Linear electromechanical actuator

Abstract

The present invention relates to a linear electromechanical actuator (1) for a welding gun for resistance spot welding of parts, the actuator comprising a housing consisting of two parts (3, 4), an electric motor (5), and an inverted roller-screw gear comprising a rod to be connected to an electrode (2) of the welding gun for converting rotational movement of the electric motor (5) into translational movement of the rod The linear electromechanical actuator (1) further comprises a sensor (15) for measuring a force of pressing the electrode (2) against a surface of the part to be welded, the force being transmitted from the electrode (2) through the inverted roller-screw gear; and a bevel gear drive for transmitting rotation of the electric motor (5) to the inverted roller-screw gear, the bevel gear drive comprising a driving gear and a driven gear, wherein an axis of the inverted roller-screw gear coincides with an axis of the rod and is arranged at an angle to an axis of the electric motor (5); and wherein the inverted roller-screw gear further comprises a nut configured for longitudinal movement with respect to the driven gear of the bevel gear drive and forming a sliding connection with the driven gear configured for transmitting a torque from the driven gear to the nut. The electromechanical actuator provides versatility, speed and high performance.

Description

[0001] LINEAR ELECTROMECHANICAL ACTUATOR

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to linear electromechanical actuators, in particular to linear electromechanical actuators for a welding gun for performing resistance spot welding.

[0004] BACKGROUND OF THE INVENTION

[0005] Linear electromechanical actuators are actively used in industry, e.g., as a part of a welding gun used to perform resistance spot welding of body parts in car manufacturing.

[0006] A C-type welding gun typically comprises a mechanism containing a frame, a stationary lever attached to the frame (hereinafter referred to as the stationary lever), a stationary electrode attached to the lever, and a movable electrode, wherein parts to welded are clamped between said electrodes during resistance spot welding. The welding gun further comprises a welding transformer with welding current supplied to said electrodes therethrough.

[0007] The movable electrode is driven by a linear electromechanical actuator, and the stationary electrode is usually fixed to the stationary lever and its position with respect to the welding gun frame remains unchanged. The positioning of the movable electrode provided by the linear electromechanical actuator allows to create the necessary force to press the movable electrode and the stationary electrode against the parts to be welded to perform the welding process.

[0008] The welding gun is mounted on a movable arm of a robot, which is an industrial manipulator controlled by a robot controller. Prior to welding, the robot brings the welding gun to the welding site such that the stationary electrode contacts one of the parts, then the movable electrode is positioned with respect to the parts to be welded at maximum speed, after which the speed of movement decreases until the electrode contacts the part to be welded. When the movable electrode contacts one of the parts, the welding gun is compressed between the part electrodes.

[0009] Electrical current is passed through the electrodes and caps at their ends contacting the parts causing local heating, melting of metal in the part compression area and the formation of a welding core. After stopping the supply of welding current, and cooling and hardening of the metal in the welding core, the parts are connected, and the welding gun opens. US9520756B2 discloses a linear electromechanical actuator with an inverted roller-screw gear (RSG) converting rotation of the electric motor into translational movement of a rod, however, the linear electromechanical actuator disclosed in US9520756B2 has large dimensions along the movement axis of the RSG rod, since the electric motor and the RSG of the linear electromechanical actuator are arranged in series, and therefore it can be used only to move one of the electrodes of a C-type welding gun.

[0010] However, for more accurate positioning of the electrodes and for creating equal force on both sides of the parts using a C-type welding gun, the electrode arranged on the stationary lever must be movable along a given trajectory as well. For executing more complex and accurate welding algorithms, it is further necessary to measure the force directly on the electrode arranged on the stationary lever. Typically, a welding gun has sufficient space for accommodating only one linear electromechanical actuator similar to the one disclosed in US9520756B2, and the space for arranging a second actuator is extremely limited or even absent, which does not allow for proper positioning of a second actuator to ensure movement of an electrode arranged on a stationary lever.

