Motors and Pulleys

[From Mystic]

Hello

I j picked up and old jointer with no motor. I looked up the manual and it oridinally came with a 3250rpm motor with pulleys to spin the knifehead at 4300rpm.


I have two 1725rpm motors. What size pulleys would I need to get 4300rpms. Anyone know of a resource for this info? Would there be a problem doing this? Thanks FM

[Ian McColgin]

If you've got the base numbers right, then you need pulleys where the circumferance of the larger, on the motor, is just shy of 2-1/2 times the diameter of the smaller. This seems pretty fast to me but I don't know much about jointer speeds. The step-up in rpm will be accompanied by a step-down in power at the blades. You might check the original specs to see if your slower replacement motors can deliver enough to keep the thing spinning when you put a plank in.

Edited to add: Boy did I mix apples and oranges above but wrote what I really meant at #4. SamSam was right. As phrased above makes no sense. I was originally writing to measure the ratio of the circumferences, like wrapping a tape around the pulleys, and then realized measuring the diameter was easier but didn't make both sides of the sentence match. [Understanding the causes of even stupid errors is always interesting.] Even when I wrote #4 I did not see the confusion of what I'd written. Ah well. Got it right eventually and David kindly emphasized the point.

[SamSam]

If you've got the base numbers right, then you need pulleys where the circumferance of the larger, on the motor, is just shy of 2-1/2 times the diameter of the smaller.

Are you sure you want to mix circumference and diameter? It seems like an impossible equation.
I think you meant the circumference of the larger is 2 1/2 times the circumference of the smaller.

[Ian McColgin]

(Pi)(larger diameter) = (2.5)(Pi)(smaller diameter)

Divide pi out of each side and you're left with diameter. It works out.

[David W Pratt]

Diameter is secondary, the ratio of diameters is the key.
Good luck.

[SamSam]

If you've got the base numbers right, then you need pulleys where the circumferance of the larger, on the motor, is just shy of 2-1/2 times the diameter of the smaller.

This is what I question, from post #2. Say you have a 2" pulley on the jointer head. It is the smaller one. 2.5 X 2" = 5". The circumference of a 2" pulley is 6.28" so that equation would make the motor pulley smaller than the jointer pulley, which wouldn't work.


(.....The ratio of the diameters and the ratio of the circumferences are the same, which surprised me, I thought using diameters was exponential, and then I realized I was using pi x R2, the formula for finding the area of a circle. Doh. The question above still seems valid, though.)

[Ian McColgin]

Ah SamSam, we were both thinking a bit off-base then. Regardless, as you'll see in my edit, you drew my eye to the error I made. Thank you.

[SamSam]

It's too bad peer checks don't work so well in politics or religion.

[Woxbox]

So there's the ratio, but as noted, the amperage of the replacement motor has to be adequate. Do you have numbers on that?

[From Mystic]

Hey

Thanks for the replys.

WOx- I have two motors one is 11amp and the other is 19amp , both at 120v.

Thanks again, FM

[Nicholas Carey]

This is pretty good: http://www.pouliesmaska.com/driveselectionnet/Main.aspx

It solves for just about anything...and then recommends specific pulleys, belts, bushings, etc.