Impact Absorbing Actuator End Stop

Abstract

1. A linear actuator comprising: a shaft; a nut that engages with the shaft such that the nut translates axially on a central axis relative to the shaft within a linear range of motion in response to relative rotation between the nut and the shaft; a locking portion located in a limited-travel position and having a rotational input portion and an axial output portion, the rotational input portion configured to rotate about the central axis relative to the shaft, the axial output portion being restrained from rotating about the central axis relative to the shaft, and configured to translate axially on the central axis relative to the rotational input portion in response to relative rotation between the axial output portion and the rotational input portion; and an axial compliance member configured to bias the axial output portion toward the limited-travel position.

Description

Technical Field

[0001] Broadly speaking, the present invention relates to the field of ball screw actuators, and more specifically, to an actuator having an improved shock absorbing end stop.

Background Art

[0002] Ball screw actuators are well known in the art and generally include a screw that mates with a nut and is driven by a motor. Relative rotation between the screw and the nut causes an axial displacement between the screw and the nut. This axial displacement typically drives the linear stroke of the operating mechanism. In such a system, one or more end stops can be used to limit the movement of the actuator to a fully retracted and / or fully extended position, thereby limiting damage to the motor, ball screw, or mechanical connections.

[0003] Patent Document 1 is directed to a flexible and fail-safe end stop for a ball screw actuator. The ball screw actuator includes a torsion spring operably arranged to absorb energy when the first and second locking portions of the ball screw actuator engage in a movement limiting position.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Means for Solving the Problems

[0005] References are made in parentheses to the corresponding parts, portions, or surfaces of the disclosed embodiments, which are for illustration only and not for limitation. The present invention provides a linear actuator (15) comprising a shaft (16) oriented about a central axis, and a nut (18) engaging the shaft (16), the nut (18) being translatable relative to the shaft (16) along a linear range of axial movement on the central axis (30) in response to relative rotation between the nut (18) and the shaft (16) about the central axis (30), a locking portion (20) located at a movement limiting position of the movement range between the nut (18) and the shaft (16), the locking portion (20) having a rotational input portion (23) and an axial output portion (21), the rotational input portion (23) being configured to rotate about the central axis (30) relative to the shaft (16), the axial output portion (21) being inhibited from rotating about the central axis (30) relative to the shaft (16), and the axial output portion (21) being configured to translate axially (27) on the central axis (30) relative to the rotational input portion (23) and the shaft (16) in response to relative rotation about the central axis (30) therebetween, and an axially flexible member (19) configured to axially bias the axial output portion (21) axially on the central axis (30) toward the movement limiting position.

[0006] The shaft (16) can comprise an outer ball track (43), the nut (18) can comprise an inner ball track (44), and a plurality of balls (45) can be disposed within the outer ball track (43) and the inner ball track (44). The axially flexible member (19) can comprise a spring acting between the shaft (17) and the axial output portion (36). The locking portion (20) can comprise a plurality of balls (22) axially disposed between the rotational input portion (23) and the axial output portion (21).

[0007] The rotational input portion (23) can include a first annular cam surface (35a, 35b), and the axial output portion (21) can include a second annular cam surface (34a, 34b) facing the first annular cam surface (35a, 35b). A plurality of balls (22) can be arranged axially between the first annular cam surface (35a, 35b) and the second annular cam surface (34a, 34b).

[0008] The nut (18) can include a torsional input locking portion (26), and the rotational input portion (23) of the locking portion (20) is configured such that when the torsional output locking portion (25) and the torsional input locking portion (26) axially overlap and abut in the rotational direction, when the nut (18) rotates in a first direction (28a) around the central axis with respect to the shaft (16), the rotational input portion (23) of the locking portion (20) rotates in a first direction (28b) around the central axis (30) with respect to the shaft (16). It can include a torsional output locking portion (25).

Brief Description of the Drawings

[0009]

Figure 1

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DETAILED DESCRIPTION OF THE INVENTION

[0010] First, it should be clearly understood that like reference numerals throughout several views are intended to identify the same structural elements, parts, or surfaces consistently, as such elements, parts, or surfaces may be further described or explained by the entire specification, which this detailed description is an integral part of. Unless otherwise specified, the drawings are intended to be read with the specification (e.g., cross-hatching, component placement, ratios, degrees, etc.) and considered as part of the overall description of the invention. As used in the following description, the terms “horizontal,” “vertical,” “left,” “right,” “up,” and “down,” and their adjectival and adverbial derivatives (e.g., “horizontally,” “rightwardly,” “upwardly,” etc.) refer to the orientation of the illustrated structure when the view of a particular drawing faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of extension or rotation, as appropriate.

[0011] FIG. 1 shows a kind of ball screw assembly known in the prior art. As shown, the ball screw assembly uses the rolling motion of balls located between a shaft and a nut to generate relative motion between the shaft and the nut. The ball bearing assembly functions as a nut, while the threaded shaft provides a helical path for the ball bearings. Thereby, the ball screw assembly can operate as a mechanical linear actuator that converts rotational motion into linear motion with less friction.

