vehicle wheels can be driven via the second differential gear, whereby the motor vehicle as a whole can be driven, for example. This means that the drive gear can be driven by the second differential gear or the second vehicle wheels can be driven by way of the second differential gear. In addition, the second axle has a second drive shaft, which is provided in particular in addition to the first drive shaft and by means of which the drive gearwheel and the second differential gear can be driven via the drive gearwheel. Thus, for example, by driving the drive gear, the second differential gear may be driven, and by driving the second differential gear, the second vehicle wheels may be driven. The drive gearwheel can be driven by means of the second drive shaft, in particular by driving the second drive shaft. Thus, the second drive shaft may drive the drive gear and drive the second differential gear via the drive gear. In particular, it is provided that the second drive shaft is connected, in particular permanently, to the drive gearwheel in a torque-transmitting, in particular rotationally fixed manner.Within the scope of the present disclosure, the feature that two components, such as the second drive shaft and the drive gear, are connected to one another in a rotationally fixed manner is understood to mean that the components connected to one another in a rotationally fixed manner are arranged coaxially with respect to one another and, in particular when the components are driven, rotate together or simultaneously about a component rotational axis common to the components at the same angular speed, in particular relative to a housing of the tandem axis.The feature that two components, such as the second drive shaft and the drive gearwheel, are connected or coupled to one another in a torque-transmitting manner is to be understood to mean that the components are coupled or connected to one another in such a way that torques can be transmitted between the components, wherein, if the components are connected or coupled to one another in a rotationally fixed manner, the components are also connected or coupled to one another in a torque-transmitting manner.The feature that two components, such as the drive gear and the second drive shaft, are permanently connected or coupled to one another in a torque-transmitting manner is to be understood to mean that there is not provided, for example, a shift element which can be switched between a coupling state which connects or couples the components to one another in a torque-transmitting manner and a decoupling state in which no torques can be transmitted between the components via the shift element, but rather the components are always or always and thus permanently connected or coupled to one another in a torque-transmitting manner, that is to say in such a way that a torque can be transmitted between the components. Thus, for example, one of the components can be driven by the respective other component or vice versa.In particular, the feature that two components, such as the second drive shaft and the drive gear, are permanently connected or coupled to one another in a rotationally fixed manner is to be understood to mean that a switching element is not provided, for example, which can be switched over between a coupling state, which connects or couples the components to one another in a rotationally fixed manner, and a decoupling state, in which the components are decoupled from one another and can be rotated relative to one another, such that no torques can be transmitted between the components, but rather the components are always or always, and therefore permanently connected or coupled to one another in a rotationally fixed manner.The tandem axle also has a clutch which is designed, for example, as a multi-plate clutch. The clutch can be switched over between a coupling state and a decoupling state of the clutch, in particular by actuating the clutch. For example, the clutch can be actuated electrically or electronically, for example by means of an electronic computing device, in order thereby to switch over the clutch between the coupling state and the decoupling state, in particular in a targeted and thus active manner, for example. In the coupled state, the second drive shaft is coupled by means of the clutch, i.e. via the clutch, to the drive shaft in a torque-transmitting manner, as a result of which the second drive shaft can be driven by the first drive shaft via the clutch. Thus, in the coupled state, the second vehicle wheels may be driven by the first input shaft via the second differential gear and the second input shaft, and, in particular, simultaneously, the first vehicle wheels may be driven by the first input shaft via the first differential gear, such that, for example, both the first vehicle wheels and the second vehicle wheels may be driven, and so on simultaneously.In the decoupling state, the second drive shaft is decoupled from the first drive shaft, so that the first drive shaft cannot drive the second drive shaft via the clutch, and therefore driving of the second drive shaft via the clutch by the first drive shaft is omitted or prevented.For example, the motor vehicle in its completely produced state has at least or exactly one drive machine, which can be designed, for example, as an internal combustion engine or as an electric machine. For example, the first drive shaft can be driven by the drive engine, as a result of which the first vehicle wheels and, in particular in the coupled state, the second vehicle wheels can be driven by the drive engine.For example, in the fully manufactured state of the motor vehicle having the tandem axle, the first vehicle wheels are arranged on sides of the motor vehicle which are opposite one another in the transverse direction of the motor vehicle.Furthermore, it is preferably provided that the second vehicle wheels are arranged on the sides of the motor vehicle that are opposite one another in the transverse direction of the motor vehicle.In order to be able to realize a particularly advantageous operation of the motor vehicle, it is provided according to the invention that the second axle has an axle housing in which the drive gearwheel which is rotatable relative to the axle housing is accommodated at least partially, in particular at least predominantly and thus at least to more than half or else completely. Since the drive gear is rotatably received, i.e. arranged, relative to the axle housing and is accommodated, i.e. arranged, in the axle housing, the drive gear is rotatably accommodated and thus arranged. In particular, the axle housing, in particular an inner circumferential surface of the axle housing, delimits a receiving space of the axle housing, in particular directly, wherein the drive gearwheel is arranged rotatably in the receiving space at least partially, in particular at least predominantly and thus at least to an extent of more than half or else completely. Thus, the second drive shaft can also rotate relative to the axle housing. The axle housing is also referred to as a carrier or axle carrier. In addition, for example, the second vehicle wheels can rotate relative to the axle housing, in particular about a respective wheel rotational axis.According to the invention, the tandem axle also has a rotational speed sensor, by means of which a rotational speed of the drive gearwheel can be detected, that is to say measured. The rotational speed sensor has a detection element which is fastened to the axle housing and is fastened to the axle housing in particular in such a way that relative movements between the detection element and the axle housing are omitted, and are therefore prevented. Furthermore, the rotational speed sensor has a sensor element which is connected, in particular permanently, in a rotationally fixed manner to the drive gearwheel and the rotational speed of which can be detected by means of the receiving element, as a result of which the rotational speed of the drive gearwheel can be detected by means of the rotational speed sensor. The sensor element can be connected directly or indirectly to the drive gearwheel in such a way that the sensor element is connected, in particular permanently, to the drive gearwheel in a rotationally fixed manner. Thus, the sensor element is co-rotatable with the drive gear relative to the axle housing and also relative to the detection element. Since the sensor element is connected to the drive gear in a rotationally fixed manner, in particular permanently, the rotational speed of the sensor element corresponds to the rotational speed of the drive gear and vice versa, so that the rotational speed of the drive gear can be detected or is detected by detecting the rotational speed of the sensor element. Since the rotational speed of the sensor element is thus detectable or detected by means of the receiving element, the rotational speed of the drive gearwheel