Synchronous belt wrap / profile modification references

[N3OX]

Hello,

I'm hoping to use a Gates PowerGrip GT2 synchronous belt drive as an approx. 8:1 reduction using a stock belt and small sprocket and a custom, large ring sprocket to drive a piece of rotating experimental equipment.

I've been in contact with an engineer at Gates who mentioned that there could be belt wrap problems with such a large sprocket (400 teeth, 14mm pitch), and he's not sure what profile modification might be necessary to get the belt to wrap properly.

I intend to continue contact with Gates but would like some advice or some good reference information so I can make an intelligent decision about the proposed drive.

The engineer at Gates has suggested a two-stage drive with stock pulley sizes but other considerations make this quite difficult. It's not impossible, but I'm at a loss to make a good decision because I have no experience with synchronous belt geometry and haven't been able to find a generic synchronous drive reference in the library or online the way you can with gear drives. I have sort of a vague idea of how a very large sprocket may affect the tooth and belt contact geometry compared to smaller stock offerings, but I don't know what the consequences would be.

I'd appreciate any advice or reference that anyone could provide.

Thanks,
Dan Zimmerman
University of Maryland Physics/IREAP

 

[dimjim]

It is in Machinery's Handbook but I think it was
in an obscure place.

 

[N3OX]

Gives me somewhere to look, anyway. Thank you.

Dan

 

[gearcutter]

I'm not sure but I think that the pulleys off the shelf aren't generated or gear cut, rather a strip of the pressed profile is shaped into a circle; so one profile serves a range of pulley sizes. I think if the teeth are generated, then the section of profile which causes the interference is automatically removed.
I'd try getting in touch with a tooling manufacturer or supplier. Who ever manufactures, either the hob or shaper cutter to cut this profile, should be able to either tell you how to design or perhaps supply the tooling to avoid this problem.

 

[dimjim]

I gave my 22nd edition away and only have the 17th
edition here at home. I know Gates Rubber had a good
design manual. They could be hobbed or shaped and there
were different shapers for different ranges of teeth.
We designed large bearings with teeth for medical
scanners that used belts. Do not remember the ratio
tho of the sprocket vs gear but the od of the gear
would have been larger than 42 inches. They may have
been under miniature pitch belts or metric pitch belts
or circular pitch belts.

 

[diskullman]

You may be able to install an idler pulley on the back side of the belt to ensure a better wrap on the small pulley. This would also make belt replacement very simple.

Russell Giuliano

 

[greenride]

Gear and belt sprockets are usually machined using hobs. These hobs cut a limited range of diameters (or tooth counts) You will want to use a cutter that is designed for your tooth count, In the the case of your 400 tooth sprocket this will be a rack form as your sprocket pd will be approaching 6 ft dia.
As for information on the actual tooth form I have two sources of information for you. The most recent publication for 8 & 14mm tooth forms is an ISO spec , I dont know the number of the spec but, its less than 3 years old. The other more cost effective (patent expired) method would be to use the HTD tooth profile (Uniroyal develop merged with Gates in the mid 80's). Since you have huge belt wrap you don't need the additional load carring capacity of the htd form on the large sprocket. This would also be in the ISO spec or in an older RMA (rubber manufactures association) spec. Depending on if synchronous speed requirement you may not need to have teeth on the large sprocket at all just friction drive it. The limitation of 5 teeth in mesh for the small sprocket if based on tooth shear and the rule of thumb is reduce load capacity by 20% for each tooth less than 5 you have meshed.