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Old 17-04-2008, 19:17   #1
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Pulley ratios or (I hate math)

Does any one know the formula for figuring out Rpms for different size pulleys? I'm installing a 6" pulley on the crankshaft which will turn a 7" electromagnetic clutch driving a high pressure pump. The pumps Rpm rating is 1750 rpm. My closet calculations are that the 6" will need to turn 2000 rpms to achive the 1750 on the 7"
I know there must be a formula on the net for driving alternators but I can't seem to find one.
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Old 17-04-2008, 19:25   #2
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The driven pully rpm equals the crankshaft pully rpm multiplied by the crank pully diameter and divided by the driven pully diameter. Obversly, the crank rpm equals the desired driven rpm multiplied by the driven pulley diameter and divided by the crank pulley diameter.

For alternators make sure that small frame alternators are not driven higher than 10,000 rpm at the highest engine rpm. For large frame alternators limit their spindle speed to 7,000 rpm. For longevity of belts do not use alternator pulley diameters less than 2.4-2.5 inches.
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Old 17-04-2008, 19:53   #3
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Thanks Rick I appreciate it.

So If my crank RPM equals the desired driven RPM (1750) mulitiplied by the driven pulley (7") equaling 12,250 divided by the crank pulley (6") my answer is 2,041.666666 RPMs

Cool beans,
PS. Ya know where I can get a REALLY accurate Tachometer?
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Old 17-04-2008, 21:01   #4
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Quote:
Originally Posted by Tellie View Post
PS. Ya know where I can get a REALLY accurate Tachometer?
Stewart & Warner is well known for tach's. Usually the more money one spends on the tach the more accurite it is. Usuall have a 4" to 6" face as well.
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Old 24-04-2008, 20:52   #5
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SW makes a good hand tach......No electrical parts.....

Or you can get a photocell type tach.......
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Old 24-04-2008, 21:17   #6
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Thanks guys. I was kinda joking about the "Really accurate tach" part. I thought the
.666666 gave it away.
But here's a serious question about the same thing. How do you determine the length of a drive belt? Again a 6" pully driving a 7" pully.
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Old 24-04-2008, 22:30   #7
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I would guess that would add half the circumfrence of each pulley and add the distance between the center of each shaft X 2.
Just a guess, I didn't pass math
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Old 25-04-2008, 01:24   #8
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Belt Length
The easy way to measure the circumference of a belt is to roll it along the wall, measuring the distance you've traveled when you get back to the same point on the belt. Subtract two inches to get the inside circumference.

If you don't have a belt, just the pulleys installed on the machine, you can run a string around the pulleys and measure that. If you don't have access to the machine, you can use a formula so royally obnoxious that Ill leave it to Norm Christopherson to explain.
Goto: Belt Length Changes

Better yet, goto Pulley And Belt Calculators:
Belt Length Calculator
Pulley And Belt Information Calculator
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Old 29-04-2008, 20:08   #9
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Thanks Gord, where do you store all this stuff? Every time we come up with a question you have a website.
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Old 29-04-2008, 22:29   #10
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Gord...You must have a massive bookmark database?
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Old 29-04-2008, 23:58   #11
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Quote:
you can run a string around the pulleys and measure that.
It is a good way to questimate, but just remember that the belt does not touch the inside base of the pulley. The belt drives by grabbing the side faces of the V section of the pulley. The belt should maintain a gap between it and the inside base of the pulley.
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