Gearboxes are found in just about any type of machinery or equipment, including your car, wind turbines, and the like. Gears turn the pieces that pump fluid through machinery and operate fan blades, or they allow transmissions to change gears and increase or decrease the RPM of a motor. Whatever the operation, gearboxes can fail—often due to improper use or care. Note a few reasons for gearbox failure in cars so you can ensure you are caring for this very important part of the transmission properly, and know what to expect by way of repairs if it does fail.
Pitting in gear teeth
Pitting in gear teeth is either very small, called micro pitting, or very large, called macro pitting. This pitting shows up as holes or erosion on the surface of the gears and especially the teeth. In turn, the teeth cannot mesh as easily and the gear doesn't turn smoothly. Eventually it will simply fail.
One common cause of this pitting is not enough fluid, fluid that is too thick, or poor quality fluid. If your car is low on transmission fluid, it will not properly lubricate the gears. If the fluid is too thick, it won't coat and cover the gear teeth as it should. When fluid is old and has dirt and debris, this can rub off onto the gears and cause this pitting or erosion. Having your transmission flushed and filled regularly can address this problem; a flush and fill doesn't just add new fluid. It removes the old fluid and all contaminants with it. Be sure you're also using the right transmission fluid for your car; some are thicker than others, and topping off your old fluid yourself without checking the right type and viscosity can put this added wear on the gear teeth.
Gears won't mesh if they're bent out of shape and improperly aligned. Bending fatigue is a common failure in gears that are pulling too heavy of a load; as they struggle to work, the resistance felt by one gear will keep it from rotating at the right speed, and the teeth will bend slightly as they mesh with another gear. If left unchecked, the teeth then bend entirely out of shape and the gears don't mesh at all.
If you cannot decrease the load of your vehicle that is putting this added weight on your transmission gearbox, upgrade to a stronger set of gears. Thicker gears can more readily handle this fatigue or resistance without having teeth get literally bent out of shape.