How crowned pulleys keep a flat belt tracking
Before V-belts were invented, machinery was usually connected
to its power source using flat canvas belts running on crowned pulleys.
These flat belts stayed tracked on pulleys without any guides or flanges.
The key to keeping a flat belt tracking on its own is the use of "crowned pulleys".
A crowned pulley is a pulley that has a slight hump in the middle, tapering off
ever so slightly towards either edge. How a crowned pulley keeps a belt tracking
on it is a mystery to most people, so I thought I'd write a small article explaining
how it actually works.
The key to understanding how a flat belt stays tracking on a crowned pulley is
to look at how a belt flexes when pulled more on one edge than another.
I'm pulling the rubber band in the photo on just the right edge.
This means there's more tension on the right
side of it, so that side stretches more, and the band overall forms
a slight arch. In an actual belt, this stretch is too subtle to be
seen just by looking at it, but with the flexibility of a rubber
band the curvature becomes obvious.
I rigged up a contraption with Lego, a flat rubber band, and an
exaggerated crowned wooden pulley to demonstrate the principle.
The higher section of the crowned pulley puts more tension on the rubber band on that side. As a result, the rubber band flexes into a slight arch towards the right. As the rubber band winds onto the pulley, this arch causes the band to always wind further up on the conical section than what was previously wound on. The higher point on the pulley always creates more tension in the belt and causes it to arch in that direction.
With this exaggerated crowned pulley, it takes just a few turns for the rubber band to wander from the edge of the pulley all the way to up the hump. Once the rubber band is on top of the crowned pulley, the maximum tension will be in the middle of it, and it no longer has any reason to arch in either direction.
The mechanism is fairly subtle in most flat belt pulley transmissions. The crowning on a crowned pulley is typically barely visible to the eye. I made the pulley at left with a very exaggerated crown on it to make it easier to see what happens.
With the much more subtle crowning on a typical pulley, the self-centering
of the belt happens more slowly. If the pulleys are misaligned, it may never
center itself. Flat belt transmissions require much more precise alignment
than a V-belt transmission does.
Here is an example application of a crowned pulley and flat belts in a more
modern application. The pulley at right is completely cylindrical and without
any flanges to guide the belt. The pulley is attached directly to a 3500 RPM motor.
The pulley at left is a crowned pulley attached to a blower. The belt drive
steps up the speed to about 11,000 RPM. The whole setup is very efficient,
and the flat belt can handle that sort of speed easily.
It may help to see how the tracking actually happens, so I made a video to illustrate the concepts covered in this article.
See also:
Back to my Woodworking website 
|
Here's a close-up of the small pulley, You can barely see how the pulley
tapers slightly towards either edge. It's this taper that keeps the
pulley aligned.
Flat belts and crowned pulleys are sometimes used in small pieces of electronics.
The old 8-track players, because they needed more power to turn the clunky
cassettes, often used flat rubber belts and crowned pulleys. The two motors at
left originally came from 8-track players. Belt driven turntables also rely on
crowned pulleys and flat belts. Cassette tape players, on the other hand,
mostly use square belts that run in a V-groove. Cassette tapes don't need much
force to turn, and the players are usually fairly small, so flat belts aren't
a good choice. CD players are always direct drive, so not much to see in those.
Bandsaws also rely on crowned pulleys to help keep the blade aligned. The blade
guides are merely there to help absorb the force of pushing the stock into the
blade. When the saw is just running without cutting any stock, the blade should
ideally not touch the back guide. The crown alone is enough to keep the blade lined up.