is or becomes detectable or detected as a result. In the invention, the rotational speed of the drive gearwheel can be detected particularly advantageously, in particular particularly robust, so that, for example, particularly advantageous operation of the motor vehicle is subsequently possible as a function of the detected rotational speed of the drive gearwheel. It has been found that in the invention the detection of the speed of the drive gear is not subject to excessive influences, in particular in that in the invention the detection of the speed of the drive gear is not excessively affected by fluids such as operating fluids. The invention is based in particular on the following findings and considerations: It is desirable to be able to capture, i.e. measure, a measured variable and to be able to determine, on the basis of the measured variable, whether the clutch is in the coupling state or in the decoupling state, in particular currently. If the clutch is in the coupling state, the clutch is closed. If the clutch is in the decoupling state, the clutch is open. In principle, it would be conceivable to integrate a sensor into the clutch and to arrange it, for example, on a component such as a clutch pack of the clutch in order to be able to record the aforementioned measurement variable by means of the sensor, on the basis of which it can be determined whether the clutch is, in particular, currently, closed or open. In particular during operation of the tandem axle, however, the coupling is supplied with a fluid, which is in particular designed as a liquid and is, for example, one of the aforementioned operating liquids, wherein the fluid can be, for example, an oil. For example, the clutch is actuated and / or lubricated and / or cooled by means of the oil. By supplying the clutch with the oil, the aforementioned sensor, which is a switch, for example, can be acted upon with an excessive amount of the oil, as a result of which the sensor can be adversely affected in its ability or possibility to record the measured variable. Alternatively or additionally, for example, by the fact that the sensor is acted upon by the oil, the sensor can be adversely affected in its possibility or capability of providing a transmission signal characterizing the measured variable and, for example, an electrical transmission signal. The aforementioned electronic computing device, which is also referred to as a control device or is designed as a control device, is basically designed to receive the sensor signal provided by the sensor, whereby the electronic computing device can determine, on the basis of the received sensor signal characterizing the measured variable, whether the clutch is, in particular, currently closed or open. For example, the sensor may be designed as a position sensor, in order to be able to detect a position of the previously mentioned component of the clutch, for example, so that the position is the measurement variable. In particular, for example, in the case of such an integration of the sensor into the clutch, a leakage can occur, as a result of which the fluid can penetrate into a housing, in particular into a switch housing, of the sensor, which can lead to a failure of the sensor.The aforementioned problems and disadvantages can now be avoided by the invention. According to the invention, the rotational speed of the drive gearwheel is used as the aforementioned measurement variable, on the basis of which it can be determined whether the clutch is, in particular, currently, closed or open. By the detection element being fastened to the axle housing and by the sensor element being connected to the drive gearwheel in a rotationally fixed manner, undesirable, excessive adverse effects on the detection of the rotational speed of the drive gearwheel can be avoided. If, for example, it is detected by means of the rotational speed sensor that the rotational speed of the drive gearwheel is different from zero, in particular is greater than zero, so that the drive gearwheel therefore rotates relative to the axle housing, it can be determined in this way that the clutch is closed, because then the drive gearwheel is driven by the second drive shaft, and the second drive shaft is driven by the first drive shaft via the clutch, which is possible only in the coupled state with respect to the coupled state and the uncoupled state. However, if it is detected, for example, by means of the rotational speed sensor that the, in particular current, rotational speed of the drive gearwheel is zero, and therefore that the drive gearwheel does not rotate, in particular while the first drive shaft rotates, in particular while the first drive shaft is driven by the drive machine and thereby rotates, it can be concluded or it can thus be determined that the clutch is currently open, because then the drive gearwheel is not driven by the second drive shaft and the second drive shaft is not driven via the clutch of the first drive shaft, which is possible, in particular with respect to the coupling state and the decoupling state, only in the decoupling state, and can therefore be the case.The rotational speed sensor is preferably designed to provide an, in particular electrical, signal which characterizes the rotational speed of the drive gearwheel detected by means of the sensor. The aforementioned electronic computing device can receive the signal and subsequently determine, on the basis of the detected rotational speed of the drive gearwheel characterized by the signal, whether the clutch is, in particular, currently, open or closed. As a result, it is possible, for example, for the motor vehicle, in particular the clutch, to be operated, in particular by means of the electronic computing device, as a function of the signal, that is to say as a function of the detected rotational speed of the drive gearwheel. In particular, an operating strategy provided for operating the motor vehicle, in particular for operating the tandem axle and, more particularly, for operating the clutch, can be carried out as a function of the detected rotational speed of the drive gearwheel. It is in particular conceivable that, for example, the clutch is operated as a function of the detected rotational speed of the drive gearwheel, in particular in such a way that the clutch is switched between the coupling state and the decoupling state as a function of the detected rotational speed of the drive gearwheel.In order to be able to detect the rotational speed of the drive gearwheel in a particularly advantageous manner and subsequently to implement a particularly advantageous operation of the motor vehicle, which operation is dependent in particular on the detected rotational speed of the drive gearwheel, it is provided in one embodiment of the invention that the sensor element is formed separately from the drive gearwheel and is connected, in particular permanently, in a rotationally fixed manner to the drive gearwheel.A further embodiment is distinguished in that the drive gearwheel has a toothing, which is also referred to as first toothing, and a shaft stub formed integrally with the toothing, on which the sensor element is provided, in particular formed or arranged. If the term "toothing" is used before and below, this is to be understood as meaning, unless otherwise stated, the first toothing of the drive gearwheel. The feature that the toothing and the shaft stub are formed integrally with one another is to be understood to mean that the shaft stub and the toothing are formed from a single piece. In other words again, the toothing and the shaft stub are preferably formed by a one-piece body, which is therefore formed in one piece and is thus formed from a single piece, and which, owing to the fact that the body is formed in one piece and is therefore formed from a single piece, is formed as a one-piece monoblock, which is therefore formed from a single piece and is thus produced integrally. This means that preferably the toothing and the shaft stub are not formed as parts formed separately from one another and joined to one another, but preferably the toothing and the shaft stub are formed from a single piece, therefore formed integrally with one another and thus formed as a monoblock or formed by a monoblock. For example, the toothing is in engagement with a second toothing of a second gearwheel of the differential gear, such that the second gearwheel and thus the second differential gear can be driven by means of the drive gearwheel. For example, the second gearwheel is a ring gearwheel. For example, the second gearwheel is connected, in particular permanently, in a rotationally fixed manner to a differential housing of the second differential gear, which housing is designed, for example, as a differential cage or is also referred to as a differential cage.Since the sensor element is provided on the shaft stub, and since the toothing is formed integrally with the