# Lawnbot Transmission

February 10, 2012 Leave a comment

Having decided to use the worm gear motors that I found at the local surplus store, my job of gearing down the drive speed became much easier. As I mentioned in an earlier post, these motors run at about 180 RPM and they already have a ton of torque. I need to gear this down to about 20 RPM, and I’ll gladly take more torque, so a gearbox is in order.

The math here is pretty easy; the motor turns at 180 RPM and I want a final output of 20 RPM. 180:20=9:1 gear ratio. The simplest route would be to use one tiny gear and one huge one, say a 10 tooth and a 90 tooth. Given that the main body is only 12″ tall, a 90 tooth gear would have to have a pretty small tooth in order to have a small enough diameter. Since I’m initially making my gears out of MDF (later to be cast in ABS plastic), I need to have a fairly large tooth, so a 90-tooth gear is out of the question. Instead, I’ll compound the gears, allowing me to use multiple gears with fewer teeth per gear. What I have decided upon is two pair of 21 and 7 tooth gears. 21:7=3:1. 3:1 + 3:1 = 9:1. Yay!

A single pair of 21:7 gears looks like this:

For a full size image, please see the pdf copy here.

The motor turns the smaller gear, which turns the larger gear at one-third the speed. At this point, the second pair is added. First large gear that is turning at one-third the motor’s speed is directly connected to the second small gear. This means the second small gear is also turning at one-third the motor’s RPM. The second small gear turns the second large gear. Since the second large gear turns at one-third the speed of the second small gear, which is already turning at one-third the speed of the motor, the second large gear turns at one-third of one-third, or one ninth the speed of the motor, hence 9:1 gear ratio.

Here is what the whole thing looks like assembled: