Sanding belt joints and running direction.

A sanding belt starts as a long strip of abrasive that is cut to length and joined into a loop. The joint is the belt’s weakest point and the reason many belts carry an arrow on the back. Knowing how belts are joined explains why the direction matters, why some belts thump, and why a belt usually breaks at the joint.

Two kinds of joint.

Klingspor, a German abrasives maker, describes two main types:

• Lap joint. The two ends are tapered and laid on top of each other, then bonded. The overlap leaves an edge on one side; if the belt runs the wrong way, that edge can catch on the work or the platen and peel the joint open.
• Butt joint. The two ends meet without overlapping and are held together by a narrow strip of tape, usually fibre-reinforced, on the front or back. The surface stays flat, and many belts with a butt joint can run in either direction.

Klingspor also notes that the joint usually runs at an angle of 45° to 80° across the belt rather than straight across, so the transition passes over the work gradually rather than all at once. The backing and the job decide which joint a maker uses.

The arrow on the back.

An arrow printed on the back shows the direction the belt is made to run. Fit the belt so the arrow points the same way as the belt travels on your machine, which is usually marked on the housing or the guard. On a handheld belt sander the belt runs backwards underneath, towards you, so check the machine’s marking rather than guessing. A belt with no arrow can generally run either way, but follow the maker’s instructions if they say otherwise.

When the joint causes trouble.

• A regular thump or mark on the work at every turn usually comes from a thick or uneven joint. A belt with a flatter joint, or a different brand, can help.
• The joint splits when the belt runs backwards, is old or dried out, or has been stored badly. See storing sanding belts.
• The belt wanders if the joint isn’t square to the edges. If one belt tracks badly but others are fine, swap it.

Check the joint before you fit a belt, especially one that has been in storage. A cracked or lifting joint can let go at speed; throw the belt away. Never try to glue or tape a broken belt back together.

Sources

• Klingspor, “Belt joint”.