If your treadmill begins to clunk or grind, one of the possible causes is a bad roller. Rollers are part of the drive belt system and there are two -- one on the front and one on the back of your machine. If you do have a bad roller, you can order a new part and replace it yourself.
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Your treadmill has a front and rear roller, sometimes referred to as a drive and idler roller, respectively. The rollers are long tube-shaped parts. The treadmill belt wraps around the rollers, which allows the belt to revolve around the deck. Small spindles on the ends of the roller are set in metal brackets on the treadmill, holding the roller in place. If your treadmill roller cracks or the bearings go bad, your treadmill belt will not run properly.
A bad roller can result in various treadmill issues. If you hear a clunking or grinding noise coming from the front of your machine, it might be a bad front roller. If the walking belt slips as you run or if it shifts to one side, this can be the result of a bad roller. If the treadmill motor runs, but the belt won't move, you might have a bad roller. However, all these issues can be symptoms of other problems, so you will need to visually inspect the rollers to see if they are damaged.
Some manufacturers encase the roller in a sleeve, which increases the grip between the roller and the belt. If this sleeve slips off to one side, it can catch at the end of the roller and cause the roller to not operate properly. If possible, trim the sleeve so it is centered on the roller. If you can't trip the sleeve, you will likely have to replace the roller part.
To access the front roller, remove the screws on the motor hood and pull the hood away from the treadmill. To access the rear roller, remove the end caps at the back of the treadmill. If you are replacing either the front or rear roller, you have to loosen the walking belt by turning the adjustment bolts on the back-end of the walking deck. All rollers are not the same. Order the new roller for your specific make and model treadmill.