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Tool Talk Discussion Forum

how to calculate difference in rpm s

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jeff vanderhors

10-20-2007 17:17:06

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hello all. question: if i have a drive shaft that is turning at 2500 rpm with a three inch diameter pulley and it runs around another pulley that is eight inches in diameter , how fast will the shaft that the eight inch pulley is mounted on spin? thanks . jeff

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Pooh Bear

10-22-2007 02:06:22

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Re: how to calculate difference in rpm s in reply to jeff vanderhorst, 10-20-2007 17:17:06

When designing something I usually try to keep pulley ratios under 3:1.

Depending on the usage I might go up to 4:1 but no bigger.

The general rule is that the belt should wrap

around the small pulley at least 120 degrees.

A 3 inch pulley driving an 8 inch pulley would be under 3:1 ratio.

2500rpm x 3/8 = 2500rpm x .375 = 937.5rpm

Pooh Bear

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Bookmark

10-20-2007 21:47:33

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Re: how to calculate difference in rpm s in reply to jeff vanderhorst, 10-20-2007 17:17:06

Gizmology

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jeff vanderhorst

10-20-2007 17:47:36

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Re: how to calculate difference in rpm s in reply to jeff vanderhorst, 10-20-2007 17:17:06

ok great. thanks. next question: is there a standard or rule of thumb that limits the size difference between two pulleys as in the example i used . in other words could i go from a three inch pulley to say a ten or twelve?

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Tim Casbolt

10-21-2007 17:42:19

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Re: how to calculate difference in rpm s in reply to jeff vanderhorst, 10-20-2007 17:47:36

I don't think there is a rule that governs the differential between the two pulleys. I have seen 4" pulleys driving 36" pulleys with no noticeable slippage.

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Bob

10-20-2007 17:50:43

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Re: how to calculate difference in rpm s in reply to jeff vanderhorst, 10-20-2007 17:47:36

AFAIK, about the only rule is "you can only go so SMALL". (There is a practical limit to how small a pulley can be made for a given belt.)

Of course, the torque output of the reduction will be determined by the "size" (cross-section) of the belt.

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Greg_Ky

10-20-2007 17:28:02

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Re: how to calculate difference in rpm s in reply to jeff vanderhorst, 10-20-2007 17:17:06

6667rpm.

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Greg_Ky

10-20-2007 17:29:41

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OOPS in reply to Greg_Ky, 10-20-2007 17:28:02

I went the wrong way - 938rpm.

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Bob

10-20-2007 17:26:53

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Re: how to calculate difference in rpm s in reply to jeff vanderhorst, 10-20-2007 17:17:06

2500 X 3 / 8 = 937.5 RPM's.

(Actually, you are supposed to use the "pitch diameter" of the pullies to get an EXACT figure, but his is close enough for MOST common "apps".)

"Pulley Pitch Diameter = The diameter as measured from the center of the belt's cross-section through the center of the pulley to the center of the belt's cross-section. This is equivalent to the root diameter of the pulley plus the belt thickness (for belts that sit on the bottom of the groove). V-Belts, which are wedged between pulley flanges, may not sit on the bottom of the groove."

(Quoted from "http://www.durabelt.com/crosssectioncalcinfo.php".)

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J Schwiebert

10-20-2007 17:25:22

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Re: how to calculate difference in rpm s in reply to jeff vanderhorst, 10-20-2007 17:17:06

Multiply your 2500 RPM times the three inch pulley size and then divide by the 8 (size of the other pulley)

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