cog belts that move when spindle changes direction

[FanucIssues]

What would cause two 60mm wide cog belts to move when the spindle changes direction? Is it, as I assume, as simple in discussion as the motor, and the spindle are not on center? If that is the case how would you verify what direction either one needs to move? Too many questions: Spindle bearings also be a part of it? What are your thoughts?

[FanucIssues]

Anyone? Please help.

[laka]

Sounds like they are not parallel to one another if the belts are moving up and down the pulleys.

[Superman]

Depends, what speed are you making the spindle reverse ?
What is the weight the spindle is carrying ? inertia considerations ?

if at 500 RPM and a small part = no jump
if at 2000 RPM and large part = yes I would expect belts to bust or at the minimum stretch badly

Get in the habit of stopping the spindle, before reversing direction,
and consider the size of the part when tapping for example