Craftsman 315.117150. The belt keeps sliding off, as soon as turned on and start to sand. It seems to be on correctly.

You need to adjust the tracking knob which is by the front handle that looks like a triangle head screw. Make sure the belt tension leaver is tight and the belt has enough tension on it. Quickly press and release the start switch and note the centering of the belt. It should be in the center of the bottom platen. If it is not, adjust the tracking knob and bump the switch again. If the belt is going in the wrong direction turn the knob the other direction. Once you are close the center of the platen, press the power switch to turn the unit on and lock the trigger. Firmly hold the unit vertically from the front handle. Fine adjust the tension knob to center the belt and make sure the belt is not hitting the case which will cause damage. You should be able to use the belt sander without the belt slipping off. Let the sander do the work, do not push down and limit the side to side motion. If the belt will not stay on then the belt is too large or the torsion spring needs to be adjusted to applying more tension on the belt and drive roller.