Linear electro-mechanical actuator

Abstract

The present invention relates to a linear electro-mechanical actuator for transferring a rotational motion to a linear motion. The actuator provides a piston having an outer load-carrying surface and being at least partly arranged inside a housing. The actuator further provides a transmission module adapted to transfer a rotational motion generated by a motor to a linear motion of the piston. The actuator includes a separating member and a lubricating member having a porous polymeric matrix and a lubricating material, the separating member and the load-carrying member being arranged adjacent to each other. Thereby, the actuator allows for lubrication of at least a portion of the outer load-carrying surface of the piston by the lubricating material upon movement of the piston. For instance, the linear electro-mechanical actuator may not require, or may at least minimize, the need of relubrication.

Claims

1. A linear electro-mechanical actuator for transferring a rotational motion to a linear motion comprising: a piston having a distal end and a proximal end, the piston extending in an axial direction and having an outer load-carrying surface, the piston being at least partly arranged inside a housing and moveable relative to the housing in the axial direction; the housing having an opening being adapted to receive the distal end of the piston and defining an inner milieu; a transmission module operatively connected to the proximal end of the piston and adapted to transfer a rotational motion generated by a motor to a linear motion of the piston in the axial direction; a separating member being arranged adjacent to the opening of the housing and in between the piston and the housing as seen in a radial direction; and a lubricating member comprising a porous polymeric matrix and a lubricating material, the lubricating member being present in the inner milieu and arranged adjacent to the opening of the housing and in between the piston and the housing in the radial direction, wherein the lubricating member is arranged adjacent to the separating member, wherein the lubricating member is not load-carrying, thereby allowing for lubrication of at least a portion of the outer load-carrying surface of the piston by the lubricating material upon movement of the piston.

2. The linear electro-mechanical actuator according to claim 1, allowing for lubrication of substantially the entire outer load-carrying surface of the piston by the lubricating material.

3. The linear electro-mechanical actuator according to claim 1, wherein said lubricating member is a separate component of said linear actuator.

4. The linear electro-mechanical actuator according to claim 1, wherein said lubricating member has the shape of a bushing.

5. The linear electro-mechanical actuator according to claim 1, wherein the separating member is arranged such that it surrounds the entire periphery of a cross-section of the piston-that forms a portion of the outer load-carrying surface of the piston.

6. The linear electro-mechanical actuator according to claim 1, wherein the separating member is a scraper.

7. The linear electro-mechanical actuator according to claim 1, wherein the separating is a sealing member.

8. The linear electro-mechanical actuator according to claim 1, further comprising a guiding member, wherein the lubricating member is arranged in between the separating member and the guiding member in the axial direction.

9. The linear electro-mechanical actuator according to claim 1, wherein said housing has the shape of a cylinder.

10. The linear electro-mechanical actuator according to claim 1, wherein the transmission module further comprises a rotating portion and a non-rotating portion being operatively engageable to each other, and wherein the non-rotating portion is operatively connected to the proximal end of the piston, and wherein the transmission module is adapted to transfer a rotational motion of the rotating portion to a linear motion of the piston in the axial direction via the non-rotating portion.

11. The linear electro-mechanical actuator according to claim 10, wherein the rotating portion is a screw and the non-rotating portion is a nut.

12. The linear electro-mechanical actuator according to claim 10, wherein the rotating portion is a nut and the non-rotating portion is a screw.

13. The linear electro-mechanical actuator according to claim 1, wherein said housing has the shape of a circular cylinder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention.

(2) FIG. 1 presents a perspective partially sectioned schematic view of a linear electro-mechanical actuator according to an example embodiment of the present invention;

(3) FIG. 2 presents a perspective partially sectioned schematic assembled view of a portion of a linear electro-mechanical actuator according to an example embodiment of the present invention;

(4) FIG. 3 presents a perspective partially schematic exploded assembly view of a portion of a linear electro-mechanical actuator according to an example embodiment of the present invention;

(5) FIG. 4 presents a chart illustrating a pressure built up in the actuator on a retracting stroke and time to reach the atmospheric pressure, respectively; and

(6) FIG. 5 presents a chart illustrating a chart illustrating a vacuum built up in the actuator on an extending stroke and time to reach the atmospheric pressure, respectively.

