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.
common sense would dictate that you shouldn't go over an angle of about 45 degrees between top and bottom belt. Less if you're transmitting anywhere near the capacity of the belt.
They have excellent downloadable PDF catalogs with good engineering sections. You have to register first. There is also a downloadable belt drive design application that does center distance, horsepower capability, belt type selection, pulley selection, etc. It's Design Flex Pro. I used it about two weeks ago. Works very well. You get a PDF report with part numbers for output.
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.
I tried it, thanks. I also tried the Maska program of similar nature. The gates program is way better but even that one leaves more questions than answers. Why are all my transmissions "overdesigned"? Why keep asking me to consider a different belt cross-section when I ticked almost all v-belts? And so on.
In the end cheerful ignorance will probably be a bliss. It will either work with the parts available or it will not. In which case I shall try something else.
Overdesigning the belt part of the drive is probably a good thing. They should wear out less frequently. As long as the required belt tension doesn't exceed the motor's overhung load capability.
I remember seeing a bunch of this stuff in an old Boston Gear catalog. Maybe you could find one on eBay? I'm not sure how much of that has made it's way online. You might also try Machinery's Handbook...
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