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Belt/Pulley length math equation. (Easy)

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slideways

06-24-2004 19:25:51

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Hey guys, Help me out here. I am making a racing tractor and i want to avoid buying and testing several belts when i change pulleys. I will be going from a 10" trans pulley to a 4". How much shorter of a belt will i need from stock? I know 3.14 comes in here somewhere.

Thanks

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farmboy

06-25-2004 19:17:05

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Re: Belt/Pulley length math equation. (Easy) in reply to slideways, 06-24-2004 19:25:51

The 10" pulley has about 15.7' of belt in contact with it (10 x 3.14 /2)and the 4" pulley has about 6.3 inches of belt in contact with it (4 x 3.14 /2).So you would need approx a 9.4" shorted belt for a 4" pulley than a 10" pulley. I say approx because if the pulleys (driver/ driven) are fairly close together, the belt will be contacting more than 1/2 of the big one and less than 1/2 of the small one.

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slideways

06-25-2004 16:31:09

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Re: Belt/Pulley length math equation. (Easy) in reply to slideways, 06-24-2004 19:25:51

Well i really need the perimeter/2. It shouldnt be a realy complicated equation.

Perimeter of the 10" minus the perimeter of the 4" divided by 2 since the belt only contacts half of it.

But how do you determine the perimeter of 10" diameter?

Thanks (im in college and im still not smart)

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R Bedell

06-25-2004 10:58:21

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Re: Belt/Pulley length math equation. (Easy) in reply to slideways, 06-24-2004 19:25:51

That sight I provided earlier was an wasy way to figure the belt length. If you want to do it "long hand" you could use the following formula:

BL = (1/2 x PD1 x 3.14) + (1/2 PD2 x 3.14) + (D x 2)

BL = Belt Length
PD1 = Pulley Diameter of 1st Pulley
PD2 = Pulley Diameter of 2nd pulley
D = Distance between the pulleys on the parrallel Diameters.

The belt only goes around 1/2 of a pulley at the Diameter point. So you would have the calculation for both pulleys plus the distance between the Diameter point times 2.

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Bob

06-25-2004 09:04:40

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Re: Belt/Pulley length math equation. (Easy) in reply to slideways, 06-24-2004 19:25:51

Find an old belt the same width, but a little longer than needed. Cut it and wrap it around the pulleys, and match up the ends, and cut a piece out until the ends butt together. Drill a couple of small holes near each cut end and tie the belt together with a piece of light wire. When you are satisfied that it fits, take the belt to your favorite belt emporium, and match it up with a new one.

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txblu

06-25-2004 09:32:25

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Re: Re: Belt/Pulley length math equation. (Easy) in reply to Bob, 06-25-2004 09:04:40

Great idea.

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txblu

06-25-2004 07:15:59

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Re: Belt/Pulley length math equation. (Easy) in reply to slideways, 06-24-2004 19:25:51

Not distracting you from Mr. Bedell's response but the 3.1417.... is the ratio of the circumference (outer round part) of a circle to the diameter. It is usually refered to using the greek letter "pi" (don't have greek on my keyboard or I'd hit that key). So that equation is pi x diameter (distance straight across circle from edge to edge.

Since your talking about a chev, you divide your answer above by 2; you're only using half the dia on length (but the whole thing for gear ratio).

Also, check where you measure length. Don't remember if it is od, id, or midway.

My 2c.

Mark

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R Bedell

06-24-2004 19:33:34

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Re: Belt/Pulley length math equation. (Easy) in reply to slideways, 06-24-2004 19:25:51

Go here, it should make it easy for you: Link

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slideways

06-25-2004 07:16:46

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Re: Re: Belt/Pulley length math equation. (Easy) in reply to R Bedell, 06-24-2004 19:33:34

Not really what i needed. I dont know the center distance nor the engine pulley size. There are several idlers along the way that will affect the length as well.

Just need to know how much shorter a belt is when you decrease a pulley by 6"

Thanks

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