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Tool Talk Discussion Forum

rpm pulley calculation

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Davidzahm

01-09-2007 08:32:43




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I put a 3450 rpm motor on a vintage wood lathe- obviously even with step pulleys this is way fast. Can someone tell me a formula to calculate rpm with various size pulleys? something along the lines of lathe=4" motor= 2"rpm = 1250 (I made these up.




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MPK

01-10-2007 16:54:55




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 Re: rpm pulley calculation in reply to Davidzahm, 01-09-2007 08:32:43  
Here"s what I use.
Motor speed x motor pulley dia. divided by driven pulley dia. = driven shaftspeed.



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sjh

01-10-2007 10:20:21




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 Re: rpm pulley calculation in reply to Davidzahm, 01-09-2007 08:32:43  
Hereis a good web site.
http://www.csgnetwork.com/pulleybeltcalc.html



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Pooh Bear

01-09-2007 19:49:33




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 Re: rpm pulley calculation in reply to Davidzahm, 01-09-2007 08:32:43  
(driver pulley diameter divided by driven pulley diameter) multiplied by RPM.

third party image

Pooh Bear



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frankiee

01-09-2007 20:07:52




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 Re: rpm pulley calculation in reply to Pooh Bear, 01-09-2007 19:49:33  
Man
How do you do the way cool diagrams
I'm jealous



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Pooh Bear

01-10-2007 04:38:03




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 Re: rpm pulley calculation in reply to frankiee, 01-09-2007 20:07:52  
I did that one in MS Paint.
If you have Windows it should be on your computer.
Start>>Programs>>Accessories>>Paint
It's real easy to learn and play with.

Pooh Bear



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frankiee

01-09-2007 10:22:04




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 Re: rpm pulley calculation in reply to Davidzahm, 01-09-2007 08:32:43  
I perfer Eric's method with a twist
I remember it by saying to myself
"DRIVER driven" relationship
So
DR = dr D= driver (motor) pulley diam R= driver (motor) pulley RPM d= driven Lathe pully diam r= driven Lathe pully RPM

Do the same as Eric said
Find r by multipling D and R then divide by d

Any question about the math and ask for more clarity

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Eric in IL

01-09-2007 09:21:44




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 Re: rpm pulley calculation in reply to Davidzahm, 01-09-2007 08:32:43  
Here is one:

D= diameter of driving pulley
S= speed of driving pulley
d= diameter of driven pulley
s= speed of driven pulley

By transposing the formula, DS=ds you can solve for the unknown. Here it is transposed:

D=ds/S S=ds/D s=DS/d d=DS/s

Using the equasion, s=DS/d I plugged in some of your numbers. s=2x3450/4 ; s=1750rpm So if you have a motor at 3450rpm with a 2 inch pulley belted to a 4 inch pulley on the spindle, the spindle will run at 1725rpm.

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Eric in IL

01-09-2007 10:50:24




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 Re: Boo-Boo, sorry! in reply to Eric in IL, 01-09-2007 09:21:44  
In my example s=1725, NOT 1750.



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JoeK(WI)

01-09-2007 08:46:18




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 Re: rpm pulley calculation in reply to Davidzahm, 01-09-2007 08:32:43  
Try this,always helps me(bookmarked) :)



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Wyokid

01-09-2007 08:41:58




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 Re: rpm pulley calculation in reply to Davidzahm, 01-09-2007 08:32:43  
Just a ratio of the pulley diameters: 2" pulley on motor, 4" pulley on lathe-lathe spins twice as fast a motor. Just use similar ratio for other sizes.



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Wyokid

01-09-2007 08:41:42




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 Re: rpm pulley calculation in reply to Davidzahm, 01-09-2007 08:32:43  
Just a ratio of the pulley diameters: 2" pulley on motor, 4" pulley on lathe-lathe spins twice as fast a motor. Just use similar ratio for other sizes.



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Gerald J.

01-09-2007 09:24:37




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 Re: rpm pulley calculation That's backwards in reply to Wyokid, 01-09-2007 08:41:42  
That's backwards. 2" on motor, 4" on lathe, lathe turns half as fast as the motor.

Actually that's not accurate for V belts, because you need to use the pitch diameters of the pulleys, typically about 0.2" less than the outside diameter.

Load speed is motor speed times motor pulley pitch diameter divided by load pulley pitch diameter.

Gerald J.



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wyokid

01-09-2007 10:38:37




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 Re: rpm pulley calculation That's backwards in reply to Gerald J., 01-09-2007 09:24:37  
yer right, shouldn't think before the tenth cup of caffine.



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