GB2048398A - Torque limiting device - Google Patents

Abstract

A torque limiting device of simple 3-piece construction reliably automatically disengages driving and driven members when the latter is subjected to a predetermined overload. The device comprises a pair of co-operating coupling members (12, 10) which respectively engage the driving and driven members, and a helical compression spring (14) interengaging the two coupling members. The direction of lay of the spring (14) is selected in accordance with the direction of rotation of the shaft (30). Overloading the driven member increases the diameter of the spring which causes it to disengage itself from the driving coupling member (12) to thus prevent torque from being transmitted to the driven member (10) while overloaded. <IMAGE>

Description

SPECIFICATION Torque limiting device This invention relates to a drive clutch or torque limiting device which automatically slips when torque tansmitted to a driven member from a driving member exceeds a predetermined value.

Flexible shafts comprise basic elements of power transmission and are designed to transmit power or control from a driving element to an element to be driven. Transmission may be over, under, or around obstacles or objects when transmission by solid shafts would be impractical or impossible.

In a typical rotatable flexible shaft, a wire mandrel has a plurality of layers of closely coiled wire wound thereover, each of the layers being successively wound over another in alternately opposing directions, i.e., right or left-hand lay. This shaft is usually covered by a flexible casing, metallic or covered, and a clearance between the shaft and casing is provided in order that the shaft may rotate freely within the casing.

Rotatable flexible shafts are of two basic types-power driven and remotely controlled.

Power driven flexible shafts are designed primarily for motor-driven or high speed operation in one direction. Remote control flexible shafts, on the other hand, are designed primarily for hand operated control in either direction of rotation.

A power driven flexible shaft has maximum torque capacity when operated in the direction which tightens up its outermost layer of wires.

When operated in the opposite direction, torque capacity if reduced by about 20 to 50%.

Therefore, for clockwise driving, a left-lay shaft should be used. Alternatively, a right-lay shaft should be employed for counterclockwise driving.

The present invention contemplates a simple, inexpensive drive clutch or torque limiter for use with power driven flexible drive shafts to provide positive fail-safe control. The device is easy to maintain, lends itself to quick coupling, and provides an audible warning of slip condition when the driven member is overloaded.

In accordance with this invention a torque limiting device is provided which comprises a first coupling member provided with means for connecting said first coupling member to a driven member for the transmission of rotational torque thereto; a second coupling member provided with means for connecting the second coupling member to a drive shaft; and spring means connecting said coupling members together in axial alignment one with the other, said spring means serving to connect said coupling members together for the transmission of rotational torque from the second coupling member to the first coupling member, the arrangement being such that increasing loading on the second coupling member progressively increases a dimension of said spring means against the inherent resilience thereof, thereby eventually to cause said second coupling member to slip relative to said spring means.

The device of this invention will be further described with reference to the accompanying drawing, in which: Figure 1 is a longitudinal cross-sectional view of the drive clutch or torque limiting device of the invention in a preferred form, and Figure 2 is a partially exploded perspective view of the device of Fig. 1.

Referring to the drawings, the torque limiting device of this invention comprises a male coupling member 10, a female coupling member 12, and a compression spring 1 4 interengaging each of the coupling members. Male coupling 10 is provided with an outwardly extending male square 1 6 for insertion into a mating orifice of a power driven hand tool (not shown), for example, grass cutters, hedge trimmers, cultivators, drills, grinders, and the like. Means for retaining male square 1 6 in the power tool during operation thereof are well known and not herein shown or described. Male coupling 10 is additionally provided with a plug member 1 8 terminating at its inner end in a shoulder 20.Plug 1 8 is provided with a helical groove 22 for its entire length, the groove being of considerable depth for receiving and maintaining spring 1 4. A central conical depression 24 is disposed in the outer end of plug 1 8.

Female coupling 1 2 is provided with a female square orifice 26 which penetrates a sufficient distance therewithin in order to receive an integrally formed square 28 of a flexible shaft 30. Integrally formed squares on flexible shafts are well known and are described, for example, in U.S. Patent No.

3,481,156. The invention, of course, is not intended to be limited to square configurations only, although preferred. Female coupling 1 2 includes a plug portion 32 having a spigot, or centralizing cone 34, which is matingly received by conical depression 24.

Male square 16, conical depression 24, female square orifice 26, and centralizing cone 34 are axially disposed. Plug portion 32 of female coupling member 1 2 is provided with a helical groove 36 of shallow configuration, the pitch and lay of the helical grooves for both plug members being the same.