shaft stub, such that the toothing and the shaft stub can rotate or rotate relative to the axle housing at the same rotational speed of the drive gearwheel, the rotational speed of the drive gearwheel can be detected particularly advantageously and robust by means of the receiving element and by means of the sensor element, such that advantageous operation of the motor vehicle can be ensured, in particular on the basis of the detected rotational speed of the drive gearwheel.A further embodiment is distinguished in that the drive gearwheel has a toothing, which is also referred to as first toothing, and a shaft stub formed integrally with the toothing, on which the sensor element is provided, in particular formed or arranged. If the term "toothing" is used before and below, this is to be understood as meaning, unless otherwise stated, the first toothing of the drive gearwheel. The feature that the toothing and the shaft stub are formed integrally with one another is to be understood to mean that the shaft stub and the toothing are formed from a single piece. In other words again, the toothing and the shaft stub are preferably formed by a one-piece body, which is therefore formed in one piece and is thus formed from a single piece, and which, owing to the fact that the body is formed in one piece and is therefore formed from a single piece, is formed as a one-piece monoblock, which is therefore formed from a single piece and is thus produced integrally. This means that preferably the toothing and the shaft stub are not formed as parts formed separately from one another and joined to one another, but preferably the toothing and the shaft stub are formed from a single piece, therefore formed integrally with one another and thus formed as a monoblock or formed by a monoblock. For example, the toothing is in engagement with a second toothing of a second gearwheel of the differential gear, such that the second gearwheel and thus the second differential gear can be driven by means of the drive gearwheel. For example, the second gearwheel is a ring gearwheel. For example, the second gearwheel is connected, in particular permanently, in a rotationally fixed manner to a differential housing of the second differential gear, which housing is designed, for example, as a differential cage or is also referred to as a differential cage.Since the sensor element is preferably provided on the shaft stub, and since the toothing is formed integrally with the shaft stub, such that the toothing and the shaft stub can rotate or rotate relative to the axle housing at the same rotational speed of the drive gearwheel, the rotational speed of the drive gearwheel can be detected particularly advantageously and robust by means of the receiving element and by means of the sensor element, such that advantageous operation of the motor vehicle can be ensured, in particular on the basis of the detected rotational speed of the drive gearwheel.It is quite preferably provided that the sensor element is formed separately from the shaft stub and thus separately from the toothing and, in particular permanently, connected to the shaft stub in a rotationally fixed manner. As a result, the rotational speed of the drive gearwheel can be detected particularly robust. This means that excessive influences that could influence the detection of the rotational speed of the drive gear can be avoided or, despite such influences, the rotational speed of the drive gear can be advantageously detected.A further embodiment is characterized in that the tandem axle has a spacer element which is formed separately from the drive gearwheel and which is connected, in particular permanently, in a rotationally fixed manner to the drive gearwheel. For example, the spacer element is a ring, and therefore annular, so that, for example, the spacer element rotates completely, and therefore completely closed, in the circumferential direction of the drive gearwheel, the axial direction of which runs perpendicular to the radial direction of the drive gearwheel. In this case, the circumferential direction of the drive gearwheel runs around the axial direction of the drive gearwheel. In particular, the drive gear is rotatable about a gear rotational axis relative to the axle housing, wherein the axial direction of the drive gear coincides with the gear rotational axis. By means of the spacer element, two bearings, by means of which the drive gearwheel is rotatably mounted on the axle housing, are held at a distance from one another in the axial direction of the drive gearwheel. For example, the bearings are designed as rolling bearings, in particular as conical bearings, in order to be able to realize a low-friction bearing of the drive gearwheel.In this case, it is further provided that the sensor element formed separately from the drive gearwheel and separately from the spacer element is connected, in particular permanently, rotationally fixedly to the spacer element and, in particular exclusively, via the spacer element, in particular permanently, rotationally fixedly to the drive gearwheel. The spacer element, the sensor element and the drive gearwheel thus rotate at the same or the same rotational speed, in particular about the gearwheel rotational axis, relative to the axle housing, such that the rotational speed of the drive gearwheel can be detected particularly robust and thus reliably and precisely.It has proven particularly advantageous here if the sensor element surrounds the spacer element at least partially, in particular at least predominantly and thus at least more than half or even completely, in the circumferential direction of the drive gearwheel extending around the axial direction of the drive gearwheel. As a result, the rotational speed of the drive gearwheel can be detected, that is to say measured, in a robust and precise manner.In a further, particularly advantageous embodiment of the invention, it is provided that the spacer element formed separately from the drive gearwheel and thus separately from the shaft stub and separately from the toothing is connected, in particular permanently, rotationally fixedly to the shaft stub and surrounds the shaft stub completely, i.e. completely circumferentially, in the circumferential direction of the shaft stub and thus of the drive gearwheel which runs around the axial direction of the drive gearwheel. In particular, it is provided that the toothing of the drive gearwheel adjoins the shaft stub in the axial direction of the drive gearwheel, wherein, for example, the toothing of the drive gearwheel has a larger diameter than the shaft stub, in particular in such a way that the largest diameter, in particular outer diameter, of the toothing is larger than the largest diameter, in particular outer diameter, of the shaft stub. The sensor element is thus provided on the shaft stub in such a way that the spacer element formed separately from the drive gearwheel and thus separately from the shaft stub and separately from the toothing is connected, in particular permanently, rotationally fixedly to the shaft stub, wherein the sensor element is connected, in particular permanently, rotationally fixedly to the spacer element via said spacer element to the shaft stub. As a result, the rotational speed of the drive gearwheel can be measured in a robust and precise manner, as a result of which the motor vehicle can be operated particularly advantageously on the basis of the measured and therefore detected rotational speed of the drive gearwheel.Finally, it has been shown to be particularly advantageous if the rotational speed sensor has a connection device, by means of which the rotational speed sensor is electrically connectable or connected to at least one line, in particular formed separately from the rotational speed sensor. In this case, the connection device is arranged outside the axle housing, in particular outside the receiving space, such that the connection device is arranged, for example, in a vicinity of the axle housing. In particular, the rotational speed sensor can provide the aforementioned, preferably electrical signal via the connection device, wherein the rotational speed sensor can transmit the signal to the line via the connection device. The line can thus receive the signal via the connection device, and the line can carry the signal, in particular to the electronic computing device, which can thus receive the signal carried by the line and provided or made available by the line. By arranging the connection arrangement direction of the rotational speed sensor outside the axle housing, it is possible to prevent the connection device from being excessively charged with an operating fluid such as the aforementioned oil, for example, so that the signal can be transmitted reliably and in a robust manner. Consequently, an advantageous operation of the motor vehicle can be represented on the basis of the signal which characterizes the detected