[0011] JP3319086B2 discloses a welding gun design comprising two electrodes, wherein each of the electrodes is moved by a separate linear electromechanical actuator. A stationary electrode is attached to a movable lever positioned by means of a second electromechanical actuator. Both electromechanical actuators are arranged parallel with respect to the welding gun frame, and the second electromechanical actuator is offset from the attachment point of the welding gun frame towards the robot base by a distance corresponding to the length of the lever part arranged perpendicular to the axis of the electromechanical actuator.

[0012] In this arrangement, the use of a second linear electromechanical actuator provides faster movement of the lever having the stationary electrode prior to the contact with the part to be welded compared to an electrode lead-in performed by a robot.

[0013] However, said structural solution leads to a significant increase in size and weight of the welding gun, while due to the large lever arm with the stationary electrode, the pressing force measurements would be less accurate, and the determination of the contact between the part to be welded and the stationary electrode would be initiated with high pressing force values, thus making said functionality ineffective. The high weight of the movable part with the electrode further limits acceleration that can be developed and, therefore, the maximum speed that the movable part with the electrode can reach. To increase acceleration and speed, it would be necessary to increase motor size of the linear electromechanical actuator for the second electrode that would lead to a further increase in weight. In addition, the welding gun weight in this case would increase, leading to a robot of greater carrying capacity being required to move such a welding gun. Robots usually comprise models with different carrying capacities, e.g., 165 kg, 210 kg, 240 kg. Moreover, the center of mass of a load in the form of a welding gun should be located at a certain distance from the movable link of the robot. Thus, if the welding gun with a movable part for the second electrode according to JP3319086B2 exceeds the permissible weight threshold when high pressing and acceleration forces are required, it would be necessary to provide a robot of greater carrying capacity in the welding cell leading to an increase in cost of such a welding cell.

[0014] In addition to the above structural disadvantages, when said arrangement of the second electromechanical actuator is used, the actuator is susceptible to damage in case of collision between the welding gun and obstacles during robot training or setup, as the second electromechanical actuator is offset from the attachment point of the welding gun towards the base of the movable link of the robot. In this case, the increase in dimensions with respect to the attachment point makes it difficult to predict movement trajectory. For instance, the operator can set the robot to move along a path containing obstacles in the form of tooling for the parts or other welding guns. In this case, the impact may fall on the frame portion being moved by the linear electromechanical actuator, thus leading to its breakage.

[0015] FR2872073 Al discloses a sensor for measuring pressing force on an electrode arranged on a stationary lever, the design partially solving the problem of controlling the pressing force between the part being welded and the electrode arranged on the stationary lever. In this technical solution, pressing force values are measured indirectly and are calculated based on measured values of stationary lever deformations. Generally, a strain gauge bridge or a piezoelectric element is used for measuring the pressing force in such manner. The manufacturing of such sensors is a very labor-intensive and expensive process and requires the use of high-tech equipment allowing to implement advanced technologies for manufacturing of high-precision measuring systems. Furthermore, such sensors are highly susceptible to electromagnetic influences caused by systems located in close proximity in operation, e.g., by a transformer or electric motors included in linear electromechanical actuators. This influence manifests itself especially during welding, when a high-intensity magnetic field is generated, rendering the operation of such sensors ineffective. During welding, noise is generated on the measurement path of the sensor due to magnetic influence, the amplitude of which is overly higher than the amplitudes of the values being measured. In particular, FR2872073 Al discloses the design of an X-type welding gun. In this design, when opening or closing the gun, the electrodes move along a circular trajectory unlike the C-type welding gun, wherein the electrodes move linearly. The X-type welding gun is generally implemented in two versions.

[0016] In the first version, two actuators are provided, wherein levers with electrodes are both movable and can move with respect to a common point. In this embodiment, for instance, two linear electromechanical actuators disclosed in the aforementioned US9520756B2 can be used allowing for direct measurements of the pressing force of both electrodes to the parts to be welded. However, such welding gun has a high weight and requires the use of a robot of greater carrying capacity in the welding cell.

[0017] In the second version, the first electrode is mounted on a stationary lever, and the second electrode is mounted on a movable lever being positioned by one electromechanical actuator, wherein the electrode moves along the circumference with respect to the attachment point of the lever on the frame. In the second version, the disadvantages described for the C-type gun in the form of size and weight limitations for placing the electrode actuator on the stationary lever are still evident. Pressing force measurement can be provided by a sensor for indirect measuring pressing force on an electrode arranged on a stationary lever, but the disadvantages of such a solution have also been described earlier.

