A belt sander is a power tool used for quickly sanding wood. They range from large belt sanders used for refinishing floors to small hand-held belt sanders used for refinishing furniture. Every belt sander, regardless of size, has at least two rollers. One provides the power to run the belt; the other keeps tension on the belt. Because the belt is not attached to the sander itself, the belt shifts slightly as it spins on the rollers. You should adjust or fix your belt when it has shifted so much it's not tracking straight, and you should also adjust it when you put on a new belt.
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Look at the diagonal seam across the belt. If it is frayed, the belt needs to be replaced. If it is not frayed, and appears in overall good condition, then the belt only needs to be adjusted.
To replace the belt, release the tensioning lever, located on the side of the sander in between the two rollers. Release it by pulling it outward, away from the sander. Once the lever is released, the belt will become slack.
Slide the belt to the side of the sander and remove it. Place a new belt on, and retighten the lever by pushing it inward, toward the sander.
To adjust an old or new belt, switch the power of the belt sander on, and note which direction the belt rides -- whether it shifts to the left or right. Turn the power off.
Locate the knob near the front roller. There should only be three controls on the sander -- the tension lever, the knob and the on/off switch. If the belt is shifting toward the side of the sander that the knob is on, then loosen the knob by turning it about half a turn counterclockwise. If the belt is shifting away from the side of the sander that the knob is on, tighten the knob by turning it about half a turn clockwise.