rotational speed of the drive gearwheel. Since the connecting device, also referred to as connecting element, connecting device or rotational speed sensor connecting element, is arranged outside the axle housing, also referred to as carrier, it is possible to prevent the connecting device from being exposed to excessive oil leakage. Failures due to lateral load oscillations can also be avoided in this way, so that the rotational speed of the drive gear can be detected in a robust and precise manner.A second aspect of the invention relates to a motor vehicle, also referred to simply as a vehicle, which has a tandem axle according to the first aspect of the invention. Advantages and advantageous configurations of the first aspect of the invention are to be regarded as advantages and advantageous configurations of the second aspect of the invention and vice versa.A third aspect of the invention relates to a method for operating a motor vehicle having a tandem axle, in particular according to the first aspect of the invention, which is also referred to simply as a vehicle. In the method, the tandem axle has a drivable first axle, which has a first differential gear, via which first vehicle wheels of the first axle can be driven. In the method, the first axle has a first drive shaft, by means of which the first differential gear and the first vehicle wheels can be driven via the first differential gear. In the method, the tandem axle has a drivable, second axle, which has a second differential gear, via which second vehicle wheels of the second axle can be driven. The second axle has a drive gearwheel, by means of which the second differential gear and, via the second differential gear, the first vehicle wheels can be driven. In addition, the second axle has a second drive shaft, by means of which the drive gearwheel can be driven. In the method, the tandem axle has a clutch which can be switched over, in particular is switched over, between a coupling state and a decoupling state. In the coupled state, the second drive shaft is coupled by means of the clutch, i.e. via the clutch, to the first drive shaft in a torque-transmitting manner, as a result of which the second drive shaft can be driven, in particular is driven, by the first drive shaft via the clutch. In the decoupling state, the second drive shaft is decoupled from the first drive shaft.In order to be able to operate the motor vehicle particularly advantageously, it is provided in the third aspect of the invention that the second axle has an axle housing in which the drive gearwheel which is rotatable relative to the axle housing is accommodated, that is to say is arranged. A rotational speed sensor is also provided, by means of which a rotational speed of the drive gearwheel is detected. For this purpose, the rotational speed sensor has a detection element fastened to the axle housing and a sensor element which is connected, in particular permanently, in a rotationally fixed manner to the drive gearwheel and is connected directly or indirectly to the drive gearwheel in such a way that the sensor element is connected in a rotationally fixed manner to the drive gearwheel. A rotational speed of the sensor element is detected by means of the receiving element, as a result of which the rotational speed of the drive gearwheel is detected by means of the rotational speed sensor, in particular in that the rotational speed of the sensor element comprises the rotational speed of the drive gearwheel. This is the case in particular because the sensor element is connected, in particular permanently, to the drive gearwheel in a rotationally fixed manner. In addition, in the method according to the third aspect of the invention, it is provided that the motor vehicle is operated as a function of the rotational speed of the drive gearwheel detected, that is to say measured, by means of the rotational speed sensor. In this case, it is provided, for example, that the clutch is switched between the coupling state and the decoupling state depending on the rotational speed of the drive gearwheel detected by means of the rotational speed sensor. Advantages and advantageous configurations of the first aspect and of the second aspect of the invention are to be seen from advantages and advantageous configurations of the third aspect of the invention and vice versa. The drive gear has a toothing and a shaft stub formed integrally with the toothing, on which the sensor element is provided.In particular, it is conceivable that the tandem axle is operated depending on the detected rotational speed of the drive gearwheel. The clutch is preferably designed as a multi-plate clutch, which preferably has a plurality of multi-plate plates, in particular successive in the axial direction of the clutch, in particular as the aforementioned plates.It is conceivable, in particular in the operating strategy, for the clutch to be switched between the coupling state and decoupling state depending on a speed of the motor vehicle which is in particular current and is also referred to as the driving speed. Alternatively or additionally, for example, in particular in the operating strategy, the clutch is switched over between the coupling state and the decoupling state as a function of a slip of at least one of the vehicle wheels. In particular, it is conceivable, in particular in the operating strategy, for the second axle to be selectively coupled to the first axle or decoupled from the first axle depending on an in particular current driving situation. For coupling the second axle to the first axle, i.e. for coupling the second axle to the first axle, the clutch is closed, and therefore the coupling state of the clutch is adjusted. In order to decouple the second axle from the first axle, the clutch is opened, and the decoupling state is thus set. It is thus conceivable that, depending on an in particular current driving situation of the motor vehicle, the clutch is switched between the coupling state and the decoupling state, in particular in the aforementioned operating strategy.For example, the sensor element is annular and thus designed as a measuring ring, which extends, for example, in the circumferential direction of the drive gear running around the axial direction of the drive gear, in particular closed and thus completely. For example, the sensor element is connected to the drive gearwheel in a rotationally fixed manner by means of a press fit. It is in particular conceivable for the sensor element to be pressed onto the drive gearwheel, in particular onto the shaft stub. In particular, it is conceivable for the sensor element to be connected to the spacer element in a rotationally fixed manner by means of a press fit, such that, for example, the sensor element is pressed onto the spacer element. It is conceivable that the spacer element is connected in a rotationally fixed manner to the drive gearwheel, in particular to the shaft stub, by means of a press fit.In this case, for example, the spacer element is pressed onto the drive gearwheel, in particular onto the shaft stub. The sensor element supplies information to the receiving element, on the basis of which the receiving element can detect the rotational speed of the sensor element and thus the rotational speed of the drive gearwheel. For this purpose, the sensor element has recesses, for example, which can be formed as blind holes or as through-openings, for example. Furthermore, the sensor element has partial regions provided, for example, in addition to the recesses, which are formed, for example, as projections, which are also referred to as a bulge. The said partial regions of the sensor element are raised in particular in the radial direction of the drive gearwheel and thus of the sensor element outwards with respect to the recesses, so that for example the recesses are set back inwards with respect to the partial regions in the radial direction of the drive gearwheel. In particular, it is provided that the recesses and the partial regions of the sensor element are arranged alternately in succession in the circumferential direction of the drive gearwheel and thus of the sensor element running around the axial direction of the drive gearwheel. The receiving element is designed to detect the rotational speed of the sensor element and thus the rotational speed of the drive gear wheel on the basis of the partial regions and on the basis of the recesses of the sensor element, in particular when the drive gear wheel and thus the sensor element rotates. The recesses and the partial regions thus form, so to speak, a knife toothing, on the basis of which the rotational speed of the sensor element and thus the rotational speed of the drive gearwheel can be detected by means of the receiving element. This ensures robust and precise detection, i.e. measurement of the rotational speed of the drive gearwheel.The axle housing is designed, for example, as a cast component