CA1233133A - Bidirectional, torque limiting, no-back clutch mechanism - Google Patents

Abstract

ABSTRACT

The bidirectional, torque-limiting, no-back clutch mechanism has an input and output drive shafts and a helical spring which is normally in interference fit with the mecha-nism housing and, in the torque transmitting mode of operation, is disengaged to permit torque transmission by a direct mecha-nical coupling between the input and output drive shafts. The torque-limiting component, which functions to interrupt trans-mission of rotation from the input drive shaft to the output drive shaft upon a predetermined torque load, is adjustable to a selected predetermined torque load in the field.

Description

31 ;~;~3~33 This invention relates to spring clutches and, more particularly, to a bidirectional, torque-limiting and no-back, clutch mechanism.
In heretofore bidirectional, torque-limiting, brake spring clutch mechanism, as is exemplified in the United States patent to Minarick et al No. 3,329,242 and Meyer et al No. 3,659,682, the driving torqwe is tmdesirably transmitted from the input drive shaft through the interference fit be-tween the helical spring and an output shaft or drum of the clutch mechanism. In addition, since torque-limiting ls achieved in such heretofore known torque-limiting, brake spring clutch mechanism by interrupting the interference fit of the helical spring, the torque-limiting load value as a function of spring force, is determined at the time of manufacture of the mechanism and is not thereafter readily adjustable in the field.
Accordingly, the present invention contemplates a bidirectional, torque-limiting and no-back mechanism of relatively simple and compact construc~ion which does not have the aforesaid dis-advantages of heretofore known spring clutch mechanisms of the bidirectional torque-limiting and brake type.
Accordingly, an object of this invention is to pro-vide a bidirectional, torque-limiting, no-back, clutch mecha-nism in which a driving torque is transmitted by a direct mecha-nical interconnection of driving parts and is not dependent upon an interference spring fit. Another object of the present in-vention is to provide a bidirectional, torque-limiting, no-back clutch mechanism in which the torque-limiting load value is readily adjustable in the field after manufacture. A still ~L~33~3 further ohject of this invention is to provide a bidirectional7 torque-limiting, no-back, clutch mechanism wherein the spring only functions as a brake and thus has a longer operative life than heretofore known spring clutch mechanisms of the same type.
~ ne present invention con-~emplates a bidirectional, torque-limiting, no-back clutch mechanism which comprises an input drive means and an output drive means supported for rota-tion in a housing having a fi~ed braking surface. A helically wound spring is disposed in the housing in normal interference fit with said braking surface. A torque limiting means is pro-vided to interconnect the input and output drive means for transmission of rotation from the input drive means to the out-put drive means and interrupt such transmission of rotation when a predetermined torque load is reached. The torque-limit-ing means also includes a drive means for engaging the output drive means and rotatively driving the latter at torque loadings below the predetermined torque load. A switching sleeve is connected to the torque-limiting means for conjoined rotation therewith and has an abutment means for engaging the opposi~e ends of the spring and thereby effect release of the spring from said interference fit wlth the braking surface and permit the drive means to rotatively drive the output drive means and, upon a predetermined torque load, allow the spring to reengage the braking surface and prevent rotation of the input and out-put drive means.
In a narrower aspect of the present invention, the torque-limiting means of the bidirectional, torque-limiting, no-back, clutch mechanism is oE the ball and inclined-ramp type consisting of a plurality o:E circumferentially spaced ball.s supported bet.ween a first collar and a second collar for move-~ ~ ~ 3~ ~ 3 m~nt in arcuate shaped inclined grooves formed in at least one of the surfaces of the first and seco~d collars. The first and second collars are supported for rotative movement and ~he second collar is also supported for axial movement away from the first collar under the urging of the balls when the first collar rotatively overrides the second collar.
A feature o this invention is an adjustment means cooperatively associated with the torque-limiting means where-by the latter is adjustable to a predetermined torque load value relatively quickly and easily in the field.
The invention will be more fully understood from the following description when considered in connection with the accompanying drawings in which:
Fig. 1 is an end view of the bidirectional, torque-limiting, no-back clutch mechanism according to this invention;
Fig. 2 is a longitudinal, cross sectional view taken along line 2-2 of Fig. 1, somewhat enlarged;
Figs. 3, 4 and 5 are cross sectional views taken substantially along lines 3--3, 4--4 and 5--5, respectively, of Fig.

