A "sticking'' belt on most treadmills is a fault that you can fix yourself. To operate smoothly, treadmills require basic maintenance. A constant, smooth action needs a balance of friction and tension between the two rollers at the front and back of the machine. Without this, the belt cannot transfer the power from the motor. It is also important to lubricate underneath the belt — the surface you run on — with the fluid recommended by the treadmill's manufacturer.
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Troubleshooting the Treadmill
The moving surface of a treadmill is known as the "belt." Before looking for mechanical problems check to ensure that you're within the manufacturer's recommended user weight limit. If you exceed the weight limit, the belt may stick. To establish that the problem is being caused by poor adjustment, rather than an electrical fault, turn on the treadmill and let it run at a slow speed. If the belt runs normally then it is unlikely you have a fault with the motor.
For a sticking belt the first step is to check the lubrication. All treadmills need some form of lubricant between the belt and the deck -- the section under the moving belt. There are many types of lubricant; the correct one is designed to work with the materials used in your machine. Check with the manufacturer or a specialty supplier. Some treadmills have a "self lubrication" system, which is a reservoir of fluid that applies the correct amount as needed. On basic treadmills the lubricant is applied manually.
The belt may stop when you step on it if the distance between the two belt rollers needs adjustment. Either the belt has stretched, causing it to lose its grip on the rollers, or the adjustable roller at the rear of the treadmill has moved. Both situations are quickly fixed by adjusting the bolts that hold the roller in place. You'll need a tool called a hex spanner, which is normally supplied with the treadmill, to loosen, move and then tighten the rear roller.
Adjusting The Rollers
The front roller is connected to the motor. The roller at the rear of the machine is adjustable. Locate the bolts at the rear of the treadmill. These may be under plastic covers. With the treadmill turned off and unplugged, turn the left and right bolts a quarter turn clockwise. Then turn the treadmill on and test it to see if the adjustment has tightened the belt and fixed the problem; if not, repeat the process of turning the machine off and turning the bolts another quarter turn on each side.
Don't be tempted to just tighten up the bolts as firmly as you can, as this may prematurely stretch the belt. Treadmills are designed to be adjusted progressively. This feature means that a belt can be adjusted a number of times before requiring replacement. Adjust both the left and right bolts equally, as turning one more than the other will eventually move the belt off center.