[0012] Referring to FIGS. 2-3, an improved ball screw end stop assembly is provided, and its first embodiment is generally indicated at 15. As shown in the drawing, the assembly 15 generally includes a ball screw shaft 16 oriented about an axis 30, a ball screw nut 18 in rotational engagement with the ball screw shaft 16, a spring 19 oriented about one end of the ball screw shaft 16, a ball ramp assembly 20 axially oriented between the spring 19 and the ball screw nut 18, and a twist lock 24 oriented between the ball ramp assembly 20 and the ball screw nut 18. As shown in the drawing, the ball ramp assembly 20 generally includes an input ramp 23, an output ramp 21, and a plurality of balls 22 therebetween. The shock absorbing end stop 15 provides a rotational stop at the end of the desired axial movement of the nut 18 relative to the ball screw shaft 16, reducing both the axial load on the shaft 16 of the ball screw assembly 15 and the torsional load on the interface of the nut 18 and shaft 16 of the ball screw assembly 15.

[0013] As shown in the drawing, the ball screw 16 includes a helical track 43, the nut 18 includes an opposing track 44, and a ball bearing 45 therebetween. As shown in the drawing, the left side of the nut 18 includes a first face 26 of the twist dog lock 24, and the right side of the input ramp 23 includes a second face 25 of the twist dog lock 24 configured to mate with the first face 26 of the twist dog lock 24 of the nut 18. Thus, when the respective mating faces 25 and 26 of the twist dog lock 24 overlap axially, oppose each other in the rotational direction, and rotate into contact as shown in FIGS. 6 and 7, the torque and rotation 28a of the nut 18 about the axis 16 is transmitted to the input ramp 23 of the ball ramp assembly 20. Since the input ramp 23 is rotatable about the end of the shaft 16 and about the axis 30, when the nut 18 rotates about the axis 30 (28a) on the shaft 16 during the mating engagement, the input ramp 23 rotates about the axis 30 (28b).

[0014] As shown, the output ramp 21 of the ball ramp assembly 20 includes an inwardly extending longitudinal spline 31 that forms an interface 33 with a similarly extending spline 32 on the outer surface of the end of the shaft 16. As a result, the output ramp 21 of the ball ramp assembly 20 is constrained and cannot freely rotate about the shaft 16 and the shaft 30, but can axially move left (27) on the shaft 16.

[0015] The ball ramp assembly 20 includes an annular ring of balls 22 disposed between the opposing inclined surfaces 35a, 35b and 34a, 34b of the input ramp 23 and the output ramp 21, respectively. As shown in the progression of FIGS. 5-7, when the input ramp 23 rotates in a first direction 28a about the shaft 16 and the shaft 30, the output ramp 23 is inhibited from such rotation by its spline interface 33 with the end of the shaft 16. Thus, the relative movement between the input ramp 23 and the output ramp 21 with the balls 22 therebetween causes an axial movement 27 of the output ramp 21 to the left. Thereafter, such axial load 27 is absorbed by a spring 19 acting between the annular retaining end flange 17 of the shaft 16 and the annular end face 36 of the output ramp 21.

[0016] In this embodiment, the spring 19 is a friction spring having opposing conical surfaces and lubricant, assembled to provide a cylindrical friction spring column oriented about the shaft 30 and on the shaft 16, having a stack of outer and inner rings. The spring 19 has a high damping potential and absorbs kinetic energy with minimal resonance. The spring 19 biases the output ramp 21 axially to the right with respect to the input ramp 23 and the annular retaining ring 38 of the shaft 16. The spring 19 functions as an axial shock absorber against the axial load 27. Although a friction spring is used in this embodiment, other types of springs or other energy absorption alternatives may be used. For example, without limitation, coil springs, Belleville washer or stack of disc springs, elastomeric springs, or hydraulic dampers can be used as alternatives.

[0017] Thus, the ball screw end stop assembly 15 includes a ball ramp assembly 20 between the torsion dog locking portion 24 and the spring 19, converting the rotational movement of the torsion dog locking portion 24 and the torsional loads 28a and 29b into linear movement and axial load 27, which is then applied to the spring 19 to absorb the energy of the impact. The axial load 27 generated by the ball ramp assembly 20 is separated from the ball bearings 45 within the nut 18 of the ball screw assembly by an annular retaining ring 38 extending from the shaft 16, which prevents the axial movement of the input ball ramp 23 to the right. Thus, fatigue damage is not introduced into the ball screw assembly.

