GB2437744A

GB2437744A - Reduction gearing with spur pinion gears having different numbers of teeth

Info

Publication number: GB2437744A
Application number: GB0606441A
Authority: GB
Inventors: Peter Shepherd
Original assignee: RECURSIVE SYSTEMS Ltd
Current assignee: RECURSIVE SYSTEMS Ltd
Priority date: 2006-03-31
Filing date: 2006-03-31
Publication date: 2007-11-07
Also published as: GB0606441D0

Abstract

Reduction gearing comprises an input shaft 1 which drives a pinion A1 that is meshed with two spur gear pinions A2 and A3. Spur pinion gear A2 has a different number of teeth than spur gear pinion A3 so that bevel crown wheel B, which is driven via bevel gear H1 by spur gear A2, rotates at a different speed and in an opposite direction than bevel crown wheel C that is driven via bevel gear H2 by spur gear A3. Crown wheels B, C are coupled to differential gears D, E and are free to rotate relative to output shaft 2. Differential gears D, E engage and drive bevel gears F which drive stub axles G that drive the output shaft 2. Crown wheels B and C are identical and a ratio of the reduction gearing may be chosen by choosing relevant standard gears.

Description

Reduction Gears

BACKGROUND TO THE INVENTION

This invention relates to reduction gears. We will describe an arrangement of reduction gears that achieve a high ratio of reduction using simple standard component gears thereby avoiding the necessity of providing specially manufactured gears at considerable cost.

Traditionally at ratios above 10:1 reduction gears use multi-stage planetary gears for light loads or worm-pinion and crown- wheel for higher loads. Multi-stage planetary gears require an expensive ring gear housing and intermediate rotors and bearings between each stage also, the number of stages and thus cost and overall size increase with the ratio.

Worm drives suffer from high friction because the worm screw wipes' across the face of the crown wheels cog tooth thus requiring lubrication, high end loads on the pinion shaft requiring thrust bearings to control the backlash, and are very difficult to control when decelerating the load because the low inertia of the pinion coupled with the friction against the overhauling crown wheel's cog causing the pinion to lock.

STATEMENT OF THE INVENTION

To mitigate or overcome these disadvantages, the present invention proposes the use of a differential gear an input being adapted to drive one bevel gear (first crown wheel) in one direction and another bevel gear (second crown wheel) in the opposite direction, an output shaft being connected to turn at a ratio determined by the difference in the speeds of the two bevel gears, characterised by a first pair of gears driving the two bevel gears and each of the first pair of gears being driven from the input by a respective one of a pair of second standard gears, whereby the ratio may be chosen by choosing relevant standard gears.

Very high ratios can be obtained by this arrangement as the smaller the difference in speed between the inputs the smaller is the displacement of the output shaft and higher the ratio.

The gears can be selected so as to rotate at speeds close to the speed of rotation of the input giving the drive momentum, the gears acting as a flywheel that assists in smoothly decelerating the drive Also there are no shaft end loads and transfer of load when the system goes from hauling a load to being overhauled by the load and it does not cause an increase in friction.

The physical size is unaffected by large changes in ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred example of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a prior arrangement of reduction gears using bevel gears, and Figure 2 shows a reduction gear set which uses standard bevel gears with standard spur gears for ratio selection.

In the prior arrangement of Figure 1 an input shaft 1 drives a bevel gear pinion A having for example 35 teeth which is in mesh with two crown wheels, crown wheel B having, for example, 140 teeth and crown wheel C having, for example, 144 teeth which provides a ratio of 3.

Crown wheel B is directly connected to a differential gear D but freely rotates with respect to an output shaft 2 with which it is coaxial. Similarly crown wheel C is directly connected to a differential gear E but freely rotates with respect to the output shaft 2.

The output shaft 2 includes a pair of stub axles Gi, G2 which extend at right angles from respective opposite sides of the output shaft 2 and rotate with the output shaft 2 about its axis, the stub axle Gi mounting a first planetary bevel gear Fl and the stub axle G2 mounting a second planetary bevel gear F2, the two planetary gears being mounted to rotate about their respective stub axles.

Differential bevel gear D is mounted to engage both first and second planetary bevel gears Fl, F2 and differential bevel gear E is similarly mounted to engage both first and second planetary bevel gears Fl, F2.

Using the example the reduction ratio for the reduction gear set can be calculated as follows:-For every four turns of input shaft A crown wheel B will rotate 360 degrees (3 5/140 = 4:1) but crown wheel C will rotate (360/((144-140)/144) = 10 degrees less, that is 350 degrees. Planetary gears F of the differential further divide this difference by two and therefore displace stub shafts G and output shaft 2 by 5 degrees. Thus the ratio of the gear train from input shaft 1 to output shaft 2 is (360/5)*4:1=288:1.