and is thus produced by casting. However, the axle housing has, for example, a fastening element, by means of which the receiving element is fastened to the axle housing. In particular, it is conceivable for the fastening element to be produced by casting, by means of which the axle housing is also produced. The sensor element is formed from sheet metal, for example.The present invention makes it possible to detect a current state of the clutch in a robust and precise manner. This is to be understood as meaning that it is possible by the invention to determine in a robust and precise manner whether the clutch is in the coupling state or in the decoupling state, in particular currently. In particular, the invention makes it possible to ascertain the following defects or causes of defects:specifying false coupling for the applicationAn algorithm results in the clutch undergoing more cycles than intendeddefects in laser weldingfailure of a vehicle air valvetoo high an input torqueFurther advantages, features and details of the invention will become apparent from the following description of a preferred exemplary embodiment and with reference to the drawing. The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention.The drawing shows in: FIG. 1 shows a schematic illustration of a tandem axle for a motor vehicle; FIG. 2 is a schematic perspective view of an axle housing of the tandem axle; FIG. 3 is a further schematic perspective view of the axle housing; FIG. 4 shows a detail of a further schematic perspective view of the axle housing; FIG. 5 shows a detail of a further schematic perspective view of the axle housing; FIG. 6 is a schematic perspective view of a receiving element of the tandem axle; FIG. 7 shows a schematic and perspective sectional view of a portion of the axle housing and of a drive gearwheel of the tandem axle; FIG. 8 shows a schematic perspective view of a sensor element of the tandem axle, which sensor element is designed as a measuring ring; FIG. 9 shows a schematic perspective view of a structural unit of the tandem axle, since the structural unit comprises the sensor element; and FIG. 10 is a block diagram illustrating a method for operating the motor vehicle.In the figures, identical or functionally identical elements are provided with the same reference numerals.FIG. 1 shows a schematic illustration of a tandem axle 10 for a motor vehicle which is also referred to simply as a vehicle and is preferably designed as a utility vehicle, in particular a heavy goods vehicle. The tandem axle 10 has a drivable, first axle 12 which has first vehicle wheels 14 and 16. The vehicle wheels 14 and 16 are arranged on sides of the motor vehicle which are opposite one another in the transverse direction of the motor vehicle, wherein the transverse direction of the vehicle is illustrated by a double arrow 18. The tandem axle 10 also includes a second axle 20 which includes second vehicle wheels 22 and 24. The vehicle wheels 22 and 24 are disposed on opposite sides of the vehicle transverse direction such that, for example, the vehicle wheels 14 and 22 are disposed on a first of the sides of the motor vehicle and the vehicle wheels 16 and 24 are disposed on a second of the sides of the motor vehicle, the first side and the second side being opposed to each other in the vehicle transverse direction of the motor vehicle. It can be seen that the vehicle wheels 14, 16, 22 and 24 are twin wheels and thus each have a twin tire. The vehicle wheels 14, 16, 22 and 24 are ground contact elements, by means of which the motor vehicle can be or is supported on a ground in the vertical direction of the motor vehicle downwards. The vehicle vertical direction of the motor vehicle runs perpendicular to the image plane of FIG. 1 and thus perpendicular to the vehicle transverse direction and is illustrated by a double arrow 26. It can be seen that the axles 12 and 20 are arranged successively in the longitudinal direction of the vehicle and thus one behind the other in such a way that the axle 20 adjoins the axle 12 to the rear in the longitudinal direction of the vehicle. The vehicle longitudinal direction runs perpendicular to the vehicle transverse direction and perpendicular to the vehicle vertical direction and is illustrated by a double arrow 28. The respective axle 12, 20 is also referred to as the respective vehicle axle.The feature that the axle 12 is a drivable axle is understood to mean that the vehicle wheels 14 and 16 and not just the motor vehicle as a whole can be driven. For this purpose, the first axle 12 has a first differential gear 30, via which the vehicle wheels 14 and 16 of the axle 12 can be driven. In addition, the first axle 12 has a first drive shaft 32, which can be driven, for example, by a drive engine of the motor vehicle, which is not shown in the figures. By driving the first drive shaft 32, the differential gear 30 can be driven by means of the first drive shaft 32, wherein the vehicle wheels 14 and 16 can be driven by driving the first differential gear 30. Thus, the first differential gear 30 can be driven by means of the first drive shaft 32 and the vehicle wheels 14 and 16 can be driven by means of the first differential gear 30.The feature that the second axle 20 is drivable, and is therefore a drivable axle, is to be understood as meaning that the vehicle wheels 22 and 24 of the axle 20 and therefore the motor vehicle as a whole can be driven. For this purpose, the second axle 20 has a second differential 34 provided in particular in addition to the first differential 30. In addition, the second axle 20 has a second drive shaft 36, by means of which the second differential gear 34 can be driven by driving the second drive shaft 36. By driving the differential gear 34, the vehicle wheels 22 and 24 can be driven. Thus, the second differential gear 34 can be driven by means of the second drive shaft 36 and the second vehicles 22 and 24 can be driven by means of the second differential gear 34.The first axle 12 has a first drive gearwheel 38 which is designed, for example, as a first bevel gear. The first drive gearwheel 38 can be driven by means of the first drive shaft 32, and for example in that the drive gearwheel 38 is connected in a rotationally fixed manner, in particular permanently, to the drive shaft 32. The second axle 20 has a second drive gear 40 provided in addition to the drive gear 38. The first differential gear 30 can be driven by means of the drive gear 38 and the first vehicle wheels 14 and 16 can be driven via the first differential gear 30, such that the drive gear 38 can be driven by driving the drive shaft 32, the differential gear 30 can be driven by driving the drive gear 38, and the vehicle wheels 14 and 16 can be driven by driving the differential gear 30. The second drive gearwheel 40 can be used as a second differential gear 34 and the second vehicle wheels 22 and 24 can be driven via the second differential gear 34. Thus, by driving the input shaft 36, the input gear 40 may be driven, by driving the input gear 40, the differential gear 34 may be driven, and by driving the differential gear 34, the vehicle wheels 22 and 24 may be driven. Thus, vehicle wheels 22 and 24 are drivable by drive gear 40 via differential gear 34, and vehicle wheels 14 and 16 are drivable by first drive gear 38 via differential gear 30. For example, the first drive gearwheel 38 is designed as a bevel gear. For example, the second drive gearwheel 40 is formed as a bevel gear.It can be seen from FIG. 1 that the respective differential gear 30, 34 has a respective input gearwheel 42, 44. The respective input gear 42, 44 is a respective gear. For example, the respective input gear 42, 44 is a respective ring gear. The drive gear 38 meshes with the input gear 42 and the drive gear 40 meshes with the input gear 44. for example, the drive gear 38 has a first toothing and the input gear 42 has a second toothing with which the first toothing is engaged or meshes, whereby the drive gear 38 is engaged or meshes with the input gear 42. The drive gear 40 has, for example, a third toothing, and the input gear 44 has, for example, a fourth toothing, with which, for example, the third toothing is or meshes, as a result of which the drive gear 40 is or meshes with the input gear 44. For example, the respective differential gear 30, 34 has a respective differential housing 46, 48 which is designed, for example, as a respective differential cage. Moreover, the respective differential 30, 34 includes differential gears 50, 52, respectively, and the respective differential 30, 34 includes output gears 54, 56, respectively. The balancer gears 50 and the driven gears 54 are gears, and the balancer gears 50 are meshed with the driven gears 54. The differential gears 52, 56 are gears, and the differential gears 52 mesh with the output gears 56. The respective differential gear 50, 52 is rotatably mounted on the respective differential