Torque-limiting device

Abstract

An actuator comprises an input shaft, a sliding collar arranged around the input shaft, and an output shaft. The input shaft, output shaft and sliding collar are arranged such that a torque applied to the input shaft can be transmitted via the sliding collar to the output shaft. The actuator comprises a torque-limiting device for limiting the transmission of torque between the input shaft and the output shaft if the torque exceeds a certain threshold. The torque-limiting device comprises one or more rolling elements arranged between the sliding collar and the input shaft to allow the sliding collar to move axially relative to the input shaft and engage with or disengage from the output shaft.

Claims

1. An actuator comprising: an input shaft; a sliding collar arranged around the input shaft; an output shaft, wherein the input and output shafts and sliding collar being arranged such that a torque applied to the input shaft can be transmitted via the sliding collar to the output shaft; and a torque-limiting device for limiting the transmission of the torque between the input shaft and the output shaft if the torque exceeds a certain threshold, wherein the torque-limiting device comprises one or more rolling elements arranged between the sliding collar and the input shaft to allow the sliding collar to move axially relative to the input shaft and engage with or disengage from the output shaft; wherein: at least one of the input shaft and the sliding collar comprises along at least part of its length one or more pits or grooves in which the one or more rolling elements are arranged; the actuator further comprises a retaining member around the input shaft and one or more rolling elements; the retaining member has one or more apertures provided therein and the one or more apertures are positioned so as to hold the one or more rolling elements in the one or more pits or grooves; and wherein the sliding collar and output shaft each comprise one or more corresponding helical ramps.

2. An actuator as claimed in claim 1, wherein the one or more rolling elements are spherical.

3. An actuator as claimed in claim 1, wherein a plurality of rolling elements are provided in each pit or groove.

4. An actuator as claimed in claim 1, comprising a plurality of pits or grooves.

5. An actuator as claimed in claim 4, wherein the pits or grooves are spaced evenly around at least one of the circumference of the input shaft and an inner surface of the sliding collar; or the pits or grooves are provided in two or more groups or clusters around the circumference of the input shaft and around an inner surface of the sliding collar.

6. An actuator as claimed in claim 1, wherein the one or more rolling elements and one or more pits or grooves are dimensioned so as to at least partially fit each other.

7. An actuator as claimed in claim 1, wherein the depth of each pit or groove is equal to the radius or half the height of the one or more rolling elements.

8. An actuator as claimed in claim 1, wherein each pit or groove has a radius of curvature equal to or larger than the radius of the one or more rolling elements.

9. An actuator as claimed in claim 1, wherein the retaining member is held in position with one or more resilient members.

10. An actuator as claimed in claim 1, further comprising a no-back device.

11. An aircraft comprising an actuation system with at least one actuator as claimed in claim 1.

12. A method of manufacturing an actuator, the method comprising: providing an input shaft, a sliding collar, and an output shaft, wherein at least one of the input shaft and the sliding collar comprises along at least part of its length one or more pits or grooves in which one or more rolling elements are arranged; positioning the sliding collar around the input shaft; arranging the input and output shafts and sliding collar such that a torque applied to the input shaft can be transmitted via the sliding collar to the output shaft; and providing a torque-limiting device for limiting the transmission of the torque between the input shaft and the output shaft if the torque exceeds a certain threshold; wherein the torque-limiting device comprises the one or more rolling elements arranged between the sliding collar and the input shaft to allow the sliding collar to move axially relative to the input shaft and engage with or disengage from the output shaft, and the sliding collar and output shaft each comprises one or more corresponding helical ramps; the method further comprising: providing a retaining member around the input shaft and one or more rolling elements, wherein the retaining member has one or more apertures provided therein and the one or more apertures are positioned so as to hold the one or more rolling elements in the one or more pits or grooves.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

(2) FIG. 1 is a cross-sectional view of part of an actuation system with an actuator according to an embodiment;

(3) FIG. 2 is a partly exploded perspective view of part of an actuation system without its housings, with an actuator according to an embodiment;

(4) FIGS. 3A and 3B are schematic diagrams illustrating part of the functioning of the actuator;

(5) FIG. 4 is a perspective view of a cage for use in an embodiment of an actuator;

(6) FIG. 5 is an end view the cage of FIG. 4;

(7) FIG. 6 is a cross-sectional view of a spherical element for use in an embodiment;

(8) FIG. 7 is a cross-sectional view of part of an input shaft with a groove therein according to an embodiment;

(9) FIG. 8 is a cross-sectional view of part of an sliding collar with a groove therein according to an embodiment;

(10) FIG. 9 is a cross-sectional view showing a no-back spring enlarged and locked within a housing;

(11) FIG. 10 is a side view illustrating how a sliding collar mates with an output shaft;

(12) FIG. 11 is a graph comparing the performance of a torque-limiting device according to an embodiment with that of a prior art torque-limiting device; and

(13) FIG. 12 is a part cut-away perspective view of a sliding collar according to an embodiment.

