Power tool

Abstract

The present specification relates to a power tool including a two-speed power transmission. A two speed powertrain includes a planetary gear set and a load responsive shift mechanism for directing torque through the planetary gear set in a high torque / low speed drive mode or across the planetary gear set in a low torque / high speed drive mode, wherein the planetary gear set is a two-stage gear mechanism comprising a first and a second planetary gear stage, where the first planetary gear stage comprises a first sun gear, where the second planetary gear stage comprises a second sun gear, where the second sun gear is a hollow sun gear, and wherein the input shaft is arranged to extend through the hollow sun gear allowing bypassing the second planetary gear stage in the low torque / high speed drive mode.

Description

Technical Field

[0001] The present invention generally relates to power tools, and more particularly to a load-responsive transmission device for a power tool. Background Art

[0002] Different types of power tools are known for use in various industries, one common type of which is a power tool for tightening screws or bolts.

[0003] One problem known to give rise to various design challenges in this field is that the working conditions and the requirements for the expected output often vary greatly during use, for example with regard to the speed and torque required for different parts of a typical working operation. This is the case, for example, for the tightening tools mentioned above, since during the tightening of a screw or nut, during the initial phase of tightening, the so-called screw-in, the required torque is low, while ideally the rotation speed should be high in order to reduce the time required for the operation, whereas during the actual tightening phase, i.e. during the actual tightening of the joint, the required torque is higher.

[0004] In order to be able to provide, for example, the desired torque levels, it is known that power tools subject to these types of varying requirements include different types of transmissions. However, a disadvantage of using transmissions to provide these higher torques is that the rotational speed provided is correspondingly reduced, resulting in an undesirably low rotational speed during screwing in. This becomes a significant problem, in particular in the field of power tools suitable for tightening large screws to very high torque values, where providing the desired high torque often results in very low RPMs, which in turn often makes the screwing-in phase unreasonably slow.

[0005] Therefore, in order to alleviate some of these problems, attempts have been made to use two-speed power transmissions which include, for example, torque-responsive shift mechanisms, i.e. transmissions in which the force flowing through the gear unit, and therefore the gear ratio that can be provided, depends on the torque level, so that higher rotational speeds can be used during screw-in and the high torque / low speed mode is only used when required.

[0006] However, there are still a number of problems associated with such transmissions. This includes problems associated with heat generation, which can be considerable, particularly at the end of tightening when the torque increases and the motor current can become very high. This excessive heat generation is not only undesirable for the motor, but also for the electronics and batteries used. Therefore, there is a need for improvement in the field of torque-responsive gear units for power tools. Utility Model Content

[0007] Therefore, it is desirable to provide an improved two-speed gear unit or power transmission for a power tool. In particular, it is desirable to provide a transmission that at least reduces heat generation. In order to better address one or more of these problems, a power tool and power transmission including a two-speed power transmission are provided.

[0008] According to a first aspect of the utility model, a power tool is provided, the tool comprising a housing, a motor, an input shaft connected to the motor, an output shaft and a two-speed power transmission device. The two-speed power transmission device comprises a planetary gear set and a load-responsive shifting mechanism, the load-responsive shifting mechanism being used to guide torque through the planetary gear set in a high torque / low speed drive mode or to cross the planetary gear set in a low torque / high speed drive mode. The planetary gear set is a two-stage gear mechanism comprising a first and a second planetary gear stage, wherein the first planetary gear stage comprises a first sun gear connected to the input shaft, a first planetary gear carrier and at least one first planetary gear meshed with the first sun gear and meshed with an inner ring fixed in the housing, wherein the second planetary gear stage comprises a second sun gear connected to the first planetary carrier, a second planetary gear carrier and at least one second planetary gear meshed with the second sun gear and meshed with an inner ring fixed in the housing, wherein the second sun gear is a hollow sun gear, and wherein the input shaft is arranged to extend through the hollow sun gear, thereby allowing the second planetary gear stage to be bypassed in the low torque / high speed drive mode.

