The belt of your treadmill is the part that rotates around its base while you run on it. Its movement is supposed to be dictated by the speed settings you input, but occasionally a treadmill belt slips when you run on it, which can be dangerous. It is caused by the loosening of the motor belt drive, which controls the movement of the belt, because the vibrations of the treadmill when it's run on can loosen the bolts holding the motor in place. Most companies that make treadmills sell replacement belts, though depending on how worn your belt is, tightening the motor belt drive yourself will stop it from slipping.
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Switch off your treadmill and unplug it.
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Remove the piece covering your treadmill's motor by unscrewing any screws. You may need to turn you treadmill on its side.
Look for any fragments of black rubber plastic in the motor. This tells you the motor drive belt is worn. If the rubber plastic is completely fragmented, you know the motor drive belt is totally worn down and you need to buy a new belt.
Tighten the motor drive belt by unscrewing the bolts holding the motor down. You may need the help of someone else to push the loosened motor down deeper into where it's sitting.
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Screw the bolts back in to keep the motor in.
Plug your treadmill back in and turn it on. Test out the belt at a low speed without stepping onto it and slowly increase the speed. If the belt is rotating normally, test it by walking on it. Slowly increase the speed to make sure it's not slipping even when you run.