As the gear pushes the filament into the hot end, sometimes the rate at which it drives the filament is faster than the rate the hot end can extrude it, so the back pressure results in the gear stripping bits of filament that end up between the teeth.

This can be caused by quite a few things:

Extruding temperature not hot enough
Partially clogged hot end nozzle
Bearing pressure too high
Bearing pressure too low
Nozzle tip too close to the print bed (first layer or so)

First thing I would do is check between the gear teeth and see if it's clogged, then watch the printer as it makes the first pass.  Usually, that is when it's most likely to strip the filament.  If this is the case, you may want to either increase the hot end temperature for the first layer, or increase the hot end to print bed gap (or both).

What tension should the clamp be? (the one that holds the bearings.)

thanks.