Pully size math

[john in la]

I know the RPM of a motor and how fast I want the object to turn.
What is the math equation to figure what 2 pullies to use.

 

[Steve in VA]

I know the RPM of a motor and how fast I want the object to turn.
What is the math equation to figure what 2 pullies to use.

I would think it's a direct ratio of the circumference of the 2 pullies. Since the circumference is Pi x D, then you could ratio the diameters. Eg, if you want to reduce the speed by 1/2, then the motor pulley should have a diameter twice that of the item pulley.

 

[moresmoke]

(input RPM * drive pulley diameter)/driven pulley diameter = output rpm.

As to designing the belt drive itself, the belt manufacturers have entire books dedicated to choosing the proper belts, number of them, and diameter of the pulleys for the required loads and speeds.

 

[DoubleO7]

I know the RPM of a motor and how fast I want the object to turn.
What is the math equation to figure what 2 pullies to use.

Easy,.... pull-up a pulley calculator on the net.

 

[Geo-TH,In]

RPM drive pulley = Motor RPM X motor pulley diameter / drive pulley diameter.

If the motor pulley diameter is 2 times larger, the drive pulley RPM will be 2 times faster.

If the motor pulley diameter is 2 times smaller, the drive pulley RPM will be 2 times slower..

 

[Butch(OH)]

Using a simple example but the math doesn't change.
Motor is 1800 you want 900 RPM 1800/900=2 to 1 ratio your driven pulley needs to be 2X the size of the motor pulley. 2" on motor, 4" on machine is an example.

Motor is 1800 you want a 1000.
1800/1000 is 1.8 to 1 ratio. Motor pulley diameter x 1.8 gives you the machine pully size. 2" on motor and 3.6 on machine.

Obviously of you want the machine to run faster than the motor the math is the same but the larger pulley goes on the motor.

 

[caterpillar guy]

5 inch pulley driving a 15 inch pulley will be about 1/3 of the motor speed. If you switch them then it would be 3 times the motor speed. I slowed down an auger one time so it didn't splash as it exited the end of the auger worked much better and still keeps up with a 10 inch going to the truck. 8 inch unloading auger.

 

[PJH]

I know the RPM of a motor and how fast I want the object to turn.
What is the math equation to figure what 2 pullies to use.

A lot of good answers here.

I accidentally learned something that I should have known - when experimenting with driven shaft speeds - if you add the pulley diameters together you can use the same belt as long as the sum of the pulley sizes stay the same. Example - A shop attic fan turned too slow to suit me. Pulleys were 7" and 2". I switched to 5" and 4" and the same belt fits perfectly.

 

[BarnyardEngineering]

If this is helpful to anyone:

 

[Butch(OH)]

A lot of good answers here.

I accidentally learned something that I should have known - when experimenting with driven shaft speeds - if you add the pulley diameters together you can use the same belt as long as the sum of the pulley sizes stay the same. Example - A shop attic fan turned too slow to suit me. Pulleys were 7" and 2". I switched to 5" and 4" and the same belt fits perfectly.

You are correct in that.
I have a Reliance/ Dodge engineering book for pulleys and belts and there are tables in it for every combination of pulleys, center distances and belt sizes. Pretty handy and I'd bet it's online now.

 

[john in la]

Thanks for the help.
I am trying to achieve a driven shaft RPM of about 200 off an electric motor.
I have an old washing machine motor and a motor off an old air compressor to choose from but I have not looked at their RPM rating.
I am under the impression that torque goes up as RPM goes down.

 

[DoubleO7]

Thanks for the help.
I am trying to achieve a driven shaft RPM of about 200 off an electric motor.
I have an old washing machine motor and a motor off an old air compressor to choose from but I have not looked at their RPM rating.
I am under the impression that torque goes up as RPM goes down.

That might involve more than two pulleys.
As "old" air compressor motors are usually no less than 1725rpm's.

 

[john in la]

That might involve more than two pulleys.
As "old" air compressor motors are usually no less than 1725rpm's.

If my calculations are correct a 1.5 inch drive pulley and a 13 inch pulley off a swamp cooler should get me in the right RPM range at a 1750 motor speed.
Now i need to check the motor specs for the motors I have.

 

[DoubleO7]

If my calculations are correct a 1.5 inch drive pulley and a 13 inch pulley off a swamp cooler should get me in the right RPM range at a 1750 motor speed.
Now i need to check the motor specs for the motors I have.

The 1.5 drive pulley might not have enough belt contact to achieve what you are driving.

 

[Bob Bancroft]

If my calculations are correct a 1.5 inch drive pulley and a 13 inch pulley off a swamp cooler should get me in the right RPM range at a 1750 motor speed.
Now i need to check the motor specs for the motors I have.

My calculations concur.

 

[john in la]

The 1.5 drive pulley might not have enough belt contact to achieve what you are driving.

Hummmm
That is something to think about.
2.5 and 14 would get me a little over 300 rpm
That might work

 

[Ray,IN]

Easy,.... pull-up a pulley calculator on the net.

Yep, https://www.omnicalculator.com/physics/belt-length
Somewhere I have a link to how to determine the length of a V-belt or serpentine belt for a vehicle.

 

[kencombs]

Hummmm
That is something to think about.
2.5 and 14 would get me a little over 300 rpm
That might work

With really small pulleys, you can incorporate an idler and longer belt to get as much 'wrap' on the small pulley as possible. Greatly increases the driving power if you can get a 180deg contact.

 

[Watthour]

If you are using a "V" belt be sure to select the pulley sizes by pitch diameter, not outside diameter. It make s a slight difference.