DETAILED DESCRIPTION OF THE INVENTION

(7) The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

(8) The present invention relates to a linear electro-mechanical actuator 100 for transferring a rotational motion to a linear motion, which is schematically shown in FIG. 1. It should be readily appreciated that the linear electro-mechanical actuator may sometimes be denoted as the linear actuator or the actuator for the sake of simplicity. The actuator comprises a piston 10, a housing 20, a transmission module 30, and a separating member 40. In FIG. 1, the example embodiment of the actuator here further comprises a guiding member 62 and a motor 70. Throughout this description, the piston extends in the axial direction A and in the radial direction R. The linear electro-mechanical actuator further comprises a lubricating member (not shown in FIG. 1) described in more detail below.

(9) The piston 10 has a distal end 14 and a proximal end 16. The piston 10 extends in an axial direction A and has an outer load-carrying surface 12. The piston 10 is moveable relative to the housing 20 in the axial direction A. The housing 20 has an opening 22 being adapted to receive the distal end 14 of the piston 10. The housing 20 defines an inner milieu 101. Here, the housing 20 has the shape of a circular cylinder.

(10) As shown in FIG. 1, the piston 10 is at least partly arranged inside the housing 20. The part of the piston 10a being arranged inside the housing is arranged in the inner milieu 101. The part of the piston 10b extending outside the housing is arranged in the outer milieu 102. In a fully retracted state, the piston 10 is mainly, such as entirely, arranged in the inner milieu 101. In a fully extended state, the piston 10 is mainly, such as entirely, arranged in the outer milieu 102. In FIG. 1, the piston is in a partly extended state.

(11) The transmission module 30 is operatively connected to the proximal end of the piston 10 and adapted to transfer a rotational motion generated by the motor 70 to a linear motion of the piston 10 in the axial direction A.

(12) Although not strictly required, the transmission module 30 here comprises a rotatable screw shaft 33 with a non-rotatable nut (not shown) running thereon. The screw shaft extends over the full length of the actuator and sets the operating length of the actuator. The nut is held in a non-rotatable state, and is displaced when the screw shaft is rotated by the motor 70. The transmission module 30 is at least partly arranged inside the piston 10.

(13) The separating member 40 is arranged adjacent the opening of the housing 20 and in between the piston 10 and the housing 20 as seen in a radial direction R.

(14) The separating member 40, herein shown as a scraper 44, separates the inner milieu 101 from the outer milieu 102 at an opening 22 of the housing adapted to receive the distal end 14 of the piston. The scraper 44 further serves to clean the outer surface 12 of the piston when retracting from the outer milieu 102 into the inner milieu 101.

(15) As mentioned above, the linear actuator may typically, but not strictly necessarily, include a guiding member. In the example embodiment shown in FIG. 1, the guiding member 62 here is arranged in the inner milieu in between the piston and the housing as seen in the radial direction R. The guiding member 62 may be arranged either closer to the proximal end of the piston or closer to the distal end of the piston. In FIG. 1, the guiding member is arranged rather in the centre part of the piston. The guiding member serves to keep the piston 10 on track during its linear movements in the axial direction A. In particular, the guiding member serves to guide the piston such that it travels efficiently as it moves in the axial direction relative the housing.

(16) When being arranged relatively close to the distal end of the piston in its fully retracted state, the guiding member 62 advantageously serves to prevent wear on the separating member 40 caused by the piston 10 as well as scrapping off of lubricating material from the outer surface 12 of the piston while passing the opening 22 and the separating member.

(17) In FIGS. 2 and 3, a portion of the linear electro-mechanical actuator 100 in FIG. 1 is shown in more detail, namely, the lubricating member 50 and its surroundings. FIG. 2 shows the lubricating member 50 and its surroundings in an assembled state, while FIG. 3 is an exploded view of the lubricating member 50 and its surroundings. All features of the actuator 100 are not explicitly shown in both FIGS. 2-3.

(18) The piston 10 having a distal end 14 and a proximal end 16 extends in the axial direction A. The distal end extends into the outer milieu 102, and the proximal end is arranged inside the housing 20 and, thus, in the inner milieu 101.

(19) The proximal end 16 of the piston is operatively connected to a nut 37 of a transmission module. The nut 37 has a threaded inner surface 38 and is operatively engageable with a screw 33 of the transmission module. The screw has a treaded outer surface 34. A rotational motion of the screw may be generated by a motor 70.

