Pulley ratios vs. the Real World

J

jschmidling

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I acquired a Power King drill press from a friend who upgraded and I am having a strange problem with speed control.

The first thing I did was to replace the 3000 RPM motor with a 1700 to get rational speeds but the slowest speed is still too high at around 800 RPM.

The sheaves on the motor and the quill seem to be the same although the quill sheave seems to be the original and the motor one is newer.

At the lowest speed, the motor is 1" and the quill is a bit under 4". Roughly speaking, a ratio of 4:1.

The measured motor speed is 1700 and the quill is about 800 or a ratio of about 2:1. This can be confirmed by manually turning the motor shaft and comparing revs on the shafts and I come up with about the same ratio.

Even considering the problems of measuring the pitch diameter of the sheaves, I don't see how I could be off by that much.

What can be going on here?

The only thing that occurs to me is the belt width and cross section but this would seem to have the same effect on both sheaves and cancel out.

Not knowing where to find the answer, I ordered a 3/8" to test this hypothesis. The belt on the machine now is 1/2".

Hopefully, someone in this group has been here before.

Thanks.

Jack Schmidling
Marengo, Il
 
In the real world a good solution will be a 3-phase motor with a VFD. The Chinese ones are so cheap now that any tool that needs variable speed should get one. JMHO

John
 
One inch (motor pulley) is an awfully small diameter for a 1/2" belt, any chance you might have made a mistake measuring it?

-frank
 
What ELHEAD said. I'm guess the 1" diameter is the bottom of the groove? I think you need to measure diameter at the midpoint of the belt—if you measure at the surface of the belt and then subtract the belt's thickness you should be pretty close. On a small pulley that will make a larger difference than on a larger one.

Sent from my Pixel XL using Tapatalk
 
The pitch diameter is measured at about 1/3ish of the way to the bottom of the groove from the outside depending on the profile. Easier is to just measure the OD which will get you real close for calculating the ratio as long as the pulleys have the same profile.
 
Interesting thread. When you say OD I assume you mean the belt surface with the belt seated?
Robert
 
I believe the relevant measurement is "circumference". Diameter x 3.1415. Can't find the Pi symbol on this keyboard....
 
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