Electromechanical actuator having sliding piston for improved hydrodynamic bearing

Abstract

The invention relates to an electromechanical linear actuator (10) which additionally has a sliding piston (50) which has a groove (55) designed in a spiral shape for the floating bearing of a spindle (40) in the electromechanical actuator. When the main shaft 40 moves relative to the sliding piston 50, the lubricating fluid between the sliding piston 50 and the main shaft 40 can support hydrodynamic lubrication between the two parts through the groove 55.

Description

Field of technology

[0001] The present disclosure relates to an electromechanical actuator having the features recited in claim 1, the electromechanical actuator having a sliding piston. Background art

[0002] Electromechanical actuators are known in the prior art. DE 102020212703 A1 thus discloses, for example, an electromechanical actuator there referred to as an "electric cylinder", in which the end of a threaded spindle is connected to a piston which divides the interior space of the actuator into a first region and a second region, the first region and the second region being separated from one another in a fluid-tight manner. The first region is there delimited only by the piston and a cantilever which projects from the housing in the direction of the axis of rotation of the threaded spindle, the housing receiving the threaded spindle and the cantilever. The piston is rotatably supported at the threaded spindle by means of a sliding bearing and is supposed to rotate as little as possible relative to the cantilever.

[0003] DE 102014213505 B4 likewise teaches an electric cylinder in which the spindle support has a support device which has a laterally movable rolling bearing (instead of the sliding bearing mentioned above). More precisely, according to this disclosure, the rotary bearing can move radially in order to enable a rotary movement.

[0004] However, the friction between the sliding piston and the spindle can still be optimized in order to waste less energy, dissipate less of the friction heat generated, and in particular in order to minimize wear. For this purpose, it is necessary in particular to prevent the piston from rotating in the cantilever.

[0005] In detail, in a linear actuator, the threaded spindle is rotatably supported at the housing about the longitudinal axis by means of a rotary bearing. The cantilever can be moved in and out by means of a nut screwed onto the threaded spindle. The threaded spindle is rotated by an electric motor. In particular, (frictional-) heat occurs at the screw engagement of the threaded spindle and the nut. Therefore, the interior space is filled with a fluid or a mixture of lubricating oil and gas. The lubrication is hermetically sealed relative to the outside atmosphere in order to prevent leaks and dirt. Since the internal space volume changes as the cantilever moves in and out in the housing, it is not suitable or even not feasible to completely fill it with an incompressible fluid (or theoretically also completely fill it with a lubricated solid material such as powder). On the contrary, satisfactory lubrication and cooling cannot be achieved with pure gas. Therefore, the fluid preferably consists of a part (incompressible) lubricating oil and a part (compressible) gas. If the nut moves, the nut displaces the fluid, which flows through the cooling channels and past the threaded nut, for example around the rolling elements supported by the threaded nut and also through the channels in the spindle.

[0006] In particular, however, when the arm is moved in and out quickly, the fluid cannot easily flow through the existing cooling channels and past the threaded nut, so that a large pressure difference occurs in the interior of the linear actuator. This pressure difference leads to a higher no-load torque of the shaft, which increases depending on the shaft speed and thus the flow speed. This leads to a lower efficiency of the shaft. Summary of the invention

[0007] It is an object of the invention to obviate or at least mitigate the disadvantages discussed above.

[0008] The invention thus realizes a linear actuator having a housing and an arm, the arm extending from the housing in the direction of a longitudinal axis, wherein the arm is movable in the direction of the longitudinal axis, wherein a threaded spindle is rotatably supported at its first end on the housing by means of a spindle rotating bearing about the longitudinal axis, wherein a second end of the threaded spindle, which is opposite to the first end in the direction of the longitudinal axis, extends into the arm regardless of the position of the arm, wherein a sliding piston is provided at the second end of the threaded spindle, the sliding piston abuts against the inner circumference of the arm in a substantially fluid-tight manner, so that the sliding piston separates a first cavity and a second cavity in the linear actuator from each other, wherein the second cavity is arranged on a side of the sliding piston facing away from the spindle rotating bearing, wherein the arm and the threaded nut are connected to each other. The invention relates to a method for manufacturing a screw threaded bearing having a plurality of roller bodies, wherein the first cavity is at least partially filled with a liquid, wherein the threaded nut is open at its two opposite ends in the direction of the longitudinal axis in such a way that the liquid can flow through the threaded nut when the arm moves, wherein a first longitudinal channel is arranged in the interior of the threaded spindle, the first longitudinal channel extending along the longitudinal axis, wherein the first longitudinal channel is fluidically connected to the first cavity at its opposite ends with respect to the longitudinal axis in such a way that the liquid can flow through the first longitudinal channel when the arm moves, bypassing the threaded nut, regardless of the position of the arm, and wherein during this flow, a sliding bearing between the sliding piston and the threaded spindle flows through. That is, the sliding piston is particularly adapted to the threaded spindle and is itself geometrically designed in such a way that a part of the flow flows between the two components and another part of the flow flows through the sliding piston itself.