[0018] Based on the above, in order to solve the disadvantages in the prior art, there is an object to provide a linear electromechanical actuator, the dimensions and weight of which allow for placing a welding gun directly on the stationary lever without the need to provide a robot of greater carrying capacity in the welding cells or retool existing welding cells, which also reduces the risk of damage to the linear electromechanical actuator during robot training or setup while allowing to determine contact by the electrode arranged on the stationary lever with the surface of the part being welded at a high speed of electrode lead-in along with the ability to measure pressing force created by the electrode on the surface of the part being welded during positioning and during the process of resistance spot welding.

[0019] SUMMARY OF THE INVENTION

[0020] In order to overcome the disadvantages in the prior art and to achieve the object of the invention, a linear electromechanical actuator for a welding gun for resistance spot welding of parts is provided, the actuator comprising: a housing consisting of at least two parts; an electric motor comprising a stator and a rotor, the stator being secured on an inner surface of one of the said at least two parts; and an inverted roller-screw gear comprising a rod connected to an electrode of the welding gun for converting rotational movement of the electric motor into translational movement of the rod, the inverted roller-screw gear being arranged in the other part of the said at least two parts. The linear electromechanical actuator further comprises a sensor for measuring a force of pressing the electrode against a surface of the part to be welded, the force being transmitted from the electrode through the inverted roller-screw gear arranged in said other part of the said two parts of the housing; and a bevel gear drive for transmitting rotation of the electric motor to the inverted roller-screw gear, the bevel gear drive comprising a driving gear (10) and a driven gear (11), the driving gear (10) being connected to the electric motor rotor, while the driven gear (11) being connected to the inverted roller-screw gear, wherein an axis of the inverted roller-screw gear coincides with an axis of the rod and is arranged at an angle to an axis of the electric motor; and wherein the inverted roller-screw gear further comprises a nut configured for longitudinal movement with respect to the driven gear of the bevel gear drive and forming a sliding connection with the driven gear configured for transmitting a torque from the driven gear to the nut.

[0021] The disclosed linear electromechanical actuator allows for using a sensor for measuring pressing force, the sensor performing direct measurement of the pressing force of the electrode on a lever, in particular a stationary lever, exerted by the welding gun against the surface of the part being welded, since the electrode connectable to the rod and the roller-screw gear through which the pressing force is transmitted to the measuring sensor are arranged on the same axis. In this case, the radial components of the force arising from non-alignment of the sensor axis and the pressing force application axis are eliminated, the weight of the structural elements between the sensor and the surface of the part being welded participating in the transfer of the pressing force is minimized, and the stiffness effect of the lever, in particular the stationary lever, and the frame structure of the welding gun when measuring the pressing force is minimized as well, as they are not part of the chain of structural elements transmitting the pressing force to the sensor.

[0022] Since the housing parts with the electric motor and the sensor for measuring pressing force are structurally separated by the bevel gear drive, this allows to further shield the sensor from the effects of electromagnetic interference induced by the electric motor during operation.

[0023] Furthermore, introduction of the bevel gear drive in the structure of the linear electromechanical actuator allows to reduce the size of the actuator along the axis of electrode movement, since in the disclosed solution, the electric motor is arranged at an angle with respect to the inverted roller-screw gear. The disclosed solution allows to place the heaviest and largest part of the linear electromechanical actuator housing wherein the electric motor is mounted such that it forms an extension of the lever, in particular the stationary lever, and the overall dimensions of the welding gun remain unchanged. The disclosed arrangement of the linear electromechanical actuator allows to minimize the weight moved thereby, said weight including only the electrode, thus requiring less operational torque and, accordingly, allows for an electric motor of smaller weight and size. As the overall dimensions of the welding gun are unchanged, the linear electromechanical actuator is arranged on the lever, in particular the stationary lever, and has the minimum possible dimensions due to the fact that the weight being moved is reduced to the weight of the electrode, the risk of damage to the linear electromechanical actuator during setup procedures carried out by the operator is much lower, unlike that in prior art solutions wherein the linear electromechanical actuator moves the entire lever. In case of the welding gun hitting the tooling or another obstacle during setup, the entire impact will be taken on by the lever, in particular the stationary lever, and not by the elements of the linear electromechanical actuator. Thus, the disclosed design does not require a robot of larger carrying capacity to be used in the welding cell, and reduces the risk of damage to the linear electromechanical actuator.

