Abstract:
An output load limiter is provided in an actuator to prevent excessive drive torque from being transmitted from a primary mover through the actuator. The actuator includes a housing, an output shaft mounted for rotation in the housing about an axis, and a drive member. The output shaft includes an external, helical spline. The drive member includes an internal helical spline engaged with the external helical spline to transmit a drive torque to the output shaft. The internal and external splines have sufficient length along the axis to allow translation of the drive member along the axis relative to the output shaft between a first position where the drive member can transmit a drive torque to the output shaft to rotate the output shaft about the axis and a second position where the drive member is restrained from transmitting additional drive torque to the output shaft.

Description:
FIELD OF THE INVENTION 
     The present invention relates to output load limiters, and more particularly relates to torque limiters used to prevent transmission of potentially damaging drive torque from a primary mover to an aircraft control surface or an actuation system for an aircraft control surface. 
     BACKGROUND OF THE INVENTION 
     In the operation of modern aircraft, flight control surfaces such as slats and flaps are powered by primary movers commonly known as power drive units. Typically, the power drive units generate drive torque which is transmitted via a variety of transmission means to move the flight control surfaces in desired directions depending on the navigational and other demands placed on the aircraft. Given the importance of the flight control surfaces to the safety of the aircraft and its passengers, it is critical that the flight control surfaces be controlled by a reliable actuation system. 
     One problem associated with many flight control systems is that if the drive line or transmission controlling the flight control surface becomes mechanically jammed, or if flight conditions prevent movement of the flight control surface in the desired direction, the power drive unit will not stop generating torque. Rather, the power drive unit, which is normally hydraulically powered, will generate relatively high stall torque. This stall torque will be transmitted to the input shaft and gearing of the flight control surface actuator, and is often sufficiently high to detrimentally affect and potentially seriously damage the flight control surface or the flight control surface actuator. 
     Examples of devices that successfully prevent the transmission of excessive drive torque by sensing an axial force on an output shaft including a ball screw that drives a flight control surface are disclosed in U.S. Pat. Nos. 4,318,304 to Lang; 4,459,867 to Jones; 4,697,672 to Linton; and 5,655,636 to Lang et al., the entire disclosures of which are incorporated herein by reference. While these devices have proven quite successful for the intended purpose, there is always room for improvement. For example, because these devices sense the total output force from the actuator, the preloaded springs that sense the output force must be of sufficient size to accommodate the total output force. This tends to impede weight and size reduction of such devices. 
     A device that successfully prevents transmission of excessive drive torques by using a ball ramp to sense torque, rather than an axial force on an output shaft, is disclosed in U.S. Pat. No. 5,299,666 to Lang et al., the entire disclosure of which is incorporated herein by reference. Again, while this device is satisfactory for its intended purpose, there is always room for improvements. For example, the use of a ball ramp tends to restrict options for arranging components within the actuator and, also tends to limit size reduction along the rotational axis of the ball ramp. 
     SUMMARY OF THE INVENTION 
     It is therefore the primary object of the present invention to provide a new and improved output load limiter to prevent excessive drive torque from being transmitted from a primary mover through an actuator. 
     It is another object of the present invention to provide a load limiter that allows for the reduced weight design. 
     It is a further object of the invention to provide a load limiter that allows for a design that requires reduced space requirements. 
     At least one or more of the above objects are achieved in an actuator including a load limiter for limiting the force that is output from the actuator. The actuator includes a housing, an output shaft mounted for rotation in said housing about an axis, and a drive member. The output shaft includes an external helical spline. The drive member includes an internal helical spline engaged with the external helical spline to transmit a drive torque to the output shaft. The internal and external splines have sufficient length along the axis to allow translation of the drive member along the axis relative to the output shaft between a first position where the drive member can transmit a drive torque to the output shaft to rotate the output shaft about said axis and a second position where said drive member is restrained from transmitting additional drive torque to the output shaft. 
