GB2234315A - Mechanical transmissions - Google Patents

Abstract

A transmission mechanism comprises a twin worm drive (106, 108) and a gear wheel transmission (110) connected in series between an input member (104) and an output member (102). With such a construction, the twin worm device (106, 108) provides irreversibility, preventing the output member (102) from driving the input member (104), while the gear wheel transmission provides a mechanical advantage or velocity ratio between the input (104) and output (102) members. The meshing worms (106, 108) rotate about axes which are almost parallel to each other, and the worm drive is rotatably coupled to the gear wheel transmission (110) by a torque limiting clutch (112). Means are provided for indicating the angular position of the output member. <IMAGE>

Description

MECHANICAL TRANSMISSIONS This invention relates to mechanical transmissions.

British Patent Specification No. 1139171 discloses a mechanical transmission in the form of two meshing worms which rotate about axes which are almost, but not quite, parallel to each other. By appropriate design of the worms, and in particular the lead angle of their threads, it is possible to obtain a transmission in which drive can be transmitted from an input shaft to an output shaft without there being any possibility of the output shaft driving the input shaft. Thus, the transmission is irreversible and can be considered as analogous to a worm and wheel transmission. However, a transmission in accordance with British Patent Specification 1139171 has several advantages over a worm and wheel transmission. For example, it can be designed to have a higher efficiency and a lower transmission ratio.Devices in accordance with British Patent Specification 1139171 can conveniently be referred to as "twin worm drives".

Although twin worm drives have been known for several years, they have not so far been used in commercially available equipment. One possible reason for this is that the slightly inclined axes of the meshing worms create difficulties in mounting the worms accurately in a housing. Also, substantial axially directed forces are generated in twin worm transmissions, which need to be taken into account.

The present invention provides a transmission mechanism comprising an input member and an output member which are drivingly interconnected by a twin worm drive and a gear wheel transmission connected in series between the input and output members.

With such a construction, the twin worm drive provides irreversibility, preventing the output member from driving the input member, while the gear wheel transmission provides a mechanical advantage or velocity ratio between the input and output members.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a sectional view of an irreversible reduction gear mechanism, taken on the line I-I in Figure 2; and Figure 2 is a view taken in the direction of the arrow I in Figure 1.

Figures 1 and 2 show a mechanism, which is suitable for turning a large rotating assembly. A section of a ring gear 102 is shown in Figure 3, and this ring gear would be secured to the large rotating assembly to be turned. Rotation of a handle 104 turns an input worm gear 106 which then turns an output worm gear 108. An output pinion 110 is rotationally connected to the output worm gear 108 by a clutch pack 112. This pinion 110 meshes with the ring gear 102. Thus, rotation of the handle 104 rotates the ring gear 102.

The input and output worm gears 106 and 108 are accommodated in a housing 114. The input worm gear 106 is formed integrally with an input shaft 116 which is supported at one end by a bearing 118 in a bore formed in the housing 114. At the other end, the input shaft 116 is supported by a bearing 120 in a bearing cap 122 which is fastened to the housing 114.

Thrust bearings 124 and 126 are provided to retain the input worm gear 106 in the axial direction.

The output worm gear 108 is supported at one end on a spigot 128 which is fastened to the housing 114.

The worm gear 108 is formed integrally with a shaft portion 130 which is supported in a support body 132 by bearings 134. Thrust loads on the output worm gear 108 are resisted by a thrust bearing 136 positioned between the end of the spigot 128 and an adjacent face of the worm gear 108, and by a thrust plug 138 which is fitted into the support body 132 and engages the end face of the shaft portion 130.

Between the worm gear 108 and the shaft portion 130, the integral body from which these elements are formed is provided with a cylindrical portion 140, a splined portion 142 and a screwthreaded portion 144.

A sealing element 146 surrounds the outer surface of the cylindrical portion 140. This sealing element is fastened to the pinion 110 and is provided with a spiral groove formation 147. This formation meshes with a drive pinion (not shown) of a gauge 149 (Figure 2) for indicating the angular position of the ring gear 102. The splines on the splined portion 142 engage alternate discs of the clutch pack 112, the other discs of this clutch pack engaging internal splines formed within the pinion 110. The screwthreaded portion 144 receives an adjusting nut, which acts on the pinion 110 through a belleville washer, and so determines the pressure applied to the clutch pack 112.

The input worm 106 has a lead angle of 5 and the output worm 108 has a lead angle of 110. The threads of the worm gears are of opposite hand, so that the axes of the input and output worm gears 106 and 108 must be inclined to each other by an angle of 60 if the worm gears 106, 108 are to mesh. In normal operation, turning of the handle 104 will rotate the input worm gear 106, this motion being transmitted to the output worm gear 108 and the pinion 110, which then rotates the ring gear 102. The transmission ratio between the input worm gear 106 and the output worm gear 108 is 2.67:1. The pinion 110 and the ring gear 102, however, introduce a further, much larger, reduction ratio.

The worm gears 106 and 108 will lock to prevent the transmission of motion from the ring gear 102 to the handle 104 because the friction angle at the cooperating tooth faces of the worm gears 106, 108 is greater than the lead angle (50) of the worm on the worm gear 106. By providing the worm gears 106 and 108 in series with the gear wheels 110 and 102 between the handle 104 and the body to be driven, the transmission provides an irreversible mechanism with a high reduction ratio, having a greater efficiency than could be achieved using a worm and worm wheel mechanism.

The large rotating body to which the ring gear 102 is secured may have a considerable mass, and consequently, even when rotating relatively slowly, its angular momentum can be very large. Since the worm gears 106 and 108 will lock to prevent further rotation of the worm gear 108 as soon as an operator stops turning the handle 104, very high inertia loads would be applied to the mechanism if the large rotating body were to be stopped almost dead. Such high loads are avoided by means of the clutch pack 112 which enables the kinetic energy of the large rotating body to be absorbed over a period of time.

The worm gears 106 and 108 are formed with multistart threads (from 2 to 8 starts) and have an axial extent which is at least three times the axial pitch of the threads. Consequently, the meshing worm gears engage each other at at least three axially spaced locations, so spreading the load between these locations.

Although various features of the disclosed embodiments are indicated in this description as being inventive, other features may also be inventive, whether taken alone or in combination. The mechanism disclosed herein is also disclosed and claimed in our copending patent application no. 8702643 (Serial No.

2200714).

Claims (4)

1. A transmission mechanism comprising an input member and an output member which are drivingly interconnected by a twin worm drive and a gear wheel transmission connected in series between the input and output members as claimed in claim 1 in which the twin worm device comprises.

2. A transmission mechanism as claimed in claim 1 in which the twim worm drive is rotationally coupled to the gear wheel transmission by a torque limiting clutch device.

3. A transmission mechanism as claimed in claim 1 or 2, in which indicating means is provided for indicating the angular position of the output member.

4. A transmission mechanism as claimed in claim 3, in which the indicating means is driven from a spiral groove formed in a sealing element secured for rotation with an output worm of the twin worm drive.