Troubleshooting a Slipping Drive Belt

Drive belt pulley slipping? Try belt dressing, and tighten it up. November 27, 2006

I just started sawing with a 40 inch R&R Howell circle saw, powered by a Minneapolis Molene power unit. I spent the last two years restoring my mill to a usable condition in my spare time. How tight should my belt be that runs the arbor coming from the power unit? (This is one of those that farmers might use, about 6 inches wide.) When I begin my cut into the log, my belt starts to slip. The motor rpms don't change, the blade is free, and all my bearings are good. And I have, or so I think, resolved all my track, lead, blade tension, true to the world problems already! No matter how slow I work into my cut, the blade slows. It's this rpm at the blade change that is messing up my cut, and my results are not good! How tight should I go on that belt, or is a belt conditioner required? Or perhaps I need to spring for a new one?

Forum Responses
(Sawing and Drying Forum)
From contributor D:
Years ago, we used what they called belt dressing. It was a black stick about 3" in diameter and we would just hold it on the inside of the belt and it would rub off and make the belt not slip. Don't know if they still use that stuff, but that is that we did.

From contributor S:
I learned to saw on my buddy's grandfather's 36" circle mill run by an IH Wheatland Special 350 connected via a 25' flat belt. We started off using the belt dressing as contributor D described. When we ran out of it, we switched to the spray stuff available at most auto shops.

From contributor R:
Just wanted to second contributor S's post. The spray belt dressing works great. I would start off with just a little, or it could become a mess. Too much of anything can build up and become a problem.

From contributor J:
Real turpentine makes a pretty good belt dressing, although you have to use it fairly
often. Also makes your belt last a long time.

From the original questioner:
Thank you - belt dressing helped a lot! And I also noticed my hub or pulley on the power unit that drives the belt is in poor shape. So I will need to give it some attention. I still am noticing slippage at that location. At least now I can cut a board with some desired results.

From contributor O:
Any chance your clutch is slipping? I've run flat belt equipment (though never a sawmill), but have yet to see a belt slip. Maybe the belt's old and needs replaced?

From the original questioner:
Today I stopped out at the mill to try a few things. My neighbor friend who happens to be a farmer also stopped by - we were looking at that pulley. He thought that the pulley's unevenness is some of the cause (not allowing the flat belt to come in proper contact - we were guessing about 50% at best contact occurring at the power unit).

After he left, I moved the motor back slightly to give it a little more tension and this cured the slippage totally. I loaded up a 16" aspen by 8' long. I never noticed any saw rpm change from the time I entered the cut till the saw came out of the log. After two long years, I finally can cut!

This pulley still has me concerned. It just doesn't look good. But before I replace it, I want to show it to a couple of wise old timers in my area to see if it is a concern to them.

From contributor O:
It sounds like you're on the right track. Most flat belt pulleys are crowned in the center, with 1-5 degrees taper toward the two outer edges. This keeps the belt centered. Some of them crown enough to actually show an apex in the middle of the pulley.

From contributor E:
We have some equipment running on lineshaft and at times have twisted the belt to get greater contact on the pulleys. Be careful to get two full twists in the drive belt, as a single twist will reverse the direction of your blade rotation. As you have already discovered, belt tension is also very important. Belt dressings seems to help for a little bit, but they never last very long. Proper tension and increasing contact area on the pulleys will usually solve slippage problems. Congrats on getting your saw up and running.