(20) The lubricating member 50 comprises a porous polymeric matrix and a lubricating material. As illustrated in FIG. 2, the lubricating member 50 is present in the inner milieu 101 and is arranged adjacent to the opening 22 of the housing 20 and in between the piston 10 and the housing 20 as seen in the radial direction R.

(21) At the interface between the inner milieu and the outer milieu, a sealing member 42 and a scraper 44, respectively, are arranged. Both the sealing member 42 and the scraper 44 are arranged in between the piston 10 and the housing 20 as seen in the radial direction. In FIG. 2, both separating members 42, 44 are arranged about the piston and surround the entire periphery 19 of a cross-section 18 of the piston.

(22) Accordingly, in this example embodiment, the linear actuator here includes a first separating member being a scraper and a second separating member being a sealing member. However, as mentioned above, the linear actuator may only include one separating member in the form of a scraper. In another example embodiment, the linear actuator may only include one separating member in the form of a sealing member.

(23) As illustrated in FIG. 3, the sealing member 42 here has the shape of a bushing. The sealing member seals the space between the outer surface 12 of the piston and the inner surface of the housing. The inner surface of the housing faces the outer surface of the piston.

(24) Moreover, the scraper 44 here has a conical shape, with a first end 45 having a periphery substantially equal to or slightly larger than the periphery of a cross-section of the piston and a second end 46 having a periphery substantially equal to or slightly smaller than the periphery of a cross-section of the housing. As is illustrated in FIG. 2, the first end 45 of the scraper faces the outer milieu and the second end 46 of the scraper faces the inner milieu.

(25) The sealing member 42 and the scraper 44 are arranged in contact to each other, or in close proximity to each other, as shown in FIG. 2. Hence, the sealing member 42 may be arranged at a minor distance from the scraper 44 without departing from the scope of the present invention as long as the functions of the arrangements are maintained.

(26) As illustrated in FIGS. 2 and 3, the actuator 100 here further includes a guiding member 62 having the shape of a sleeve, surrounding the entire periphery of a cross-section of the piston. The guiding member 62 is entirely arranged in the inner milieu, in between the piston and the housing as seen in the radial direction R. As mentioned above, it is to be noted that the guiding member is only an optional component of the actuator.

(27) As illustrated in FIG. 2, the lubricating member 50 is arranged adjacent to the separating member 40. That is, the lubricating member 50 is arranged adjacent to the separating member 40 as seen in the axial direction A. Further, the lubricating member 50 of the actuator here is arranged in between the guiding member 62 and the sealing member 42 as seen in the axial direction A.

(28) In FIG. 2, the lubricating member has the shape of a bushing 52, surrounding the entire periphery of a cross-section of the piston. The lubricating member 50 is arranged such that it allows for lubrication of the outer surface of the piston, the sealing member and the scraper.

(29) It should be readily appreciated that in all of the embodiments of the present invention, the lubricating member may not necessarily be a bushing. Accordingly, the lubricating member can be provided in several different forms as long as the lubricating member can include a porous polymeric matrix and a lubricating material while fulfilling the required function of the lubricating member.

(30) As illustrated in FIG. 2, the separating members 42, 44 here are arranged about the piston. Analogously, the lubricating member 50 here is arranged about the piston. Analogously, the guiding member 62 here is arranged about the piston.

(31) Typically, the guiding member 62 is load-carrying, while the lubricating member 50 is not. Optionally, although not strictly required, also at least one of the separating members 42, 44 is load-carrying.

(32) In order to ensure a smooth operation of the linear actuator, the piston 10 should be freely moveable in the axial direction relative to the guiding member 62, the lubricating member 50 and the separating members 42, 44.

(33) The arrangement of the linear electro-mechanical actuator, shown in general in FIG. 1 and more in detail in FIGS. 2 and 3, allows for lubrication of at least a portion of the outer load-carrying surface 12 of the piston 10 by the lubricating material of the lubricating member 50 upon movement of the piston.