[0009] According to a first aspect, a two-speed transmission and therefore a power tool provides an inventive solution to the above problems by means of a design incorporating a load-responsive shift mechanism that automatically directs torque through the planetary gears in a high torque / low speed drive mode or across the planetary gears in a low torque / high speed drive mode depending on the torque level, wherein the above problems associated with heat generation are mitigated by means of a two-stage gear mechanism comprising a first and a second planetary stage, wherein the second stage comprises a hollow sun gear through which the input shaft extends. When the gear unit is shifted on two stages with a combined higher gear ratio, the motor torque at the end of tightening is correspondingly reduced, which in turn reduces heat losses, since heat losses are proportional to the square of the current level.

[0010] Those skilled in the art will appreciate that a two-stage planetary gear mechanism works equally well with any type of load-responsive mechanism for directing torque through a planetary gear set in a high torque / low speed drive mode or across a planetary gear set in a low torque / high speed drive mode, both manually controlled shift mechanisms and automatic mechanisms including mechanical actuation mechanisms and mechanisms actuated by actuators based on sensor data, etc.

[0011] According to one embodiment, the second sun gear and the first planet carrier are integrally formed. In such an embodiment, the second sun gear may be described as forming part of the first planet carrier and may, for example, be formed in and extend from a middle portion of the carrier. According to one embodiment, the first sun gear forms part of the input shaft.

[0012] Furthermore, since the first and second planetary gears mesh with the same internal gear ring, the number of teeth on the corresponding sun gear will play a decisive role in the gear ratio. For example, according to one embodiment, the gear ratio of the first gear stage is greater than or equal to 5. In one embodiment, the gear ratio is in the interval 5.5-6.5. Furthermore, according to one embodiment, the gear ratio of the second gear stage is in the range 2-5, preferably in the range 3-4. In one embodiment, the gear ratio is in the range 2.75-3.25.

[0013] The term load-responsive is to be understood as a transmission whose operating mode depends on the load under which the transmission is operated. For example, according to one embodiment, the load-responsive shift mechanism is a speed-responsive shift mechanism. In one embodiment, such a speed-responsive mechanism may include a centrifugal coupling or clutch. According to one embodiment, the load-responsive shift mechanism is a torque-responsive gear mechanism.

[0014] According to one embodiment, the torque-responsive transmission is a first-stage gear unit. That is, in such an embodiment, the input shaft is directly connected to the motor, or even arranged in some cases in the motor module of the tool. This is particularly advantageous because a more compact and slim design of the tool can be achieved.

[0015] According to one embodiment, the power tool further comprises a front gear unit or a second stage gear unit. In such an embodiment, the torque responsive transmission can be arranged between the motor and the front (or second stage) gear unit. This is advantageous because the front gear unit for such a tool can be a replaceable front gear unit, so that the front gear unit can be changed according to the desired torque, while the torque responsive transmission remains in or on the tool.

[0016] Regarding the power tool itself, according to one embodiment, the motor is an electric motor. The tool can be, for example, a handheld power tool selected from the group including a screwdriver, a nut runner, a drill and a grinder. However, those skilled in the art will appreciate that use with a stationary or fixed tool requires only slight modifications to the structure. In some embodiments, the power tool can be a battery-powered tool. In one embodiment, the power tool is a tool that provides a higher tightening torque, for example in the range 3500-4500Nm, in some cases up to 8000Nm.

[0017] According to one embodiment, the power tool further includes a device for monitoring the amount of torque transmitted by the tool and a control unit that is operated to control the rotational speed of the motor based on the monitored amount of the indicated torque. In one embodiment, the device for monitoring the amount of torque transmitted by the tool may include at least one sensor for sensing the amount of torque transmitted. For example, the inner ring gear can be fixed in the housing by means of a torque sensor so that a reading of the transmitted torque can be obtained. In addition, the device for monitoring the amount of indicated torque may include a circuit (i.e., a circuit arrangement) suitable for monitoring the motor current or other internally provided data related to the motor performance.

[0018] Thus, the control unit may be configured to control the tool by comparing the value of the quantity obtained from the sensor with a predetermined threshold value and to reduce the rotation speed of the motor when the measured value approaches the threshold value.Thereby, changing between drive modes of the transmission may be facilitated.