The disadvantage of this arrangement is that because the two crown wheels are the same diameter and have different numbers of teeth, the relative numbers of teeth determining the overall ratio, one or other of the crown wheels B or C will always have to be custom made and must be varied to change the ratio. This is expensive and there is a considerable technical benefit in designing a reduction gear that uses standard stock gears. Such an arrangement of reduction gear embodying the invention is shown in Figure 2.

In Figure 2 the crown wheels B and C are identical standard 5:1 ratio bevel gear pairs.

Instead of being driven by the bevel gear pinion A of Figure 1, they are driven by two spur gears A2 and A3 which in turn are driven by a spur gear pinion Al on the input shaft 1.

Thus the input shaft 1 mounts the pinion Al that engages with two spur pinions A2, A3. Each spur pinion A2, A3 is mounted on a respective shaft with a respective bevel gear HI, H2.

The input shaft 1 drives (rotates) the pinion Al that in turn drives (rotates) the spur pinions A2, A3 in opposite directions. The spur pinions A2, A3 drive their respective bevel gears Hi, H2. The bevel gears thereby drive their respective crown wheels B and C in opposite directions as with Figure 1. The remaining parts operate as in Figure 1 and as described above.

The 5:1 ratio plus the 2:1 division of the differential multiply any difference in the speed of A2 to the speed of A3 by ten. The input shaft 1 drives spur gear pinion Al, with for example 12 teeth, which in turn turns spur gear A2 with 35 teeth and spur gear A3 with 36 teeth. For each full rotation of spur gear A2, spur gear A3 rotates 1/36 *360 =10 degrees less. This displacement when multiplied by factor often to the output shaft 2 results in a displacement of 1 degree. Thus if input shaft 1 were to drive A2 and Al is an idler gear the ratio would be 360:l.However because of the ratio A1=12: A2=35 the ratio is 360*35/12 =1050:1.

A third non-integer ratio could also be obtained by applying input drive 1 to spur gear A3 resulting in a ratio of 370.285+ :1. Three different ratios may be obtained depending on whether the input shaft 1 is directly coupled to drive spur gears Al or A2 or A3. For each revolution of the output shaft 2 input shaft A2 must rotate 360 times input shaft Al must rotate 1050 times and input shaft A3 must rotate 370.285 times. Thus by choosing which shaft Al or A2 or A3 we connect to the input shaft, we can select one of these three inherent ratios.

The very high ratio of this drive train means that it would probably not be possible to back drive, that is, force applied to output shaft 2 will not cause input shaft 1 to turn.

However in some applications it is desirable to allow the release of the output shaft.

This can be achieved in the arrangement of Figure 2 by simply moving any one of the spur gears out of mesh and thus negating the need for an additional clutch.

The invention is not restricted to the details of the foregoing example. For example, the reduction gear described will usually be mounted in a gearbox.

The reduction gear is scaleable and so may be used with small gears of, for example, plastic material for small appliances up to large gears of steel for use in lift (elevator) drives.

The invention is not restricted to the foregoing example.

The input shaft 1 may directly drive any one of the spur gears Al or A2 or A3.

Claims

Claims 1. A reduction gear comprising an input (1) adapted to drive a

first bevel gear (B) (first crown wheel) in one direction and a second bevel gear (C) (second crown wheel) in the opposite direction, an output shaft (2) being connected to turn at a ratio determined by the difference in the speeds of the first and second bevel gears, characterised by a first pair of gears (Hi, 112), one (Hi) of which drives the first bevel gear and the second (H2) of which drives the second bevel gear, each of the first pair of gears being driven from the input by a respective one of a pair of second standard gears (A2, A3), whereby the ratio of the reduction gear may be chosen by choosing relevant standard gears.

2. A reduction gear as claimed in claim 1 in which the standard gears are spur gears.

3. A reduction gear as claimed in claim 1 or 2 in which there is provided an input gear (Al) driven by an input shaft (1) mounted to engage each of the standard gears and to drive them in opposite directions.

4. A reduction gear as claimed in claim 3 in which the output can be disengaged from the input by disengaging the input gear from the one or both standard gears.

5. A reduction gear as claimed in any of claims 1 to 4 in which first bevel gear B is directly connected to a differential gear D but freely rotates with respect to the output shaft 2 with which it is coaxial, second bevel gear C is directly connected to a differential gear E but freely rotates with respect to the output shaft, the output shaft 2 including a pair of stub axles GI, G2 which extend at right angles from respective opposite sides of the output shaft 2 and rotate with the output shaft 2 about its axis, the stub axle Gi mounting a first planetary bevel gear Fl and the stub axle G2 mounting a second planetary bevel gear F2, the two planetary gears being mounted to rotate about their respective stub axles, the differential bevel gear D being mounted to engage both first and second planetary bevel gears Fl, F2 and differential bevel gear E is similarly mounted to engage both first and second planetary bevel gears Fl, F2.

6. A reduction gear as claimed in any of claims 1 to 5 in which the input is directly connected to drive one of said standard gears.

7. A reduction gear as claimed in claim 3 or claim 3 and either of claims 4 or 5 in which the input is directly connected to drive the input gear (A 1).