[0009] In particular, the invention provides a sliding piston in which rotation relative to the cantilever is prevented in that the sealing friction at the piston is higher than the torque occurring as resistance in the hydrodynamic bearing in every operating state. A break in the lubricating film is prevented so that "corrosion" or increased wear that would damage the bearing does not occur. This is particularly important in outdoor applications with changing temperatures and the accompanying changing viscosity of the lubricant.

[0010] Elastomeric plain bearings cannot be used in outdoor applications due to temperature requirements. Since the coefficient of thermal expansion of the elastomer and that of steel differ too much, large clearances will occur at higher temperatures. Although solutions similar to DE 102014213505 B4 can be used, here the floating bearing must be very delicate due to the additional flow channels. Therefore, this concept is considered uneconomical for cost and durability reasons.

[0011] This is achieved according to the invention by a sealed piston with optimized geometry and optimized material selection. The second region of the electromechanical actuator is partially filled with lubricating oil similarly to that in DE 102020212703A1. A first fluid flow path is provided, which includes the bearing clearance of the piston plain bearing, wherein the first fluid flow path is so extended that the movement of the cantilever causes the lubricating oil to flow in the first fluid flow path, in particular through the piston plain bearing. However, what is particularly important in connection with the present invention is that the lubricating oil displaced on one side of the threaded nut can reach the other side of the threaded nut through the spindle channel.

[0012] The piston runs on the threaded spindle on a surface that is cylindrical with respect to the longitudinal axis of the threaded spindle. More precisely, the spindle rotates in the piston while the piston rotates as little as possible.

[0013] The spindle is preferably arranged such that it completely penetrates the piston. On the side of the threaded spindle that is opposite to the (actual) thread for pushing the cantilever by the threaded nut, the nut can be screwed onto the external thread of the threaded spindle, and this nut axially holds the piston on the threaded spindle. Thus, the threaded spindle (and thus also the spindle channel present in the threaded spindle) penetrates the nut. Thereby, the lubricating fluid flowing out of the spindle channel can reach the side of the piston that is opposite to the threaded spindle through the piston.

[0014] Through helically arranged grooves provided on the side facing the spindle in the region of the sliding contact part in the piston, when the spindle rotates and the sliding piston is fixed, the vast majority of the fluid flow can flow in the first fluid flow path. Starting from these grooves, the lubricating oil can easily diffuse into the actual bearing clearance. The grooves preferably extend continuously from the end face of the piston abutting against the threaded spindle within the inner peripheral surface of the piston, so that no constrictions for oil flow are generated anywhere. Thereby, the hydrodynamic lubrication of the plain bearing is improved.

[0015] Preferably, the grooves extend helically such that the oil flow is supported in the first fluid flow path.

[0016] The second fluid flow path can be formed by a bore parallel to the axis in the piston. This second fluid flow path extends parallel to the first fluid flow path. The flow resistance (in other words the diameter) through the bore parallel to the axis can control what proportion of the total oil flow will flow through the sliding bearing. Advantageously, the ratio between the cross-sectional area of the groove and the cross-sectional area of the through-hole lies between 50% and 200%, in other words the cross-sectional area of the through-hole lies between half and twice as large as the cross-sectional area of the groove.