2, but on a somewhat smaller scale;
Fig. ~ is a top plan view of the clutch mechanism with the upper half of the housing cut-away and the other assem-bly components shown in elevation, the spring being broken away for illustration purposes only;
Fig. 7 is a perspective view of the clutch mechanism according to this invention with parts cut-away to more fully dis-close the component parts of the clutch mechanism;
Fig. 8a is an exploded view of some of the component parts of the clutch mechanism; and Fi.g. 8b i9 an exploded vie~ o:E the other component parts of the ~ ~ 3 ~ ~ 3 clutch mechanism not shown in Fig. 8a.
Now, referring to the drawings ancl, more specifically, to Figs. 1, 2 and 7, the bidirectional, torque-limiting, no-back clutch mechanism, according to this invention, is generally designated by the reference number 10. The clutch mechanism 10, while having broad and varied application, is particularly suited for torque transmission systems such as wing flap and slat actuation systems or door actuation systems for aircraft, where interruption of torque transmission upon a temporary overload condition is required and where normal torque transmission is automatically restored upon elimination of the overload con-dition.
The clutch mechanism 10 in gPneral comprises a housing 12 in which an input drive shaft 14 and an output drive shaft 16 are supported for rotation. The input drive shaft 14 is con-nected to a source (not shown) of bidirectional rotary power, while output drive shaft 16 is connected to a member to be rotated (not shown), such as a wing flap or slat. A torque limiting subassembly or means 18 is disposed within the housing to interconnect the input and output drive shafts 14 ~nd 16 for to~^que transmission and interruption of such transmission when a predetermined torque loading occurs. The torque limiting means 18 (see Figs. 2 and 7) includes a drive means 20 for engaging the output.drive shaft 16 and rotatively diring the latter. A switching sleeve 22 overlies and is connected to the torque limiting means 18 so that the switching sleeve 22 rotates together with rotation oE the torque limiting means. A heli-cally wound ~pring 24 iæ disposed withi.n hous-Lng 12 ~o surro~md and lie substantially coaxially to switching sleeve 22. T~e spring 24 iæ so dimensioned relative to the inner peripheral _ 5 _ ~L~33~L33 surface of the housing (hereinafter referred to as the braking surface 26) to be normally in interference fit therPwith. The switching sleeve 22 has abutment means in the form of two axially spaced, radially extending ears or tabs 28 and 28A
which ~unction to rotatively connect spring 24 to ou~put drive shaft 16 and to engage the spring to effect release of spring 24 from its interference fit with braking surface 26 and, upon a predetermined torque load, allow the spring 24 to re-engage braking surface 26 and thereby prevent rotation of input and output drive shafts 16. An adjusting means 30 is provided on input drive shaft 14 to permit adjustment of the torque limiting means 18 to selected predetermined torque loads.
The housing 12, as best shown in Figs. 1, 2, 6, 7, 8a and 8b, comprises a cup-shaped portion 32 and a disk-shaped end wall or cap 34. The cap 34 is fixed to the open end of portion 32 by a plurality of circumferentially spaced, radially extending tongues 36 on portion 32 which mate with grooves 38 in cap 34. A split ring 40 seats within registered, annular grooves 42 and 44 in portion 32 and cap 34, respectively, to secure cap 34 to portion 32. To facilitate insertion oE split ring 40 in grooves 42 and 44, cap 34 is provided with an open-ing (not shown~ communicating with the grooves 42 and 44 through which split ring 40 is fed into the grooves. The cup-shaped portion 32 has a plurality of radially extending ears 46 at the end opposite from cap 34 by which ears the clutch mechanism 10 is mounted to a structure (not shown), such as a wing spar. The inner peripheral surface of portion 32 serves as the braking surface 26. The cap 34 and portion 32 have coaxially aligned openings 48 and 50, respectively J in whlch bearings 52 and 5~ are mo~mted :Eor rotatively supporting input drive shaft 14 and output drive shaf~ 16.
The input drive shaft 14 is shown as a solid elon-gated body while output drive shaft 16 is a hollow member having a cylindrical portion 56 projecting through opening 50 of the housing and an enlarged diameter, flanged portion 58 dimensioned for a close running fit within housing portion 32.
A pair of juxtaposed, arcuate-shaped drive fingers 62 project coaxially from flanged portion 58 toward cap 34. The dr:ive fingers 62 are spaced radially inwardly of the outer peri-pheral surface of flanged portion 58 so as to provide an arcuate space between the outer surfaces of the drive fingers 62 and braking surface 26 for swi~ching sleeve 22 and spring 24 which overlie and surround drive fingers 62. The inner end portion of input drive shaft 14 is also journaled in a bearing 63 mounted in an internal hub portion 64 of output drive shaft 16. Also constituting part of the output drive shaft 16 are two spaced retaining rings 66 and 68 (see Figs.
2 and 8a). For ease of manufac~uring and assembly, these retainer rings are made as separate members from the output drive shaft 16J but are keyed to rotate with the drive fingers 62. As best shown in Figs. 3, 5 and 8a, each retaining ring has an arcuate shaped portion of increased radial thickness extending inwardly, hereinafter referred to as a key portion 70, and which fits within the space formed between drive fin-gers 62 and abut the longitudinal edges of ~he fingers. The retaining rings 66 and 68 not only function to transmit torque but also serve to retain spring 24, collars 72 and 74 and drive cam 80 in a desired circumferential relationship to each other and the spring 2~ in a desired longitudinal pasition in hou~ing 12. An anti-~riction bearing 7~ is interposed between ~33~33 the inner surface of cap 34 and retaining ring 68 The torque limitin~ means 18 includes the drive means 20, which consists of two radially extending tangs which are formed integrally with each of two axially spaced, identical collars 72 and 74. Each o these collars 72 and 74 have an axially extending hub portion 76 and is secured for conjoined rotation to input drive shaft 14 by any suitable means, as for example a plurality of pins 78. Also, hub portions 76 provide a seat or bearings 52 and 63. Disposed adjacent collar 74 is an output drive cam 80 which is connected for conjoined rota-tion to switching sleeve 22 by two diametrical, radially ex-tending tongues 82 which engage complementary diametrically opposite grooves 84 in the inner surface of switching sleeve 22 (see Figs. 4 and 8b). The tongue 82 and groove 84 connec-tion (splined interconnection) permits drive cam 80 to main-tain rotational engagement with switchîng sleeve 22 upon axial movement of the drive cam 80. Interposed between the juxtaposed faces 36 and 88 of the respective collar 76 and drive cam 80, are a plurality of balls 90 which are held in circumferentially spaced relationship to each other by a retaining ring 92. The faces 86 and 88 of collar 74 and drive cam 80, respectively, have for each ball 90 juxtaposed semi-spherical recesses 94 in which each ball seats or torque transmission. At least one of the faces, as sho~n face 86 of collar 74, has arcuate shaped inclined-ramp grooves 96 extending in opposite directions rom each of the ball recesses. The drive cam 80 is biased axially toward collar 74 by Belleville sprin~ washers 98 which are disposed over input drive shaf~ 14 and between an anti-riction bearing lO0 and a pressure washer 102. It is apparent that, where the biasi.ng force is not sufficie.nt to retain balls 90 ~3~