[0024] The disclosed design, wherein the nut of the inverted roller-screw gear is not connected to the driven gear in the axial direction, during the measurement of the pressing force on the electrode allows to minimize the effect on the sensor of measuring the pressing force occurring in the axial direction on the driven gear when transmitting a torque from the electric motor, allowing to measure the pressing force with high accuracy when moving the electrode, reducing the time taken by determining the moment of contact of the electrode with the part being welded, and increasing repeatability of measurement results. This is particularly important when determining the moment of contact of the electrode with the surface of the part being welded, since the force acting on the electrode at the moment of contact in this case is comparable to the force arising in the axial direction due to transmission of torque in the bevel gear drive, and is close to the minimum measured value of said force.

[0025] The provided degree of freedom for the RSG nut with respect to the driven gear of the bevel gear drive provided by the sliding connection between the nut and the driven gear, allows to perform direct measurements of the pressing force of the electrode against the surface of the part being welded, provides additional shielding, eliminates error caused by auxiliary elements when transferring the pressing force to the sensor, allows to measure the pressing force with high accuracy, speed and repeatability of results even at low pressing forces, which, for instance, is a significant problem inherent to the prior art solution which includes providing a separate sensor for measuring pressing force on the lever, in particular the stationary lever, of a welding gun for measuring the pressing force of the electrode indirectly based on deformation of the lever, in particular the stationary lever.

[0026] The ability to perform accurate measurements at high speed in part of the range with low values of pressing forces allows to increase the speed of welding gun lead-in to the parts being welded and to work with high acceleration values achievable by the linear electromechanical actuator. This allows to reduce the duration of the welding cycle at the electrode lead-in stage to the part being welded. Increased speed and acceleration of electrode movement further allows to speed up the process of determining zero point of the electrode and the spatial coordinates of the welding gun, thus reducing the duration of the welding gun setup process during initial start-up, and reduces welding cell downtime during welding gun maintenance, e.g., when determining zero point of the electrode after sharpening or replacing worn caps.

[0027] It should be noted that due to achieving increased speed, the duration of welding gun downtime during regular calibration thereof is reduced.

[0028] In one of the embodiments of the present invention, the inverted roller-screw gear further comprises a bearing support with a bearing, and the force measuring sensor is mounted with a preload provided by biasing a spring of an outer ring of the bearing. The preload allows to select play in the connections transmitting the force to the sensor for measuring pressing force, and further provides a stable pressing force value on the sensor for measuring pressing force prior to measurement.

[0029] In one of the embodiments of the present invention, the driven gear has a through hole configured to receive the nut therein, wherein the through hole has a non-circular shape at a portion of the hole depth or over the entire hole depth, and wherein the nut has a portion of a non-circular shape on its outer surface, the portion engaging with said through hole of a non-circular shape to transmit the torque.

[0030] In one of the embodiments of the present invention, the linear electromechanical actuator can be mounted on a C-type welding gun or on an X-type welding gun. Therefore, the disclosed design is versatile and can be mounted on different types of welding guns.

[0031] In any of the embodiments of the present invention, the linear electromechanical actuator can be mounted on a stationary lever of the welding gun. Although in a preferred embodiment, the disclosed electromechanical actuator is mounted on a stationary lever of the welding gun, it can also be mounted on the movable lever of a welding gun with the same advantages as in the case of mounting on the stationary lever.

[0032] A further object of the invention comprises a welding gun provided with a linear electromechanical actuator accordingly to one or more of the previous characteristics above indicated.

[0033] It is a further object of the invention the use of a welding gun as above indicated, for performing a resistance spot welding.

[0034] For further explanation of the present invention, embodiments thereof will be disclosed and discussed below with reference to the accompanying drawings.