housing 46, 48 about a first axis of rotation relative to the respective differential housing 46, 48, namely at least indirectly. The respective output gearwheel 54, 56 is rotatable about a second axis of rotation relative to the respective differential housing 46, 48, wherein the respective second axis of rotation runs perpendicular to the respective first axis of rotation. By driving the respective differential housing 46, 48, which can be driven by driving the respective differential gear 30, 34, the differential gears 50, 52 can be driven, wherein the output gears 54, 56 can be driven by driving the differential gears 50, 52. By driving the output gears 54, the vehicle wheels 14 and 16 may be driven, and by driving the output gears 56, the vehicle wheels 22, 24 may be driven. In the exemplary embodiment shown in FIG. 1, the respective differential gear 30, 34 is designed as a respective bevel gear differential.The first axle 12 has first side shafts 58 and 60, wherein the side shaft 58 is drivable by a first of the output gears 54 and the side shaft 60 is drivable by a second of the output gears 54. For example, the side shaft 58 is permanently torque-transmittingly coupled to the first output gear 54, and the side shaft 60 is permanently torque-transmittingly coupled to the second output gear 54, for example. By means of the side shaft 58, the vehicle wheel 14 can be driven by driving the side shaft 58, and by means of the side shaft 60, the vehicle wheel 16 can be driven by driving the side shaft. In this case, the side shaft 58 can be driven by means of the first output gearwheel 54 by driving the first output gearwheel 54, and the side shaft 60 can be driven by means of the second output gearwheel 54.The second axle 20 has a third side shaft 62 assigned to the vehicle wheel 22, which is assigned to a first of the output gearwheels 56. In addition, the second axle 20 has a fourth side shaft 64 which is assigned to the vehicle wheel 24 and is assigned to a second of the output gearwheels 56. The vehicle wheel 22 can be driven by driving the side shaft 62 by means of the side shaft 62, and the vehicle wheel 24 can be driven by driving the side shaft 64 by means of the side shaft 64. By means of the first output gearwheel 56, the side shaft 62 can be driven by driving the first output gearwheel 56, with the result that the vehicle wheel 22 can be driven by means of the drive gearwheel 56 via the side shaft 62. By means of the second output gearwheel 56, the side shaft 64 can be driven by driving the second output gearwheel 56, with the result that the vehicle wheel 24 can be driven via the side shaft 64 by means of the second output gearwheel 56. The axle 20 has a first clutch device 66 assigned to the first output gearwheel 56 and to the side shaft 62, and a second clutch device 68 assigned to the side shaft 64 and to the second output gearwheel 56. The respective clutch device 66, 68 can be switched between a respective connection state and a respective release state, in particular by electrically or electronically controlling the respective clutch device 66, 68. In the respective connection state of the respective clutch device 66, 68, the respective side shaft 62, 64 assigned to the respective clutch device 66, 68 is coupled via the respective assigned clutch device 66, 68 in a torque-transmitting manner to the respective assigned and thus assigned output gearwheel 56, so that in the connection states of the clutch device 66 and 68, the output gearwheels 56 can drive the side shafts 62 and 64 and thus the vehicle wheels 22 and 24 via the clutch devices 66 and 68. In the respective release state of the respective clutch device 66, 68, however, the respective, assigned and thus associated side shafts 62, 64 are decoupled from the respective assigned and thus associated output gearwheel 56, so that in the release states of the clutch devices 66 and 68, the output gearwheels 56, the side shafts 62 and 64 and thus the vehicle wheels 22 and 24 cannot drive.The tandem axle 10 also has a clutch 70, which is provided in particular in addition to the clutch devices 66 and 68 and is preferably designed as a multi-plate clutch. Most preferably, the clutch 70 is designed as a multi-plate clutch, which comprises a plurality of multi-plate plates, which are arranged consecutively in the axial direction of the clutch 70, as the aforementioned plates. The clutch 70 is switchable between a coupling state and a decoupling state. In the coupled state, the second drive shaft 36 is coupled to the first drive shaft 32 in a torque-transmitting manner by means of the clutch 70, as a result of which the second drive shaft 36 can be driven by the first drive shaft 32 via the clutch 70. In the decoupling state, the second drive shaft 36 is decoupled from the first drive shaft 32, as a result of which the drive shaft 36 cannot be driven from the drive shaft 32 via the clutch 70.In the exemplary embodiment shown in FIG. 1, a third drive gearwheel 72 is connected, in particular permanently, in a rotationally fixed manner to the drive shaft 32. A fourth drive gearwheel 74 is connected, in particular permanently, in a rotationally fixed manner to an input element 76 of the clutch 70. For example, the drive gears 72 and 74 are formed as spur gears. The driving gears 72 and 74 are meshed with each other, thus meshed with each other. For example, the input element 76 is or comprises a first panel carrier. By means of the laminated disks, the input element 76 can be connected to an output element 78 of the clutch 70 in a torque-transmitting, in particular rotationally fixed manner, wherein, for example, the output element 78 comprises a second laminated carrier or is a second laminated carrier. The drive shaft 36 can be driven by an output element 78 of the clutch 70, in particular in such a way or in that the drive shaft 36 is connected to the output element 78 in a torque-transmitting, in particular torque-proof manner, in particular permanently. Thus, in the coupled state of the clutch 70, the output member 78 can be driven by the input shaft 32 via the input member 76 and the drive gears 72 and 74, whereby the input shaft 32 can drive the input shaft 36 via the clutch 70. In the decoupling state, the output element 78 is decoupled from the input element 76, such that the input element 76 and thus the drive shaft 32 cannot drive the output element 78 and thus not the drive shaft 36.For example, the respective clutch device 66, 68 is designed as a respective claw clutch, so that the respective side shaft 62, 64 can be connected in a positive-locking manner to the respective associated output gearwheel 56 by means of the respective clutch device 66, 68.It can be seen particularly well in conjunction with FIG. 2 that the second axle 20 has an axle housing 80, also referred to as a carrier or axle carrier, wherein the respective vehicle wheel 22, 24 is rotatable relative to the axle housing 80, in particular about a respective wheel rotational axis. The axle housing 80 can also be seen particularly well from FIGS. 3 and 4. The axle housing 80 can also be seen particularly well from FIGS. 5 and 7. It can be seen particularly well from FIG. 7 that a receiving space 84, in particular directly, is delimited by the axle housing 80, in particular by an inner-periphery-side lateral surface 82 of the axle housing 80. The second drive gear 40 is accommodated in the accommodation space 84 and thus in the axle housing 80. The drive gear 40 is rotatable about a gear rotation axis 86 relative to the axle housing 80, wherein the gear axis 86 extends in the axial direction of the drive gear 40, the radial direction of which extends perpendicular to the axial direction of the drive gear 40. In the present case, the axial direction of the drive gearwheel 40 coincides with the gearwheel rotation axis 86. Furthermore, it can be seen particularly well from FIGS. 7 to 9 that the tandem axle 10 has a rotational speed sensor 88, by means of which a rotational speed of the drive gearwheel 40 can be detected. If the drive gear 40 is driven by the drive shaft 36, the drive gear 40 rotates at the aforementioned rotational speed of the drive gear 40 about the gear rotational axis 86 relative to the axle housing 80, which is, for example, the aforementioned housing of the tandem axle 10. From FIGS. 7 and 5, a coupling element 90 formed separately from the drive gearwheel 40 and connected, in particular permanently, in a rotationally fixed manner to the drive gearwheel 40 can be seen, by means of which coupling element, for example, the drive gearwheel 40 is coupled, in particular permanently, to the drive shaft 36 in a torque-transmitting manner, in particular in a rotationally fixed manner.The rotational speed sensor 88 has a receiving element 92 which is formed separately from the axle housing 80 and is fastened to the axle housing 80 and a sensor element 94 which can be seen particularly