belts, pulleys, bandsaw blades and bandsaw wheels

I see you problem, but they are not the same situation at all.

A V groove has a V belt which matches it and the tension pulls the belt down into it's groove like a wedge giving good grip.

The other sort of belt is a flat belt which will centralise on a barrelled pulley (or may even skip up onto the 'cheeks' of a pulley which has 'cheeks')...I can't offer a very good explanation of this effect without waving my paws about and yowling loudly as it is somewhat counterintuitive .

Del

Hello BruceFla,

Also a drive belt is some what forgiving to alinement's that are not perfect.

Bandsaw blades and sanding belts are not.

The reason your Bandsaw doesn't wander like your sander is the blade has guides that limit deflection. Belt sanders, the sanding belt limits deflection. I have some cheap sander belts that wander at random.

Brad

Sure sounds right to me! Thanks for the simplification.

The belt sander has flat rollers because it's more important to have a flat abrasive surface than it is to prevent wandering. In order to avoid whorls and scarring of the work piece, it's critical to have constant tension across the width of the belt.

A convex roller would, over time, cause the belt to form a similar shape. The end result would be a somewhat heavier abrasion along the edges of the belt than in the center. Your hoped-for flat, smooth work piece would end up with grooves cut into the surfiace on both edges of the belt.

Higher-end sanders have belt guides on the return (non-contact) side of the belt rotation to help control wander.

I was told that the belt would ride down in the groove.

In fact if you watch carefully the belt (rope or whatever) keeps trying to climb out of the groove but keeps slipping back down. A belt on a convex or barrel shaped pulley is a better way providing feed back in a more controlled way.

As a child I watched in awe as the flat belts in the shearing sheds were driven around by the old single cylinder kerosene or diesel engines. 6" wide belts flying at god knows what speed, spanning 12 feet to the overhead drive system for all the shearing gear, and those belts never came off.

I asked my father "Why?" and his reply was very simple. "Look at the shape of the pulleys." He said. "The circumference is greater there and that means the centre is going faster than the outside edges. That means that the belt will always be pulled back to the centre because that part is moving faster than the edges."

My dad was pretty smart. (For a shearer.)

I tried to answer this, but I'm left scratching my head and saying, that's a very good question!

That is a nice practical way of describing it. Well done. Back in the seventies when I was an apprentice I was taught how to "lace" a flat belt. These were 2" - 6" wide and made leather. It was something of an art as the torque had to be close to constant as the lacing passed over the drive pulley.

I think it is the only thing that I was taught in my apprenticeship that I haven't had to use somewhere along the way.

BAB

Bandsaw and belt sander pulleys are CROWNED in the middle to help make the blade (or belt) center up during operation. The belt tends to stay put in the middle because any tendency to move off center is resisted because the trailing edge (side) of the blade or belt has to move "uphill" (and over the crown).

If the pulleys are flat, there is no such opposing force. The blade or belt would always walk off in one direction or the other, because you can't get perfect alignment of all pulleys.

I used to work for a company which made conveyor belt systems. Some of the belts were as much as 30" wide. If the machinist screwed up and omitted the crown, even these wide flat belts would walk off the machine. There were no "guides" on the edges of the pulleys to keep the belt centered. They make the belt harder to install. And the edges of the belt can become torn or glazed in use.

V belts and pulleys are the same principle in reverse. The leading edge of the belt has to move "uphill" if it is going to move. So they stay centered. You can't keep a bandsaw blade centered in a "V" because it will cut it's way out.

No! Even a belt which starts like this:-

Will move back to the centre of the pulley. Think about the way the belt tries to curve "down" out of the page in this diagram, and, so pointing up towards the centre. Or see Rogerzz's explanation above.