[0035] BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Fig. l is a cross-sectional side view of a linear electromechanical actuator.

[0037] Fig. 2 is a cross-sectional side view of a highlighted section of the linear electromechanical actuator of Fig. 1.

[0038] Fig. 3 shows a Type C welding gun with a linear electromechanical actuator according to the present invention mounted thereon.

[0039] Fig. 4 shows a Type X welding gun with a linear electromechanical actuator according to the present invention mounted thereon.

[0040] DETAILED DESCRIPTION OF THE INVENTION

[0041] Fig. 1 shows a linear electromechanical actuator 1 to be mounted on a lever, in particular a stationary lever, of a welding gun, for positioning the electrode 2 of the welding gun, and Fig. 2 shows a portion of the linear electromechanical actuator of Fig. 1 enlarged for a clearer understanding of the described structure. In the illustrated embodiment, but not limited thereto, the linear electromechanical actuator 1 comprises a housing comprised of two parts 3 and 4. The linear electromechanical actuator 1 further comprises an electric motor 5 disposed in one housing part 4. The linear electromechanical actuator 1 further comprises an inverted roller-screw gear (RSG) 8 disposed in the other housing part 3. The RSG 8 comprises a rod 9 to be connected to the electrode 2, as well as comprises threaded rollers 13, a bearing support 17, bearings 16 and 18 and a nut 20. The linear electromechanical actuator 1 further comprises a bevel gear drive 12 and at least one sensor 15 for measuring a force of pressing the electrode 2 against the surface of the part being welded, the sensor 15 connected to the RSG 8. The electric motor 5 comprises a stator 6 and a rotor 7 with permanent magnets, the rotor being arranged on two bearing supports thereof.

[0042] A driving gear 10 is press-fitted on the axis 14 of the rotor 7 but the gear 10 can be attached to the rotor 7 using another method, e.g. by pinning, threaded connection or a combination of several attachment types.

[0043] In the housing part 4, a driven gear 11 is mounted, the driven gear 11 connected to the nut 20 and prevented from longitudinal movement with respect to the other housing part 3 by means of at least one bearing support 19 with bearings.

[0044] The gears 10 and 11 are engaged with each other forming a bevel gear drive 12, and transmit rotational movement and torque from the electric motor rotor 7 to the nut 20. The gear ratio of the bevel gear drive 12 is selected so as to provide the necessary linear movement speed of the welding gun electrode and the pressing force against the part on said electrode.

[0045] In the described embodiment, but not limited to, the gear 11 has a through hole providing engagement with the nut 20 and the transfer of torque from the gear 11 to the nut 20, wherein the through hole can have a non-circular shape either over the entire depth of the hole or only at a portion of the depth. An example of the hole shape, but not limited thereto, is a splined hole or a hole of a complex shape obtained by modifying a cylindrical hole with at least one or more key slots or flats.

[0046] The nut 20 has an outer surface shape complementary to the shape of the inner surface of the through hole of the gear 11 and has a non-circular shape along the entire length or at a portion of the length corresponding to the above-described shape of the through hole. An example of the outer surface shape of the nut 20, but not limited thereto, is a toothed shaft or cylindrical shaft surface with at least one or more keyslots or flats.

[0047] Thus, the gear 11 and the nut 20 form a sliding connection, wherein the torque from the gear 11 is transmitted to the nut 20 but the nut 20 has a degree of freedom in the axial direction with respect to the gear 11, as a result of which the axial force arising in the bevel gear drive 12 on the driven gear 11, when transferring rotational motion and torque from the driving gear 10 thereto, has a minimum value and does not affect the pressing force of the electrode 2 during operation of the electric motor 5. Thus, the nut 20 is not prevented from longitudinal movement with respect to the gear 11 of the bevel gear drive 12 and can move during operation when a pressing force occurs on the electrode 2 of said welding gun.

