belt oscillation probs

[JMarkWolf]

Can anyone direct me to a source of information relating to methods of controlling belt oscillation on a low RPM, high torque, cog belt system? I suspect all that is needed is idlers and/or guides but have heard horror stories about standing waves turning V-belts inside out, etc., and I think there is more to it than meets the eye.

 

[micjk]

jmarkwolf,
Try

http://www.gates.com.

I downloaded a pretty good troubleshooting guide from their website. Hope this helps. Roy Gariepy
Maintenance and Reliability Dept.
Bayer Corporation Dorlastan Fibers Div.
Goose Creek, South Carolina USA

 

[VibEngr]

I recently performed a field test on a motor driven mud pump system. At certain speeds within the operating range, the belts experience a lot of slapping. From the strain gage telemetry system on the motor shaft it was determined to be a torsional resonance. Does you problem occur at a specific speed? Have you performed a torsional analysis of the system? If your interested in learning more about torsional vibration go to

http://www.engdyn.com/torsanal.htm

 

[JMarkWolf]

Thanks for the responses.

My application is a 2.5in wide kevlar/aramid cogbelt, 8mm tooth pitch, on a secondary drive shaft between a 150HP reciprocating engine and the main rotor shaft on an experimental class helicopter. The cog belt pinion sprocket is 3.5in diameter and it drives a 14in main sprocket.

The small sprocket is on a 30mm shaft, overhung by about 2.25in, alloy unknown, Rockwell C hardness 0f 34. Main shaft turns at 520RPM, secondary shaft turns at 2080RPM. Dimension between the main rotor shaft and secondary shaft is about 20in.

This secondary shaft snapped just below the small cog sprocket and just inside the bearing race, at 155 hours operation.

Break appears to be from bending fatigue. Cog belt tension has been maintained loose enough not to assert undue side load on the shaft, but tight enough so that belt teeth don't climb sprocket teeth.

I suspect that belt oscillation is inducing cyclical side load on the secondary shaft at some frequency, eventually resulting in fatique and failure, but need to discuss this with an expert.

 

[GregLocock]

Sorry I'll have to go metric.

I get that the reciprocating bending moment on the shaft is about 670 Nm, with a shaft stress of 4 MPa, and 20 million cycles.

That is way under the endurance limit for steel, if you have no stress raisers... but it is such a high number of cycles that you are operating on the limits of the curves I've got, even for spring steels.

If you think about it you are doing the same job as the supercharger belt drive, and I would argue that they tend to be 20-30 mm diameter shafts, but only 50 hp or so, at much higher speeds.

I'd guess your problem is likely to be simple high cycle fatigue - get a power transmission guy to check the design properly and use known materials for the shaft next time. I hope no one got hurt. Cheers

Greg Locock

 

[JMarkWolf]

Thanks for the response Greg

Nobody was hurt but I wasn't amused at the damage to my aircraft after a "nearly" perfect autorotation from 700 feet.

The alloy is known by the subsystem manufacturer, but I'm trying to get "independant scrutiny" in to this issue.

I've been trying to find a "power transmission guy" with no luck. Can you refer me to anyone?

 

[GregLocock]

Sounds like you are getting into a quasi legal situation - get a good PE from your 'hood!.

I'm in Australia - not much help to you. Incidentally when I said that it was under the endurance limit that is a good thing, not a bad thing.

Cheers

Greg Locock

 

[VibEngr]

The motor driven mud pump system that I was telling you about also uses Kevlar belts. I've heard that this type of belt is very strong and can actually be tightened to the point of bending the shaft before the belt will break.

If you feel that the belt was not overly tight, then another possibility is alignment. If the shafts are not parallel and the outside of the pulley is carrying the majority of the load, this would cause a higher bending stress.

In our case the shaft failures looked like bending as well. However, I feel that torsional vibration may have helped initiate a crack and then the bending stress finished the shaft off.

Good luck to you and be safe. I hope you don't kill yourself before you figure out what is wrong with your equipment.

 

[ace334]

Mark,
At first I thought you may have been talking about belt vibration in a friction or Vbelt drive. The horror stories you have heard are related to what is known as belt creep and the required torque at the driven shaft. Usually as the load changes in a Vbelt drive the slippage of the belt changes proportionally in the sheaves and will cause a change in belt tension. This induces a change in tension on the "loose" side of the belt. That tensional change will travel the length of the loose side of the belt as a sine wave as the belt travels through the drive. If the that wave comes into phase with the normal bending stress in the belt as it travels the belt will "flop" wildly due to the two waves amplifiying, conversely they can also cancel each other out.

Adressing design concearns for the type of belt that you are using or commonly refered to a synchronous or HTD style timing belt. These belts use very brittle type cords for carrying the tensional loads required for power transmission. The cords are either fiberglass, steel or aramid. They all carry loads with very little or no stretch. This is a key feature in the cord structure of building a timing belt. Unfortunately they are bad a carrying shock loads. In addition due to the fact they are very brittle they do not like to be bent "backwards" as you suggested in adding a tensioning pulley to eliminate the vibration. Reducing the tension in the belt may cause the belt to jump teeth which throws the whole thing out of time.

Therefore the entire design of the drive should be reviewed. If the drive can handle some slippage between the two shafts a friction belt would be better to carry shock type loads. If no slippage can be tolerated then the timing belt is the way to go. Changing pulley diameters can change the dynamic load on the shaft. Changing the width of the belt can also reduce ot increase the tension in the drive.

If you would like to look into this further contact me at e.w.steele@worldnet.att.net. I am the technical manager for Optibelt Corporation. We are the largest maker of power transmission belts in Europe and I work for the United States division.