[0017] At the outside of the piston there is a seal consisting of a guide ring and a dynamically acting sealing ring. These components are commercially available standard goods, which reduces the overall cost. The friction of the sealing ring must be so high that relative movement occurs at the working surface of the main shaft and no relative movement occurs at the guiding and sealing parts of the piston. Therefore, the piston should not rotate.

[0018] Advantageously, contrary to the closest prior art, instead of using plastic, an aluminum-forging alloy with a high silicon content is used as the piston material. Thereby, the coefficient of thermal expansion of the piston is closer to the coefficients of the other components of the electromechanical actuator, in particular the sleeve and the main shaft, which are also made of metallic material and interact with the piston. Thereby, more stable lubrication can be achieved over a larger temperature range. The emergency operation performance is improved by the high silicon content.

[0019] Compared to DE 102020212703 A1, with the structure having the features explained above, it is achieved that fluid can flow in the tightly sealed internal space of the electromechanical actuator from the internal space of the sleeve or the cantilever into the deep bore in the main shaft and vice versa.

[0020] Although in this description "through-hole", "deep bore" etc. are mentioned, it should be obvious to the person skilled in the art that the corresponding connecting channels between the fluid inlet and the fluid outlet on the surface as well as blind holes can be manufactured not only by drilling but also by any other manufacturing process (etching, additive manufacturing, erosion... should be mentioned by way of example). For the sake of simplicity, nevertheless "drilling" is described in this description. Description of the Drawings

[0021] Wherein:

[0022] Figure 1 shows a linear actuator according to the invention for elucidating the fluid circulation.

[0023] Figure 2 shows an isometric view of a sliding piston according to a first embodiment of the present disclosure;

[0024] Figure 3A full-scale view of the sliding piston from Figure 2 is shown from the other side, and

[0025] Figure 4 from Figure 1 the enlarged portion shows a view of the sliding piston and its surroundings in the electromechanical actuator in the installed state. DETAILED DESCRIPTION

[0026] Embodiments of the present disclosure will be described below with reference to the relevant drawings.

[0027] Figure 1 A linear actuator 10 is shown, which has a housing 20 and a cantilever 60 that extends from the housing 20 (upward in the figure) along a longitudinal axis 13. The cantilever 60 is movable in the direction of the longitudinal axis 13. A threaded spindle 40 is rotatably supported about the longitudinal axis 13 at or in the housing 20 by means of a spindle rotary bearing 43 at its first end 41 and can be driven by a motor connected to the first end 41. The second end 42 of the threaded spindle 40 opposite the first end 41 in the direction of the longitudinal axis 13 always extends into the cantilever 60. The second end 42 of the threaded spindle 40 is provided with a sliding piston / piston / (sliding-)piston 50, which will be explained in more detail later. The sliding piston abuts against the inner peripheral surface of the cantilever 60 in a substantially fluid-tight manner such that the sliding piston separates the first cavity and the second cavity 11; 12 in the linear actuator 10 from each other. Here, the second cavity 12 is arranged on the side of the sliding piston 50 facing away from the spindle rotary bearing 43. The cantilever 60 is firmly connected to a threaded nut 62, which is in screw engagement with the threaded spindle 40 (directly or preferably via a plurality of rolling elements, such as planetary gears, not shown here). The first cavity 11 is at least partially filled with a liquid. More precisely, in this embodiment, the first cavity includes a fluid that is a mixture of a liquid, such as lubricating oil, and a gas, such as nitrogen. The threaded nut 62 is open at its two ends opposite in the direction of the longitudinal axis 13 such that the fluid can flow through the threaded nut 62 when the cantilever 60 moves. A first longitudinal channel 81 is arranged inside the threaded spindle 40, which extends along the longitudinal axis 13. The first longitudinal channel 81 is fluidly connected to the first cavity 11 at its ends opposite with respect to the longitudinal axis 13 (e.g., at the first end 41 through a transverse hole that leads into the longitudinal channel, which is illustrated in the figure by a number of points connected into a "thick line") such that the fluid can also flow through the first longitudinal channel 81 bypassing the threaded nut 62 regardless of the position of the cantilever 60 when the cantilever 60 moves, wherein this flow passes through the sliding bearing between the sliding piston and the threaded spindle and thus lubricates it.

[0028] Figure 2 Shown from the side against the threaded spindle of the electromechanical actuator, i.e., from the Figure 1 and Figure 4 left side in Figure 1 and Figure 4 is the sliding piston 50 shown as being inserted. The multiple inlets of the through-hole 52 and, in this embodiment, three circumferentially spaced helical grooves 55 can be seen. These grooves 55 form a first fluid flow path for the lubricating oil. The grooves 55 extend continuously from the piston end side against the threaded spindle 40 within the inner peripheral surface of the sliding piston 50, so that there are no narrow parts for the oil flow anywhere. The grooves 55 extend in such a helical shape that they support the oil flow when the threaded spindle 40 rotates and thus when the cantilever 60 is simultaneously moved in or out. The through-hole 52 forms a second fluid flow path.

[0029] Figure 3 The same sliding piston 50 is shown in an isometric view from the other side (from the Figure 1 and Figure 4 "right side" in Figure 2 and Figure 3 ). In addition to the through-hole 52 and the grooves 55, four threaded blind holes 58 distributed on the periphery can also be identified. Based on the comparison of the output ends of the through-hole 52 in Figure 4 , however, it can be better identified in the Figure 4 section that the through-hole 52 extends obliquely through the sliding piston. The aforementioned threaded blind holes 58 are used to fasten the (

[0030] shown in Figure 3 ) axial seal 20 to the sliding piston 50 by means of threaded elements 30. The axial seal 20 can be configured as, for example, a check valve based on a diaphragm (more detailed in the parallel application) or include such a check valve, and in this case, the fluid entering the cantilever 60 is conveyed to the side of the threaded spindle again through the spindle channel 81 in the threaded spindle 40 and through the through-hole 52. The fluid, especially incompressible lubricating oil or grease, entering the cantilever due to the negative pressure occurring during the retraction could otherwise impede the re-insertion if the fluid cannot leave the tightly sealed cantilever. Figure 4 ) is realized in the sliding piston. After the sliding piston 50 is applied to the threaded spindle 40, the threaded nut 45 is screwed onto the external thread of the spindle 40 and axially holds the sliding piston 50 on the cylindrical surface. The spindle channel 81 passes through the external thread and thus also through the threaded nut 45.

[0031] Unlike pistons from the prior art in which POM plastic material is used, it has proven to be advantageous in the present invention that the piston 50 is made of an aluminum-wrought alloy with a high silicon content. This material has a similar thermal expansion to the metal cantilever 60 and spindle 40, so that lubrication can be maintained stably over a wide temperature range.

[0032] As from Figure 4 As can be seen in FIG. 1 , the piston is supported on the cantilever 60 by means of a guide belt 90 and cooperates with a seal 100 so as to seal the first cavity 11 with the second cavity 12 (at Figure 4 The guide strip 90 and the seal 100 are used to support the piston in the cantilever 60 as rotationally fixed as possible. Relative rotation should only take place between the main shaft 40 and the sliding piston 50. This is achieved by selecting the friction of the seal 100 at the cantilever 60 and the sliding piston 50 to be so high that relative movement occurs at the main shaft running surface 42 and no relative movement occurs at the guide strip and the seal of the sliding piston 50. This is facilitated by generating lower friction at the outer diameter of the sliding piston 50 in order to compensate for the torque occurring between the main shaft running surface 42 and the associated running surface 57 of the sliding piston 50, because the running surface 57 of the sliding piston 50 is located further inward than the seal 100.

[0033] The guide strip 90 serves as a guide element that is common in corresponding cylinders and absorbs radial loads that occur during operation. Usually, the guide strip 90 is made of plastic or soft metal in order to avoid contact between the metal components of the linear actuator. Compared with the metal components "sliding piston 50" and "cantilever 60", the material of the guide strip 90 provides lower friction and a larger contact area due to its better elastic deformation. In addition, self-lubricating materials, such as corresponding plastics, can be used. Such standard components can be obtained cost-effectively in any size.

[0034] The sliding piston 50 is thus held rotationally fixed relative to the cantilever 60 by the guide strip 90 and (in particular) the seal 100 and thus slides on the spindle running surface 42. The lubricating groove 55 in the sliding piston 50 enables the lubricating fluid to flow from the spindle channel 81 into the first region both through the through-opening 51 and also through the groove 55, and vice versa. Due to the groove 55, the lubricating fluid forms a lubricating film between the sliding piston 50 and the spindle running surface 42.