[0048] An axis of the inverted RSG 8 coincides with an axis of the rod 9 and an axis of the electrode 2 with said elements moving along said axis. It should be noted that the positional angle of the axes of gears 10 and 11 can be arbitrary so the electric motor 5 can be arranged both at a right angle and at a non-right angle with respect to the axis of the inverted RSG 8 and, therefore, to the electrode 2 movement axis. The placement of the electric motor 5 at an angle to the movement axis of the rod 9 of the RSG 8 allows to reduce the dimensions of the linear electromechanical actuator 1 in axial direction and to arrange the housing part 4 as an extension of the lever, in particular the stationary lever, for the electrode 2 while maintaining overall dimensions of the welding gun, e.g. the welding guns 21 and 23 shown in Figs. 3 and 4.

[0049] By using the sensor 15, the linear electromechanical actuator 1 can directly measure the force of pressing the electrode 2 against the surface of the part being welded, both at the moment the electrode 2 touches the surface of the part being welded and throughout the welding operation.

[0050] The sensor 15 for measuring the pressing force can be a sensor based on a strain gauge or a piezoelectric element, structurally more simple compared to sensors mounted separately on a stationary electrode because, in the case of mounting the sensor directly into the actuator, the force of pressing the electrode against the surface of the part being welded acts on the same axis as the sensor, thereby eliminating radial components of said force. In contrast to the disclosed design of the electromechanical actuator 1 with the sensor 15, wherein the force of pressing the electrode against the surface of the part being welded is mechanically transmitted directly to the measuring sensor, if a separate sensor for measuring the force of pressing the electrode against the surface of the part being welded is mounted on the stationary electrode, the deformation of the lever, in particular the stationary lever, is measured by transmitting the deformation to the measuring sensor, thus introducing an additional degree of error into measurements. The shape of the sensor is not structurally limited, allowing for the use of low-cost conventional solutions.

[0051] The error in measuring the pressing force of the electrode 2 against the surface of the part being welded is minimized by the fact that the sensor 15 for measuring pressing force is arranged on the same axis as the electrode 2 and, therefore, on the same axis as the RSG 8 and the rod 9, and the pressing force arising at the moment of contact between the electrode 2 and the surface of the part being welded is transmitted to the rod 9, then from the rod 9 through the contact threads of the rollers 13 to the nut 20, then through the nut 20 to the bearing 16, then to the bearing support 17 and eventually to said sensor 15 for measuring pressing force. It should be noted that the sensor 15 for measuring pressing force is mounted in the housing, in particular in the housing part 3, with a preload, providing for a selection of play in the above connections transmitting the force to the sensor 15 for measuring pressing force. In addition, preloading provides a stable pressing force value to the sensor 15 for measuring pressing force prior to measurement which is registered by the system as the zero value. Preloading is provided by means of a bearing (16) having an outer ring with a spring, wherein the spring is biased to provide said preloading.

[0052] As previously described, the nut 20 and the gear 11 are not mechanically connected to each other in axial direction, thus minimizing the negative effect of the axial force arising on the gear 11 during operation of the motor 5 and rotation of the bevel gear drive 12 on the results of pressing force measurement by the sensor 15, which also contributes to reducing error in measuring pressing force of the electrode 2 against the surface of the part being welded.

[0053] Reducing error in measuring pressing force of the electrode 2 due to the above factors allows for measuring the minimum values of the pressing force of the electrode 2 against the surface of the part being welded, and the measurement results have high repeatability, which in the first case allows to determine the moment of contact at the initial stage of the electrode 2 lead- in to the surface of the part being welded and to apply more complex positioning algorithms with an increased electrode lead-in speed, and in the second case ensures the stability of the pressing force of the electrode against the surface of the part being welded during welding, as the subsequent application of pressing force of the electrode is carried out based of the values measured by the sensor, allowing to reduce the number of welding point defects.

[0054] Fig. 3 shows a C-type welding gun 21 with a linear electromechanical actuator 1 arranged on the lever 22, in particular the stationary lever 22, and Fig. 4 shows an X-type welding gun 23 with a linear electromechanical actuator 1 arranged on the lever 22, in particular the stationary lever 22. As one example of the design, but not limited thereto, the electromechanical actuator 1 is arranged within the lever, in particular the stationary lever 22, of the welding gun using conventional actuator mounting areas of an actuator instead of which the disclosed electromechanical actuator 1 is mounted.