in ~heir respective recesses 94, the collar 74 will angularly override drive cam 80 and the drive cam will be axially moved away from collar 74 as the balls ride on the inclined-ramp grooves 96. Coacting with the herein described torque limit-ing mean 18 is an adjusting means 30.
The adjusting means 30 comprises a castellated nut 104 which is threaded on a threaded portion 106 of input drive shaft 14. The nut 104 applies axial force against pres-sure washer 102 which, in turn, places washers 98 under com-pression. The washers 98, in turn, via anti-friction bearing 100, exerts an axial force on drive cam 80 toward collar 74 to thereby squeeze balls 90 in their associated recesses 94.
Thus, by turning nut 104 on the threaded shaft portion 106, the torque limiting means 18 can be adjusted to interrupt torque transmission a~ a wide variety of selected predeter-mined torque loads. To ensure retention of the adjustment, a locking washer 108 is provided adjacent nut 104 (see Figs. 2 and 8a). The locking washer 108 is keyed to input drive shaft 14, via an internal tang 110 radially extending into a keyway 112 in input drive shaft 14. The locking washer 108 also has a tab 114 radially extending from its outer periphery~ which tab is folded into the grooves of the casteIlations of the nut.
Thus, the locking washer 108 prevents, through tang 1~0 and tab 114, relative rotation between the nut 104 and input drive shaft 14 and, thereby, assures retention of the torque-limit-ing torque load adjustment. As previously set forth, drive cam 80 is in constant rotational engagement with the switching sleeve 22 through which torque is transmitted to the output dr-lve shaft 16 in either direction of rotation of input drive shaft 1~.

