Electromechanical actuator with sliding piston for improved hydrodynamic bearing

Abstract

Linear actuator (10) with a housing (20) and a cantilever (60) which projects out of the housing (20) in the direction of a longitudinal axis (13), wherein the cantilever is movable in the direction of the longitudinal axis (13), wherein a threaded spindle (40) is rotatably mounted on the housing (20) at its first end (41) by means of a spindle pivot bearing (43) with respect to the longitudinal axis (13), wherein a second end (42) of the threaded spindle (40), opposite the first end (41) in the direction of the longitudinal axis (13), projects into the cantilever (60), regardless of the position of the cantilever (60), wherein the second end (42) of the threaded spindle (40) is provided with a sliding piston (50) which bears substantially fluid-tight against an inner circumferential surface of the cantilever (60), so that it defines a first and a second cavity (11; 12) within 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 pivot bearing (43),wherein the boom (60) is fixedly connected to a threaded nut (62) which is in screw 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 opposite ends in the direction of the longitudinal axis (13) in such a way that the liquid can flow through the threaded nut (62) upon movement of the boom (60), characterized in that a first longitudinal channel (81) is arranged inside the threaded spindle (40), which extends along the longitudinal axis (13), wherein the first longitudinal channel (81) is fluidically connected to the first cavity (11) at its ends opposite the longitudinal axis (13) in such a way that upon movement of the boom (60), the liquid flows through the first longitudinal channel (81), bypassing the threaded nut (62), regardless of the position of the boom (60),and wherein this flow passes through a sliding bearing between the sliding piston (50) and the threaded spindle (40).

Description

Technical FieldThe present disclosure relates to an electromechanical actuator with a sliding piston having the features listed in claim 1.Electromechanical actuators are known in the prior art. For example, DE 10 2020 212 703 A1 discloses an electromechanical actuator referred to there as an "electric cylinder", in which one end of a threaded spindle is connected to a piston which divides an interior space of the actuator into a first and a second region, which are delimited from one another in a fluid-tight manner. The first region is exclusively bounded there by the piston and a cantilever which projects in the direction of an axis of rotation of the threaded spindle from a housing which accommodates the threaded spindle and the cantilever. The piston is rotatably mounted on the threaded spindle via a slide bearing and is to be prevented from rotating as far as possible with respect to the extension arm.DE 10 2014 213 505 B4 likewise teaches an electric cylinder in which the spindle bearing has a support device with a transversely movable rolling bearing (instead of the aforementioned sliding bearing). More specifically, according to this disclosure, a pivot bearing is radially movable to enable rotational movement.However, the friction between the sliding piston and the spindle can still be optimized in order to waste less energy, to have to remove less friction heat that arises, and in particular in order to minimize wear. For this purpose, in particular a rotation of the piston in the boom must be prevented.In detail, in a linear actuator, a threaded spindle is mounted on the housing such that it can rotate with respect to the longitudinal axis by means of a pivot bearing. The extension arm can be extended and retracted by means of a nut screwed onto the threaded spindle. The threaded spindle is rotated by an electric motor. Above all, at the screw engagement of the threaded spindle and nut, (frictional) heat arises. Therefore, the inner space is filled with a fluid or a mixture of lubricating oil and gas. The lubrication has a hermetic seal against the external atmosphere to eliminate leakage and contaminants. Because the cantilever extends and retracts in the housing, so that the interior volume changes, complete filling with an incompressible fluid (or theoretically also with lubricating solids such as powder) is disadvantageous to impossible. Conversely, 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. As the nut moves, it displaces the fluid flowing through the cooling channels and past the nut, for example, rolling elements supporting the nut, as well as through channels in the spindle.In particular, when the boom is being moved in and out rapidly, the fluid cannot easily flow through the cooling channels present and past the threaded nut, so that a strong differential pressure is produced in the interior of the linear actuator. This differential pressure leads to a higher idling torque of the axle, which increases as a function of the axle speed and thus the flow speed. This results in a lower efficiency of the axle.It is the object of the invention to eliminate or at least alleviate the disadvantages discussed above.Therefore, the present invention provides a linear actuator having a housing and a cantilever which protrudes from the housing in the direction of a longitudinal axis, wherein it is movable in the direction of the longitudinal axis, wherein a threaded spindle is mounted on