V-belt slipping - material selection? 2

[schaplan]

We have a rotational scanner which uses a v-belt drive (2L size v-belt, 0.25in wide), and we're getting slippage on the drive sheave. The current design is at maximum tension, although we can modify the design to increase the tension on the belt. Before modifying the geometry of the tension system (bearing slides perpendicular to belt on one side between sheaves), I'd like to get some feedback on materials or other solutions.

The drive sheave diameter is 3in and the driven sheave is 9in, and we used the Gates design manual to spec the grooves.

The driven sheave (9in) requires 18 lb-ft to move at 21rpm (max speed).

Both sheaves are sealed anodized aluminum, and the belt (34in long) is rubber with polyester reinforcing cords. We understand we can increase the traction of the drive sheave by separating the pulleys more (as suggested by Gates) but this also isn't an option in the short term, and if we move the drive pulley it has to go on another platform where the belt will need to be 75-80in and I haven't seen 2L belts that length.

Is the anodized aluminum a poor sheave material? Can I improve the traction with another material?

 

[MikeHalloran]

Derivations in the typical undergraduate machine design book will demonstrate that v-belts absolutely _must_ slip in order to transmit power.

If slippage is important, and you are the manufacturer, it may be time to help your chief designer find a job elsewhere.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

The only reason increasing the center distance between your pulleys would help is by providing a slightly increased circumferential length the belt wraps around the drive pulley. Adding tensioner and idler pulleys would provide the same effect.

As for hard anodized aluminum sheaves, they should work just fine.

 

[gruntguru]

Mike. Why not assume the OP is talking about "excessive" slippage? There is a difference.

schaplan.
I assume the tensioner presses on the outside of the belt.
What is the wrap angle on the drive pulley?
If the tensioner diameter can be reduced and the adjustment allows, the wrap angle will increase.
Spring loaded tensioners must be on the "slack" side of the belt.

je suis charlie

 

[MikeHalloran]

Gruntguru, I would prefer to assume nothing, but few queries are that complete.

Since schaplan called the product a rotational scanner, I assumed that precise, repeatable positioning of the driven element was important, as in most/all scanners, no matter what they scan or how they do it.

I.e., IMHO, they've followed the proper procedures for sizing the drive; they just picked the worst possible drive in the first place.

I could be wrong; it happens.


Mike Halloran
Pembroke Pines, FL, USA

 

[dvd]

What is a rotational scanner? It seems like there must be some occasions when slip is required or a different arrangement would have been chosen, like a synchronous belt, or chain, or drive shaft, or servo, or, or, or...

 

[schaplan]

Thanks for the replies everyone.

I know a scanner can can be many things. In this case, the slippage wasn't an issue for scanning accuracy because we encoded the scan data to the driven sheave rather than using motor position. The slippage is an issue because we're adding z-axis scanning and the rotational and z-axis movement need to work in unison.

It sounds like it will be worthwhile to change the drive to a timing belt. The v-belt was convenient because it allowed the drive to slip if the part on the turntable collided with scanning hardware or foreign objects, but slipping under regular operating conditions wasn't acceptable. Instead we'll add a clutch or set the motor to stall above a design torque load.

 

[tbuelna]

If you truly require precise synchronization between the drive shaft and the driven shaft, then even a timing belt may not be sufficient. A rubber timing belt can only transfer force in tension. So if your drive must maintain precise synchronization in both directions of rotation, you might consider using something other than a timing belt.

 

[arbreen]

tbuelna, can you elaborate on that last point? Do you mean because of backlash? Tensile modulus?

 

[tbuelna]

arbreen- Let's say you have a simple timing belt drive with a driver pulley, a driven pulley and a belt that is tensioned by adjusting the pulley center distance. The system is required to maintain precise timing when it's being driven in either direction of rotation by the drive pulley. When the direction of drive rotation reverses, it will alter the tension force in each side of the belt. And this will change the angular phasing between the drive and driven pulleys. The only way this is not a problem is if no torque is being transferred by the belt drive.

 

[lukin1977]

I had a similar problem on a machine (not a rotational scanner) where the slippage was excessive and I solve the problem by replacing the V belts with cogged belts

Cogged belts have slots that run perpendicular to the belt’s length. The slots reduce the bending resistance of the belt. Cogged belts can be used with the same pulleys as equivalently rated V-belts. They run cooler, last longer, and have an efficiency that is about 2 percent higher than that of standard V-belts