(34) In all of the embodiments of the present invention, there is provided a linear electro-mechanical actuator which is capable of improving the application of the lubrication in terms of precision and functionality, while providing a precise amount of a lubricating material. In this context, the linear electro-mechanical actuator according to the present invention may not even require relubrication. More specifically, by the arrangement of the linear electro-mechanical actuator as described above, it becomes possible to assemble the actuator easily in a dry state of the lubricating member, i.e. with no smeary grease, or other form of liquid or semi-liquid lubricating material, present except in the porous polymeric matrix of the lubricating member. In addition, the linear electro-mechanical actuator may easily be used due to a relatively controlled consumption of lubricating material causing substantially no leakage of lubricating material as well as due to its tolerance to e.g. washing as well as the linear electro-mechanical actuator may allow for environmentally friendly handling of the lubricating member including the unconsumed lubricating material at end of service life, in particular when provided as a separate member.

(35) Examples

(36) Performance tests have been performed by the inventors in order to support the inventiveness of the present invention. As will be shown below, the performance tests showed surprisingly good results. It is to be noted that the performance tests include some further components of the linear actuator which are considered only optional for the present invention.

(37) In more detail, the performance tests focusing on wear under normal circumstances related to lubrication of a piston, a guiding member and a sealing member in a so-called actuator of the type SKF ActSys GBG 0407538 (SKF, Sweden). A conventional SKF ActSys GBG 0407538 actuator comprising grease as lubricating material was compared to a modified SKF ActSys GBG 0407538 actuator comprising a lubricating member according to the invention, namely, a so-called solid oil.

(38) In the both tested actuators, the piston was guided by a guiding member and sealed by a sealing member to the housing. The guiding member had the shape of a sleeve. The sealing member had the shape of a bushing (a so-called lip sealing).

(39) In the tested modified actuator, the lubricating member was arranged in between the sealing member and the guiding member in the axial direction. The lubricating member had the shape of a bushing arranged to surround the periphery of a cross-section of the piston. In the test, the bushing had an outer diameter of 33 mm, an inner diameter of 28 mm and a width of 10 mm, giving it a volume of 2.4 cm3. The lubricating member, the scaling member and the piston were assembled in a dry state (i.e. in a non-lubricated state).

(40) Tests were set up to run for some 500 000 cycles, both to pronounce the wear for regular usage as well as to give an indication of usability of the SKF ActSys GBG 0407538 actuator where an L.sub.10 value of 1 500 000 cycles are used. The L.sub.10 value gives an indication of the service life of the actuator by stating that less than 10% of the actuators break down before the present number of cycles is reached.

(41) To accentuate a radial load over the guiding member the actuator was dislocated with up to 3 cm thereby applying an uneven load. No other extra artificial wear condition, such as dirt, heat or moist, was introduced during the tests.

(42) Test 1: A Comparative Test Between a Conventional SKF ActSys GBG 0407538 Actuator (SKF, Sweden) and a SKF ActSys GBG 0407538 Actuator Comprising a Lubricating Member

(43) The actuators were mounted vertically with a pushing load during the test.

(44) During the initial 50 000 cycles, a forced radial load of 15 mm and a 50 kg load, which corresponded to a radial load of approximately 50 N over the guiding member, was used. The speed was set to approximately 75 mm/s.

(45) During the subsequent 450 000 cycles, a forced radial load of 35 mm and a 10 kg load, which corresponded to approximately the same radial load as above (but with less back-driving force), was used. The speed was set to approximately 85 mm/s.

(46) The length of the stroke was approximately 220 mm, which resulted in a total travel distance of the sealing member relative the piston of 220 km during the test.

(47) Wear and cleanliness on the piston and the sealing member on each of the actuators were observed during the test. At some points during the tests and eventually when reaching 500 000 cycles, the actuators were disassembled and the sealing member, the piston and the guiding member were further visually observed for wear.

(48) During the initial 75 000 cycles, the piston of the conventional actuator run drier and started to build up debris at the sealing member and showed typical blackening of its surface. During the subsequent cycles up to a total of 500 000 cycles, the actuator showed increased wear of the sealing member and the guiding member as well as increased discoloring of the piston.

(49) During the initial 75 000 cycles, the piston of the modified actuator comprising a lubricating member became quite oily with a pronounced wet ring at the end of each stroke and remained clean. During the subsequent cycles up to a total of 500 000 cycles, the wear of the sealing member and the guiding member as well as the discoloring of the piston remained marginal.

(50) The weight of the lubricating member, therein a bushing of solid oil, was measured before and after the test, respectively, in order to quantify the consumption of the lubricating material of the lubricating member.