[0055] With this arrangement of the linear electromechanical actuator 1, the overall dimensions of the C-type welding gun 21 and the X-type welding gun 23 do not increase, thus retaining the ability of the welding gun to perform welding in hard-to-reach places, e.g., around bent areas of a car body. At the same time, the overall weight distribution and the center of gravity of the C-type welding gun 21 and the X-type welding gun 23 are preserved, thus eliminating the need to replace the industrial manipulator used to move the welding guns 21 and 23 with one having a greater carrying capacity when using the disclosed linear electromechanical actuator 1.

[0056] At the end of the electrode 2 a cap 24 is attached, which directly contacts with the surface of the part being welded. Therefore, in the present description of the invention, the contact between the electrode and the part being welded is understood to mean the contact between the cap and the part being welded. Over the course of the welding operation, the cap 24 wears out, the material becomes softer due to heating during welding and loses its shape; after a critical loss of shape, the procedure for restoring the shape of the cap is performed, e.g. by sharpening, but after each sharpening the height of the cap decreases up to the minimum permissible height, after which the cap is replaced. Thus, the height of the electrode contact point on the lever, in particular the stationary lever, continuously changes; however, as the linear electromechanical actuator 1 can determine the contact between the electrode 2 and the surface of the part being welded, additional calibration of contact point height is not required, thereby reducing the total welding cycle time and leading to an increase in productivity of the car body assembly line utilizing resistance spot welding.

Claims

CLAIMS1. A linear electromechanical actuator (1) for a welding gun for performing resistance spot welding of parts, the actuator comprising: a housing comprising at least two parts (3, 4), an electric motor (5) comprising a stator (6) and a rotor (7), the stator (6) being secured on an inner surface of one (4) of the said at least two parts (3, 4) of the housing; and an inverted roller-screw gear (8) comprising a rod (9) to be connected to an electrode (2) of the welding gun for converting rotational movement of the electric motor (5) into translational movement of the rod (9), the inverted roller-screw gear (8) being arranged in the other part (3) of the said at least two parts (3, 4) of the housing, the actuator being characterized by further comprising: a sensor (15) for measuring a force of pressing the electrode (2) against a surface of the part to be welded, the force being transmitted from the electrode (2) through the inverted rollerscrew gear (8), the sensor (15) being arranged in said other part (3) of the said at least two parts (3, 4) of the housing; and a bevel gear drive (12) for transmitting rotation of the electric motor (5) to the inverted roller-screw gear (8), the bevel gear drive comprising a driving gear (10) and a driven gear (11), the driving gear (10) being connected to the rotor (7) of the electric motor (5), while the driven gear (11) being connected to the inverted roller-screw gear (8), wherein an axis of the inverted roller-screw gear (8) coincides with an axis of the rod (9) and is arranged at an angle to an axis of the electric motor (5), wherein the inverted roller-screw gear (8) further comprises a nut (20) configured for longitudinal movement with respect to the driven gear (11) of the bevel gear drive (12) and forming a sliding connection with the driven gear (11) configured for transmitting a torque from the driven gear (11) to the nut (20).

2. The linear electromechanical actuator according to claim 1, wherein the inverted roller-screw gear (8) further comprises a bearing support (17) with a bearing (16), and wherein the force measuring sensor (15) is mounted with a preload provided by biasing a spring of an outer ring of the bearing (16).

3. The linear electromechanical actuator according to claim 1, wherein the driven gear (11) has a through hole configured to receive the nut (20) therein, wherein the through hole has anon-circular shape at a portion of the hole depth or over the entire hole depth, and wherein the nut (20) has a portion of a non-circular shape on its outer surface, the portion engaging with said through hole of a non-circular shape to transmit the torque.

4. The linear electromechanical actuator according to claim 1, which is mounted on a C- type welding gun (21) or on an X-type welding gun (23).

5. The linear electromechanical actuator according to any one of claims 1-4, which is mounted on a stationary lever of the welding gun.

6. A welding gun provided with a linear electromechanical actuator accordingly to one or more of the previous claims.

7. The use of a welding gun, accordingly to claim 6, for performing a resistance spot welding.

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