[raymo]

Individual gears and gear trains are always less than perfect, the more gears in a given train the greater the imperfection. Even horrendously expensive
gear systems are not perfect, that is one of several reasons that guiding of long exposures is necessary. The imperfections in gears causes a phenomenon called periodic error which naturally is seen with each revolution of any given gear[hence periodic]. There are PEC features which can be trained to largely
nullify PE by causing the tracking motor to adjust to the error, but even these cannot completely eliminate the error. Because of the imperfection the gears
cannot be in constant mesh, so a tiny amount of backlash is necessary; the
higher the quality of the gear train, the smaller the necessary backlash, and the better the tracking will be.
There is a point where trying for gear perfection becomes pointless because other things which affect tracking accuracy can cause larger tracking
inaccuracy than gear imperfections do, so back to guiding.
raymo