(51) Before the test, the lubricating member measured 1.94 gram. After 160 000 cycles, the lubricating member measured 1.92 gram. At the end of the test, i.e. after 500 000 cycles, the lubricating member measured 1.89 gram. The lubricating member in the modified actuator allowed for an at least almost linear consumption of lubricating material during the cycles run by the actuator. This result implies that a more efficient lubrication was enabled in the modified actuator than in a conventional actuator both with regard to location and amount of lubricating material.

(52) Test 2: A Test to Observe the Wear of the Sealing Member

(53) In Test 2, the same actuators as in Test 1 were used. The actuators were mounted horizontally without any external load during the test. A total of 500 000 cycles was run.

(54) The initial 250 000 cycles were run at 85 mm/s, and the final 250 000 cycles were run at 100 mm/s. The forced radial load was kept at 35 mm, which corresponded to a radial load of approximately 50 N. The length of the stroke was approximately 230 mm, which resulted in a total travel distance of the sealing member relative the piston of 230 km during the test.

(55) The pressure inside the actuator was measured after a certain numbers of cycles. The time to reach atmospheric pressure was also measured. In an ideal state, the pressure difference in the inner milieu corresponds to the time for balancing the pressure difference in the extended state of the actuator. Thus, the variance in the measured parameters was presumed to correspond to the wear of the sealing member and the guiding member, respectively.

(56) The pressure inside an actuator is build up on a retracting stroke. Vacuum in an actuator is build up on an extending stroke.

(57) In the test, it was seen that the pressure built up on a retracting stroke and the time to reach atmospheric pressure slightly increased over the run cycles.

(58) FIG. 4 presents a chart 200 illustrating a pressure built up in the actuator on a retracting stroke and time to reach the atmospheric pressure, respectively.

(59) In FIG. 4, the solid line 202 corresponds to the pressure, measured in MPa, built up in the actuator on a retracting stroke, and the dotted line 204 corresponds to the time, measured in seconds, to reach the atmospheric pressure.

(60) Both results indicate that the leakage of air through the sealing member and the guiding member, respectively, decreased over the run cycles.

(61) In the test, it was seen that the vacuum built up on an extending stroke and the time to reach atmospheric pressure slightly increased over the run cycles.

(62) FIG. 5 presents a chart 210 illustrating a vacuum built up in the actuator on an extending stroke and time to reach the atmospheric pressure, respectively.

(63) In FIG. 5, the solid line 212 corresponds to the vacuum, measured in MPa, built up in the actuator on an extending stroke, and the dotted line 214 corresponds to the time, measured in seconds, to reach the atmospheric pressure.

(64) Both results indicate that the leakage of air through the sealing member and the guiding member, respectively, decreased over the run cycles.

(65) Before the test, the lubricating member measured 1.95 gram. At the end of the test, i.e. after 500 000 cycles, the lubricating member measured 1.90 gram.

(66) To conclude, the results of the tests were that the actuator comprising a lubricating member showed less signs of wear, had a more persistent and controlled lubrication as well as a neater appearance than the actuator comprising conventional grease.

(67) These advantages may lead to less problems relating to excessive grease leakage from the actuator onto a door mechanism compartment as well as a facilitated assembly process of the actuator due to a solid lubricating member instead of loose grease.

(68) Additionally, variations to the disclosed example embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

REFERENCE NUMBERS

(69) 100 linear electro-mechanical actuator 101 inner milieu 102 outer milieu A axial direction R radial direction 10 piston 10a part of piston in the inner milieu 10b part of piston in the outer milieu 12 outer load-carrying surface of the piston 14 distal end of the piston 16 proximal end of the piston 18 cross-section of the piston 19 periphery of the cross-section of the piston 20 housing 22 opening being adapted to receive the distal end of the piston 30 transmission module 33 screw 34 threaded outer surface 37 nut 38 threaded inner surface 40 separating member 42 sealing member 44 scraper 50 lubricating member 52 bushing 62 guiding member 70 motor 200 pressure build up chart 202 pressure built up in the actuator on a retracting stroke (solid line) 204 time to reach the atmospheric pressure (dotted line) 210 vacuum build up chart 212 vacuum built up in the actuator on an extending stroke (solid line) 214 time to reach the atmospheric pressure (dotted line)