the housing at its first end rotatably with respect to the longitudinal axis by means of a spindle rotary bearing, wherein a second end of the threaded spindle, which second end is opposite the first end in the direction of the longitudinal axis, protrudes into the cantilever, at which position the cantilever is at which position the second end of the threaded spindle is provided with a sliding piston which bears substantially fluid-tightly against an inner circumferential surface of the cantilever such that it delimits a first and a second cavity within the linear actuator from one another, wherein the second cavity is arranged on the side of the sliding piston remote from the spindle rotary bearing, wherein the cantilever is fixedly connected to a threaded nut, which is in screw engagement with the threaded spindle (for example via a plurality of rolling bodies), wherein the first cavity is at least partially filled with a liquid, wherein the threaded nut is open at its two ends opposite in the direction of the longitudinal axis in such a way that the liquid can flow through the threaded nut during a movement of the boom, wherein a first longitudinal channel is arranged in the interior of the threaded spindle, which channel extends along the longitudinal axis, wherein the first longitudinal channel is fluidically connected to the first cavity at its ends opposite in relation to the longitudinal axis in such a way that the liquid can flow through the first longitudinal channel during a movement of the boom, bypassing the threaded nut, is at the same position as the boom, and wherein a sliding bearing between the sliding piston and the threaded spindle is flowed through during this flow. This means that in particular the sliding piston is matched to the threaded spindle and itself is geometrically designed such that a part of the flow flows between these two components and another part of the flow flows through the sliding piston itself.More particularly, the present invention provides such a sliding piston in which rotation relative to the boom is prevented by the seal friction on the piston being higher than the torque which occurs as resistance in the hydrodynamic bearing in any operating state. The lubricant film is prevented from being torn off so that no "seizure" or increased wear occurs which would destroy the bearing. This is important above all at changing temperatures in external use and, associated therewith, changing viscosity of the lubricant.An elastomer slide bearing cannot be used on account of the temperature requirements in external use. Since the thermal expansion coefficients of elastomer and steel are too different, too large a clearance would be produced at a higher temperature. Although a solution similar to DE 10 2014 213 505 B4 could be used, the movable bearing would have to be very delicate because of the additional flow channels. This idea is therefore considered uneconomical for reasons of cost and durability.This is achieved according to the invention by a sealing piston with optimized geometry and optimized material selection. The second region of the electromechanical actuator is partially filled with lubricating oil, similar to DE 10 2020 212 703 A1. A first fluid flow path is provided, which comprises the bearing gap of the piston slide bearing, wherein the first fluid flow path runs such that a movement of the boom brings about a lubricating oil flow in the first fluid flow path, in particular through the piston slide bearing. What is important in connection with the present invention is above all that the lubricating oil, which is displaced on one side of the threaded nut, can pass via the spindle channel to the other side of the threaded nut.The piston runs on the threaded spindle on a circular cylindrical surface with respect to its longitudinal axis. More specifically, the spindle rotates within the piston while the piston is not rotating as much as possible.The spindle is preferably arranged so as to pass completely through the piston. On the side of the threaded spindle which is remote from the (actual) thread for displacing the extension arm via the threaded nut, a nut can be screwed onto an external thread of the threaded spindle, which nut holds the piston axially on the threaded spindle. Thus, the threaded spindle (and thus also a spindle channel present in the threaded spindle) penetrates the nut. Lubrication fluid flowing out of the spindle channel can thereby pass through the piston onto the side of the piston facing away from the threaded spindle.By helically arranged grooves in the piston provided on the side facing the spindle, more precisely in the region of the sliding contact, most of the fluid flow can flow in the first fluid flow path when the spindle and the fixed sliding piston are rotated. Starting from these grooves, the lubricating oil can easily spread into the actual bearing gap. The grooves preferably extend continuously from the piston end face, which abuts the threaded spindle, over the piston inner circumferential surface, so that nowhere is there a constriction for the oil flow produced. Therefore, hydrodynamic lubrication of the slide bearing is improved.Preferably, the grooves are helical such that the oil flow in the first fluid flow path is assisted.A second fluid flow path can be formed by axially parallel bores in the piston. This second fluid flow path runs parallel