Hi, I have a Wilhelm Bruder organ whose keyframe drive is by round
leather belt.  The belt is driven from the crankshaft and then it goes
across the back of the instrument before turning right angles in a most
bizarre way.  It is certainly original and the belt system has an
in-built tensioning device so the belt is very tight and certainly not
loose.

But the belt, although new, seems to slip on the pulley that is the
drive pulley on the crankshaft.  I have roughened it up with some
sandpaper and it stops slipping for a while.

Can anyone suggest something that needs to be applied to the pulley
or belt to stop or lessen this problem?  Any suggestions would be
gratefully received.

Thanks,
Jonathan Holmes