# Spur gears

1. Nov 8, 2007

### travisr34

I am building a robot for a design class. Part of the design includes a wheel base to drive the robot forward. To accomplish this I was going to attach a spur gear system to a wheel axel and drive the input gear with an electric motor. The gears themselves will be made out of acrylic and cut out via a laser cam so I have a lot of options as far as how I make it. My question relates more to the design of the spur gears. I would like to know if the number of teeth that the output gear has, will in anyway impact on the speed/torque of the axel. Also while I know a greater diameter will allow for greater torque on the output gear it will also require the input gear to have a greater diameter which means more "back torque" on the input gear. Is there a way to figure out the ratio of diameters that will give me the greatest amount of torque? My goal is to get this to go as fast as possible with the given motors that I have.

2. Nov 8, 2007

### FredGarvin

Tooth size is dictated by the loading that will be applied to them, i.e. the torque you have to transmit. The actual number of teeth will impact the efficiency of the gear in one way or another but it will not effect the speed. The speed is dictated by the overall diameter.

3. Nov 8, 2007

### Danger

Welcome to PF, Travis.
Essentially, you just compare the number of teeth, or the diameter, of the two mating gears to find the ratio. If the output gear is twice the size of the input, it will also have twice as many teeth, since they have to be the same pitch. That will reduce the speed in half and double the torque.
'How Stuff Works' has a great section on gears if you want to check it out.

4. Feb 25, 2010

### ansysking

as per the formula (d1/d2)=(n2/n1)=(t1/t2) where d-dia of gears,n-speed in rpm,t-no of teeth, definitely the no of teeth on output gear will affect the speed.

5. Mar 2, 2010

### ansysking

as per the formula (d1/d2)=(n2/n1)=(t1/t2) where d-dia of gears,n-speed in rpm,t-no of teeth, definitely the no of teeth on output gear will affect the speed.