[0019] For example, in the low torque / high speed drive mode, the sensed amount can be a motor current indicating torque, so that when the current approaches a threshold value and therefore when the torque increases in this case, the rotational speed of the motor can be reduced, so that it can be beneficial to switch to the high torque / low speed drive mode. Similarly, in the high torque / low speed drive mode, the sensed amount can be a torque value from a torque sensor, so that when the torque value approaches a threshold value and therefore when the torque decreases in this case, the rotational speed of the motor can be reduced, so that it can be beneficial to switch back to the low torque / high speed drive mode. Therefore, at least one sensor can also be a sensor or torque sensor (i.e., a torque sensor) for sensing the motor current, or generally even an internal feature of a control unit that monitors current or other motor parameters. In one embodiment, the torque sensor is activated only in the high torque / low speed drive mode. The activation of such a sensor can be used as an indicator of the control unit, indicating that the transmission has been switched to the high torque / low speed mode.

[0020] According to one embodiment, the shift mechanism comprises a driving member, a driven member and a plurality of engagement elements, wherein the driving member is connected to the sun gear of the first planetary gear stage, and the driven member is connected to the output shaft; the plurality of engagement elements are arranged to engage the driving member and the driven member with each other in a first position and to engage the second planetary gear carrier and the driven member with each other in a second position. The driven member comprises a plurality of axially extending grooves, the grooves being arranged to support the engagement elements for axial displacement of the engagement elements between the first position and the second position, and the driving member comprises an axially acting first cam device for cooperating with the engagement element in the first position of the engagement element. A first axially acting spring device is arranged to bias the engagement element towards a first position of the engagement element, whereby the action of the first spring device counteracts the axial force generated by the first cam device on the engagement element, so that the engagement element is retained in the first position at torque values ​​below a predetermined level, but is displaced out of the first position by the first cam device at torque values ​​above the predetermined level, and the driven member further comprises a second axially acting cam device, the second axially acting cam device being arranged to exert an axial displacement force on the engagement element towards the second position of the engagement element, overcoming the biasing action of the spring device, as the engagement element has left the first position at torque values ​​above the predetermined level. A second planetary wheel carrier is engaged to an engagement sleeve, the engagement sleeve providing radial support for the engagement element in the second position of the engagement element and being provided with a plurality of axially extending tracks for cooperating with the engagement element in the second position of the engagement element, and the drive member comprises a plurality of recesses, wherein each of the first axially acting cam elements respectively forms part of one of the recesses, and wherein each of the recesses is adapted to accommodate and radially support one of the engagement elements in the first position of the engagement element.

[0021] One advantage of this type of transmission is that, because the drive member of the transmission of the present invention includes a plurality of recesses which are suitable for accommodating and radially supporting the coupling element in a high speed / low torque mode, the coupling element is effectively isolated (i.e., disengaged) from components rotating at different speeds without providing additional components.

[0022] More specifically, in some embodiments, the engagement sleeve may provide radial support for the engagement element only in the second position of the engagement element, i.e., such that each of the engagement elements is isolated and rotationally disengaged from the engagement sleeve in the high speed / low torque mode. In one embodiment, the axially acting cam means may be provided in an axially extending slot.

[0023] According to one embodiment, the engagement sleeve is arranged to be axially movable. Thereby, a slight clearance can be achieved, which in turn can be used to handle possible angular misalignments of the engagement element and the engagement sleeve. In particular, when the engagement element is moved from the first position to the second position, any misalignment between the engagement element and the axially extending groove in the driven member can be handled by the axially movable sleeve, rather than causing a jam between the components.

[0024] According to one embodiment, the two-speed power transmission device further comprises a second axially acting spring means for biasing the axially movable engagement sleeve against the engagement element. Thereby, misalignment can be handled more effectively as the engagement sleeve is slightly pushed against the engagement element, thereby facilitating return into the recess by a slight force acting on the engagement element in the correct direction. The engagement sleeve normally remains in a default position, i.e. adjacent to the drive member, but if the engagement element becomes misaligned, the engagement sleeve can be sprung or slightly moved in an axial direction under the bias of the spring, thereby allowing the engagement element to slide into the correct position.

[0025] According to one embodiment, the second planet wheel carrier comprises an outer sleeve, which extends in the axial direction and is rotationally locked to the engagement sleeve to engage the second planet wheel carrier and the driven member with each other in the second position. In one embodiment, the outer sleeve is rotationally locked to the engagement sleeve by means of a plurality of balls or other rolling elements arranged in a track, the track being formed on the outside of the engagement sleeve.

