GB2106215A - Worm drive - Google Patents

Abstract

A worm drive comprises a worm 1 connected, for example, to a flexible shaft 2, and a toothed gear 4 meshing with the worm, the teeth of the gear being constituted by the coils of an annular coil spring 5 stretched around the periphery of the gear. <IMAGE>

Description

SPECIFICATION Worm drive This invention relates to worm drives.

In many applications there is a need for an extremely low cost worm drive which can be easily and cheaply produced on a mass production basis. In many such applications, particularly those in which rotary movement is transmitted via flexible drive shaft from a power source to a remote output at which the rotation is to be changed from one axis to another via a worm drive, there is a need for a simple low cost drive which can operate satisfactorily at the relatively low speeds and at relatively low torques that are suitable for transmission by such drive shafts. Of course, conventional solid construction worms and worm wheels can be used, but as such, tend to be relatively expensive in comparison with the rest of the drive, or its particular application.

In accordance with the present invention a low cost worm drive is provided suitable for low torque applications comprising a worm connected or connectable to a drive shaft and a driven member mounted on a shaft for rotation about the axis of said shaft upon rotation of said worm, said member having a plurality of peripheral teeth meshing with said worm, wherein said teeth are constituted by the coils of a helical coil spring mounted around the periphery of the driven member.

In a particularly low cost construction, the worm can also be constituted by a helical coil spring mounted for rotation about its longitudinal axis with its coils meshing with the coils of the spring on the periphery of the driven member.

The driven member usually will be a worm wheel with an endless coil spring stretched around its periphery, a particularly preferred construction comprising a conventional V-pulley wheel with an endless coil spring stretched around the periphery of the pulley wheel and seating in the V-groove.

The angle of the V-groove can be selected so as to provide a variety of torsional characteristics, including for example, a degree of slip between the pulley wheel and the surrounding spring.

Altematively, where rotation of the driven member through only a limited angular range is needed, the driven member can be the sector of a circle with a length of helical coil spring secured along its arcuate periphery. Other shapes, including irregular shapes can easily be devised to suit particular applications.

The invention is iliustrated by the accompanying drawings in which: Fig. 1 is an elevation, part cut away, of a worm drive according to the invention; Fig. 2 is a section on line 2-2 of Fig. 1; and Fig. 3 is an elevation of another embodiment of the invention illustrating a simple application of the principles thereof; Fig. 4 is an alternative form of worm construction for use in the present invention.

Referring to Fig. 1, the worm drive illustrated comprises a conventional worm 1 secured to the end of a flexible drive shaft 2. Mounted on a shaft 3 at right-angles to the axis of the worm 1 is a V pulley 4 in whose groove seats an endless helical coil spring 5. The endless spring is formed from a straight length of conventional helical coil spring bent into a loop and welded, or otherwise connected together at its opposite ends to form the loop. The diameter of the loop formed by the coil spring is such that it is a tight fit within the groove 6 of the V-pulley 4. As will be appreciated the act of bending the coil spring to form an endless loop will cause the individual coils to splay open around the outer circumference of the loop, thereby to form a series of teeth which mesh with the thread on the worm.

Because spring 5 is a tight fit within the groove 6, rotation of the worm will drive the pulley 4 about the shaft 3. At the same time, although this is not an essential feature of the invention, pulley 4 can be made to slip relative to the spring 5. This may be useful in some cases where it is desired to rotate the pulley 4 by hand whilst the worm is stationary. The torque transmission characteristics of the worm drive can be varied considerably by selecting the diameter of the spring loop relative to the diameter of the pulley, by selecting the diameter of the spring coil relative to the width of the V-groove, and by selecting the angle of the Vgroove itself.

In the embodiment of Fig. 3, the driven member is in the form of a sector 7 mounted for rotation on a shaft 8 with the pointed end of the sector acting as a pointer 9 against a scale 10. Secured in a groove in the arcuate edge of the sector is a length of helical coil spring II. The coils of the spring II mesh with the coils of a second, straight length of helical coil spring 1 2 secured to the end of a flexible drive shaft 1 3 and acting as a worm to drive the sector 7 about the shaft 8. As will be appreciated, this represents a particularly simple construction in which the two parts of the worm drive simply comprise a length of helical coil spring.

A modified form of worm is shown in Fig. 4. In this case the worm comprises a mandrel 14 secured coaxially to the end of the flexible drive shaft 13, with a helical coil spring 12 resiliently or otherwise held about the mandrel.

Other embodiments and applications of the principles of this invention will be apparent to the reader. Particular applications, which are envisaged for this invention, are in low torque, low cost drives for a range of domestic and industrial machines powered by a flexible drive cable; in the operating mechanisms for opening fan lights, ventilation flaps etc. in remote locations by means of a flexible drive cable; adjustable aerial mountings; adjustable mountings for wing mirrors on cars and many other applications.

A particular feature of the worm drives in accordance with this invention, besides their low cost, is their ability to operate with the input and output shafts slightly skewed relative to each other, i.e. in conditions where the input and output shafts are not exactly perpendicular. Also of significance is the ready and rapid replacement of worn parts. These and many other advantages will be readily apparent. The one disadvantage is in the limited torque capacity of the drive due to the inherent flexibility in the coils of the spring of the driven member, and of the worm, it a second coil spring is used, which act as the intermeshing gear teeth. However, as already stated, this is not a significant disadvantage in many applications where the requirement is for a low cost, low torque device.

Claims (4)

1. A worm drive comprising a worm connected to or connectable to a drive shaft and a driven member mounted on a shaft for rotation about the axis thereof, said driven member having a plurality of teeth around the periphery thereof meshing with the thread of the worm, wherein said teeth are constituted by the coils of a helical coil spring mounted around the periphery of the driven member.

2. A worm drive according to claim 1, wherein the driven member is in the form of a V-pulley and the coil spring is an endless helical coil spring resiliently engaging in said groove.

3. A worm drive according to claim 1 or2, wherein the worm is also in the form of a helical coil spring.

4. A worm drive according to claim 3, wherein the worm drive comprises a helical coil spring mounted around a central mandrel.