Pulley - Lawnmower

Pulley - Lawnmower

Nov 01, 2022

James Laumeyer

Continuing with the race mower project one of the last major issues was the thing just keeps eating its belts. When we would run it it would go for a few minutes before the belt heated up to a point it was melting, stretching, and tearing itself apart.

This tells us a few things, the belt we are using is not strong enough for the power we are sending through it and that, in all likelihood, the system is geared too high and will continue to eat belts if the ratio is left as it is. As an additional precaution the pulleys on the back half of the mower were swapped out for steel ones in this process as research indicates that the cast-iron bullies have a bad habit of blowing up at high RPM.

While this is not something we have witnessed yet, it is also something we would like to avoid at all cost to the inherent danger of shrapnel. To find the proper belt and pulley sizes the following short MATLAB script was used to plug in available pulley sizes and insure that the required belt length would be one that it readily available in the material and size we need.

clear; clc; close all;

L = 7.125 % axle to axle distance Dl=3; %dimension of the pulley on the transmission Ds= 5;% Diameter of the pulley on the jack shaft BL= (Ds*(pi/2))+(Dl*(pi/2))+(L*2)+[((Dl-Ds).^2)./(L*4))] %Belt length This was based off the equation found in the book Desk Ref by Thomas J. Glover as shown in the image below.


By utilizing this equation and installing the correct parts we were able to get the mower back up and running and have it successfully complete some trial runs clocking up to 15mph at idle.

This is inline with the initial expectation of 50mph as at full throttle the engine would be going about three times faster and put us at about 45mph, though admittedly none of us have been daring enough to try that yet.