[0035] In summary, the present invention realizes an electromechanical linear actuator 10 which additionally has a sliding piston 50 having a helically designed groove 55 for the floating bearing of the spindle 40. During the relative movement between the spindle 40 and the sliding piston 50, the lubricating fluid reaching between the sliding piston 50 and the spindle 40 through the groove 55 can support the hydrodynamic lubrication between these two components.

Claims

1. A linear actuator (10) comprising a housing (20) and an arm (60) which projects from the housing (20) in the direction of a longitudinal axis (13), wherein: The cantilever arm is movable in the direction of the longitudinal axis (13), wherein the threaded spindle (40) is rotatably supported at its first end (41) on the housing (20) by means of a spindle rotary bearing (43) about the longitudinal axis (13), wherein, regardless of the position of the cantilever arm (60), the second end (42) of the threaded spindle (40) opposite to the first end (41) in the direction of the longitudinal axis (13) extends into the cantilever arm (60), wherein the second end (42) of the threaded spindle (40) is provided with a sliding piston (50), which is substantially fluid-tightly abutted against the inner circumference of the cantilever arm (60), so that the sliding piston A movable piston separates a first cavity and a second cavity (11; 12) in the linear actuator (10) from each other, wherein the second cavity is arranged on the side of the sliding piston (50) facing away from the spindle rotary bearing (43), wherein the cantilever arm (60) is firmly connected to a threaded nut (62) which is in helical engagement with the threaded spindle (40), wherein the first cavity (11) is at least partially filled with a liquid, wherein the threaded nut (62) is open at its two ends which are opposite in the direction of the longitudinal axis (13) in such a way that the liquid can flow through the threaded nut (62) when the cantilever arm (60) is moved, Characterized in that a first longitudinal channel (81) is arranged in the interior of the threaded spindle (40), which first longitudinal channel extends along the longitudinal axis (13), wherein the first longitudinal channel (81) is fluidically connected to the first cavity (11) at its end opposite to the longitudinal axis (13) in such a way that, regardless of the position of the cantilever arm (60), the liquid flows through the first longitudinal channel (81) while bypassing the threaded nut (62) when the cantilever arm (60) moves, and wherein, during this flow, the sliding bearing between the sliding piston (50) and the threaded spindle (40) is flowed through.

2. The linear actuator (10) according to claim 1 comprises a sliding piston (50) having a through hole (52) which is provided for flow to flow through in parallel to the threaded spindle (40) and for achieving a fluid connection between the first cavity and the second cavity (11, 12).

3. The linear actuator (10) according to claim 2, wherein: The through hole (52) extends toward the center axis of the sliding piston (50).

4. The linear actuator (10) according to claim 2 or 3, wherein: The cross-sectional area of ​​the through hole (52) is between 50% and 200% of the cross-sectional area of ​​the groove (55).

5. The linear actuator (10) according to claim 4, wherein: Furthermore, when the cantilever (60) moves, more fluid flows through the through hole (52) than through the sliding bearing and the groove.

6. The linear actuator (10) according to any one of the preceding claims, wherein: The sliding piston (50) is made of an AlSi wrought alloy.

7. The linear actuator (10) according to any of the preceding claims, further comprising a guide belt (90) and a seal (100) which are arranged on the outside of the sliding piston (50) and which hold the sliding piston (50) on the cantilever arm (60) in a rotationally fixed manner by means of friction.

8. The linear actuator (10) according to any one of the preceding claims, further comprising an inner space which is enlarged relative to the working surface (57) of the sliding piston (50) and is used to receive a threaded nut (45).

9. The linear actuator (10) according to any one of the preceding claims, further comprising a threaded hole (58) for fastening the axial seal (20) by means of a screw (30).

10. The linear actuator according to claim 9, further comprising an axial seal (20) comprising a check valve, and the fluid entering the cantilever (60) is delivered to the side of the threaded spindle (40) through the spindle channel (81) in the threaded spindle (40) and through the through hole (52).

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