to the first fluid flow path. By means of the flow resistance (in other words the diameter) of the axially parallel bores, it is possible to control which proportion of the total oil flow flows through the plain bearing. Preferably, the ratio between the cross-sectional area of the grooves and the through-holes is between 50 and 200%, in other words, the cross-sectional area of the through-holes is between half and twice as large as the cross-sectional area of the grooves.On the outside of the piston, a seal is provided consisting of guide rings and dynamically acting sealing rings. These components are standard commercial goods, which reduces the overall costs. The friction of the sealing rings must be so high that the relative movement takes place on the spindle running surface and not on the guides and seals of the piston. The piston should therefore not rotate.In contrast to the closest prior art, it is also advantageous not to use plastic, but rather an aluminum wrought alloy with a high silicon content as piston material. As a result, the coefficient of expansion of the piston approximates the coefficient of other parts of the electromechanical actuator, in particular the coefficient of the spindle and spindle, which likewise consist of metallic materials and cooperate with the piston. This enables more stable lubrication in a larger temperature range. The high silicon content improves the emergency running properties.The construction having the features explained above achieves, in comparison with DE 10 2020 212 703 A1, that the fluid is hermetically sealed.The inner space of the electromechanical actuator can flow from the inner space of the spindle sleeve or of the extension arm to the deep hole bore in the spindle and vice versa.Although "through holes", "deep hole holes", etc. are mentioned in this illustration, it should be obvious to a person skilled in the art that the corresponding connecting channels between fluid inlets and outlets on surfaces and blind holes can be produced not only by drilling, but also by any other desired production methods (etching, additive production, Senkerodieren,.etc. may be mentioned as examples). For simplicity, "bores" are nevertheless written in this illustration.Brief Explanation of the FiguresFIG. 1 shows a linear actuator according to the invention for illustrating the circulation of fluid. FIG. 2 is an isometric view of a sliding piston according to a first embodiment of the present disclosure; FIG. 3 is an isometric view of the slide piston of FIG. 2 viewed from the other side; and FIG. 4 is an enlarged view of the slide piston and its surroundings in an electromechanical actuator in the installed state from FIG. 1.DESCRIPTION OF THE EMBODIMENTSHereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.FIG. 1 shows a linear actuator 10 having a housing 20 and a cantilever arm 60 which protrudes from the housing 20 ("upward" in the figure) along a longitudinal axis 13. The boom 60 is movable in the direction of the longitudinal axis 13. A threaded spindle 40 is mounted on or in the housing 20 at its first end 41 by means of a spindle rotation bearing 43 such that it can rotate about the longitudinal axis 13 and can be driven by a motor connected to the first end 41. A second end 42 of the threaded spindle 40 opposite the first end 41 in the direction of the longitudinal axis 13 always protrudes into the extension 60. The second end 42 of the threaded spindle 40 is provided with a sliding piston / piston / (sliding) piston 50, which is explained in more detail below, which bears substantially fluid-tightly against an inner circumferential surface of the extension arm 60 in such a way that it delimits a first and a second cavity 11; 12 in the linear actuator 10 from one another. The second cavity 12 is arranged on the side of the sliding piston 50 facing away from the spindle pivot bearing 43. The extension arm 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, e.g. planets) not shown here. The first cavity 11 is at least partially filled with a liquid, more specifically it contains in this embodiment a fluid which is a mixture of liquid, e.g. lubricating oil, and gas, such as nitrogen. The threaded nut 62 is open at its two ends opposite in the direction of the longitudinal axis 13 in such a way that the fluid can flow through the threaded nut 62 during a movement of the boom 60. In the interior of the threaded spindle 40 a first longitudinal channel 81 is arranged, which extends along the longitudinal axis 13. The first longitudinal channel 81 is fluidically connected to the first cavity 11 at its ends opposite with respect to the longitudinal axis 13 (at the first end 41, for example, by transverse bores which open into the longitudinal channel, which is illustrated in the figure by some points connected to a "thick line") in such a way that the fluid can also flow through the first longitudinal channel 81 during a movement of the boom 60, independently of its position, while bypassing the threaded nut 62, wherein this flow flows through a slide bearing between the slide piston and the threaded spindle and thus lubricates it.FIG. 2 shows the sliding piston 50 illustrated in a built-in manner in FIGS. 1 and 4 from the side bearing against the threaded spindle of the electromechanical actuator, i.e. viewed from the left-hand side in FIGS. 1 and 4. A plurality of inlets of