Wobbly Coote

C

CDH

Joined
Jun 8, 2007
Messages
283
Well I finally got a real belt grinder, a 6" 2x72 Coote. I hooked it up to a 1 Hp TEFC motor I had bartered off a guy a year ago in anticipation of just this...

I mounted the motor similar to the pictures on the Coote website, with two commercial grade (heavy!) 3.5" door hinges on a doubled 3/4" plywood base. The weight of the motor provides belt tension to the 3 step (2-3-4") pulleys. That means the motor is slightly floating in the air except for the hinges. The rest of the base is another doubled 3/4" plywood base across two 4x4's to get the height up a bit.

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Now the problems:

1. On the slowest speed the vibration is pretty bad. The idler pulley end is vibrating a good 1/8" and there is no way I could grind with that much shaking. With the belt off the motor is smooth and quiet, and spinning the grinder by hand feels fine.

2. When I put a 1" diameter spring on the floating end of the motor platform to try and increase belt tension the motor started making a rattling, bad bearing type sound. Does this type of motor (NEMA 56C) not handle axial loading (direct drive bearings only)?

2a. The belt is quite warm to the touch after a minute of running (no grinding loads). I suspected slipping and put a spring in to increase belt tension but it didn't help. I have spent the better part of 2 days tweaking and aligning...it is as square as it's going to get.

Suggestions???
 
I had the same issues with my old coote 10x72. The problem was cheap step pulleys that were bored slightly off-axis. If you have a dial indicator and a mag stand it is easy to check. Mount the mag base off the side plate or uprght on the grinder and indicate off of the outer ring of the pulley. You can also indicate off of the end of the shaft to determine bearing slop (should be less than 0.001" in a full rev). Compare that to your reading off of the pulley OD and it should become pretty apparent if that is the problem.

If your pulleys are true (and I suspect they're not) the vibration may be due to flex in you setup, probably under the grinder mount. I mounted my coote to a tapped 1" block welded to a piece of 3/8" plate that extended under the motor mount. The one and only real problem with the coote design is the small footprint of the mount and the height of the machine itself, which exacerbates vibration issues. If you have a really rigid setup, no pulley misalignment and good step pulleys, it is a good solid machine.

Hope it helps,

Nathan
 
Did you get your pulleys from Norm? The ones hes sent me were very well centered and balanced. How does it run without a belt? Kind of off topic, but I change the angle of the grinder a lot when I'm working. I'll tilt it all the way back when using the wheel, and bring it up for the platen or slack area. The placement of your motor prohibits this, but if you don't move the grinder it won't matter.
 
I have a second hand coote as well. I just picked it up a few months ago and have both the 6" and 10" wheels. I have it set up pretty much the same as you do but I only have a 1/2 hp motor. I noticed some shaking as well when on the big pulley, so instead of letting the motor hold all the tension I put wedges under the base of the motor. Not huge, just enough to to take a bit of the load off. I think the problem is if the motor is completely square with the grinder, the belt holds tension but the weight of the motor still pulls a little. I found that the wedges support the motor just enough to reduce vibration without reducing the tension on the belt.
 
My Coote vibrated pretty badly until I replaced the v-belt with a Fenner Power Link belt. Problem solved. You can get them on Ebay last I checked.
 
I want to switch to a power link as well. You can get them from Lee Valley.
 
One thing I would suggest is to mount the motor on the other side, unless you never plan to tip the unit to the rear. Use good pulleys- machined are better than cast, IMHO.

Milt
 
tattooedfreak said:
I want to switch to a power link as well. You can get them from Lee Valley.
Click to expand...

I remember when I first got my Coote turning, my excitement at finally having a real belt grinder turned to disappointment and frustration when it started bouncing. I think it is the join in the V-belt that causes the vibration, that spot dosen't flex as easily as the rest of the belt and when it goes around the pulleys it creates a tension spike, which pulls at the machine and causes the vibration. The problem went away instantly with the Power link belt.

After experiencing this I almost replaced the v-belts on my my drill press and micro lathe too. The Fenner belts work, that would be the first tree I barked up if I were you.
 
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Thanks guys!

I can tilt it back about 30 degrees...and since I have no plans to hollow grind I'm good with that. I was more interested in compactness...as you can see I don't have a lot of space to work with. :(

The pulleys are cast and yes, I got them with the grinder from Coote. I tried to localize a out of round with a marker lightly touched to the rim of a running pulley, and I think one is slightly out of round but not much.
 
I should add that I'm also using a Fenner belt. They aren't cheap, but they work great. You may want to start there before replacing your pulleys. They are directional, so pay attention to the little arrows marked on the belt. Another nice thing is that you can fine tune them by removing or adding a link or two. This eliminates any spacing issues you might have with a fixed length belt.
 
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