AU706829B2 - Improved gear arrangement - Google Patents

Description

-2- This invention relates to an improved gear arrangement for the transmission of power through a large reduction.

There are applications in a number of machines, and particularly in power driven wheelchairs, for a relatively high speed electric motor to drive wheels at a relatively low speed. The gear reduction required is so great that at least two stages are frequently used, and consequently there is a considerable expense entailed in the gearing. Bearing loads are high, tooth meshing speeds are high, and noise and wear are consequential problems.

1 0 This expense can be largely reduced, and the noise can also be reduced if the gearing is of a resilient plastics material such as nylon instead of metal, but one of the difficulties encountered with the use of nylon gears is that the tooth strength is sometimes insufficient to provide the required factor of safety, and even with helical teeth the noise exceeds acceptable levels.

1 5 The main object of this invention is to provide a reduction gearing which avoids the need for two stages of reduction, which is of relatively low cost, and wherein the effective tooth strength is increased without increasing the tooth size of a gear and its mating pinion.

Thus there is provided according to the invention a gear 20 arrangement comprising an elongate driving pinion having no more than teeth, two intermediate idler pinions, and a toothed driven wheel, said driving pinion being located between said idler pinions with teeth on opposite sides of the driving pinion engaging teeth of said idler pinions, and the teeth of both ~of said idler pinions engaging the teeth of said driven wheel, the axes of rotation of the driving pinion and the idler pinions being coplanar, and further comprising bearings supporting the driving pinion at spaced locations straddling the driving pinion engagements with the idler pinions, the driving *oeeo and idler pinions being journaled independently of each other to maintain the coplanar relationship whereby the load transmitted by the idler pinions to the driven wheel is equally shared between the said idler pinions.

An embodiment of the invention is described hereunder with respect to and is illustrated in the accompanying drawings wherein iI -2a- Fig 1 is an end view showing the motor driven pinion, the intermediate idler pinions and the driven gear; Fig 2 is a fragmentary section taken on plane 2-2 of Fig 1; and *o S o Fig 3 is a fragmentary section taken on plane 3-3 of Fig 2, drawn to a larger scale.

In this embodiment, motor shaft 10 couples through a coupling key 11 into a motor driven metal driving pinion 12, as illustrated, the driving pinion 12 having four only teeth but the teeth are helical, and also elongate as shown in Fig 2. In some embodiments fewer than four teeth may be used on pinion 12, but in one of its embodiments the invention is limited to a driving pinion having no more than ten teeth (to limit the size of the gearbox).

Pinion 12 engages the teeth of two spaced intermediate idler pinions 13, which in this embodiment are of nylon or other suitable plastics material for driving purposes, and the teeth of both the pinions 13 engage the teeth 14 of a driven gear wheel 15. They are mechanically synchronised and the teeth 14 are of metal, but in some applications can also be of plastics material for noise level control. The use of nylon in idler pinions 13 imparts the advantage that any minor variation in dimension of the teeth can be accommodated by the resilience of the teeth. This way, a very high degree of equality of load sharing of the teeth is achieved, in turn reducing the probability of fracture to almost half what occurs with prior art gear trains.

:Fig 3 illustrates the mounting arrangement for the idler pinions 13. Each pinion 13 is rotational on a respective support shaft 16, both ends of each shaft being supported by plates 17. Projecting ends of the support shafts 16 are S*halved diametrically as illustrated, and enter corresponding half circular apertures in the frame at 18 where the frame supports shaft bearings 19, this arrangement locking the shafts 16 against rotation.

The gear wheel 15 transmits the torque to a pair of aligned output shafts 16 which can be coupled to the wheels of a wheelchair either directly, but preferably through a differential gearing arrangement, by which the torque is at least partially transmitted also to the other wheel of the wheelchair. One -3convenient arrangement illustrated in Fig 1 is for the torque transmitting pinions 22 of the differential gearing to be carried in the gear wheel 15, and this is achieved conveniently by having slots in the gear wheel 15 which receive bearing plates 23 which themselves carry the pinions 22. This latter arrangement is well known and forms no part of this invention.

A consideration of the above embodiment will indicate the invention to be very simple, but there is no need for separate bearings for idler pinions 13 because of the short chordal distance between the teeth of pinions 12 and 13, pinions 13 in being nylon running directly on their shafts. Furthermore, bending moment otherwise imparted to pinion 12 is avoided, since the driving pinion 12 and idler pinions 13 have coplanar axes of rotation. Still further, the surface meshing speeds are low, in consequence both the volume of noise and its frequency are low. Most of these advantages are achieved in this invention even if the axes of the driving and intermediate idler pinions are not coplanar.

In all instances, the invention will be seen to achieve an inexpensive, quietrunning mechanically strong gear train, particularly suitable for various applications where a large degree of speed reduction is required associated with a strong but silent gear train.

4 i 4 -4- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A gear arrangement comprising an elongate driving pinion having no more than 10 teeth, two intermediate idler pinions, and a toothed driven wheel, said driving pinion being located between said idler pinions with teeth on opposite sides of the driving pinion engaging teeth of said idler pinions, and the teeth of both of said idler pinions engaging the teeth of said driven wheel, the axes of rotation of the driving pinion and the idler pinions being coplanar, and further comprising bearings supporting the driving pinion at spaced locations straddling the driving pinion engagements with the idler pinions, the driving and idler pinions being journaled independently of each other to maintain the coplanar relationship whereby the load transmitted by the idler pinions to the driven wheel is equally shared between the said idler pinions.

1 5 2. The gear arrangement of claim 1, wherein the idler gears are of resilient polymeric material rotatively mounted on fixed shafts.

3. The gear arrangement of claim 1, wherein the idler gears are rotatively mounted on fixed shafts.

a 04. A gear arrangement according to claim 1, wherein said idler pinions each comprise teeth of resilient polymeric material.

5. A gear arrangement according to claim 4, wherein teeth of said idler a pinions are nylon.

6. A gear arrangement according to claim 4 wherein said driving pinion is of metal.

7. A gear arrangement according to claim 4 wherein said driven wheel is metal and comprises a pair of power transmitting bevel gears in a configuration which, in use, is a differential gear box.

8. A gear arrangement substantially as hereinbefore described with

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