     In one form, the actuator includes a housing, an output shaft mounted for rotation in the housing about an axis and including an external helical spline, and a drive gear including an internal helical spline engaged with the external helical spline to transmit a drive torque to the output shaft. The internal and external splines have sufficient length along the axis to allow translation of the drive gear along the axis relative to the output shaft between a first position and a second position. The actuator further includes a first stop surface secured against rotation about the axis relative to the housing, and a second stop surface moveable into and out of interference engagement with the first stop surface and secured for translation along the axis with the drive gear and against rotation about the axis relative to the drive gear. The second stop surface is out of interference engagement with the first stop surface with the drive gear in the first position. The second stop surface is in interference engagement with the first stop surface with the drive gear in the second position to restrict rotation of the drive gear about the axis. 
     In one form, the actuator includes a housing, an output shaft, a drive gear, and first, second, third, and fourth stop surfaces. The output shaft is mounted for bi-directional rotation in the housing about an axis and includes an external helical spline. The drive gear includes an internal helical spline engaged with the external helical spline to transmit a drive torque to the output shaft. The internal and external splines have sufficient length along the axis to allow translation of the drive gear along the axis relative to the output shaft between first, second, and third positions with the first position located axially between the second and third positions. The first and third stop surfaces are secured against rotation about the axis relative to the housing. The second stop surface is moveable into and out of interference engagement with the first stop surface and secured for translation along the axis with the drive gear and against rotation about the axis relative to the drive gear. The second stop surface is out of interference engagement with the first stop surface with the drive gear in the first position. The second stop surface is in interference engagement with the first stop surface with the drive gear in the second position to restrict rotation of the drive gear in one direction about the axis. The fourth stop surface is moveable into and out of interference engagement with the third stop surface and secured for translation along the axis with the drive gear and against rotation about the axis relative to the drive gear. The fourth stop surface is out of interference engagement with the third stop surface with the drive gear in the first position. The fourth stop surface is in interference engagement with the third stop surface with the drive gear in the third position to restrict rotation of the drive gear in the other direction about the axis. 
     In one form, the helical splines are part of a ball spline and are engaged to each other by a plurality of ball spline balls. 
     In one form, the drive gear is rotatably mounted to the housing through the ball spline and the output shaft. 
     In one form, the actuator includes a first spring that is preloaded between the drive gear and the housing with the drive gear in the first position to generate a first desired trip force against the drive gear that must be overcome to translate the drive member from the first position toward the second position. 
     In one form, the actuator includes a second spring that is preloaded between the drive gear and the housing with the drive gear in the first position to generate a second desired trip force against the drive gear that must be overcome to translate the drive member from the first position toward the third position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional view of an actuator embodying the present invention; 
     FIG. 2 is a transverse sectional view taken generally along line  2 — 2  in FIG. 1; 
     FIG. 3 is a rollout view of a helical ball spline taken generally along line  3 — 3  in FIG. 2; 
     FIG. 4 is a roll-out view of a drive gear and a pair of reaction plates taken generally along FIG. 2, with the drive gear shown in a first position; 
     FIG. 5 is a roll-out view of a drive gear and a pair of reaction plates taken generally along line  3 — 3  in FIG. 2, with the drive gear shown in a second position; 
     FIG. 6 is a roll-out view of a drive gear and a pair of reaction plates taken generally along line  3 — 3  in FIG. 2, with the drive gear shown in a third position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, an actuator  10  includes a housing  12 , an output shaft  14 , a drive member in the form of a helical drive gear  16 , a helical ball spline  17  operably coupling the drive gear  16  to the output shaft  14  to transmit a drive torque to the output shaft  14  from the drive gear  16 , an input member in the form of a worm gear  18  meshed with the drive gear  16  to transmit a drive torque to the drive gear  16  from a primary mover  19 , a pair of reaction plates  20  and  22  located on opposite sides of the drive gear  16  and including a plurality of respective clutch teeth  24  and  26 , a stack of springs  28  located on one side of the drive gear  16  between the drive gear  16  and the housing  12  to resist translation of the drive gear to the left in FIG. 1, and a stack of springs  30  located on the other side of the drive gear  16  between the drive gear  16  and the housing  12  to resist translation of the drive gear to the right in FIG.  1 . The drive gear  16  includes a plurality of clutch teeth  31  on one side of the drive gear for selective interference engagement with the clutch teeth  24  of the reaction plate  20 , and a plurality of clutch teeth  32  on the other side of the drive gear  16  for selective interference engagement with the clutch teeth  26  of the reaction plate  22 . 
