Attention smart woodworkers, engineers Need help with pulley sizes

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Forum topic by Built2Last posted 11-15-2011 04:44 AM 2842 views 0 times favorited 7 replies Add to Favorites Watch
View Built2Last's profile


234 posts in 3680 days

11-15-2011 04:44 AM

Topic tags/keywords: resource question trick tip walnut pine cherry willow cedar ash oak alder bandsaw milling

Ok, here’s the thing I need some smart person to help me with. I’m changing the motor on the sawmill I built a while back from a gas powered to electric. Really I have already changed it and now I am changing the electric motor. The electic motor I have on it now is a 7.5 hp that turns 3600 rpm’s. With this motor I’m running a 4 inch pulley on the motor (drive pulley) and a 14 inch on the driveshaft (driven pulley). This gives me the blade speed I need. The motor I’m changing to is a 7.5 hp also but it turns 1750 rpms. Now I know that I can run say a 3.25 inch diameter drive pulley and a 4.87 inch driven pulley to get the right blade speed. My question< is the horsepower transfered by the belts affected by the size of the pulley’s. What I mean is, if I run say a 6 inch drive and a 9 inch driven will it transfer more horsepower or will the smaller pulleys on the drive and driven shaft transfer more. If so, how much differance is there. Since the cost of pulleys can be outrageous, smaller would be better for me. I know smaller pulleys will wear out a belt faster but it also puts less strain or load on the motor bearings and belts would be cheaper to replace than bearings plus the savings using smaller pulleys is substantial. My idea now is to go with a 2 groove 3.25 on the drive pulley and a 4.9 or so 2 groove driven pulley. My center distance between pulleys will be somewhere around 18 inches or so. I think I can space then so that I can use BX56 belts since I already use then in another application and keep then on hand.

If anyone knows for sure about this or just has a common sense idea about this, I would apprecaite your input.

7 replies so far

View sandhill's profile


2128 posts in 4127 days

#1 posted 11-15-2011 05:15 AM

No the HP is a constant dictated by the motor but the load changes effecting the required HP. The number of belts you use (gang) will help you with the torque. The diameter of the pully will change the speed I guess you know that).

View patron's profile


13640 posts in 3544 days

#2 posted 11-15-2011 05:46 PM

how about a hand here guys

with all the ‘shop built’ tools on here

there must be many
that know about gears and pulleys

all i know is
pulleys rotate in the same direction

and gears counter-rotate

but size ratio
and speed
are over my head

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View PurpLev's profile


8547 posts in 3851 days

#3 posted 11-15-2011 05:54 PM

I think you’ll be fine with the 3.25 pulley on the motor if you had good results with the 4” on the previous motor, the size difference is not that big.

As for the HP and torque – as long as the motor runs at full speed you’ll get all the HP and torque it can produce regardless of pulley arrangement. the pulley arrangement will just control the driven shaft’s speed in relation to the motor rpm.

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View DS's profile


3033 posts in 2623 days

#4 posted 11-15-2011 06:07 PM

The size of the pulleys will affect the amount of force exerted on the belt to transfer 7.5hp from the motor to the blade. A larger pulley will have more surface area in contact with the belt and the increased leverage from the larger pulley will exert less force on any given part of the belt. That will equate to less wear.

It seems you have the blade speed part worked out. This is simply a ratio of the Motor RPM to the Blade RPM—The pulley diameter would need to match this ratio.

In your first setup 3600RPM with 4” gives 1028.6RPM on the blade with a 14”—a ratio of 14:4.

With the second setup, the ratio is now known by the RPM—1750 : 1028. Any combination of pulleys with this same ratio will give you the desired blade speed. e.g. If you keep the 4” Motor pulley, a 6.8” Blade pulley would be needed.

What is needed, is to know the amount of force the belt will handle, so you can decide if larger pulleys, or perhaps a second belt is needed.

Some information can be gleened from your previous setup. How well did the belt perform with the 4” drive pulley and the 7.5hp motor? If it was satisfactory, you could derive that a 4” drive pulley is sufficient. If it wore badly, you might consider a larger drive pulley to reduce the wear on the belt – just be sure to maintain the 1750:1028 size ratio to the blade pulley.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View Built2Last's profile


234 posts in 3680 days

#5 posted 11-15-2011 06:35 PM

Thanks guys!!!!!!! My little brain is going to explode!!!! Since I started this I’ve had to teach myself to wire a three phase converter, a three phase motor, find out pulley speeds and all sorts of other stuff and to think all I wanted to do was saw some lumber. What got this latest problem going was, while trying to save money and being ready to saw lumber, I was researching pulleys, sheaves, bushings etc and came across a Masta or Gates website about how certain belts, pulleys and they diameter of the pulleys could affect the loss of horsepower being transfered from motor to the driven pulley. I just wanted to make sure I was giving myself the best setup because I’m tired of the electrician, engineer or whatever you call it, hat I’ve been wearing. I think I’m going with the 4” drive pulley for now since I already have it and all I need is a bushing to use it on the new motor since the other one has a smaller shaft.
I appreciate the input and help so much!!!!!! It will be take a few days to get a bushing and when I do, I’ll repost how it worked out.
Thanks again!!!

View devann's profile


2246 posts in 2895 days

#6 posted 11-15-2011 08:11 PM

No I’m not going to do the math for you, but I will tell you about a book that I’ve found for the shop math questions that’s worth it’s weight in gold. It even has a whole chapter on pulleys and how to figure out which size to use.

-- Darrell, making more sawdust than I know what to do with

View EEngineer's profile


1117 posts in 3816 days

#7 posted 11-16-2011 02:49 AM

Classic physics definition for work in a rotational system:

work = torque X speed

The work (HP) you deliver with different size pulleys will be a constant. Power in = Power out (minus any losses due to friction, belt slippage, etc). However, as you speed up or slow down the shaft speed from the motor speed you will get more or less torque.

For example, if you use a 2:1 pulley ratio to drop the 3600 RPM speed of the motor to 1800 RPM, you will see approximately 2X the motor torque at the driven shaft.

-- "Find out what you cannot do and then go do it!"

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