Convex Roller Theory 1

[fcesar]

I'm looking for an old theory that explains why belts on a belt sander stays on the center of the convex roller. The same theory applies to flat belt on a crowned pulley (on old farm equipment), some paper mill applications, vacuum cleaned belts etc.
Does anyone know where I can find the theory behind this?
Thanks
Kiko

 

[MintJulep]

Because for one side of a belt to run off the edge of a crowned roller the other side has to get up over the crown. Trying to run over the crown increases the tension in that side, which tends to pull it back where it it supposed to go.

 

[Barry1961]

http://www.belttechnologies.com/designinfo/crown.html

Barry1961

 

[fcesar]

It makes sense, but it doesn’t explain why a conveyor belt “walks” to the tighter side. When adjusting conveyor belt alignment, we need to tighten the side we want the belt to move to (or vice-versa).
I imagined the convex roller as well as conveyor belts used the same theory. “The belt seeks the highest point.”
Where is my thought wrong?

 

[Barry1961]

When the belt starts to track off the crown it will put a slight curve in the belt when looking from above. This causes the belt to feed onto the pulley crooked which moves it back toward the top of the crown.

If you picture the back of you hand as the belt with your fingers pointing in the direction of travel. As the belt tracks off to the right it will cause a curve in the clockwise direction. If you think of this curve being an extreme 45 degree angle to the direction of travel it becomes obvious which way the belt will track.

Crowned pulleys are not good for all systems or belts. You will need to have enough flex in the system to allow the curve to take place. How much flex is needed will depend on pulley OD, belt type and center distance for starters. Try to make your crown round instead of trapezoid.

Barry1961

 

[Compositepro]

Barry, is correct. I read an excellent description in a 1920's vintage physics book (they don't writeabout practical subjects anymore). A crowned pulley stretches a belt more in the center than the edges. When the belt is centered it is straight. If it wanders to one side the belt approaching the pulley curves slightly toward the centered tracking position and will ride back up onto the crown of the pulley. It is the elasticity of the belt and how it is distorted by a crowned pulley that makes it work. I've seen cases where designers have tried to use crowned pulleys on inelastic materials and it doesn't work.

 

[Barry1961]

Years back I saw a small vertical demo conveyor that had a series of lines drawn across the width of the belt when it was relaxed. As the belt was tensioned the crown deformed the lines into chevrons. It was an interesting demo and was used to shown the effects of different types of tracking among other things. If I remember correctly it was a Siegling demo and was in their Mobile, Al office. I think I just saw a video of it but my mind is fuzzy on it.

You might contact a local rep to see if they still have it.

I think it may have been meant to train their field service techs.

A general rule is that the belt tracks toward where it first touches.

Barry1961