A properly adjusted treadbelt should never slip. Most treadmills will provide the user with easy access to belt tracking. Treadmill belts may stretch slightly under initial use and may need tightening. Using the allen wrench supplied with your treadmill, or another of the right size, tighten both belt adjustment screws clockwise. Belt adjustment screws are usually located at the rear of the treadmill.
Some belts require looser running than others, so check the user's guide before making adjustments.
Make sure belt is properly aligned. Improper belt tracking may result in roller knocking. A knocking noise may indicate a defective roller, however, it is usually a result of the belt placing too much force on the roller from side to side. In this case, the knocking noise will be at a much slower rhythm than roller rotation. Proper tracking reduces the load on belt guides and ensures that the front and rear rollers are parallel.