[0018] Thus, the locking portion 26 of the nut 18 of the ball screw end stop assembly 15 can be rotated to move left until it engages with the locking portion 25 of the input ramp 23. At this point, the input ramp 23 begins to rotate 28b by the rotation 28a of the nut 18 about the axis 30 and the shaft 16. However, the output ramp 21 cannot rotate as it is rotationally prevented with respect to the shaft 16 via the spline interface 33. The relative rotation between the input ramp 23 and the output ramp 21 forces the ramps to axially separate by a distance 48 based on the angles 40, 41 of the opposing ball ramp pockets 34a, 34b and 35a, 35b. The input ramp 23 is prevented from moving axially to the right on the shaft 16 by the annular retaining ring 38, which forces the output ramp 21b to move axially 27 to the left, thereby compressing the spring stack 19. The spring stack 19 absorbs the kinetic energy of the system 15 during compression and prevents the generation of excessive torque spikes in the actuator's gear train at the actuator's movement limit position.

[0019] The ball screw end stop assembly 15 can be arranged, for example, but not limited to, to act between the airfoil and the fuselage of an aircraft to adjust the orientation of the airfoil relative to the fuselage. The motor drives the relative rotation between the ball screw 16 and the ball nut 18 to cause an axial relative movement between the ball screw 16 and the ball nut 18. As a non-limiting example, the motor can be an electric motor or a hydraulic motor.

[0020] The ball screw end stop assembly 15 protects the ball screw from axial loads generated by contact with the end stop 17 and absorbs the torsional kinetic energy of the drive motor within the actuator. The ball screw end stop assembly 15 provides a rotational stop at the moving end of the ball screw and prevents additional axial loads from being generated by the lead or shaft of the ball screw via the torsion dog tooth locking portion 24. The ball screw end stop assembly 15 is an improvement over a system that includes only a torsion dog locking portion and a spring. The problem with using only a torsion dog locking portion and a spring is that when the actuator in question is equipped with a high-speed hydraulic or electric motor, the kinetic energy of the drive motor may not be sufficiently absorbed by the locking portion and the gear train between the motor and the locking portion. The kinetic energy can be very high, and the load path between the motor and the ball screw locking portion can be very stiff, so the torque spike resulting from a full-speed impact on the torsion dog tooth locking portion can damage the gear train or require a much more robust design to handle the load. And when the locking portion is frequently impacted, especially in a telescoping ball screw where the intermediate ball screw locking portion is impacted with each operation, this can lead to fatigue problems in the ball screw and the components of the drive system. This system is improved in this regard.

[0021] Some additional changes can be made to the disclosed embodiments. For example, but not limited to, the opposing ball ramp pockets 34a, 34b and 35a, 35b of the input ramp 23 and the output ramp 21 may have alternative configurations, angles, and cam surfaces to provide the desired range and speed of axial separation depending on the application. Also, alternative shaft thread configurations or profiles, and helical tracks between the ball shaft and the nut, and different ball recovery systems may be used. As yet another alternative, lead screws and nuts may be used without ball bearings.

[0022] Accordingly, the form of the ball screw end stop assembly has been shown, described, and several modifications have been discussed, but those skilled in the art will readily understand that various additional changes can be made without departing from the scope of the present invention.

Claims

1. a shaft oriented around a central axis, and a nut that engages with the shaft, wherein the nut translates axially on the central axis relative to the shaft within a rectilinear range of motion in response to relative rotation between the nut and the shaft around the central axis, a locking portion located at a movement limiting position within the range of motion between the shaft and the nut, the locking portion having a rotational input portion and an axial output portion, the rotational input portion configured to rotate around the central axis relative to the shaft, the axial output portion being inhibited from rotating around the central axis relative to the shaft and configured to translate axially on the central axis relative to the rotational input portion and the shaft in response to relative rotation around the central axis between the axial output portion and the rotational input portion, a linear actuator comprising an axially flexible member configured to axially bias the axial output portion axially on the central axis toward the movement limiting position.

2. The linear actuator according to claim 1, wherein the shaft comprises an outer ball track, the nut comprises an inner ball track, and the linear actuator comprises a plurality of balls disposed within the outer ball track and the inner ball track.

3. The linear actuator according to claim 1, wherein the axially flexible member comprises a spring acting between the shaft and the axial output portion.

4. The linear actuator according to claim 1, wherein the locking portion comprises a plurality of balls axially disposed between the rotational input portion and the axial output portion.

5. The linear actuator according to claim 4, wherein the rotational input portion comprises a first annular cam surface and the axial output portion comprises a second annular cam surface facing the first annular cam surface.

6. The linear actuator according to claim 5, wherein the plurality of balls are axially disposed between the first annular cam surface and the second annular cam surface.

7. the nut comprising a torsional input locking portion, the rotational input portion of the locking portion comprising a torsional output locking portion, The linear actuator according to claim 1, wherein when the torsional output locking portion and the torsional input locking portion overlap in the axial direction and come into contact with each other in the rotational direction, when the nut rotates in a first direction around the central axis with respect to the shaft, the rotational input portion of the locking portion is configured to rotate in the first direction around the central axis with respect to the shaft.

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