To replace a drive belt on a Bosch 1274DVS belt sander you need to unscrew the toothed belt wheel from the rear roller (843). To do that: 1) remove any sand paper, 2) unscrew the torx overall-head screw (819) and remove the black plastic hood (37) that covers the drive belt, 4) remove the old drive belt (33) if it is damaged, 5) wrap a strip of leather, canvas, or vinyl around the toothed belt wheel (843), 6) shove a wooden wedge into the front slot between the roller and the housing (838) to keep the roller from turning, 7) grab the toothed belt wheel with a large channel lock wrench and turn the belt wheel in a clockwise direction, 8) unscrew the roller from the toothed belt wheel, 9) replace the drive belt, screw the toothed belt wheel back onto the roller, and replace the hood. You should be back in business.
Pry the top cover off in the middle with a slotted screw driver and the front cover with a torx head bit. Remove rear and forward handles. Remove 4 cheese head screws and and separate the top half of the plastic housing, just give it a little wiggle and it will come. Lift out motor being careful not to chip brushes. You may gently pry the brushes back with a utility knife as you lift, I just gently pull it out. Keep an eye on what bearings go in what slot and where the wires go for reassembly. The flange bearing is on the front of the armature and has a rubber cover on it. When I took mine off all the bearings just fell out so watch out where they land so one doesn't get caught in the motor. Note which way the flange bearing and the cover face. The cover goes over the open side of the bearing housing. The bearings are held in with grease so don\'t drop or bang the bearings before putting them on the armature shaft. The bearings slide right on easily. Put the bearings into their appropriateworn slots, you may have to wiggle the magnetic field housing a little bit to get it in. The wires tuck in to the various slots. Carefully align the brushes in their seats, the right side wire goes around the post and the left wire continues across the front. Reassemble in reverse order. Ahhh, sounds so fine.