_ g _ ~3~33 The switching sleeve 22J as is best shown in Figs.
2, 5 and 6, has tabs 28 and 28A which are so dimensioned that each tab 28 and 28A projects radially outwardly into a stepped groove 116 (see Fig. 6) formed in the inner face of key por-tion 70 of the associated retaining ring 66 or 68 and in arcuate alignment with and adjacent to tangs 118 and 118A
formed at each end of spring 24. The switching sleeve 22 is also dimensioned and positioned relative to collars 72 and 74 so that each of the tangs of the drive means 20 project radial-ly outwardly between the end of spring 24 and the adjacent re-taining ring 66 or 68 and in rotative alignment with a shoulder 120 formed by a first step in the stepped groove 116 o the associated retaining ring.
I~hen the torque-limiting means 18 functions to inter-rupt torque at a predetermined torque load~ an indicating means 122, provided in the clutch mechanism 10, is actuated. The indicating means 122 consists of a sleeve 124 which has an out-wardly flanged end portion 126 and a longitudinal groove 128 extending from one end to a point adjacent the flanged end por-tion 126. This sleeve 124 is supported for axial reciprocative movement on collar 72 with the flanged end portion 126 disposed adjacent cap 34 of housing 12. It is dimensioned in length so that the end o~posite from flanged portion 126 is spaced ~rom drive cam 8Q, but sufficiently close to be engaged by the drive cam 80 when the latter is axially moved to the right as viewed in Fig. 2. When the predetermined torque load occurs to cause axial movement of drive cam 80 and spring 24 to reengage braking sur~ace 26 and thus arrest rotation of input drive sha~t 14, drLve cam 80 abuts ~leeve 124 and axially moves it to t:he right, as viewed :Ln F'ig. 2, and into abutment against indicator pin 128.