well from FIG. 8 and which, in the exemplary embodiment shown in the figures, is annular and is therefore formed as a ring. The ring is also referred to as a measurement ring. The sensor element 94 is connected, in particular permanently, in a rotationally fixed manner to the drive gearwheel 40, it being possible for a rotational speed of the sensor element 94 to be detected by means of the receiving element 92. By detecting the rotational speed of the sensor element 94, the rotational speed of the drive gearwheel 40 can be detected by the rotational speed of the sensor element 94 corresponding to the rotational speed of the drive gearwheel 40 by the sensor element 94 being connected to the drive gearwheel 40 in a rotationally fixed manner. It can be seen from FIG. 8 that the sensor element 94 has recesses 96, which are formed in the present case as through openings, for example. Between each of two recesses 96, which are directly consecutive and thus adjacent to one another, in particular exactly, in each case two circumferential directions of the drive gearwheel 40 and therefore of the sensor element 94 running around the axial direction of the drive gearwheel 40, a respective further partial region 98 of the sensor element 94 is arranged, wherein the recesses 96 and the partial regions 98 are arranged alternately consecutive in the circumferential direction of the drive gearwheel 40 and therefore of the sensor element 94. The circumferential direction of the drive gearwheel 40 and thus of the sensor element 94 is illustrated in FIG. 7 by a double arrow 100; if the drive gearwheel 40 and thus the sensor element 94 rotate about the gearwheel rotational axis 86 relative to the axle housing 80, then for example the alternately successive recesses 96 and partial regions 98 cause changes in a measurement signal provided for example by the rotational speed sensor 88, it being possible for the respective rotational speed, and therefore the rotational speed of the drive gearwheel 40, to be detected as a function of these changes.The aforementioned second toothing of the drive gearwheel 40 is partially recognizable in FIG. 7 and is denoted by 102. It can be seen that the drive gearwheel 40 has the toothing 102 and a shaft stub 104, wherein the toothing 102 and the shaft stub 104 are formed integrally with one another, and are therefore formed from a single piece via the shaft stub 104, the drive gearwheel 40 is mounted on the axle housing 80 such that it can rotate about the gearwheel rotational axis 86 relative to the axle housing 80, in the present case such that the shaft stub 104 and therefore the drive gearwheel 40 are mounted on the axle housing 80 such that it can rotate via two bearings 106 and 108. The bearings 106 and 108 are designed in the present case as rolling bearings, in particular as conical bearings. The bearings 106 and 108 are arranged in succession in the axial direction of the drive gearwheel 40 and are in particular spaced apart from one another. In this case, a spacer element 110 is provided which is formed separately from the drive gearwheel 40 and thus separately from the shaft stub 104 and separately from the toothing 102, is formed as a ring and therefore completely surrounds at least one longitudinal region of the shaft stub 104 in the circumferential direction of the drive gearwheel 40. The spacer element 110 is arranged in the axial direction of the drive gearwheel 40 between the bearings 106 and 108, in particular between inner bearing rings of the bearings 106 and 108, wherein the bearings 106 and 108, in particular their inner bearing rings, are held at a distance from one another in the axial direction of the drive gearwheel 40 by means of the spacer element 108. The spacer element 110 formed separately from the drive gearwheel 40 and thus separately from the shaft stub 104 and separately from the toothing 102 is connected, in particular permanently, in a rotationally fixed manner to the shaft stub 104 and thus to the drive gearwheel 40, for example in that the spacer element 110 is pressed onto the shaft stub 104 and thus onto the drive gearwheel 40.FIG. 9 shows a structural unit 112 which comprises the sensor element 94 and the spacing element 110. The sensor element 94 is formed separately from the drive gearwheel 40 and in the present case is connected in a rotationally fixed manner, in particular permanently, to the drive gearwheel 40 in such a way that the sensor element 94 is formed separately from the spacing element 110, is arranged on the spacing element 110 and is connected in a rotationally fixed manner, in particular permanently, to the spacing element 110. Since, in the exemplary embodiment shown in the figures, the sensor element 94 is designed as a ring, the sensor element 94 surrounds at least one respective length region of the spacer element 110 and of the shaft stub 104 in the circumferential direction of the drive gearwheel 40 in a completely encircling manner. For example, sensor element 94, which is formed separately from drive gear 40 and separately from spacer element 110, is connected to spacer element 110 in a rotationally fixed manner, in particular permanently, by means of a press fit, so that sensor element 94 is connected to drive gear 40 in a rotationally fixed manner, in particular permanently, via spacer element 110. The axial direction of the drive gear 40 coincides with the gear rotation axis 86 and is illustrated by an arrow 114.It can be seen particularly well from FIG. 6 that the receiving element 92 and thus the rotational speed sensor 88 has a connection device 116, by means of which the rotational speed sensor 88 is electrically connectable or connected to a line, not shown in the figures and formed separately from the rotational speed sensor 88. The receiving element 92 can provide an electrical signal, for example, which characterizes the rotational speed of the drive gearwheel 40 detected by means of the rotational speed sensor 88. Via the connection device 116, the preferably electrical signal can be electrically transmitted from the receiving element 92 to the line, which can lead the signal, for example, to an electronic computing device of the tandem axle 10, also referred to as a control device. The line may provide the signal, wherein the electronic computing device may receive the signal provided by the line. As a result, for example, the electronic computing device can operate the motor vehicle, in particular the tandem axle 10, as a function of the received signal, in particular as a function of the rotational speed of the drive gearwheel 40 detected by means of the rotational speed sensor 88. It can be seen particularly well from FIG. 7 that the connection device 116 is arranged outside the receiving space 84 and thus outside the axle housing 80 and thus in a surrounding area 118 of the axle housing 80. In particular, the connection device 116 is arranged on an outer side 122 of the axle housing 80 facing the environment 118 and facing away from the receiving space 84 and from the outer circumferential surface 82 on the inner circumference.FIG. 10 shows a block diagram for illustrating a method for operating the tandem axle 10. In FIG. 10, blocks B 1, B 2, B 3, and B 4 illustrate different operating modes of the tandem axle 10. The operating mode B1 is a 6x4 traction mode in which the clutch 70 is in its coupled state and the clutch devices 66 and 68 are in their connected states. For example, depending on the driving situation, the clutch 70 is switched between a decoupling state and a coupling state, and for example the respective clutch device 66, 68 is switched between the connection state and the release state depending on the driving state. The operating mode B 1 is activated, for example, when an in particular excessive slip of the vehicle wheels 14 and 16 of the first axle 12 is detected. The operating mode B 2 is, for example, a 6x2 standby mode, which is set, for example, in all slip-critical driving situations. In the second operating mode B 2, the clutch 70 is in the decoupling state, while the clutch devices 66 and 68 are in their connection states. As illustrated in FIG. 10 by the arrows, starting from the operating mode B 2, it is optionally possible to switch to the operating mode B 1 or the operating mode B 3. The operating mode B3 is a 6x2 mode in which the clutch 70 is in its decoupling state, and in the operating mode B3 the clutch devices 66 and 68 are in their release states. The operating mode B 3 is switched, for example, into non-critical driving situations with regard to a respective slip of the vehicle wheels 14 and 16, for example when the motor vehicle is driven on a freeway.The operating mode B 4 is, for example, a synchronization mode in order to switch, for example, into the operating mode B 1 or the operating mode B 2. In the operating mode B 4, the clutch 70 is in its coupling state, while the clutch devices 66 and 68 