[0026] Therefore, in the low speed / high torque mode, the torque will be directed to the driven member via the planet carrier, the transfer balls, the engagement sleeve and further via the engagement element. In addition, in some embodiments, the outer sleeve can be coaxially arranged with the driven member and the driving member. In one embodiment, there is an axial overlap between the outer sleeve and the driving member, the portion where the recess is arranged and / or at least a portion of the engagement sleeve.

[0027] According to one embodiment, a first axial bearing is provided to support the drive element on the second planetary carrier and a second axial bearing is provided to support the outer sleeve on the housing, so that the force from the first spring device acting on the engagement element can be absorbed into the tool housing via the first and second bearings. Thereby, an advantageous disengagement between the spring (i.e. the engagement element) and the motor shaft is achieved, thereby protecting the motor shaft (i.e. the input shaft) from the axial force applied by the spring. Alternatively, this force can be absorbed by the tool housing. Further advantages include providing a design with a shorter tolerance chain (i.e. less accumulation of tolerances).

[0028] According to one embodiment, the drive member comprises an axial flange, which is arranged as a radial support engagement element. This is an advantageous design, since improved radial support and rotational disengagement are provided. In one embodiment, the sum of the axial extent of the flange and the depth of the recess is greater than the radius of the ball. Thus, sufficient radial support and complete radial disengagement between the ball and components rotating at different rotational speeds are ensured in high speed mode.

[0029] In one embodiment, the first spring means comprises a first coil spring arranged in a coaxial relationship with the driven member and exerting a biasing force on the ball. This is advantageous because, for example, a more compact design can be achieved. In one embodiment, the first coil spring is supported directly on the engagement element. In another embodiment, the first coil spring is supported on the engagement element via a contact element (examples include flat ring elements) that is in continuous contact with the engagement element. In addition, in some embodiments, a device for changing the setting of the biasing force exerted by the spring can be provided.

[0030] According to a second aspect of the present invention, there is provided a power transmission device for a power tool according to any one of the above embodiments.

[0031] According to a third aspect of the utility model, a method for controlling a power tool including a gear unit according to any one of the above-mentioned embodiments is provided. According to one embodiment, the method comprises the following steps: monitoring a quantity indicating a first torque value, comparing the first torque value with a predetermined threshold torque value, and controlling the rotational speed of the motor when the measured value approaches the threshold. According to one embodiment, the step of controlling the rotational speed of the motor comprises: reducing the rotational speed of the motor when the measured value approaches the threshold. According to one embodiment, the monitored quantity is at least one of a motor current and a torque sensor value.

[0032] The objects, advantages and features of the methods conceivable within the scope of the second and third aspects of the present invention will be readily understood by reference to the foregoing discussion of the second aspect of the present invention.

[0033] Further objects, features and advantages of the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art appreciate that different features of the present invention may be combined to create embodiments other than those described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be described in the following illustrative and non-limiting detailed description of exemplary embodiments with reference to the accompanying drawings, in which

[0035] Figure 1ais a cross-sectional view of a portion of an exemplary power tool including a dual-speed power transmission for a power tool according to one embodiment, shown in a low torque / high speed drive mode.

[0036] Figure 1b is a cross-sectional view of a portion of the same exemplary power tool including a dual-speed power transmission for a power tool according to one embodiment, shown in a high torque / low speed mode.

[0037] All the figures are schematic, not necessarily to scale and generally show only parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested. DETAILED DESCRIPTION

[0038] Figure 1a 1 is a cross-sectional view of a portion of an exemplary power tool, in this case a handheld battery-powered tool, according to one embodiment. The tool includes a housing 10, an input shaft 20, a motor (not shown) connected to the input shaft, an output shaft 15, and a two-speed transmission arranged between the input shaft and the output shaft.

[0039] Figure 1a The two-speed power transmission 1 of the illustrated embodiment includes a planetary gear set 18 and a torque-responsive shift mechanism 19 for directing torque from an input shaft 20 (i.e., from the motor) through the planetary gears 18 to the output shaft 15 in a high torque / low speed drive mode or over the planetary gears 18 in a low torque / high speed drive mode. Figure 1a Shown in low torque / high speed drive mode.

[0040] The planetary gear set 18 of the illustrated embodiment is a torque-responsive two-stage gear mechanism that includes a first planetary gear stage 28 and a second planetary gear stage 38 .

