Motor rpm question?

pedervisti

pedervisti

Joined
Oct 31, 2008
Messages
249
How many rpm, should a motor for a 3 step pulley have?
its going on a 6" coote?
is 1450 rpm too slow? or should i aim at around 3000 rpm.:confused:
Thanks
 
Motor speed does not matter. It is the gear ratio from the pulleys that you want to figure out. Decide on the highest desired belt speed in FPS and calculate backward from there.
 
Here's one way of looking at it, using common 2,3,4 in. pulley stack both sides for a three - speed setup since it's easy to figure out the ratios here.

Motor speed (powered axle) at 1725 rpm, speed changes calculated for 3 step pulley set, diameters of 2“, 3“, and 4”.

Driven Axle r.p.m. results of pulley pairings.
862.5 @2” x 4” (half speed) ratio of 0.5
1725 @3” x 3” (full speed) ratio of 1
3450 @4” x 2” (double speed) ratio of 2

Drive wheel diameter calc:
6”x3.14 = 18.84 in. dia.

Rpm of driven axle x belt drive wheel diameter = belt speed in surface inches per minute.

Belt speed in surface inches per minute w/6” drive wheel
16249.5 in./min. @ 862.5 rpm. Half speed
32499 in./min @ 1725 rpm. Full speed
64998 in./min @ 3450 rpm. Double speed

Divide by 12” to find SFPM.

Belt Sfpm., Calcs w/6” drive wheel
1354 Half speed
2708 Full speed
5416.5 Double speed


So if you use these diameter pulley sets and a 6" drive wheel on your grinding belt, you have a fairly low speed, a decent regular speed, and a really fast hogging speed. If you used a 3450 rpm motor with the same pulleys, you'd get 2708, 5416.5, and 10833 sfpm, or WAY TOO FAST.

The above is intended only as an example of how to figure out your axle speeds and sfpm. There are pulley configurations that can yield acceptable speeds from a wide range of motors, including 3450 rpm motors.

Be aware that increasing speed mechanically decreases torque.

Also, here is a link to a page that can help you figure out pulley and belt ratios.
http://www.csgnetwork.com/pulleybeltcalc.html
 
Last edited:
Using the above setup, your 1450 RPM motor is going to be fine. What size and type of motor is it?

If you use a 3400 RPM motor you would need to have a 2" drive pulley and a 2-3-4 step pulley on the grinder to get the same results.
 
Actually, I think with a 3450 rpm motor a motor pulley of 3" with a 1.5-3-4.5 step pulley would yield the same results,
1354 Half speed
2708 Full speed
5416.5 Double speed

correct me if I'm wrong.

One thing, if using a single pulley on the motor and a 3 step pulley on the grinder shaft, the motor will have to have a decently long belt for alignment and a way to easily adjust for differing belt length, such as being mounted under the table on a hinge, then letting the motor weight tension the belt.
 
bladsmth said:
Using the above setup, your 1450 RPM motor is going to be fine. What size and type of motor is it?

If you use a 3400 RPM motor you would need to have a 2" drive pulley and a 2-3-4 step pulley on the grinder to get the same results.
Click to expand...
Its a 1450 rpm 2 horse motor, and it runs on 380 volts if that makes any difference
Thanks :)
 
Salem,
If the motor was 3400RPM and you used a 3" drive and 1.5-3-4.5" step pulley - the ratios would be 2:1, 1:1, 2/3:1. At 3400 RPM that would give 6800RPM,3400RPM, and 2265RPM. With a 6" drive that would yield the top belt speed of a bit over 10,600FPS. I don't want to stand next to that when it is running.

Also, I have never seen a commercial step pulley with that small sizes in a 3/4" shaft . You would probably have to go with a 2-3-4" step pulley and have a 4" drive pulley.

Peder -
Is this a 3PH motor? If it is the best solution is to get a VFD and make the grinder VS.
 
I think a VFD takes away from the biggest attraction to a Coote, cost. I think that step pulley's will be more than adequate, so many folks start out on a Grizzly and they aren't variable speed. They're also going pretty darn fast when they get upgraded to a 10" drive wheel. The nice thing with a Coote is that VS doesn't really cost you anything above fixed speed.

I think his motor and drive shaft are 5/8" and step pulleys will be available. For what it's worth, the sizes don't matter anyways, it's the ratio. (Yes, I said it - it's not the size, it's how you use it :D)
 
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