Tuesday, May 14, 2013

V4.19 - V-Belts and heat

As with most writers, it is sometimes difficult to come up with a subject for a column. What I have found that seems to be working is to just stay in touch with my friends on the internet forums and use their discussions as a basis for my next column… it seems to be working out fairly well. This week’s column fits in real nice because I was once the Technical Service Manager for the group that owns Powermatic. So, let’s start with the issue and then we will determine the resolution.

The question from a poster was: “I have a Powermatic PM15 planer with 3 drive belts. The belts seem to run very hot. The pulleys on the cutter head and motor get hot enough that you can’t hold your hand to them. The pulleys are aligned within 1/16 and are tensioned with about 1/2 inch deflection, as per the manual. I called Powermatic and talked to a tech and he said it was normal for the belts to run hot. Anybody have any input on this? This is an older used machine, not a new one. I would appreciate any ideas on this, thanks”

'Too hot to touch' is rather meaningless when it comes to stuff like this, but it's used all the time. I say “meaningless” because what might be ‘too hot to touch’ for Bob, might only feel mildly warm to Sam. Without actually giving a measurement, who really knows? Yet, people say that to describe something that they perceive to be a problem. I was curious as to just how much heat is generated by a v-belt, as I'd never actually looked into it. I couldn't find any data on it from the major belt manufacturers, so the physics of 'how hard is the pull, and how fast is it going' seems to give a reasonable answer and it seems to be in the general neighborhood of what you'd expect, at around 10% power overhead loss from friction and flexing, give or take, though 10% would depend on how big the motor is, and the belts and pulleys don't know that. They just generate heat when running at speed and when under load.

Just running with that and thinking out loud, with a 4500 rpm cutterhead and 3" OD cutterhead pulley (2.85" pitch diameter), I get around 3350 ft/min belt speed. If it takes a 5 lb pull on the belts to move the system at a steady speed, that's about .5 hp [375W or 1270 BTU/hr] lost in the belts and bearings (mostly the belts, assuming ball bearings). Scale that up or down if 5 lb isn't right, but that's a pretty big power loss running under no-load conditions (1/10th of the 5 hp I think that machine's motor is rated for).

Send your questions or comments to:
Toolsmartz@bellsouth.net and we’ll see what we can do to help you.

1 comment:

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