     The output shaft  14  is mounted for rotation in the housing  12  about an axis  33  by a pair of bearing assemblies  34  and  36  located on opposite sides of the drive gear  16 . The bearing assemblies  34  and  36  include respective bearing mount rings  38  and  40  that mount the output shaft  14  to respective ball bearings  42  and  44 . Preferably, the output shaft  14  is secured against translation relative to the housing  12  along the axis  33  by the bearing assemblies  34  and  36 . While there are a number of ways to achieve this result, in the illustrated embodiment an annular shoulder  46  on the output shaft  14  that reacts loads on the output shaft  14  is directed to the right in FIG.  1  through the bearing assembly  36  into the housing  12 , and a shoulder  48  on the output shaft  14  reacts loads on the output shaft directed toward the left in FIG.  1  through the bearing assembly  34  to the housing  12 . 
     While any suitable type of spring can be used in the actuator  10 , in the illustrated embodiment, the springs  28  and  30  are bellville springs. Further, while there are a number of acceptable ways to arrange the springs  28  and  30 , in the illustrated embodiment the bellville springs  30  are piloted on the output shaft  14  and located axially on the output shaft  14  by a shoulder  50  of the output shaft  14 . An annular spacer  52  is piloted on the output shaft  14  and sandwiched between the drive gear  16  and the bellville springs  30 . The bellville springs  28  are piloted on the output shaft  14  and located axially on the output shaft  14  by the bearing mount ring  38 . A needle thrust bearing assembly  56  is piloted on the shaft  14  and sandwiched between the gear  16  and the springs  28  to transmit loads between the springs  28  and the gear  16  while minimizing the rotational friction drag on the gear  16  about the axis  33  relative to the output shaft  14 . The springs  28  and  30  are preloaded between the gears  16  and the housing  12  by a lock nut  58  that is threaded onto the output shaft  14 . The respective preloads in the springs  28  and  30  defines the respective trip forces required to translate the drive gear along the axis  33 . Depending upon the particular application, the preload in the springs  28  can be of a different magnitude than the preload of the springs  30 . This can be done in a number of ways. For example, in the illustrated embodiment, the springs  28  are designed to have a different spring rate than the springs  30 . By way of further example, in the illustrated embodiment, the preload on the springs  30  can be limited by the engagement of an annular shoulder  60  on the shaft  14  with an annular bearing race  62  of the needle thrust bearing assembly  56 . This allows for a higher preload on the springs  28  than on the springs  30 . This is often desirable because the desired load limit from the actuator  10  may depend on the direction of actuation from the output shaft  14 . 
     While the output shaft could transfer torque to any type of mechanical element or component, in the illustrated embodiment the output shaft  14  includes a ball screw assembly  66  that can be attached to a flight control surface  68  either directly or through additional actuation mechanisms. The housing  12  includes a mount flange  70  with a spherical bearing  72  for connection with a frame or other member for reaction of forces on the housing  12 . 
     As best seen in FIGS. 2 and 3, the helical ball spline  17  includes a plurality of helical spline grooves  74  in the drive gear  16 , a plurality of helical spline grooves  76  in the output shaft  14 , and a plurality of ball spline balls  78  engaging the helical spline grooves  74  and  76 . The helical grooves have a helix angle φ relative to the axis  33 . The balls  78  are retained in the grooves  74  and  76  by the spacer  52  and the bearing race  62 . The helical grooves  74  and  76  have sufficient length along the axis  33  to allow translation of the drive gear  16  between a first position shown in FIG. 4 and a second position shown in FIG. 5, and between the first position and a third position shown in FIG.  6 . In the first position, the drive gear is substantially centered between the reaction plates  20  and  22 , with the teeth  31  and  32  being out of interference engagement with the teeth  24  and  26 . In the second position, the drive gear  16  is shifted to the left in FIGS. 1 and 5 with the clutch teeth  31  in interference engagement with the clutch teeth  24 . In the third position shown in FIG. 6, the drive gear  16  is shifted to the right in FIGS. 1 and 6 with the clutch teeth  32  in interference engagement with the clutch teeth  26 . While the translation of the drive gear  16  between the first, second, and third positions can be accommodated in a number of ways, in the illustrated embodiment the helical spline grooves  76  in the output shaft  14  have an extended length to accommodate this translation. 
