I don't know what the simple explanation is, but flat belts will typically
self-center on a crown or high spot on a wheel, spindle etc.

This same crown effect is used on many machines, not just tape drive
mechnisms, where they're often seen in electronic repair, but as you mention
not usually made up of flat angles.

--
Cheers,
WB
..............


"N_Cook" wrote in message
...
I don't remember seeing this before. Instead of baluster/bulbous pulley or
capstan, two angled faces with a thin ridge in the centre on the drive
pinion. So the belt must ride on the ridge but why does it not hunt about?
There is a limiting flange either side of the angled flats and the width
of
the rubber band is greater than the distance between a flange and the
central ridge, but still why no riding about?. Then what I thought was
crude
construction is probably part of the dynamics. The capstan is 2 discs
fitted
together but one is 49.94mm diameter and the other 50.04mm so this step
change must make sure the band rides over the ridge but biased to one
side,
lightly touching one angled face, but not touching a limiting flange. When
I
get the mechanism working again I will try viewing with a xenon strobe
light
. Anyone any input on this ? more or less prone to changes in rubber
restitution/stretch etc, more/less likely to come of when PTO is engaged,
greater/less accuracy in speed of drive with varying driven-side back
torque
etc.