are in their release states. For example, the operating mode B 3 is switched from the operating mode B 3 to the operating mode B 1. For example, the operating mode B 1 is switched to the operating mode B 2 and vice versa, and for example the operating mode B 2 is selectively switched to the operating mode B 1 or the operating mode B 3, wherein the operating mode B 1 is switched to the operating mode B 3 or the operating mode B 2, for example. For example, the operating mode B 3 is switched to the operating mode B 4, wherein for example the operating mode B 4 is selectively switched to the operating mode B 3 or the operating mode B 1. In order to switch, for example, from the operating mode B 3 into the operating mode B 2 or the operating mode B 1, switching is effected, for example, from the operating mode B 3 via the operating mode B 4 into the operating mode B 1 and, for example, optionally and if required, switching from the operating mode into the operating mode B 2.It can be seen from FIG. 4 that the axle housing 80 has at least one fastening element 120, by means of which the receiving element 92, in particular in a non-destructively detachable manner, is fastened in the axle housing 80.List of reference characters10 Tandem axle 12 First axle 14 Vehicle wheel 16 Vehicle wheel 18 Double arrow 20 Second axle 22 Vehicle wheel 24 Vehicle wheel 26 Double arrow 28 Double arrow 30 First differential gear 32 First drive shaft 34 Second differential gear 36 Second drive shaft 38 First drive gear 40 Second drive gear 42 Input gear 44 Input gear 46 Differential housing 48 Differential housing 50 Differential gear 52 Differential gear 54 Driven gear 56 Driven gear 58 Side shaft 60 Side shaft 62 Side shaft 64 Side shaft 66 Clutch device 68 Clutch device 70 Clutch 72 Third drive gear 74 Fourth drive gear 76 Input element 78 Output element 80 Axle housing 82 Inner circumferential surface 84 Receiving space 86 Gear rotation axis 88 Rotational speed sensor 90 Clutch element 92 Receiving element 94 Sensor element 96 Recess 98 Partial region 100 Double arrow 102 Toothing 104 Shaft stub 106 Bearing 108 Bearing 110 Spacer element 112 Structural unit 114 Double arrow 116 Connection device 118 Environment 120 Fastening element 122 Outer side

Claims

Tandem axle (10) for a motor vehicle, having: - a drivable, first axle (12), which has: ◯ a first differential gear (30), via which first vehicle wheels (14, 16) of the first axle (12) can be driven; and ◯ a first drive shaft (32), by means of which the first differential gear (30) and the first vehicle wheels (14, 16) can be driven via the first differential gear (30); - a drivable, second axle (20), which has: ◯ a second differential gear (34), via which second vehicle wheels (22, 24) of the second axle (20) can be driven; ◯ a drive gearwheel (40), by means of which the second differential gear (34) and the second vehicle wheels (22, 24) can be driven via the second differential gear (34); and ◯ a second drive shaft (36), by means of which the drive gearwheel (40) can be driven; and - a clutch (70) which can be switched over between: ◯ a coupling state in which the second drive shaft (36) is coupled to the first drive shaft (32) in a torque-transmitting manner by means of the clutch (70), as a result of which the second drive shaft (36) can be driven from the first drive shaft (32) via the clutch (70); and ◯ a decoupling state in which the second drive shaft (36) is decoupled from the first drive shaft (32); characterized in that: - the second axle (20) has an axle housing (80) in which the drive gearwheel (40), which can be rotated relative to the axle housing (80), is accommodated; - a rotational speed sensor (88) which is designed to detect a rotational speed of the drive gearwheel (40) and has: ◯ a receiving element (92) fastened to the axle housing (80) is provided; and ◯ a sensor element (94) which is connected to the drive gearwheel (40) in a rotationally fixed manner and the rotational speed of which can be detected by means of the receiving element (92), as a result of which the rotational speed of the drive gearwheel (40) can be detected by means of the rotational speed sensor (88); and - the drive gearwheel (40) has a toothing system (102) and a shaft stub (104) which is formed integrally with the toothing system (102) and on which the sensor element (94) is provided.Tandem axle (10) according to Claim 1, characterized in that the sensor element (94) is formed separately from the drive gearwheel (40) and is connected to the drive gearwheel (40) in a rotationally fixed manner.Tandem axle (10) according to Claim 1 or 2, characterized in that the sensor element (94) surrounds the shaft stub (104) at least partially in the circumferential direction (100) of the shaft stub (104).Tandem axle (10) according to one of the preceding claims, characterized in that: - a spacer element (110) formed separately from the drive gearwheel (40) is connected to the drive gearwheel (40) in a rotationally fixed manner, wherein two bearings (106, 108), by means of which the drive gearwheel (40) is rotatably mounted on the axle housing (80), are held at a distance from one another in the axial direction (114) of the drive gearwheel (40); and - the sensor element (94) formed separately from the drive gearwheel (40) and separately from the spacer element (110) is connected in a rotationally fixed manner to the spacer element (110) and is connected in a rotationally fixed manner to the drive gearwheel (40) via the spacer element (110).Tandem axle (10) according to Claim 4, characterized in that the sensor element (94) surrounds the spacer element (110) at least partially in the circumferential direction (100) of the drive gearwheel (40).Tandem axle (10) according to Claim 4 or 5, characterized in that the spacer element (110), which is formed separately from the drive gearwheel (40) and thereby separately from the shaft stub (104) and separately from the toothing (102), is connected to the shaft stub (104) in a rotationally fixed manner and completely surrounds the shaft stub (104) in the circumferential direction (100) of the shaft stub (104).Tandem axle (10) according to one of the preceding claims, characterized in that the rotational speed sensor (88) has a connecting device (116), by means of which the rotational speed sensor (88) can be electrically connected or is connected to at least one line, wherein the connecting device (116) is arranged outside the axle housing (80).Motor vehicle having a tandem axle (10) according to one of the preceding claims.Method for operating a motor vehicle having a tandem axle (10), in which the tandem axle (10) has: - a drivable first axle (12), which has: ◯ a first differential gear (30), via which first vehicle wheels (14, 16) of the first axle (12) can be driven; and ◯ a first drive shaft (32), by means of which the first differential gear (30) and the first vehicle wheels (14, 16) can be driven via the first differential gear (30); - a drivable second axle (20), which has: o a second differential gear (34), via which second vehicle wheels (22, 24) of the second axle (20) can be driven; ◯ a drive gearwheel (40), by means of which the second differential gear (34) and the second vehicle wheels (22, 24) can be driven via the second differential gear (34); ◯ a second drive shaft (36), by means of which the drive gearwheel (40) can be driven; and - a clutch (70), which can be switched over between: ◯ a coupling state, in which the second drive shaft (36) is coupled to the first drive shaft (32) in a torque-transmitting manner by means of the clutch (70), as a result of which the second drive shaft (36) can be driven from the first drive shaft (32) via the clutch (70); and ◯ a decoupling state, in which the second drive shaft (36) is decoupled from the first drive shaft (32); characterized in that: - the second axle (20) has an axle housing (80), in which the drive gearwheel (40), which can be rotated relative to the axle housing (80), is accommodated; a rotational speed sensor (88) is provided, by means of which a rotational speed of the drive gearwheel (40) is detected, wherein the rotational speed sensor (88) has: ◯ a receiving element (92) fastened to the axle housing (80); and ◯ a sensor element (94), which is connected to the drive gearwheel (40) in a rotationally fixed manner and the rotational speed of which is detected by means of the receiving element (92), as a result of which the rotational speed of the drive gearwheel (40) is detected by means of the rotational speed sensor (88); - the drive gearwheel (40) has a toothing system (102) and a shaft stub (104), which is formed integrally with the toothing system (102) and on which the sensor element (94) is provided; and - the motor vehicle is operated as a function of the rotational speed of the drive gearwheel (40) detected by means of the rotational speed sensor (88).

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