RPM calculation

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Spanish Determining RPM of given size pulley. May 17, 2003

Question
Can anyone tell me how to calculate the RPM of a given size pulley? If I had a motor with a 1" shaft with a 4" pulley mounted on the shaft and this was driving another 10 inch pulley, what would the RPM be of the 10 inch pulley? I am building a bandsaw mill and I am trying to get the correct band speed, which I am not sure of either. Any suggestions on the correct band speed (11/4" band X 13'8" long)?

Forum Responses
(Sawing and Drying Forum)
From contributor R:
The 4" pulley will turn 2.5 times to the 10" pulley's one. That is called a 2 1/2 to 1 ratio. Divide the big one by the small one and this will always give you the gearing ratio. So if your motor is turning at 2500 rpm, your driven pulley would only turn 1000. If for some reason you put the 10" on the motor and drove the 4", the 4" would turn at 6250 rpm (with the motor running at 2500 rpm). 2500 x 2.5.

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I think the desired speed is close to 5,000 ft. per minute.

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The 4 inch pulley will have 2.5 times the rotational speed of the 10 inch pulley. The pulley diameter times the RPM rate will be constant. So, as the pulley size gets larger, the pulley speed gets lower.

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From contributor R:
I'm sure that the above post meant that as the driven pulley gets larger the RPM gets slower. I just wanted to clarify to make sure no one got confused. If the drive pulley got bigger the driven pulley would get proportionally faster. Let us know when your mill is making sawdust. Boards may be a couple of days after that. Endeavor to persevere!

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I just found the info that you need. It is for the Suffolk low tension blade. Here is the formula for finding pulley size for you particular engine. You are shooting for a *minimum* of 4500 surface feet per minute up to 5800 for a maximum. This is for Timberwolf blades from Suffolk Machinery.

Motor RPM X motor pulley diameter divided by driven pulley diameter X band wheel diameter X 3.1416 divided by 12 = SFPM

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Before any calculation is made, you need to know what the motor RPM is at the shaft (either stated on the motor label or taken with a tacho or whatever other means). Once you get that, multiply this RPM by 4 and divide the result by 10 to get the bandwheel RPM (I presume the bandwheel and the 10 inch pulley are on the same shaft). Next, find the circumference of the bandwheel in feet and multiply this by the bandwheel RPM. There you have what you're looking for - the number of feet (blade) per minute, or what you call "band speed".

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The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

From contributor A:
RPM is important to calculate, but also the amount of force you will need in relation to the material. Take into consideration, for example, that a 20hp motor could have 1760 rpm and a 40 hp motor could have the same rpm. Speed is important, but also the force or power.