[0041] The first planetary gear stage 28 comprises a first sun gear 281 connected to the input shaft 20 and, in the embodiment shown, three planet gears engaged with the sun gear 20 and a first planet gear carrier 282 meshing with an internal gear 22 fixed in the housing 10 .

[0042] On the other hand, the second planetary gear stage 38 includes a second sun gear 381 integrally formed with the first planet carrier 282, and the planetary gears engaged with the second sun gear and the second planet carrier 382 mesh with the inner ring gear 22. In the illustrated embodiment, the second sun gear 381 and the first planet carrier 282 are integrally formed, as shown in Figure 1a As can be seen in FIG. 2 , the second sun gear portion extends in the forward direction in the middle of the carrier 282. In addition, the first sun gear 281 forms a part of the input shaft 20.

[0043] As from Figure 1a It can be seen that the second sun gear 381 is more specifically a hollow sun gear, which is designed so that the input shaft 20 can extend through the hollow sun gear. Thus, the planetary gears can be bypassed in the low torque / high speed driving mode.

[0044] In the high torque / low speed drive mode, the torque is directed to the first and second gear stages, thus achieving a larger combined gear ratio - i.e. shifting gears on two planetary stages (simultaneously). In the exemplary embodiment shown, the gear ratio of the first gear stage is 6, while the gear ratio of the second stage is 3.25.

[0045] Figure 1a An exemplary power tool includes a torque-responsive gear mechanism in which the coupling elements (three balls 30 in the illustrated embodiment) are arranged to engage the driving member 26 and the driven member 27 with each other in a first position (i.e., the low torque / high speed drive mode referred to above) and are arranged to engage the second planetary carrier 382 and the driven member 27 with each other in a second position (i.e., the high torque / low speed drive mode referred to above).

[0046] Furthermore, the driven member 27 comprises a plurality of axially extending grooves 36 arranged to support the balls 30 for axial displacement between the first position and the second position, while the drive member 26 comprises an axially acting first cam device arranged in equally spaced recesses 39 for cooperating with the engagement element 30 in its first position.

[0047] A first axially acting helical spring 31 is arranged coaxially with respect to the driven member for biasing the ball 30 towards the first position, whereby the action of the helical spring 31 counteracts the axial force generated by the first cam device on the ball 30. As a result, the ball 30 is held in the first position at torque values ​​below a predetermined level, but is forced out of the first position by the first cam device at torque values ​​above a predetermined level. In the embodiment shown, the spring 31 bears directly on the ball 30.

[0048] The driven member 27 in turn includes a second axially acting cam device 36b, which is arranged to: as the ball 30 has left the first position at a torque value above a predetermined level, overcome the biasing action of the coil spring 31 and apply an axial displacement force on the ball 30 toward the second position of the ball 30.

[0049] Furthermore, in the embodiment shown, the second planetary wheel carrier 382 is engaged to an axially movable engagement sleeve 29, which provides radial support for the balls 30 in the second position. Thus, a plurality of axially extending tracks for cooperating with the balls 30 are arranged on the inner surface of the engagement sleeve 29. In the embodiment shown, the number of tracks of the engagement sleeve 29 is twice the number of the balls 30, i.e. six in the embodiment shown. Furthermore, in the embodiment shown, the engagement sleeve 29 is biased against the balls 30 by a second axially acting helical spring 40, which is arranged coaxially relative to the driven member 27 and the first helical spring 31.

[0050] In order to engage the second planet carrier 382 and the driven member 27 with each other in the second position, the planet carrier 382 comprises an outer sleeve 32. The sleeve 32 extends in the axial direction and is rotationally locked to the engagement sleeve 29 by means of a plurality of smaller balls 32a. Furthermore, in order to handle the force from the first coil spring 31 acting on the balls 30, a first axial bearing 33 is provided to support the drive element 26 on the second planet carrier 382, ​​and a second axial bearing 34 is provided to support the outer sleeve 32 on the housing 10 so that the force can be absorbed by the housing.

[0051] In operation, the input shaft 20 is connected to the electric motor and the output shaft 15 is engaged via a nut seat to a screw joint to be tightened. The function of the transmission and therefore the function of the power tool is achieved by a transmission that selectively provides a connection between the drive member 26 and the driven member 27 (bypassing or via a two-stage planetary gear mechanism depending on the torque level).

