Cog Belt Drives 2

[JJPellin]

We have been having a rash of failures in cog belt drives on fin-fans. I found an issues that I think may be the basic problem. Our supplier has been providing a mixuture of two different patterns of cog belt. The original design used cogs that were round; basically half-circles. Some new designs use cogs that are shaped like gear teeth; rounded sides but flat on the tip. We are ordering belts and sprockets of the old round-tooth type. But the supplier is sometimes substituting the gear-tooth shaped pattern. They claim that the belt of one pattern can be run on the sprocket of the other pattern. Does anyone have experience with this. I would like to have some ammunition to go back on our supplier and force them to sell us what we want. By the way, we have a corporate alliance with this supplier, so I can't go to a different supplier to get what I want. Any help will be greatly appreciated.

 

[TheTick]

Do the different styles have the same part number? If so, not, then you definitely have a big problem.

I would say the short-term answer is immediately return and not pay for any and all incorrect belts. Make it a headache for your vendor to do this to you.

 

[JJPellin]

The part numbers are the same. We have two sprockets on he shelf that have the following part numbers stamped on the box, engraved into the parts and in the warehouse description for the parts:

P30-14M-85-2517
P32-14M-85-2517

The first one has the rounded grooves. The second one has the gear-tooth shaped grooves. These parts should be identical except for the fact that one has 30 teeth and the other has 32 teeth.

I am hoping there is a belt expert out there who can say with absolute certaintly that these different patterns cannot be run against one another. Any vendor contact that I have would have a conflict of interest. Of course the belt manufacturer wants me to buy their sprockets.

 

[mog69]

If both belts are the same length, the different numbers of teeth must equate to different pitch dimensions. The sprockets will have pitch suitable for 30 OR 32 teeth belts, they can't accommodate both different types without altering sprocket centres.

 

[patdaly]

Go to Gates or any other manufacturers websites. This is a confusing issue, and only the Mfr. can tell you for certain if they recommend mixing. We have been through similar issues with suppliers who mistakenly believed "parts is parts". We also had a nationwide contract with a supplier, but after shown their mistake, they immediatly corrected the problem.

What Manufacturer of belts and pulleys are you using?

 

[ornerynorsk]

Corporate alliance = communism. Do you really need to buy from the "preferred" supplier, or could, say, the maintenence dept get ahold of some correct belts and sprockets for you?

 

[Philrock]

There are many timing belt tooth forms on the market. Some are interchangeable; some are not. It is essential to use compatible belt and pulley tooth forms. Manufacturers should be able to give you all the help you need with this. Dealers and distributors are not usually as knowledgeable as manufacturers, and certainly may be biased toward lines they carry. Your experience of success with one type of belt and failure with a different one is a clear indication that the failing belts are either of the wrong tooth form or defective.

I would hope that your "corporate alliance" would allow you to go to another supplier if your "corporate ally" can not supply you with equipmnet that works.

 

[tygerdawg]

All correct points above:
(1) verify pitch of pulley (BOTH pulleys), use same pitch on belt
(2) verify tooth form of pulley, use compatible form on belt. Some manufacturers claim certain different forms are compatible.

ALSO:
(3) verify correct application:
(3a) correct belt tension (too little will destroy your belts, too much will destroy your bearings)
(3b) suitable pulley spacings, idlers, wraparounds, etc.
(3c) horsepower loads and belt sizings
(3d) verify correct alignment & parallelism of pulleys to avoid uneven loading on tooth forms

TygerDawg

 

[JJPellin]