333~33 After contact of sleeve 124 against the end of indicator pin 128, the continued movement of the sleeve 124 moves the indicator pin to the right until a hairpin shaped spring 130 snaps into an annular groove 132 in the pin. The spring 30 functions to hold indicator pin in the indexed position, even after the clutch mechanism returns to norma:L function~ so that maintenance inspectors will become aware that an overload condition occurred and perform a more thorough inspection to ascertain the reason for the overload occurrence. A retaining plate 133 is receivable in a recess in the outer surface of cap 34 and is secured therein by a screw 135. This plate 133 serves to keep the indicator pin assembly within cap 34.
The clutch mechanism 10 functions to transmit rota-tion of input drive shaft 14 to output drive shaft 16 below a preseIected torque load in the following manner. Assuming that input drive shaft 14 is being driven by a source of bi-directional rotary power, such an electric, pneumatic or fluid motor, in a counter-clockwise direction as shown by the arrows in Figs. 2, 3, 5 and 6, collars 72 and 74 which are pinned to 2~ input drive shaft 14 are rotated i~ a counter-clockwise direc-tion. This rotation is transmitted from collar 72 to drive cam 80, via balls 90, of the torque-limiting means L8. Since , drive cam 80 is rotationally connected to switching sleeve 22 by tongues 82 and grooves 84, switching sleeve 22 is rotated.
As best seen in Fig. 6~ rotation of switching sleeve 22 eause tab 28, adjacent retaining ring 66, to move into abutment against tang 118 of spring 24 and to cause, wi~h further rota-tive movement, the coils of spring 24 to contract~ the oppo-site end of the spring being held against movement by the tang 118A and tab 28A o:E coLlar 72 coming into abutment against a ~3~33 radial shoulder 134A formed by stepped groove 116 in retaining ring 68. This contraction o:E the coils of spring 24 moves the spring out o interference fit with braking surface 26. After sufficient rotative movement to release the springs grip on housing 12, the tang of drive means 20 on collar 74 abuts a radial shoulder 120 of stepped groove 116 of retaining ring 66.
Since retaining ring 66 is ~eyed to output drive shaft 16 by the key portion 70 engaging the longitudinal edges of fingers 62 of the output drive shaft 16, the latter is thus directly : 10 rotatively driven in the counter-clockwise direction. At this time, with spring 24 disengaged from braking surface 26, it is rotatively ca~ried by the retaining rings 66 and 68 as output drive shaft 16 is rotated.
; The sequence of operation as herein set forth is achieved because a lost-motion clearance C between tang 11~
and intermediate shoulder 136 of stepped groove 116 of retain-ing ring 66 is sufficient to allow enough arcuate movement of spring tang 118 relative to retaining ring 66 to effect con-traction of the coils of spring 24 and complete disengagement ; 20 from braking surface 26 before abutment against shoulder 136.
Also the clearance C' between tang drive means 20 and shoulder 120 is greater than clearance C so that unbraking of spring 24 is achieved before driving of retaining ring 66 and output drive shaft 16.
When input drive shaft 14 is rotated in the clockwise direction (opposite from the arrows shown in Figs. 2, 3, 5 and 6), the spring 24 is caused to disengage from braking surface 26 by tab 28A of switching sleeve 22 engaging tang 118.A of spring 24 adjacent retaining ring 68, the opposite end oE the sprlng being held by action of tang 118 abutting tang 28 which, ~3~33 in turn, abuts shoulder 134 formed by stepped groove 116 in retaining ring 66. With the sprlng 34 ou-t of its braking position, th.e output drive shaft 16 is rotated in the clock-wise direction, via the tang drive means 20 of collar 72 abut-ting radial shoulder 120A of retaining ring 68 and rotatively driving the latter.
Assuming a counter-clockwise transmission of torque from input drive shaft 14 to output drive shaft 16 during which ~ime a torque load develops which is a~ or greater than the present torque load of torque limiting means 18, collar 74 rotatively overrides output drive cam 80. In so doing, balls 90 ride out of the recesses 94 and along the arcuate, inclined-ramps 96 thereby moving output drive cam 80 to the right, as viewed in Fig~. 2, 6 and 7 ! against the biasing force of Belleville spring 98. The tangs 28 of switching sleeve 22 ceases to rotate with tang drive means 20 (the switching sleeve having stopped rotative movement because rota-tion is no longer transmitted to the output drive cam 80 ~nd from the latter to switching sleeve 22). ~t the opposite end of spring 24, tang drive means 20 of collar 72 continues to move as does tang drive means 20 of collar 74 and with the rota-tive force on tang 118 thus relieved because the spring rotates away from abutment with tang 28, the coils of spring 24 are allowed to re-expand into interEerence ~it wi-th braking surface 26 and thereby block rotation of input drive shaft 14, through tang drive means 20, and also reverse rotation through output drive shaft 16 by reason of the abutment oE shoulder 136 of re-taining sleeve 66 against spring tang 118 in a direc~ion to expatld the coils o:E the spring.
~s prev:iously described, axial movement o:E O~lt~Ut 33~33 drive cam 80, upon a predatermined torque load being reached, axially moves indicator sleeve 124 which, in turn, axially moves indicator pin 128 outwardly of the outer surface of cap 34. This extension of pin 128 from the cap 34, where it is held by a spring 130, alerts inspectors to ~he fact that the clutch mechanism was subject to a torque overload condition.
It is believed now readily apparent that the present invention provides a bidirectional, torque limiting, no-back, clutch mechanism which is capable o~ field adjustment of the torque load at which inter-ruption of torque transmission will occur. It is a clutch mechanism lO wherein rotation in each direction of rotation is transmitted from an input drive shaft to an output drive shaft by a direct mechanical connection and not through a helical spring. It is a clutch mechanism which is relatively simple and compact in construction with all of ~he parts and components thereof within a housing and thereby protected against dirt, damage and malfunction.
It is not the intention hereof to restrict applica-tions of the invention by the figures and description thereof, but rather it should be understood that the present disclosure is to illustra~e the concepts and principles o~ the present invention and that changes or alterations thereto obvious to one skilled in the art would still come within the scope of this disclosure. Also, it should be understood that the figures are deliberately not drawn to scale and are exaggerated in some respects for clarity. Therefore, although a preferred embodiment of the apparatus o~ the present invention has been herein disclosed, it should be obvious that the present dis-closure is made by way o~ example only and that variations are possible wlthout departing from the swbJect matter comin~ within the scope o~ ~he ~ollowlng c.laims J which claims are regarded as the invention.

