[Al Skierkiewicz]

Salik,
Dave presented a great explanation but I would like to weigh in on the motors. The bearing and brush friction can be considerable when the motor is new or if the output shaft should get bent during handling or running. If you don't measure force with the motors in place but disconnected from anything, add in a little fudge factor just to be on the safe side.
In my business, a real world example of stiction (we have always thought of it as "STicky frICTION") occurs between the rotating video head and the polished surface of the tape. Although every attempt is made to force a cushion of air between the two surfaces to overcome the stiction, other factors and Murphy will intervene. A fluid, for instance, in the form of high humidity or a liquid will actually turn into an adhesive under such tight tolerances. The result is tape damage as the tape sticks to the rotating head and is sucked out of the cassette at light speed. That is why all consumer video tape recording devices have humidity sensors on the surface of the video head. When the humidity gets too high, the transport is disabled.