# Maximum transmission ratio of a spur gear pair

1. Aug 4, 2013

### siddharth23

WHat is the permissible maximum transmission ratio of a Spur gear pair? I read somewhere that it is 1:5. What wil happen if this is exceeded?

2. Aug 4, 2013

### Baluncore

Depending on tooth profile, if a simple spur gear has less than about 12 teeth it will obstruct. That means a 1:5 ratio requires 12:60 tooth gear wheels which will be quite large. For a 1:6 ratio it is better to use two stages with ratios of 1:2 and 1:3 which becomes 12:24 and 12:36 teeth.

There will be times when only one tooth on each gear wheel is working. Since power = torque * RPM, the faster stage gear teeth can be smaller and lighter since those teeth handle less torque more often. That can reduce the weight so when multi-stage reduction is used for more than a 1:5 ratio, the gearbox can be smaller, lighter and lower cost.

So yes, if the ratio is greater than about 1:5 you should consider multi-stage reduction for economic reasons. There is no physical ratio limit if there is no weight or cost limit.

3. Aug 5, 2013

### siddharth23

Thanks! I know minimum teeth on the pinion should be 18 to avoid interference. What's the minimum RPM at which a pair of Spur gears can be used?

4. Aug 5, 2013

### Baluncore

There is no minimum speed. They can stop and reverse.
There may be a backlash problem when significant power is not being transmitted.
There are ways of reducing backlash if it is a problem.