On Oct 24, 9:17*pm, "Michael Koblic" wrote:
I have a vague memory of reading about rules that govern the design of
pulley and belt transmission, specifically the optimum distance between
centres of the pulleys and the maximum recommended ratio of pitch diameters.
I cannot find the reference and I am beginning to wonder if I dreamt it.

Clearly there is going to be some power loss if the angle of belt contact
with the small pulley is much less than 180 degrees but are there any math
formulae which would help in optimum design? I have been through Google
(again!) as well books and all I am finding is old stuff *related to flat
belts made of cowhide and such.

Michael Koblic,

If you pay attention to vee belts you notice all sorts of successful
misuse. I try to avoid pulleys smaller than 2" diameter and if forced
to use a 1-1/2" pulley I put a notched belt on it. It it slips,
tighten it. You can increase the contact angle of the smaller pulley
with an idler. They are common on old riding mowers.

The engine pulley on my sawmill wouldn't slip when the belt was loose
until I sanded the paint off the groove. I needed it to slip to
disconnect the engine when starting it. The belt driving the hydraulic
pump on the tractor slipped until I reduced the pump speed enough.
This one was difficult because both the pump and the motor shaft
required custom-machined hubs. In both cases the belt was transmitting
at least 5HP. For a fractional HP electric motor, don't sweat it.
Build the reduction you think you need and try it. If a pulley slips
it will heat up quickly and be easy to find. Tighten the belt or try
the next larger size pulley. You can mount the motor and intermediate
shaft on hinged plates and use only one tensioner.

For loosely spec'ed and sometimes casually made items like hardware-
store vee belts I do a rough calculation of the acceptable range and
try it. Sometimes a different brand of belt in the same size or a cast
iron vs zinc pulley solves the problem.

Jim Wilkins