     One or more cylindrical pins  80  anchored in the housing  12 , extending through respective apertures  82  and  84  in the plates  20  and  22 , secure the reaction plates  20  and  22  against rotation about the axis  33  relative to the housing  12 , while allowing translation of the reaction plates  20  and  22  along the axis  33  relative to the housing  12 , the output shaft  14 , and the drive gear  16 . A spring  86  is piloted on the pin  80  and interposed between the plates  20  and  22  to normally bias the plates out of interference engagement with the teeth  31  and  32  on the gear  16 . As best seen in FIG. 5, the teeth  24  and  31  are provided with rake angles that draw the reaction plate  20  and the drive gear  16  together when the drive gear  16  is rotated in a counter-clockwise direction in FIG. 2, and that force the reaction plate  20  and the drive gear  16  apart when the drive gear  16  is rotated in a clockwise direction. Similarly, as best seen in FIG. 6, the teeth  26  and  32  are provided with rake angles that draw the reaction plate  22  and the drive gear  16  together when the drive gear  16  is rotated in a clockwise direction in FIG. 2, and that force the reaction plate  22  and the drive gear  16  apart when the drive gear  16  is rotated in a counter-clockwise direction. This helps to insure engagement of the reaction plates  20  and  22  with the drive gear when required, and disengagement by reversing rotation after an excessive torque from the primary mover has been reacted. Thus, it can be seen that the teeth  24  and  26  define one or more stop surfaces that are secured against rotation about the axis  33  relative to the housing, while the teeth  31  and  32  define one or more stop surfaces that are movable into and out of interference engagement with the stop surfaces defined by the teeth  24  and  26  and secured for translation along the axis  33  with the drive gear  16  and against rotation about the axis  33  relative to the drive gear  16 . 
     In operation, the drive gear  16  transmits a drive torque from the worm gear  18  to the output shaft  14  through the helical ball spline  17 . The helix angle φ of the helical ball spline  17  generates an axial force on the drive gear  16  as a result of the drive torque. Depending on the direction of the drive torque, the axial force will either be toward the right or toward the left in FIG.  1 . If the drive torque exceeds its desired upper limit in either direction of rotation, the axial trip force on the drive gear  16  will be sufficient to overcome the preload of the associated spring  28 ,  30  and will move the drive gear  16  either to the left or right depending upon the direction of rotation of the drive gear  16 . This will result in the engagement of either the teeth  24  and  31  or the teeth  26  and  32 , which will then assists in translation of the drive gear to either the second position or the third position, again depending upon the direction of rotation. The interference engagement of the teeth  24  and  31  in the second position, or the interference engagement of the teeth  26  and  32  in the third position, reacts excessive drive torque to the housing  12  and prevents further rotation of the drive gear  16 , until the direction of rotation is reversed. After reversal of the direction of rotation of the drive gear  16 , the spring  86  in combination with either the teeth  24  and  31  or the teeth  26  and  32 , force separation of the drive gear  16  from either the reaction plate  20  or the reaction plate  22 , again depending on the direction of rotation. 
     It should be understood that while the invention has been described herein in connection with one highly preferred embodiment in the form of a ballscrew actuator, the invention will find use in many forms of actuators, and accordingly, no limitation to use in connection with ballscrews, or any of the detailed features of the actuator, are intended unless expressly stated in the appended claims. For example, while the helical ball spline  17  is highly preferred for generating the axial translation force on the drive gear  16 , other structures, such as a simple helical spline, can be used on the output gear  16  to generate the axial translation force. In this regard, while it is preferred that the drive gear  16  be mounted for rotation by the helical ball spline  17  and the output shaft  14 , other rotational mount configurations, such as through a separate pair of bearings directly mounting the gear  16  to the housing  12 , can be used. By way of further example, while it is preferred that the teeth  31  and  32  be formed as a unitary part of the drive gear  16 , other arrangements are acceptable as long as the teeth  31  and  32  are secured against rotation about the axis  33  relative to the gear  16  and arranged to translate into interference engagement with the teeth  24  and  26  in response to the gear  16  translating from the first position to the second position or from the first position to the third position. As yet another example, while the clutch teeth  24 ,  26 ,  31 , and  32  are preferred, other structures can be used to define stop surfaces that will react excessive drive torque from the drive gear  16  to the housing  12 .