Anthony wrote:

SBAER wrote in news:h8ydnQ2GgYKOzWXcRVn-
:



I am an engineer, I do machine design on a contract basis mostly
automation equipment. I have to design a flat belt conveyor (something
the customer normally just purchases) about 10" wide and 24" long.
My question concerns the crown on the pulleys. Does the tail pulley
have to be crowned as well as the head pulley? I prefer to approximate
the crown by turning a slight angle on each end of the pulley so that
the machining can be done on a simple lathe. Does anybody have a
suggestion as to the angle and length of this chamfer?
My four year engineering education did not cover this (or anything else
that might be of use to a automation designer) and none of my handbooks
have do either.

Any help would be appreciated.

stan



Cross posted to AMC where Koz may be able to help you out. He may even be
able to get you the exact conveyor you need, without you having to design
it.





Although there are several factors involved, the actual answer to the
amount of crows is"as little as possible". Yea, that doesn't answer the
question at all. Assuming the belt is nearly perfect and mostly
non-elastic, crown would be approximately 1/5 of the belt thickness
relative to the radius of the pulley. In addition to providing some
minimal tracking, a crown is really there because most of the thickness
errors and wavyness in the belt strip material occur in the outer 20% of
each edge. By relieving the pressure against the pulley on these outer
edges (smaller pulley diameter under the outer edges), errors in the
strip at this point have a reduced effect on causing tracking errors.

Regarding doing the crown via a taper, that's called a "narrow bodied
roll". Typically, 60% flat in the center, 20% tapered at each edge by
reducing the radius 1/5 of the belt thickness, with some flat left over
on each end so that if the belt wanders, it doesn't wander off the edge
of the pulley.

Reality is, however, that belting strips tend to be elastic and require
slightly greater crown than the 1/5 of the belt thickness mentioned.
This becomes a subjective matter depending on so many factors that it
can become a guess. It tends to be relative to the belt thickness and
width. Crown should usually never be more than the thickness of the
belt. However, this rule falls into the dumpster for really wide belts.
Same goes out the window the other direction for small diameter
pulleys: crowning the thickness of the belt may make it act weird if
the pulley is a 1" diameter nosebar.

As to crowning the tail pulley, In theory you should. On most conveyors
the only place to provide any tracking adjustment is at the tail and
having a little crown here can improve the system so that it isn't so
"touchy" to adjustment. The goal is to get the belting to come of the
tail heading the right direction (square to the frame) and enter the
head pulley square to the frame. People tend to neglect the return
sides of the belting when designing conveyors (belt goes on a roller
coaster rode) so the tail pully crown tends to be needed to compensate
for errors induced by the return side.

Tracking is NOT accomplished by tensioning the belt. The only tension
on the belt should be enough pressure on the drive pulley so that
Friction factor of the pulley x Pressure overcomes Friction factor x
Pressure of the rest of the conveyor. Anything more than that just
wears things out. To track an unruly belt, Think in terms of RELIEVING
tension. That is, don't tighten one side to get it to track, loosen the
other side. Once it starts appearing to track OK, then start tightening
BOTH sides evenly to make sure there is enough pressure on the drive to
overcome the friction on the rest of the conveyor. When doing this, you
may have to make some slight adjustments in tracking angle of the
pulley...kind of a process of apply a little more tension to both,
adjust tracking via reducing tension on one side...apply a little more
tension to both process.

Even when doing all the right tracking adjustments, they sometimes seem
to have a mind of their own. Assuming that everything is square in 3
dimensions and the belt is good quality, sometimes you just have to
consult the voodoo gods and do whatever works.




Koz