The present invention relates generally to electromechanical actuation of aircraft control surfaces, and more particularly to torque limiters designed to prevent transmission of excessive torque and load after an electromechanical actuator for moving an aircraft control surface has encountered a hard mechanical stop.
Aircraft control surfaces, for example flaps located on the trailing edge of a fixed wing, slats located on a leading edge of a fixed wing, spoiler panels, aileron surfaces, and the like, have traditionally been actuated by hydraulic actuation systems. More recently, electromechanical actuators (“EMAs”) have gained acceptance in the aviation industry for adjusting the position of control surfaces. EMAs are designed to sweep through a given stroke, linear or rotary, but must have definite points where the stroke must start and end. In practice, two sets of endpoints are defined: one set defines the electrical stroke and the other the mechanical stroke. In normal operation, EMAs are controlled by sophisticated integral or remote electronics over the electrical stroke. However, conditions may arise where an errant command results in the EMA being driven beyond the normal electrical stroke endpoint into a mechanical stroke endpoint. The endpoints that define the mechanical stroke are usually hard mechanical stops. Aircraft manufacturers require that the EMA contain the EMA stroke to prevent possible damage to the airframe or control surfaces. Because of usual space constraints in aircraft, extra room to include “soft” mechanically cushioned stops is not available. If an EMA is driven at sufficient rate into a mechanical end stop either during an in-flight event or as a result of a rigging error during assembly, significant damage usually occurs. After a “shearout” device is employed, and after an event, the EMA is rendered inoperative. A costly overhaul process is required to replace parts and return the unit to service.
It is known to use a rotary ball detent mechanism in an EMA system to limit the torque transmitted from an input gear to an output gear to a chosen maximum torque. The input and output gears are axially aligned on a drive shaft. After a stop is encountered, the rotary ball detent mechanism disconnects the driving inertia from the load path at levels that prevent damage. Conventional ball detent mechanisms employ a series of metal balls all in the same plane that are equally spaced around a circumference about the drive shaft. The balls are held between two circular plates each having an array of pockets to hold the balls. The spacing between the plates is therefore the ball diameter less the depth of the opposing ball pockets. A cage between the plates having a thickness slightly less than the plate spacing is usually employed to maintain even angular ball spacing. The plates and balls are held on the drive shaft by relatively heavy axial spring loading. Under normal operation, all parts rotate together at a commanded speed. The magnitude of the spring loading, the size and number of balls, and depth and shape of pocket dictate the torque limit of the device.
The breakout load or torque limit is selected to be greater than the maximum operating load so that it never “trips” during normal operation, but less than loads that would cause damage to the EMA. With the conventional ball detent mechanism described above, after a breakout or hard stop condition is encountered, one plate is brought to an abrupt stop while the other continues to rotate as the set of balls, in unison due to the cage, roll out of the pockets and onto the flat opposing surfaces of the two circular plates. The shaft is usually rotating at least several hundred—and often up to several thousand—revolutions per minute. The control electronics cannot sense a problem or act on a problem instantaneously, so the EMA's motor is driven for some fraction of a second after breakout. For example, if initial speed is 2400 RPM and six balls are used, with an assumed time of 200 msec before the motor can be turned OFF, 8 revolutions occur. Therefore, the balls that breakout of the initial pockets then encounter 48 more events of rolling into and out of subsequent pockets in the direction of rotation. With the high spring force and the abrupt shape of the pockets, the continued motion of the balls rolling into and out of pockets results in a very violent series of events. The balls experience very high and repeated impact loading and may fracture. Also, the edges of the pockets in the plates may generate harmful debris. Tests have shown significant damage to ball pockets after several encounters. The audible noise from the conventional approach is a loud chatter that may be described as “machine-gun-like.”
The present invention solves the damage and noise problems associated with a breakout event experienced by a conventional torque-limiting assembly. Moreover, the present invention provides a torque-limiting assembly that is easily reset for continued operation after a breakout event.
The present invention provides a ball-detent torque-limiting assembly with breakout means for maintaining an axial separation distance between opposing pocketed surfaces of the assembly once the balls have rolled out of their pockets, wherein the axial separation distance maintained by the breakout means is at least as great as the diameter of the balls. The breakout means assumes the axially directed spring load that urges the opposing pocketed surfaces together, thereby preventing the balls from entering and exiting the pockets in quick and violent succession following breakout and avoiding damage to the torque-limiting assembly.
In one embodiment, the breakout means comprises an angular array of cooperating pairs of ramp members respectively protruding from one of the pocketed surfaces and from a facing surface of a cage retaining the balls. In another embodiment, the breakout means includes a plurality of rollers in an angular array spaced radially relative to the balls and opposing ball pockets to avoid alignment with the ball pockets. In both embodiments, the breakout means is reversible to reset the assembly by commanding a reverse rotation in an angular direction opposite the breakout direction.
Features and advantages of embodiments of the present disclosure will become apparent by reference to the following detailed description and drawings, in which:
Assembly 10 generally comprises an elongated shaft 12 supporting an input gear 14 and an output gear 16. Shaft 12 includes a splined end 18 provided with a circumferential retaining groove 19. Assembly 10 further comprises a spring 20, washers 22, a collar 24, retainer clips 26, and a backing plate 28 all mounted on shaft 12.
Output gear 16 is mounted on shaft 12 for rotation with the shaft. In the context of the present specification, “mounted on” is meant in a broad sense to include a part that is separately manufactured and slid onto shaft 12, as well as a part that is integrally formed on shaft 12.
Input gear 14 is mounted on shaft 12 so as to be rotatable about the shaft axis relative to the shaft, and axially displaceable along the shaft in first and second opposite axial directions. Input gear 14 includes a driving surface 38 facing in a first axial direction toward splined end 18 of shaft 12. Driving surface 38 may be an integral surface of input gear 14 as shown in
Backing plate 28 includes a toothed opening 46 enabling the backing plate to be mounted on splined end 18 of shaft 12 such that the backing plate rotates with the shaft about the shaft axis. Backing plate 28 is constrained against axial displacement along shaft 12 in the first axial direction by C-shaped retainer clips 26 received in retaining groove 19. Backing plate 28 includes a detent surface 48 opposing driving surface 38 and having a plurality of ball pockets 50 angularly spaced about the shaft axis.
Spring 20, which may be embodied as a Belleville spring pack, may be mounted over cylindrical sleeve 34 of input gear 14 for partial receipt within annular recess 36 for an axially-compact biasing arrangement. One end of spring 20 bears against axially-fixed output gear 16 by way of washers 22 and collar 24, while the other end of spring 20 bears against axially-displaceable input gear 14. As may be understood, spring 20 is arranged to provide an axially-directed load urging input gear 14 in the first axial direction toward backing plate 28.
Assembly 10 further comprises a cage 32 having a central mounting hole 52 for mounting the cage on shaft 12. Cage 32 is mounted on shaft 12 between driving surface 38 and detent surface 48. Cage 32 includes a driven surface 54 facing driving surface 38, and a braking surface 56 facing detent surface 48. Cage 32 further includes a plurality of ball openings 58 therethrough. Ball openings 58 are angularly spaced about the axis of shaft 12. Assembly 10 also includes a plurality of balls 30 of uniform diameter received in ball openings 58. The diameter of balls 30 is greater than the axial thickness of cage 32 (i.e. the distance from driven surface 54 to braking surface 56), such that protruding spherical caps of each ball 30 project into a ball pocket 40 in driving surface 38 and an opposing ball pocket 50 in detent surface 48. Under normal torque loading conditions, the bias of spring 20 maintains the assembly in the described arrangement.
When a hard mechanical stop event results in abrupt rotational stoppage of shaft 12 and output gear 16, the motor of the EMA momentarily continues to drive input gear 14. When this occurs, assembly 10 is designed to allow slippage between input gear 14 and shaft 12 to prevent torque transmission to shaft 12 in excess of a predetermined torque limit. In accordance with the present invention, assembly 10 comprises breakout means for causing and maintaining axial separation of driving surface 38 from detent surface 48 by a distance at least as great as the diameter of balls 30 during a mechanical stop event, whereby balls 30 are not repeatedly slammed into pockets 40 and 50 as input gear 14 continues to rotate.
