What's causing my belt-sander belts to break?

I recently bought a new belt sander and a supply of new belts. Four of those, including the one that came with the machine, fractured at the seam. Could heat and humidity cause these failures? Is there a method to repair these broken belts?
—Bob Mathews, Nokomis, Fla.

Belt seams rarely fail, despite the friction and high-speed flexing that sanding belts endure. Heat, humidity, and age can contribute to belt seam failures, but they're not the only causes. Check that your belts are turning in the direction indicated on the inside face. When not using your sander, release the tension on the belt. If the problem persists, exchange the sander or have a warranty service center check the tension setting. To rule out belt age as a possible problem, purchase what you expect to use within eight months to a year and store unused belts in a temperature-controlled place. As for repairing those broken belts, there's no safe way to duplicate the heat-activated adhesives used by manufacturers, so carve up those old belts for your hand-held sanding pad.