Which motor for my no-weld grinder

E

elasmonut

Joined
Feb 15, 2006
Messages
662
O.K. guys I have built my 2x72 no weld grinder,well I'm still waiting on a drive shaft, but am ready to choose a motor. I can't really afford a variable speed, and know I should probably go with at least 2hp. I'm running a four step pulley system and a four inch drive wheel. My options here for a motor are 2hp at 1440 rpm, 2.5 or 3 hp at 2800 rpm, or a 3.2 hp at either 1440 rpm or 2800 rpm. I have seen some of the belt speed calcs and formulas here but they make my feeble brain hurt, so what I'm after is some simple advice.
 
Let's see if I can get this right...

It's all about the ratio of the drive pulley to the shaft pulley.
If the drive is 1/2 the size of the shaft, then the shaft turns 2x as fast as the drive.
If the drive is 2x the size of the shaft, then the shaft turns 1/2x as fast as the drive.
If the drive and shaft have the same size pulley, then they turn at the same rate.

Part two:
If you use a 4" drive wheel, the belt speed in inches is RPM of the shaft times 4 times pi (3.14).
So, roughly, the belt speed in feet equals the RPMx(4x3.14)/12. What does that mean? If my math is right, with a 4" drive, the belt speed in feet is roughly equal to the shaft RPM.

And so...
set up your pulleys at the ratios of .5, 1, and 2, use a 4" drive wheel, and your SFPM will be just about the same as your shaft RPM.
 
5 4 3 2
2 3 4 5


With that 4 step setup, the ratios are not as useful 3
The center 2 are very close together and the outer 2 are very drastic.

I would get this motor
2hp at 1440 rpm

and

line up the pulleys so

5 4 3 2
...2 3 4 5

that would give you on the 3 speeds 2x, 1x and 1/2 speed


Leave your shafts a bit longer so you can set them up that way & room to play.



what motor is this?
all TEFC?
a "real" HP rating? or some creative rating?

3.2 hp is an unusual rating.


let's see all the data plate info for the motors if you can
do you have 220v wiring available ?
 
I run 5432 pulleys with a 6" drive on a 1750rpm motor. I rarely use 4th gear, if I recall, it is around 6700 SFM. The low speed is around 1000 SFM. The only thing I would like is a slower speed on the low end for some work. I will most likely get a 4" drive for occasional 650 SFM.
 
Make sure whatever motor you pick is fully enclosed, not open. That will limit your choices.
 
Thanks guys very helpful as always. To answer 12345678910 what you say about the 4 step pulley system makes sense, though I didn't think about that when I bought them, offsetting them is probably a better system. This is the first thing I've built really, that is much more complex than a knife and I admit I'm making mistakes but learning a lot as I go. The motor is TEFC but the horsepower rating may be creative which is why I was leaning toward a larger capacity than the usual 2hp that most seem to use. The power source is a lead for 240V but that will plug straight into the wall here in Oz. I'll try and post some more info but the site I'm looking at doesn't provide much more info.
 
elasmonut,

When I had Tracy's plans for the NWG, he had the equation to determine belt speed on a 2 pulley set up. I don't know where I put the plans and forgot the equation, would you give it to me from your plans? Anyone else who knows it?

Thanks, E
 
I found the formula and did some calcs. (I actually have a 1725 rpm motor with a 2-3-4-5 pulley set up and 6" drive wheel)

5-2 = .262 * 6 * 1725 * 2.5 = 6779 SFPM
4-3 = .262 * 6 * 1725 * 1.33 = 3607 SFPM
3-4 = .262 * 6 * 1725 * .75 = 2034 SFPM
2-5 = .262 * 6 * 1725 * .4 = 1085 SFPM

If I used a 4" drive wheel, I would get:

5-2 = .262 * 4 * 1725 * 2.5 = 4520 SFPM
4-3 = .262 * 4 * 1725 * 1.33 = 2404 SFPM
3-4 = .262 * 4 * 1725 * .75 = 1356 SFPM
2-5 = .262 * 4 * 1725 * .4 = 723 SFPM

If I offset the 2-3-4-5 and used a 4" drive, I would get:

4-2 = .262 * 4 * 1725 * 2 = 3615 SFPM
3-3 = .262 * 4 * 1725 * 1 = 1807 SFPM
2-4 = .262 * 4 * 1725 * .5 = 904 SFPM

Since I rarely use 4th gear, I will most likely get a 4" wheel but stick with the 4 pulley set up and achieve a slower speed.

I hope my ramblings help you, elasmount.
 
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