through-holes 52 and, in this embodiment, three helical grooves 55 spaced apart over the circumference can be seen. The grooves 55 each extend continuously from the piston end face, which abuts a threaded spindle 40, over the inner circumferential surface of the sliding piston 50, so that nowhere is there a constriction for the oil flow produced. The grooves 55 extend in a helical manner such that the oil flow during rotation of the threaded spindle 40 and thus simultaneous engagement or engagement. Thus, extension of a boom 60 is supported. The through holes 52 form a second fluid flow path.FIG. 3 shows the same sliding piston 50 in isometric view from the other side (from "right" in FIGS. 1 and 4 ). In addition to the through-bores 52 and the grooves 55, four threaded blind holes 58 can be seen distributed over the circumference. From the comparison of the outputs of the through-bores 52 in FIGS. 2 and 3, but better still in the section of FIG. 4, it can be seen that the through-bores 52 run obliquely through the sliding piston. The aforementioned blind tapped holes 58 serve to secure an axial seal 20 (shown in FIG. 4) to the slide piston 50 by means of screws 30. This axial seal 20 can be designed as a non-return valve (explained in more detail in a parallel application), for example on a membrane basis, or can comprise such a non-return valve and in this case can convey fluid that has entered the extension 60 again via the spindle channel 81 in the threaded spindle 40 and via the through-bores 52 to the side of the threaded spindle. Fluid, in particular incompressible lubricating oil or grease, which reaches the boom as a result of a negative pressure occurring during the extension could otherwise prevent the renewed retraction if it could not leave the hermetically sealed boom.It can also be seen from FIG. 3 that space for a threaded nut 45 (cf. FIG. 4 ) is created in the sliding piston. After the slide piston 50 has been applied to the threaded spindle 40, this threaded nut 45 is screwed onto an external thread of the spindle 40 and holds the slide piston 50 axially on a circular cylindrical surface. The spindle channel 81 passes through the external thread and thus also the threaded nut 45.In contrast to prior art pistons using POM plastic material, it has been found advantageous in the present invention to produce the piston 50 from a wrought aluminum alloy with a high silicon content. This material has a similar thermal expansion to the metallic cantilevers 60 and spindle 40, so that lubrication can remain stable over a wide temperature range.As can be seen from FIG. 4, the piston is supported on the extension arm 60 via guide bands 90 and interacts with seals 100 in order to carry out a seal between the first cavity 11 and the second cavity 12 (not shown again in FIG. 4 for reasons of clarity) on the left and right of the sliding piston 50. The first cavity 11 is a region around the spindle 40 filled with lubricating fluid, while the second cavity 12 on the right of the axial seal is a region largely free of lubricating fluid in the boom 60, the guide bands 90 and seals 100 serving to support the piston in the boom 60 in a manner as rotationally fixed as possible. Relative rotation should only occur between the spindle 40 and the slide piston 50. This is achieved by selecting the friction of the seals 100 on the arm 60 and on the sliding piston 50 to be so high that the relative movement takes place on the spindle cam surface 42 and not on the guide bands and seals of the sliding piston 50, which is facilitated by the fact that a lower frictional force is to be generated on the outer diameter of the sliding piston 50 in order to compensate for the torque occurring between the spindle running surface 42 and an 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 seals 100.The guide belts 90 take over the radial loads occurring during operation as guide elements customary in corresponding cylinders. The guide belts 90 are usually made of plastic or a soft metal in order to avoid contact between the metallic components of the linear actuator. The material of the guide bands 90 offers less friction and, owing to its better elastic deformation, a greater contact area than the metallic components "sliding piston 50" and "cantilever 60". In addition, self-lubricating material, e.g. corresponding plastic, can be used. Such standard elements are of any desired size at low cost.The sliding piston 50 is accordingly held rotationally fixedly relative to the boom 60 by the guide bands 90 and (above all) the seals 100 and accordingly slides on a spindle running surface 42. A flow of lubricating fluid from the spindle channel 81 via both the through-bores 51 and the grooves 55 into the first region and vice versa is made possible by the lubricating grooves 55 in the sliding piston 50. Thanks to the grooves 55, the lubricating fluid forms a lubricating film between the sliding piston 50 and the spindle running surface 42.In summary, the present invention provides an electromechanical linear actuator 10 having, among other things, a sliding piston 50 with helically configured grooves 55 for a floating bearing of a spindle 40. During a relative movement between spindle 40 and sliding piston 50, a lubricating fluid passing through the grooves 55 between sliding piston 50 and spindle 40 can support the hydrodynamic lubrication between these two parts.

