Pulley size when changing motor speed

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Forum topic by Don W posted 11-16-2013 01:33 AM 1417 views 0 times favorited 3 replies Add to Favorites Watch
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Don W

19045 posts in 2805 days

11-16-2013 01:33 AM

I’d like to buy a 3450 rpm motor for my refurbed lathe.

Its just a price thing, and I don’t have the original pulley anyhow.

The motor that was on it was a 1726 RPM, with a 275-A Pulley. What pulley would I use on the new motor to get the same speeds?

I’ve searched for an online converter but didn’t dig one up. Logic would tell me it would need to be just about half the diameter, but I’d like another opinion.

-- - Collecting is an investment in the past, and the future.

3 replies so far

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#1 posted 11-16-2013 01:37 AM

DELETE. I was trying to chat on one page and read on a few more and i got messed up.

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#2 posted 11-16-2013 02:25 AM

Don’t rely on online calculators, many of them give wrong numbers which I learned after an hour or so of frustration. Rely on your own calculations.

If I remember correctly, it’s the diameter of the driven pulley (d2) times final rpm (r2) divided by motor rpm (r1); so motor pulley diameter = d2 (r2/r1). But the short answer is the new motor should have a pulley 1/2 the size of the old one.

Edit: I checked this one and it is working correctly.

-- Rick M,

View JohnnyB's profile


105 posts in 2627 days

#3 posted 11-16-2013 03:11 AM

Yep, the new pulley needs to have a diameter half the size of the existing pulley. There could be other factors to consider. If your original pulley is 2 3/4 inch diameter and uses an A belt (guessing this from the pulley number you gave), the new pulley would be 1 3/8 inch diameter. Pulleys for A belts might not be available in a diameter this small because there is a limit to how tight a given size belt can “wrap”. You might need to go to a fractional horsepower belt if your motor is less than 1 HP. I believe that there is a fractional belt that is just smaller than an A belt and can be used with an A pulley. You can probably find all the info you need in the McMaster-Carr catalog. What kind of lathe is it?

-- JohnnyB - - Sometimes determination can substitute for skill.

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