GB2126686A

GB2126686A - Rotatable toothed drive element

Info

Publication number: GB2126686A
Application number: GB08322078A
Authority: GB
Inventors: Brian Robert Duke
Original assignee: DAVALL MOULDED GEARS
Current assignee: DAVALL MOULDED GEARS
Priority date: 1982-09-09
Filing date: 1983-08-16
Publication date: 1984-03-28
Also published as: GB8322078D0; GB2126686B

Abstract

A rotatable toothed drive element, such as a gear wheel (1), comprising a circular, generally disc- shaped body and a plurality of teeth (6) spaced regularly around the periphery of the body, the body having a plurality of regularly angularly spaced members (7) extending radially of the body intermediate the centre and periphery thereof. Preferably, the number of members (7) is equal to the number of teeth (6) or a multiple thereof, or vice-versa, in which case, each member (7] can be in register with one or more of the teeth (6), or vice-versa. Alternatively, the number of members (7) may not be equal to the number of teeth (6) or a multiple thereof, or vice-versa, in which case, one of the members can be in register with one of the teeth, with one or more of the other members being out of register, either just or completely, with respective ones of the other teeth. The inventive element has particular application in reducing substantially the mechanical noise generated in high speed applications, such as, when used as an intermediate drive element of a rotary lawn mower. <IMAGE>

Description

SPECIFICATION Rotatable toothed drive element This invention relates to a rotatable toothed drive element, such as, a gear wheel, and particularly one of a plastics material.

With known gear wheels, it has been found that, in high speed applications particularly, a certain amount of noise is generated due to mechanical vibration of the rotating wheel.

Various means have been tried to eliminate or reduce such noise, one being to place a resilient member between the gear wheel and its associated shaft, hub or other appropriate transmission element. Another attmept to reduce mechanical noise in rotating gear wheels has been to form recesses in the body of the gear wheel, although this has been found to result in differential shrinkage of the material from which the wheel is manufactured, particularly in the case of plastics gear wheels, and, as a consequence, undesirable distortion of the gear pitch circle is produced.

It is an object of this invention to substantially reduce this nose generated by a rotating toothed drive element, such as, a gear wheel, whereby the invention provides a rotatable toothed drive element comprising a circular, generally disc-shaped body and a plurality of teeth spaced regularly around the periphery of the body, the body having a plurality of regularly angularly spaced, members extending radially of the body intermediate its centre and periphery.

In the case where the number of members is equal to the number of teeth or is a multiple thereof, or vice versa, that is to say, n NT=Ns or NT=n N5 where NT= the number of teeth, N5 the number of radially extending members and n is an integer, then each member may be in regsiter with one or more of the teeth, or vice versa. Alternatively, each member may not be in register with the associated tooth or teeth, or vice-versa. Moreover, where the number of members is not equal to the number of teeth or a multiple thereof, or viceversa, that is to say, n NT+Ns or NTsin N5 then one or more of the members can be just out of register with an integral number of teeth, or vice versa.The use of the expression "just out of register" in this sense is intended to mean "out of precise register". For example, for gear wheels having a substantial number of teeth and radially extending member, the radially extending centre lines of the majority of the members will be very slightly offset angularly with respect to the radial centre lines of the corresponding teeth around the periphery of the wheel body.

It has been found that these various angular configurations of the body with respect to the teeth provide equal shrinkage thereof during rotation, particularly high speed rotation, whereby little, if any, distortion of gear pitch circle occurs.

In order that the invention may be more fully understood, two embodiments of gear wheel, in accordance therewith will now be described by way of example and with reference to the accompanying drawings in which: Fig. 1 is a partial sectional view of one embodiment of gear wheel; Fig. 2 is a plan view of the gear wheel shown in Fig. 1; Fig. 3 is a partial sectional view of another embodiment of gear wheel; and Fig. 4 is a plan view of the gear wheel shown in Fig. 3.

Referring firstly to Figs. 1 and 2 of the drawings, a moulded plastics gear wheel 1 is mounted on a splined, metallic hub 2 of an associated drive shaft 3.

The wheel 1 has a body, comprising an outer body portion 4 and an inner body portion 5, and fifty four teeth 6 are regular angular intervals around the periphery of the outer body portion.

The outer body portion 4 consists of fifty four radially extending members 7 which lie in angular register with respective ones of the teeth 6.

The inner body portion 5 is moulded with a pair of annular grooves 8, 8' to reduce the thickness of the moulded section of the inner body portion.

In use of the gear wheel 1 as, say, an intermediate drive element of a rotary lawn mower, high speed rotation thereof results in substantially even flexing of the wheel, due mainly to flexing of the members 7, whereby any distortion of the gear pitch circle of the wheel is eliminated or at least substantially reduced. This elimination or at least substantial reduction in any distortion of the gear pitch circle results in greatly reduced mechanical noise which would otherwise be generated by known forms of gear wheel without the radially extending members 7.

Referring now to Figs. 3 and 4 of the drawings, a moulded plastics gear wheel 1' is mounted on a splined, metallic hub 2' of an associated drive shaft 3'.

The wheel 1' is similar to the wheel 1 described above with reference to Figs. 1 and 2, in that it has a body, comprising an outer body portion 4' and an inner body portion 5', and fifty teeth 6 extending at regular intervals around the periphery of the outer body portion 4'.

However, this outer body portion 4' consists of forty nine radially extending members 7' of which one lies in exact angular register with one of the teeth 6' and the remainder lie just or completely out of register with the other forth eight teeth, depending upon their angular spacing around the periphery of the body portion 4' with respect to the one, in-register member.

The inner body portion 5' is also moulded with a pair of annular grooves 8", 8"' to reduce the thickness of the moulded section of that body portion.

In use of the gear wheel 1' as, say, a high speed intermediate drive element of a rotary lawn mower, rotation thereof results in substantially even flexing of the wheel, whereby any distortion of the gear pitch circle of the wheel, whereby any distortion of the gear pitch circle of the wheel is eliminated or at least substantially reduced. As in the case of the first embodiment of wheel 1 , this elimination or at least substantial reduction in any distortion of the gear pitch circle results in greatly reduced mechanical noise.

Although in the two particular embodiments described above, the adjacent pairs of members 7, 7' are separated by respective tapered slot 9, 9', this part of the outer body portion 4, 4' could be substantially solid with the slots 9, 9' being replaced by much narrower slits, whereby the angular extent of each radially extending member 7, 7' would be increased.

For a particular mechanical system, there is an optimum torsional stiffness which can be established by calculation when the radially extending members 7, 7' are considered as geometrical shapes.

If, for example, these are of rectangular form, then well known Encastre beam formulae may be added to enable the precise stiffness to be predicted in advance of manufacture.

Claims

1. A rotatable toothed drive element comprising a circular, generally disc-shaped body and a plurality of teeth spaced regularly around the periphery of the body, the body having a plurality of regularly angularly spaced members extending radially of the body intermediate the centre and periphery thereof.

2. A drive element as claimed in claim 1, wherein the number of members is equal to the number of teeth or is a multiple thereof, or viceversa.

3. A drive element as claimed in claim 2, wherein each member is in register with one or more of the teeth, or vice-versa.

4. A drive element as claimed in claim 1, wherein the number of members is not equal to the number of teeth or a multiple thereof, or viceversa.

5. A drive element as claimed in claim 4, wherein one of the members is in register with one of the teeth, with one or more of the other members being out of register with respective ones of the other teeth.

6. A drive element as calimed in any preceding claim including an annular groove in at least one of the opposed surfaces of the disc-shaped body.

7. A drive element as claimed in any preceding claim made of a plastics material.

8. A drive element as claimed in claim 7, wherein the plastics material is flexible or semirigid.

9. A drive element as claimed in claim 7 or 8 mounted upon a metallic hub.

10. A moulded plastics gear wheel substantially as hereinbefore described with reference to Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.