Claims

Linear actuator (10) having a housing (20) and a cantilever (60) which projects out of the housing (20) in the direction of a longitudinal axis (13), wherein it is movable in the direction of the longitudinal axis (13), wherein a threaded spindle (40) is mounted on the housing (20) at its first end (41) rotatably with respect to the longitudinal axis (13) by means of a spindle rotary bearing (43), wherein a second end (42) of the threaded spindle (40), which second end is opposite the first end (41) in the direction of the longitudinal axis (13), projects into the cantilever (60), regardless of the position in which the cantilever (60) is located, wherein the second end (42) of the threaded spindle (40) is provided with a sliding piston (50) which bears substantially fluid-tightly against an inner circumferential surface of the cantilever (60), such that it delimits a first and a second cavity (11; 12) within the linear actuator (10) from one another, wherein the second cavity is arranged on the side of the sliding piston (50) facing away from the spindle rotary bearing (43), wherein the arm (60) is firmly connected to a threaded nut (62) which is in screw 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 opposite in the direction of the longitudinal axis (13) in such a way that the liquid can flow through the threaded nut (62) during a movement of the arm (60), 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 ends opposite in relation to the longitudinal axis (13) in such a way, the liquid flows through the first longitudinal channel (81) during a movement of the boom (60), bypassing the threaded nut (62), regardless of the position of the boom (60), and wherein a slide bearing is flowed through between the slide piston (50) and the threaded spindle (40) during this flow.Linear actuator (10) according to claim 1, comprising a sliding piston (50) having through-holes (52) which are provided to be flowed through parallel to the threaded spindle (40) and to provide a fluid connection between the first and the second cavity (11, 12).Linear actuator (10) according to Claim 2, wherein the through-bores (52) converge towards a central axis of the sliding piston (50).Linear actuator (10) according to Claim 2 or 3, wherein helically arranged grooves (55) are provided in the sliding piston (50) on the side pointing towards the threaded spindle (40), namely in the region of a sliding contact towards the threaded spindle (40).The linear actuator (10) of claim 4, wherein the cross-sectional area of the through-holes (52) is between 50% and 200% of the cross-sectional area of the grooves (55).The linear actuator (10) of claim 5, wherein upon movement of the boom (60), more fluid also flows through the through-holes (52) than through the plain bearing and the grooves (55).The linear actuator (10) according to any one of the preceding claims, wherein the slide piston (50) is made of a wrought AlSi alloy.Linear actuator (10) according to one of the preceding claims, further comprising guide bands (90) and seals (100) which are provided on the outer side of the sliding piston (50) and hold the sliding piston (50) on the boom (60) in a rotationally fixed manner by means of friction.Linear actuator (10) according to one of the preceding claims, further having an interior space which is enlarged in relation to a running surface (57) of the sliding piston (50) for receiving a threaded nut (45).Linear actuator (10) according to one of the preceding claims, further comprising threaded holes (58) for fastening an axial seal (20) by means of screws (30).Linear actuator according to Claim 10, further comprising an axial seal (20) which comprises a nonreturn valve and conveys fluid which has entered the boom (60) again via the spindle channel (81) in the threaded spindle (40) and via the through-bores (52) to the side of the threaded spindle (40).

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