DETAILED DESCRIPTION

(14) FIG. 1 is a cross-sectional view of part of an actuation system 1 comprising a linear actuator with a torque-limiting device according to an embodiment of the invention. FIG. 2 is a partly exploded perspective view of part of the actuation system 1 without its housings 6, 12, 13. FIG. 10 is a side view illustrating how a sliding collar 3 mates with an output shaft 7. FIG. 12 is a part cut-away perspective view of the sliding collar 3.

(15) As shown in FIGS. 1, 2, 10 and 12, the actuation system 1 comprises an input shaft 2, a sliding collar 3 and an output shaft 7. The input shaft 2 is coupled to a power drive unit (not shown) which generates a torque to drive the input shaft 2. This torque is transmitted from the input shaft 2, through the sliding collar 3, to the output shaft 7, which then in turn uses the torque to move a further component (e.g. an aircraft wing flap or slat, not shown). These components are provided within housings 6, 12, 13.

(16) The actuation system 1 comprises a torque-limiting device in the form of the sliding collar 3 with helical ramps 3ba and 3bb, which can mate with corresponding helical ramps 7ba and 7bb, respectively, provided on the output shaft 7. This is illustrated in FIG. 10, which shows a side view of part of the actuation system 1 with the helical ramp 3ba of the sliding collar 3 mating with the helical ramp 7ba of the output shaft 7. Ramps 3ba and 7ba mate when the input shaft 2 rotates in one rotational direction (clockwise) and ramps 3bb and 7bb mate when the input shaft 2 rotates in the opposite rotational direction (anti-clockwise).

(17) A contrast spring 5 is installed in the actuator housings 6, 12, 13 around part 3c of the sliding collar 3 and mating against radially-extending surface 3d. The contrast spring 5 applies a force to the sliding collar 3, pushing its teeth 3a into engagement with the teeth 7a of the output shaft 7 (as discussed below).

(18) A no-back spring 22a is provided around the contrast spring 5. A further no-back spring 22b is provided further along the output shaft 7, as shown in FIG. 2. The no-back springs 22a and 22b each have coils with a rectangular cross-section section. Each end of the springs 22a and 22b has a radially inwardly projecting tooth, 23a, 23b, as shown in FIG. 9.

(19) In FIG. 2, the sliding collar 3, contrast spring 5 and their surrounding no-back spring 22a are shown in exploded view, so that the spherical elements 25 and cage 9 (described in more detail below) can be seen more clearly.

(20) The torque-limiting device in the form of the sliding collar 3 is arranged around the input shaft 2 such that it can transmit torque from the input shaft 2 to the output shaft 7 but prevent the transmission of excessive input torques (i.e. input torques above an activation threshold). This torque transmission process is illustrated schematically in FIG. 3 and described in more detail below.

(21) Spherical (rolling) elements 25 are provided between the input shaft 2 and the sliding collar 3, and ten grooves 4 are provided along part of the length of the input shaft 2 to hold or receive the spherical elements 25.

(22) As described above, by providing these spherical elements 25 between the input shaft 2 and the sliding collar 3, this reduces the friction between the sliding collar 3 and the input shaft 2, thereby improving the repeatability (reducing the variability) of the torque-limiter activation setting. This point is illustrated in FIG. 11, which is described below.

(23) The ten grooves 4 are arranged circumferentially around the input shaft 2 in two groups of five grooves 4. Other numbers and/or groupings of grooves 4 are also possible. In an alternative embodiment, the grooves 4 are not grouped but are simply spaced evenly around the input shaft 2.

(24) The spherical elements 25 and grooves 4 are dimensioned so that the spherical elements 25 can partially fit in the grooves 4. The depth 4a of each groove 4 (as indicated in FIG. 7) is equal to the radius 25a (or half the height) of the one or more rolling elements 25 (as indicated in FIG. 6). However, in alternative embodiments, the depth 4a of each groove 4 is slightly more or slightly less than this.

(25) Each groove 4 has a radius of curvature 4a which is equal to the radius 25a of the spherical elements 25.

(26) Each groove 4 is 25 mm long and, in the embodiment shown, three spherical elements 25 are provided in each groove 4.

(27) As illustrated in FIGS. 2 and 12, the sliding collar 3 also comprises ten grooves 8, which are provided along part of its inside length. These grooves 8 are provided at positions corresponding to those of the grooves 4 of the input shaft 2. Thus, the rolling elements 25 are located partially within both the grooves 4 of the input shaft 2 and the grooves 8 of the sliding collar 3. This arrangement allows for torque to be transmitted from the input shaft 2, through the one or more rolling elements 25, to the sliding collar 3.

