Saturday, May 18, 2013

V4.20 - V-Belts and heat; part 2


Last week, we started talking about a fellow’s Powermatic planer that has a 3 V-belt drive system and he thinks the belts/pulleys are getting too hot. From the tone of his post, it reads as if he is amazed that there is any heat at all. My contention is that, yes, there is heat, there will BE some heat and it really isn’t unusual… for these reasons…picking up where we left off:

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).

Not surprising, as there are three A-section belts in there, but pretty big nonetheless. Additional power is lost through air movement and sound generation, but I suspect that's minor. But losing 1/2 hp (more or less) to heat in the drive should make for a noticeable increase in temperature, and with little skin area on those sheaves, they'll have a hard time shedding that heat, at least until they're hot enough to reach equilibrium.

Power loss in the belts may actually be higher at high speeds due to the viscoelasticity of synthetic rubbers, where the faster you deform it, the harder it resists deformation, so at high speeds it's harder to flex, and therefore wastes more energy. But for sure, the bend/unbend events are very fast, and with a 30" pulley to pulley center distance (eyeball estimate), each belt bends and straightens over 1000 times per minute (twice per round trip). Cogged AX-series belts won't heat as much, and serpentine belts far less still (they're so thin, there's little stretching/compressing of the 'rubber'). But the sliding of the vee into and out of the grooves will still be there for all belts, so some heat is generated there too, no matter what.

A possible resolution would be to reduce the belt tension a little. As long as they don't slip under heavy cutting, there's no harm, but frying the belts if the cutterhead stalls wouldn't be a good thing. I'd just run them as-is, and if they need replacing some day, use AX belts for the higher efficiency and lower heat generation.

The bottom line is that heat is going to happen. How much is too much? If the belts start melting – yep, that’s too much.

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

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