[0052] When the tightening operation starts, the motor starts to transmit torque through the transmission. In the first stage, when the shift mechanism 19 occupies the high speed / low torque drive mode, the ball 30 is seated in the recess 39 of the drive member 26, and the torque transmitted to the drive member 26 via the input shaft 20 is transmitted to the driven member 27 via the recess, the ball 30 and the groove 36, that is, directly from the drive member 26 to the driven member 27 by the input shaft 20 extending through the hollow sun gear 381, bypassing the two-stage gear set and therefore not being affected by it.

[0053] As the torque resistance in the screw joint increases, the first axially acting cam element exerts an increasing axial force on the ball 30, and when a predetermined torque level is reached, this force replaces the biasing force of the spring 31 and the ball 30 will begin to move axially through the slot 36, and eventually the cam means 36b will also exert an auxiliary axial force on the ball 30, again eventually replacing the force of the spring 31, thereby allowing the ball 30 to complete its axial movement and occupy its second position. Examples of such cam means 36b include inclined sides or diverging portions of the corresponding slot 36.

[0054] The shift mechanism 19 has now put the transmission into its high torque / low speed drive mode - ie torque is now directed to the two-speed transmission. Figure 1a The same implementation is in Figure 1b 36b exceeds the biasing force of the spring 31. When the torque has been reduced to this level, i.e. when the predetermined drive mode switching point is reached, the force applied by the cam device 36b will no longer exceed the spring force and the balls 30 are displaced back to their first position.

[0055] In order to facilitate this mutual engagement, and more particularly to facilitate gear shifting, the power tool as described above comprises a sensor (not shown), in this case a torque sensor. A control unit consisting of the tool or a separate unit is operative to receive sensed data from the torque sensor and to control the rotational speed of the motor accordingly. More specifically, as the measured torque value approaches a predetermined threshold torque value (i.e., the value at which a gear shift will occur), the control unit reduces the rotational speed of the motor.

[0056] As in the illustrated embodiment, the torque sensor is arranged between the housing 10 and the inner ring gear 22, and therefore, the sensor is active (i.e., gives meaningful readings) only in the second drive mode (i.e., high torque / low speed drive mode) when the torque is actually directed to the inner ring gear, and therefore, the above-mentioned process of controlling the speed using data from the sensor is only meaningful when determining when to switch from the high torque / low speed drive mode to the low torque / high speed drive mode.

[0057] When the transmission is operating in the low torque / high speed drive mode, the control unit instead monitors the motor current by means of a suitable circuit arrangement (not shown) in order to determine that the torque is approaching a threshold value and that the rotational speed should therefore be reduced to facilitate the gear shift. As an additional function, the intention of the torque sensor to start transmitting torque data can be used by the control unit to confirm that the transmission has switched to the high torque / low speed drive mode and is operating in the high torque / low speed drive mode.

[0058] Although the utility model has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description should be considered illustrative or exemplary rather than restrictive; the utility model is not limited to the disclosed embodiments. It is understood by those skilled in the art that many modifications, changes and variations can be imagined within the scope defined by the appended claims. In addition, those skilled in the art can understand and implement variations of the disclosed embodiments when practicing the claimed utility model based on a study of the drawings, the specification and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "one" or "an" does not exclude pluralities. The fact that certain measures are recorded only in mutually different dependent claims does not indicate that the combination of these measures cannot be used to benefit. Any figure mark in the claims should not be interpreted as limiting the scope of the claims.