(28) The grooves 8 on the sliding collar 3 have the same dimensions and shape (i.e. length, depth and/or radius of curvature) as the grooves 4 of the input shaft 2 (i.e. as described above).

(29) FIG. 8 illustrates the cross-section of such a groove 8 provided on an inner surface of the slighting collar 3. The groove 8 has a radius of curvature 8a.

(30) The sliding collar 3 also has a radially projecting surface 3d against which the contrast spring 5 mates.

(31) The spherical elements 25 are held in place by a cage 9. In some embodiments, the cage 9 may be held in place by a spring. In other embodiments (such as the one shown in the figures), such a spring is not necessary.

(32) The cage 9 is illustrated more clearly in FIGS. 4 and 5. The cage 9 is a cylindrical sleeve with a plurality of circular apertures 9a provided therein.

(33) The number of apertures 9a provided in the cage 9 equals or exceeds the number of spherical elements 25 such that an (at least one) aperture 9a is provided for each rolling element 9. The apertures 9a are positioned so as to hold the spherical elements 25 in the grooves 4. Thus, the apertures 9a are arranged in rows corresponding to each groove 4, each row containing the same (or more than the) number of apertures 9a as there are spherical elements 25 provided in that groove 4.

(34) The radius 9b of each aperture 9a (as indicated in FIG. 4) is slightly larger than the radius 25a of each spherical element 25. This means that the apertures 9a can act to axially retain the spherical elements 25 in the grooves 4.

(35) The cage 9 has a radius 9c (as indicated in FIG. 5), which is larger than that of the input shaft 2 and less than that of the sliding collar 3. In this way, the cage 9 fits between the input shaft 2 and the sliding collar 3. The cage 9 is slightly longer than the grooves 4.

(36) During assembly, grease is provided in each groove 4 to help keep the spherical elements 25 within the grooves 4 before the cage 9 is positioned around them.

(37) In the embodiment shown, all of the components of the actuation system 1 described are made of steel, with the exception of the housings 12 and 13, which are made of aluminum. However, other materials may alternatively be used.

(38) The spherical elements 25 allow the sliding collar 3 to move axially relative to the input shaft 2 and the output shaft 7, thereby allowing the sliding collar 3 (via its radially projecting surface 3d) to compress the spring 5, axially disengaging the ramps 3b and 7b, as described below.

(39) FIGS. 3(a) and (b) are schematic diagrams illustrating the torque transmission process and the functioning of the torque-limiting sensing elements. In these figures, the parts shown rotate about a horizontal axis.

(40) As illustrated in these figures, interlocking frontal teeth 3a, 7a are provided on the sliding collar 3 and on the output shaft 7, respectively. Each of these interlocking teeth 3a, 7a, has two sloping sides providing helical ramps 3ba, 3bb, 7ba, and 7bb, respectively. Pairs of helical ramps 3ba, 7ba or 3bb, 7bb (depending on the direction of rotation of the input torque), mate with each other, allowing torque to be transmitted from the sliding collar 3 to the output shaft 7.

(41) The helical ramps 3ba, 3bb, 7ba and 7bb each have an angle with respect to the direction normal to the tangential force from the applied (input) torque TA, such that whenever a torque is transmitted, an axial force component is generated which tends to separate the sliding collar 3 and the output shaft 7. Thus, the axial loading force on the spring 5 is caused by the transmission of torque between a helical ramp 3ba or 3bb on the sliding collar 3 and the corresponding helical ramp 7ba or 7bb, respectively, on the output shaft 7.

(42) When the input (applied) torque TA exceeds a certain threshold, this generated axial force component exceeds a preload force FS of the spring 5 (which force pushes the teeth 7a and 3a together, via the radially extending surface 3d or the sliding collar 3) such that a relative rotation occurs between the sliding collar 3 and the output shaft 7, resulting in an axial sliding movement of the sliding collar 3 on a mating surface (helical ramp) 7ba or 7bb (depending on the direction of rotation of the input torque, i.e. clockwise or anti-clockwise) of the output shaft 7. This sliding movement eventually results in a relative angular displacement between the sliding collar 3 and the output shaft 7, whereby the teeth 7a and 3a are no longer engaged and torque is no longer transmitted from the input shaft 2, via the sliding collar 3, to the output shaft 7. Thus, the transmission of excessive input torques (i.e. above a certain activation threshold) from the input shaft 2 to the output shaft 7 is prevented.

(43) This movement of the output shaft 7 also causes one of the two no-back springs 22a or 22b (depending on the rotational direction of the input torque, i.e. clockwise or anti-clockwise) to be engaged via a projection tooth 23b and enlarged and locked in a seat 6a in the housing 6 (as illustrated in FIG. 9), so as to obtain a required braking function of the input shaft 2. In other words, when the torque limiting function is activated (preventing transmission of torque from the input shaft 2 to the output shaft 7) a no-back spring 22a or 22b is also engaged, thereby acting as a brake on the input shaft 2.

