Drive Belt Tension vs Temperature 1

[ttdmt]

I would like to talk to someone with indepth belt drive design experience.

I am working on a 50hp drive design using Gates GT2 Polychain cog belt. One of the problems of the design that Gates Engineering has not been able to address adequately is the effect of thermal expansion on belt tension. The design does not allow for a tensioner. The belt uses a Kevlar cord as its back bone. Kevlar has a backward coeficcient of thermal expansion. Gates confirms that the molded belt assembly changes length in sink with plain Kevlar. Therefore, when my machine sees 40 below zero the steel frame will shrink (center to center dist of drive)and the belt will grown in length. From first calculations this will remove all tension from the belt. Gates says a cog belt without tension will probably fail immediately. They also say they do not have a problem in real applications with temperature driven tension changes. I don't see how that can be. Have any of you had experience to shed light on this? If not experience then a theory of why cog belts with Kevlar would work over wide ambient temp conditions.

 

[evelrod]

While I was at US Borax in '95-'97, we used several Kevlar Gilmer type drive belts, mostly 4 inch and 6 inch wide(don't recall the hp). They required very high preload (tension) and were very allignment critical but, for the mild temps in the L.A. harbour (40f to 100+f) there were no problems that I can recall. Our main problem was heat so the situation is apparantly the opposite of yours. Sorry. That's about all the experience I can relate.

Rod

 

[ttdmt]

Thanks for your note. You mentioned alignment issues. It just so happens that alignment is one of our big concerns. Maybe I could ask a couple more questions?

What is a Gilmor drive?

How bad were your alignment problems and what did you do about them? Our design will be 37mm wide (about 1.5&quot so I hope that will make belt life less sensitive to alignment. What total anglular tolerance did you work to during installation? How difficult was it to attain your alignment tolerance? What tools did you use to do so?

Did your alignment vary after installation in a transient or permanent manor? These changes could be due to vibration, external frame loads, belt tension variations, or different torque loads on the drive. How were you able to see this and how did you adapt your design to these problems?

 

[evelrod]

Gilmer belts were just one of the first 'toothed' belt drives years past.
The allignment was allways critical but once in a good setup, it seemed to stay that way. As to special tools---the factory sent all sorts of diagrams, papers, angles etc. but none were any better than 'eyeballing' with a straight edge and then running at low speed to adjust for any minor variations. Much like lining up the conveyors that these particular drives were operating.
The one tool that we did use regularly was the 'tension guage' as these belts require very high tension preloads.
Sorry to say that your small belt will still require good allignment and a bit of "tuning" after you get it running.
We also found that pulleys that had 'retainer' fences on the edges only served to destroy the belt if it got out a bit. The motor base can be set up with tensioner bolts in most cases and often the driven end also. Not rocket science but, considering the high cost of these belts it sure pays to be cautious. Work safely.

Rod

 

[dvd]

I have used Polychain GT2 belts on numerous conveyor projects. The high tension required for these belts definitely requires planning on your part to avoid problems. I have seen output shafts and roller shafts break because of the high tension. So be careful with your overhung loads and make sure that your gearbox and driven shafts can handle the higher loads. We don't experience too many problems with alignment, although you do need to align the sheaves carefully. We generally use flanged sheaves on both driver and driven sheave.

I don't understand your comment that the design does not allow for a tensioner. Are you referring to the Polychain belt or your machine? You can use a tensioner with these belts, but will have better luck if you do not "backbend" the belt.

As far as your machine use at -40° F or C, I would suspect that this machine will not see these temperatures in a rapidly fluctuating environment. It will get cold and stay cold for several months. You have to be used to maintaining gearboxes and bearings at these temperatures, so it won't be that much of an extra effort to check your belt tension every month.

Also be careful if you have a high starting inertia or the possibility of your machine suddenly stopping or jamming. These belts won't slip and something will have to give, unlike v-belts which will slip.

 

[ttdmt]

Thanks for the advise.

 

[dvd]

One additional thought on this subject is that if you locate a tensioner device from one side of the machine frame it could provide temperature compensation for the belt. You would need to look at the shrinkage of the bearing centers and then calculate a support length for the tensioner which would shrink back to one side and provide additional tensioning.