In assembling the torque limiting device, spring 1 4 is screwed onto plug 1 8 of the male coupling member until an end of the spring abuts shoulder 20. The spring if of sufficient length to yet threadedly receive plug portion 32 of the female coupling member.

Alignment of plug 18 and plug portion 32 is insured by registration between centralizing cone 34 and conical depression 24. The end spring 14, which is seated in the shallow groove 36 af the female coupling is not restrained, as opposed to the restraint offered by shoulder 20 to the other end of spring 1 4.

Spring 14 is pitched so as to constantly urge centralizing cone 34 into tight engagement with conical depression 24.

In operation, assume the driven power tool is to be used in such a manner that its output shaft rotates in a clockwise direction. Thus, flexible shaft 30 will be provided with a left lay, i.e., the outermost layer of wires will tighten when the flexible shaft is rotated in a clockwise direction; and spring 14 will be provided with a right lay, i.e., its diameter will increase when urged to rotate in a clockwise direction when one end is in abutting relationship with shoulder 20. Now, as torque progressively increases upon transmission thereof from the driving member to the power tool, the diameter of spring 1 4 will also progressively increase, and the spring will tend to lift out from the shallow groove 36 disposed around the femal coupling member.I have discovered that the spring will initially lift out of the shallow groove at the interface area of the coupling members before finally snapping out entirely from the shallow groove if torque is of sufficient magnitude. So long as the torque continues to exceed a predetermined limit, the spring will slide over the plug portion 32 while emitting an audible clicking sound each 360 of revolution of the female coupling member, the clicking sound occurring when the spring "clicks back" into the groove. Of course, the spring will re-seat itself in groove 36 when the load on the driven member is sufficiently reduced. Regardless of load, there is little relative movement between plug 18 of male coupling member 10 and spring 1 4. Plug portion 32 is desirably surface hardened for maximum wear resistance.

Infinite predetermined torque ratings may be obtained by control of certain variables.

For example, in spring 14: type, size, and cross-section of the wire used, spring pitch, length and amount of spring governing slipping friction; in plug portion 32: type of material and its coefficient of friction, groove diameter and outside diameter of the plug portion, and the like.

The invenion has been described for use with flexible shafts. There may be applications where rigid connections or members may be used in lieu thereof and such non-flexible means are intended to fall within the scope of the invention. Further, it is understood that the flexible drive shaft employed and described herein may be protected by a flexible casing, as is well known in the art, and that the use of end fittings, adapters, ferrules, and the like, associated with conventional flexible shaft applications are contemplated by the present invention.

Claims (9)

1. A torque limiting device for rotary drive shafts comprising: a first coupling member provided with means for connecting said first coupling member to a driven member for the transmission of rotational torque thereto; a second coupling member provided with means for connecting the second coupling member to a drive shaft; and spring means connnecting said coupling members together in axial alignment one with the other, said spring means serving to connect said coupling members together for the transmission of rotational torque from the second coupling member to the first coupling member, the arrangement being such that increasing loading on the second coupling member progressively increases a dimension of said spring means against the inherent resilience thereof, thereby eventually to cause said second coupling member to slip relative to said spring means.

2. A device according to claim 1, wherein said first and second coupling members each comprise an axially extending circular plug, which plugs are oppositely opposed one to the other, and said spring means comprise a helical coil spring into the opposite ends of which engage respectively the axially extending plugs on said first and second coupling members.

3. A device according to claim 2, wherein the axially extending plug on the first coupling member comprises a deep helical groove in the surface thereof and in which seat the coils of the helical coil spring adjacent the first end thereof, said first coupling member also having a shoulder against which abuts the end of said coil spring, and wherein the axially extending plug on the second coupling member comprises a shallow helical groove in the surface thereof and in which seat the coils of the helical coil spring adjacent the second end thereof.

4. A device according to claim 2 or 3, wherein the end faces of said axial plugs are in abutting relation and urged into mating engagement by said spring.

5. A device according to claim 4, wherein one of said end faces is provided with a centering boss and the other with a complementary screening recess in mating engagement with said boss.

6. A device according to any one of the preceding claims including a drive shaft connected to said second coupling member.

7. A device according to claim 6, wherein said shaft is a flexible shaft.

8. A device according to claim 6, as de pendent upon any one of claims 2-5, wherein the flexible drive shaft is a helically wound shaft, the hand of the outermost layer of which is opposite to the hand of the coil spring.

9. A device according to claim 1, substantially as described with reference to the accompanying drawings.