Claims

1. A power tool comprising: Housing (10); motor; Input shaft (20); The output shaft (15) and Two-speed power transmission, The dual-speed power transmission device comprises a planetary gear set (18) and a load-responsive shift mechanism (19), wherein the load-responsive shift mechanism (19) is used to direct torque through the planetary gear set (18) in a high-torque / low-speed drive mode or to pass over the planetary gear set (18) in a low-torque / high-speed drive mode; The planetary gear set is a two-stage gear mechanism including a first planetary gear stage (28) and a second planetary gear stage (38); The first planetary gear stage (28) comprises a first sun gear (281) connected to the input shaft (20), a first planetary gear carrier (282) and at least one first planetary gear meshing with the first sun gear and meshing with an inner gear ring (22) fixed in the housing (10). The second planetary gear stage (38) comprises a second sun gear (381) connected to the first planet carrier, a second planet carrier (382), and at least one second planet gear meshing with the second sun gear (381) and meshing with the inner ring gear (22) fixed in the housing (10); The second sun gear is a hollow sun gear, the input shaft being arranged to extend through the hollow sun gear so as to allow the second planetary gear stage to be bypassed in the low torque / high speed drive mode; The shift mechanism (19) comprises a driving member (26), a driven member (27) and a plurality of engaging elements (30), wherein the driving member (26) is connected to the sun gear (281) of the first planetary gear stage (28); the driven member (27) is connected to the output shaft (15); and the plurality of engaging elements (30) are arranged to engage the driving member (26) and the driven member (27) with each other in a first position and to engage the second planetary wheel carrier (382) and the driven member (27) with each other in a second position.

2. The power tool according to claim 1, characterized in that: The second sun gear and the first planet carrier are formed integrally.

3. The power tool according to claim 1, characterized in that: The first sun gear forms a part of the input shaft.

4. The power tool according to claim 1, characterized in that: The gear ratio of the first gear stage is greater than or equal to 5.

5. The power tool according to claim 1, characterized in that: The gear ratio of the second gear stage is in the range 2-5.

6. The power tool according to claim 1, characterized in that: The load-responsive shift mechanism is a speed-responsive shift mechanism.

7. The power tool according to claim 1, characterized in that: The load-responsive shift mechanism is a torque-responsive shift mechanism.

8. The power tool according to claim 1, It is characterized in that The driven member (27) comprises a plurality of axially extending slots (36) arranged to support the engagement element (30), the slots (36) being used for axial displacement of the engagement element (30) between the first position and the second position, The drive member (26) comprises first axially acting cam means for cooperating with the engagement element (30) in the first position of the engagement element (30); a first axially acting spring device (31) arranged to bias the engagement element (30) towards the first position of the engagement element (30), whereby the action of the first spring device (31) counteracts the axial force generated by the first cam device on the engagement element (30), so that the engagement element (30) is maintained in the first position at torque values ​​below a predetermined level, but is forced out of the first position by the first cam device at torque values ​​above the predetermined level; and The driven member (27) further comprises a second axially acting cam device (36b) arranged to exert an axial displacement force on the engagement element (30) towards the second position of the engagement element (30) against the biasing action of the spring device (31) as the engagement element (30) has left the first position at a torque value above the predetermined level; wherein the second planetary wheel carrier (382) is engaged to an engagement sleeve (29), the engagement sleeve (29) providing radial support for the engagement element (30) in the second position of the engagement element (30) and being provided with a plurality of axially extending tracks for cooperating with the engagement element (30) in the second position of the engagement element (30), and wherein the drive member comprises a plurality of recesses (39), wherein each of the first axially acting cam elements respectively forms part of one of the recesses, and wherein each of the recesses is adapted to accommodate and radially support one of the engagement elements (30) in the first position of the engagement element (30).

9. The power tool according to claim 8, characterized in that: The engagement sleeve is arranged to be axially movable.

10. The power tool according to claim 9, characterized in that: Further included is a second axially acting spring means (40) for biasing the axially movable engagement sleeve (29) against the engagement element (30).

11. The power tool according to any one of the preceding claims 8 to 10, characterized in that The second planet wheel carrier (382) comprises an outer sleeve (32) which extends in the axial direction and is rotationally locked to the engagement sleeve (29) to engage the second planet wheel carrier (382) and the driven member (27) with each other in the second position.

12. The power tool according to claim 11, characterized in that: A first axial bearing (33) is provided to support the drive element on the second planetary carrier, and a second axial bearing (34) is provided to support the outer sleeve on the housing (10), so that the force from the first spring device acting on the engagement element can be absorbed into the housing via the first axial bearing and the second axial bearing.

13. The power tool according to claim 8, characterized in that: The drive member (26) comprises an axial flange arranged to radially support the engagement element.

14. The power tool according to claim 1, characterized in that: Further included is a device for monitoring a quantity indicative of torque transmitted by the tool and a control unit operative to control the rotational speed of the motor based on the sensed quantity indicative of said torque.

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