capstan drive sizing problem

[tdw]

I want to creat a capstan drive on an engine dynomometer. The drive wheel is aluminum 2.5" dia X 1" wide, running at 12,000 RPM and the driven wheel is 7.5" dia X 1" wide with bonded buna tire shore D 70.
The wheels need to resist a 27# shear load at the area of contact.
The question is how much do I have to load these wheels together to do this?

TIA
Terry

 

[eromlignod]

Why don't you design the capstan as an adjustable, spring-loaded pressure mechanism and dial it in while you're running?

Don
Kansas City

 

[tdw]

"Why don't you design the capstan as an adjustable, spring-loaded pressure mechanism and dial it in while you're running?"

I need a ballpark number so I can be sure the system bearings will handle the load.

 

[eromlignod]

Well, the coefficient of friction of rubber on just about any solid surface is at least 1.0 or better. That would mean that your normal force would have to be at least 27 lbs. Virtually any bearing can handle this radial load.

It's the rpm that I would be more concerned about. You have to pick a bearing that can handle that speed (4000 rpm). That puts you in the realm of ball or needle bearings, which have load capacities many times what you're needing here.

Also, your two cylindrical surfaces had better not have very much runout or you might be bouncing around and/or tearing up the rubber. Good luck!

Don
Kansas City

 

[Philrock]

I agree with eromlignod that bearing loads will not be a problem.

Friction with worn rubber can be significantly lower than with clean, new rubber. To be conservative, I'd assume a friction coefficient of .3. You could increase friction considerably by knurling the aluminum wheel.

Your loads and speeds seem pretty high for a rubber wheel.

 

[tdw]

Thanks for the input.
I've decided this isn't a very robust design, so I'm going to change it to a .08 pitch MXL belt drive on the 2.5" dia. pulley driving a smooth face 7.5" dia. wheel.
Terry

 

[Philrock]

I like the change to a timing belt, but MXL belts typically have a nylon fabric jacket on the faces of the teeth, specifically designed for low friction. Why not use a 7.5" toothed pulley?

 

[tdw]

MXL toothed pulley stock at 7.5" is hard to find.
Terry

 

[Philrock]

Probably impossible to find in stock, but not at all hard to have one custom made.