Claims (15)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A bidirectional, torque-limiting, no-back, clutch mechanism comprising:
a) a housing having a fixed braking surface;
b) an input drive means supported for rotation by said housing and connected to a source of bi-directional power to rotate the input drive means;
c) an output drive means supported for rotation by said housing;
d) a helically wound spring in said housing and constructed and arranged to be in normal inter-ference fit with said braking surface;
e) torque limiting means interconnecting in one operative position said input and output drive means for transmitting rotation from the input drive means to the output drive means and in another operative position interrupting such transmission of rotation when a predetermined torque load is reached; and f) a switching sleeve disposed in said housing and connected to said torque limiting means for con-joined rotation with the latter and having a spring engaging means for engaging opposite ends of the spring and effect relase of the spring from said braking surface and thereby permit transmission of rotation from the input drive means to the output drive means and, upon a pre-determined torque load and when said transmission of rotation between the input and output drive means ceases, allows said spring to reengage the braking surface and thereby prevent rotation of the input drive means and the output drive means.

2. The apparatus of claim 1 wherein an adjustment means is provided which coacts with the torque-limiting means to effect adjustment of the latter for a selected predetermined torque load value at which rotary transmission is interrupted.

3. The apparatus of claim 1 wherein said torque-limiting means includes a first means rotatable with the input drive means, a second means rotatably connected to the switching sleeve and axially slidable relative to the switching sleeve and the first means, a camming means disposed between said first and second means to cam said second means axially upon a predeter-mined torque load.

4. The apparatus of claim 3 wherein a spring biasing means is disposed adjacent said second means to urge said second means in a direction of the first means.

5. The apparatus of claim 4 wherein an adjustment means is disposed to coact with said biasing means to adjust the force exerted by the latter against said second means in accord-ance with the selected predetermined torque load at which trans-mission of rotation is to be interrupted.

6. The apparatus of claim 3 wherein input drive means is a drive shaft and said first means comprises two collars secured in axial spaced relation to the drive shaft for conjoined rota-tion with the latter.

7. The apparatus of claim 6 wherein each of the two collars has a radially extending tang which rotatively engages the output drive means to transmit rotation to the latter.

8. The apparatus of claim 1 wherein said output drive means comprises a drive shaft having a radial flanged portion from which axially extend two diametrically spaced fingers.

9. The apparatus of claim 8 wherein said output drive means includes two retaining rings supported on the fingers, one at each end of said helically wound spring, and keyed to said fingers for conjoined rotation with the latter.

10. The apparatus of claim 9 wherein each of said re-taining rings are identical and each has a stepped groove for confining the adjacent spring end and spring engaging means to limited rotative movement

11. A bidirectional torque-limiting, no-back clutch mechanism comprising a) a housing having a braking surface;
b) an input drive means connected to a source of bidirectional rotary power and an output drive means supported for rotation in said housing;
c) a helically wound spring in said housing and surrounding said input and output drive means and in normal interference fit with the braking surface of said housing;
d) torque limiting means, including a first and a second part, disposed between said input drive means and said output drive means for transmission of torque from the input drive means to the output drive means and, in another operative position, interruption of such torque transmission when a predetermined torque load is reached;
e) said first part of said torque limiting means being connected to said input drive means for con-joined rotation with the latter and having a first abutment means for transmitting torque to the out-put drive means in the same direction as the direction of rotation of said input drive means;
f) a switching sleeve surrounding and substantially coaxial with the input and output drive means and within and substantially coextensive with said spring; and g) said switching sleeve being rotatively connected to said second part of said torque limiting means for conjoined rotations with said second part and having a second abutment means engaging the op-posite end of the spring and connecting said spring to the output drive means and to cause the spring to contract and disengage from said braking surface and thereby release said first abutment for rotation of said output drive means and, upon a predetermined torque load, allowing said second abutment to permit the spring to expand and re-engage the braking surface and thereby prevent rotation of the input and output drive means.

12. The apparatus of claim 11 wherein an adjustment means is provided which coacts with the torque-limiting means to adjust the latter for selected predetermined torque load values at which torque transmission is interrupted.

13. The apparatus of claim 11 wherein said torque limiting means is of the ball and inclined-ramp type consisting of a plurality of circumferentially spaced balls supported between the first and second parts of the torque-limiting means for movement in arcuate shaped inclined ramps formed in at least one of the surfaces of the first and second parts and wherein said second part is supported for rotative movement and for axial movement under the urging of the balls when the first part rotatively overrides the second part.

14. The apparatus of claim 11 wherein said output drive means includes a retaining ring disposed at each of the oppo-site ends of the helical spring and rotatively carried by the output drive means.

15. The apparatus of claim 14 wherein each of the retain-ing rings has spaced radially extending shoulders located so as to be engaged by said first abutment means and the ends of the spring.