Reference is made to
When a hard mechanical stop is encountered, backing plate 28 stops rotating along with shaft 12 and output gear 16. However, input gear 14 continues to be driven momentarily due to delay in stopping the EMA motor, and toque is transmitted to shaft 12. When the torque limit is exceeded, input gear 14 will rotate relative to shaft 12 and backing plate 28. As this happens, balls 30 will roll out of pockets 40 in driving surface 38; this is best seen in
If a breakout occurs, the control electronics will eventually command the EMA's motor to stop. The present invention will then allow a simple reset of the assembly 10 by commanding a reverse rotary motion of input gear 14 to cause balls 30 to roll back into the original pockets 40, 50. The invention handles a breakout event with little or no damage to the system.
Backing plate 128, shown in
Reference is now made to
Inner cage 135 has a central mounting hole 164 for mounting the inner cage on shaft 12. Inner cage 135 also has a plurality of roller openings 166 angularly spaced about the shaft axis for receiving a plurality of rollers 131. In the figures, rollers 131 are illustrated as being cylindrical rollers to readily distinguish them from balls 30, however rollers 131 may also be embodied as spherical rollers (balls). Regardless of the shape that rollers 131 take, the diameter of rollers 131 is selected to be the same as or slightly greater than the diameter of balls 30. Finally, inner cage 135 includes a plurality of coupling tabs 168 each projecting radially outward for receipt within an associated coupling recess 159 of outer cage 133.
Operation of the breakout means of the second embodiment will now be explained with reference to
The breakout event continues in
As may be understood, input gear 114 will continue to rotate in the CW direction until the EMA's control electronics have received a signal that actuator output is not moving and sent a motor stop command to cease driving input gear 114. This may take on the order of 100-200 msec. Assuming an initial speed of 2400 RPM (40 revs per second), approximately eight revolutions of input gear 114 may be expected. During these revolutions, outer cage 133 and inner cage 135 will also rotate about shaft 12 such that rollers 131 will periodically reenter roller pockets 151 and spring loading will be momentary transferred back onto balls 30. Thus, balls 30 and rollers 131 will alternate in taking up the spring load during post-breakout rotations. In order to prevent damage or at least reduce the risk of damage, it may be advantageous to use special non-galling stainless steel (Nitronic 60) or another material suitable for braking or sustained frictional heating for inner cage 135, which is spring loaded against the backing plate 128 with about 600 pounds of force. An oil bath lubrication of assembly 110 may also be used to prevent or minimize damage to moving parts.
It will be appreciated that the present invention prevents repeated events in which the balls roll out of their pockets and are then slammed back into another pocket. This improvement is accomplished in a very compact space envelope. Other approaches may accomplish the same functionality, but they use mechanisms requiring larger physical volume.
10 torque-limiting assembly, first embodiment
12 shaft
14 input gear
16 output gear
18 splined end of shaft
19 retaining groove of shaft
20 spring
22 washers
24 collar
26 retainer clips
28 backing plate
30 balls
32 cage
34 input gear mounting sleeve
36 input gear annular recess
38 input gear driving surface
40 input gear ball pockets
42 input gear ramps
44 input gear slots
46 backing plate toothed opening
48 backing plate detent surface
50 backing plate ball pockets
52 cage mounting hole
54 cage driven surface
56 cage braking surface
58 cage ball openings
60 cage ramps
62 cage slots
110 torque-limiting assembly, second embodiment
114 input gear, second embodiment
128 backing plate, second embodiment
131 rollers
132 composite cage
133 outer cage
135 inner cage
138 input gear driving surface, second embodiment
140 input gear ball pockets, second embodiment
148 backing plate ball detent surface, second embodiment
150 backing plate ball pockets, second embodiment
151 backing plate roller pockets
152 outer cage axial hole
158 outer cage ball openings
159 outer cage coupling recesses
164 inner cage mounting hole
166 inner cage roller openings
168 inner cage coupling tabs
The present application claims priority of U.S. Provisional Patent Application No. 61/724,989 filed Nov. 11, 2012, the entire contents of which are incorporated herein by reference.
Number | Date | Country | |
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61724989 | Nov 2012 | US |