Thank you for all the replies, especially tygerdawg and Philrock. The belts and both sprockets in each installation are the same pitch (14 mm in the example), same width (85 mm in the example) and are verfied to be properly desined for the horsepower, speed and torque. We use laser equipment to align the belts. We still tension our belts by "feel". But I should note that we have about 150 fans with cog belt drive. Our mechanics are very good at installing, aligning and tensioning these belts. And the vast majority of them have been working very well. It is only recently and only on a couple of fans where we are having the problem. In one particular fan, we failed multiple belts, so they replaced the smaller sprocket and then we broke the motor shaft. We replaced the motor and broke another motor shaft three months later. The mechanics admitted that they were tensioning the belt tigher than ususal. But they stated that the belts were slipping (jumping a tooth) and they had to increase the tension to keep the belt in the sprocket. Everything still points to a tooth pattern problem. My fear with vendor information is that they have a strong incentive to tell me that I need to upgrade all my belts and sprockets (150 fans times two sprockets equals a lot of money) to the "newer", "better" pattern. This corporate aliance is locked solid. I cannot go elsewhere so I have to work this out with my belt supplier. I have no options. I will check with Gates, Uniroyal and Goodyear, our primary manufacturers to give them a chance to set me straight. The sprockets we have been getting are Martin. I am not familiar with that company but will try to contact them, as well. Thanks again.

 

[rorschach]

It's too bad you are locked in solid like that, I would have suggested that it might pay to look at Goodyear's Eagle PD series belts, they are herringboned and the teeth are 180 degrees out of phase on opposite sides of the centerline. the pulleys don't need flanges and they are much quieter than conventional cog belts. and sing you have more tooth contact area, the HP rating is higher for a given width.

As to your issue I'm a little confused, has the belt profile changed or has the pulley changed? or is it both?

I've dealt with Martin Sprocket and Gear for years and have had pretty good luck with them. The two part numbers you list are standard off-the-shelf numbers in their catalog. you can contact them from their web site:

http://www.martinsprocket.com/

 

[MikeHalloran]

I'm not aware of any manufacturers who claim compatibility among different tooth profiles, and I'd immediately cease doing business with one who did.

There are several different profile shapes out there, and there are also a few pitches that are close, but not exact matches.

Anytime you need extra tension on a toothed belt to keep it from jumping teeth, something is _very_ wrong. Your techs should be instructed to replace both pulleys and the belt when that happens.

If you are getting mismatched parts under the same number, you need to replace your distributor immediately.




Mike Halloran
Pembroke Pines, FL, USA

 

[Jordonlaw]

JJPellin,

I used to do FEA on synchronus (ie toothed) belt profiles for one of the companies mentioned in this thread. The simple fact is that a belt of one profile running on a sprocket of another will wear out quicker than if both profiles are the same. Tension is very important as the belt pitch is only "correct" for a given tension and other environmental variables (eg temperature).

Different profiles are designed for different loading conditions so changing to something that looks "similar" need not work sucessfully.

Go with what works rather than changing. The belt manufactures mentioned will be able to check the profiles you've been using and give advice accordingly.

 

[JJPellin]

I witnessed another fan today with the same problem. They replaced the belt and the smaller sprocket. The belt in the warehouse was a Goodyear. The sprocket was a Martin. Both the belt and the sprocket appeared to have very similar (if not identical) trapezoidal shaped cogs. The belt seemed to fit very nicely in the 192 tooth larger sprocket. That is, until you rotated it by hand. Then the belt would start crawling up and eventually be running tooth to tooth rather than tooth in groove. After a little more rotation, the belt would jump into the next tooth and skip. In order to get it to stop doing this, they had to set the tension so tight that they were bending the bracket that the motor mounts on. These parts are obviously incompatible. I instructed our planner to purchase identical replacement belts and sprockets to go back to the original design. Then I plan to change our warehouse stocking descriptions so that no substitutions are allowed. I spoke to the supplier and he told me we are saving about $5 (US) per belt by going with Goodyear. I had one fan fail last week that costs me over $40,000 per day in the summer. I can buy a lot of belts for that. No more substitutions. No more mismatched sets. And I am sure that Goodyear makes a fine product when it is used properly, but I plan to go back to Gates, where we have had the most success.

 

[Jordonlaw]

JJPellin,

The other aspect I should have mentioned is that the same belt geometry in different materials can have a big effect. Some elastomers are much stiffer than others leading to a different amount of stretch on tensioning. If you replace a belt with one from a different material you could also get the problems you have experienced. The suppliers know all this and will advise accordingly.

Incidentally I have simulated the belt climbing you have witnessed using FEA by simply changing the belt tension.