Belt Grinder- Pulley size question

J

jakeotter

Joined
Aug 29, 2014
Messages
32
Hi I'm getting either a 6" or 8" contact wheel 2x72 Coote Grinder. I'm planning to use a step pulley on motor and 3" pulley on driven wheel (grinder shaft). Does the size of motor bore shaft diameter affect the overall SFPM? So is there a real difference between 5/8" or 7/8" shaft on motor. Sorry first grinder and just want to set up little slower.

Also just for kicks does anyone have any opinion on the 6" vs 8" contact wheel. I never do any hollow grinding ...
Thanks !
 
Shaft size does not affect speed. The diameters of pulleys and drive wheels will determine speed.
 
The motor shaft just dictates what pulley wheel you get. It doesnt affect the size of the wheel, just the ID. I use a 10" coote but have an 8" wheel as well. you can hollow grind with the smaller wheels but it takes a bit of practice and mostly for smaller or thinner blades: less than 1" wide.
 
Would it matter tho if I don't ever do any hollow grinding ? I'm usually working with 3/32" to 5/32 thick stock, 1.5" wide max. Doing flat, saber, and scandi grinds
 
I would suggest you use step pulley on both the motor and driven shaft. When you move the belt it should be straight in line with the driven pulley not angled which will cause significant belt and pulley wear. It may not seem that extreme when you look at it but at speed that belt is wearing into the pulley.

On the size of Wheels Bigger is better even if you never intend to do hollow grinds you may find a need when building jigs or fixtures and the larger wheel will run cooler at all speeds.
 
Ok that makes sense. I wondered about that but was not sure. Good thing you said that lol. I had planned on the 8" wheel but was back and forth. Why does the larger wheel run cooler? I thought the larger wheel would run faster thus increase heat.
 
The larger diameter actually runs slower.....think of how fast a 1" will revolve and what little bit of surface area is covered by the belt and how fast it heats up....for instance its always recommendation to run the small wheels at slower speed to keep from melting the rubber and burning up the bearings...the larger the rubber surface the easier it dissipates heat. Hope that makes sense?
 
I'm not familiar with Coote grinders. Googling for "Coote Grinder" images seems to mostly show a 2-wheel grinder with a drive/contact wheel and an idler wheel.

If I've understood it right, the belt speed on such a setup is directly proportional to the drive/contact wheel diameter, so the 6" wheel will only give 3/4 of the belt speed given by the 8" wheel, all else being equal.

This is different to a 3- or 4-wheel grinder, where the drive wheel is a fixed size, so the belt speed is constant and the contact wheel is driven by the belt at a constant surface speed. In this case, the 8" wheel would only turn at 3/4 the RPM of the 8" wheel, all else being equal. I think this is the situation Busto is describing in post #7.

If I've misunderstood, I apologize.

As Busto says on post #5, you want step pulleys on both shafts. The classic step pulleys are 2", 3", 4" and they give a driven shaft speed of 1/2, 1 and 2 times the motor shaft speed. The 3 steps keep the belt straight, so minimize transmission losses, and also keep the centre-centre distance between the motor and driven shafts constant.

If you only put a step pulley on one shaft, you usually have to engineer in a huge amount of adjustment to keep the belt tensioned and aligned. In the end it costs much more money and is much more effort than going for the second step pulley.

Other step spacings are available and may give better results in your application. Engineering-wise, it is just important that the pulleys are the same on both shafts.
 
^^^ You're right on the money for the speed difference and on the pulleys....:thumbup:
 
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