(44) Thus, when the input torque exceeds a certain activation threshold, the torque-limiting device is activated preventing transmission of the input torque from the input shaft to the output shaft 7. In addition, the input torque is grounded to the housing 6 through one of the logarithmic no-back springs 22a and 22b, depending on the direction of rotation of the input torque.

(45) Once the spring 22a or 22b is locked inside its seat 6a in the housing 6 in this way, the only way to unlock the spring 22a or 22b is to reverse the (rotational) motion of the input shaft 2. The torque-limiting device automatically disengages (i.e. stops limiting the transfer of torque) when the input torque is reversed. When the input torque is reversed in its rotational direction, the sliding collar 3 rotates in the opposite direction resulting in its teeth 3a re-engaging with the teeth 7a of the output shaft, thereby allowing the input torque to be transmitted to the output shaft 7.

(46) The angle(s) of the helical ramps 3ba, 7ba and 3bb, 7bb with respect to the direction normal to the tangential force from the applied (input) torque TA can be selected so as to provide a desired activation threshold of the torque limiting device, i.e. to set at which input torque the torque limiting device would be activated. As different ramps 3ba, 7ba and 3bb, 7bb are involved in this function depending on the rotational direction of the input torque, the angles may be set differently to provide different activation thresholds of the torque limiting device for the two different rotational directions, if desired. Alternatively, the angles may be the same. However, each pair of corresponding ramps 3ba, 7ba, and 3bb, 7bb, should have the same angle to ensure smooth mating.

(47) With regards to external loads on actuators (e.g. a ballscrew type actuator as illustrated in FIG. 1), these can be tension or compression loads. The output shaft 7 is normally geared to a wormwheel 26 that is normally splined (installed) on the nut of a ballscrew (not shown). As a result, external loads applied to the ballscrew are transferred to the output shaft 7 through the wormwheel 26, and can cause clockwise or anti-clockwise rotation of the output shaft 7 (and, as a consequence, of the input shaft 2) unless a no-back device is provided. The two no-back springs 22a and 22b act to prevent such clockwise and anti-clockwise rotation, respectively, of the output shaft 7 as a result of external loads.

(48) Thus, a braking function against excessive external loads is also provided by the logarithmic no-back springs 22a and 22b. When an external load is transmitted from the output shaft 7 to the input shaft 2, the rectangular cross-section coils of one of the two no-back springs 22a or 22b (depending on the rotational direction of the torque from the external load, i.e. clockwise or anti-clockwise) are enlarged in their seat 6a and lock with the housing 6, which is firmly fitted within the actuator body (housing) 13, so preventing the reversibility of the actuator (i.e. torque being transmitted back to the input shaft 2). This enlarged and locked state of a no-back spring 22a is illustrated in FIG. 9.

(49) As discussed above and shown in FIG. 2, there are two no-back springs 22a and 22b. One no-back spring 22a brakes (locks) when an external load acts in compression (retraction) and the other one 22b brakes (locks) when the external loads acts in tension (extension). The no-back spring 22a performs the no-back function of preventing the input shaft 2 from rotating in an anti-clockwise direction. The other no-back spring 22b prevents rotation caused by external loads in a clockwise direction.

(50) When they are released, the no-back springs 22a and 22b rotate with minimum drag torque. When they are activated, the no-back springs 22a and 22b develop a very large braking torque (T.sub.B), which is a linear function of its preload torque (T.sub.0) and an exponential function of the number of coils (N) and of the friction factor (f) of the spring, as defined by:
T.sub.B=T.sub.0e.sup.2Nf

(51) FIG. 11 is a graph comparing the performance of a torque-limiting device according to an embodiment of the invention with that of a prior art torque-limiting device. The dashed line 31 shows the torque-limiter setpoint (the torque value in Nm at which the torque-limiter is activated) as a function of external torque rate for a prior art torque-limiter (i.e. without such rolling elements provided between the sliding collar and the input shaft). The solid line 30 shows the torque-limiter setpoint as a function of external torque rate for a torque-limiter with rolling elements according to an embodiment of the present invention.

(52) As can been seen from the graph, the setpoint for the prior art torque-limiter varies as a function of torque rate. However, for the torque-limiter according to the embodiment of the present invention, the setpoint is nearly constant as a function of torque rate, as the torque rate varies between 5 and 45 Nm/s, i.e. the torque-limiter is nearly insensitive to the torque rate. The difference between minimum and maximum setpoint for the torque-limiter according to the present invention is around 10% while the normal dispersion for prior art torque limiters can be greater than 30%.