Mechanics often view the gearbox as the truck's most reliable component and treat it as if it's indestructible.

David Levin, vice president of Continental Manufacturing Co., a Houston mixer-drum manufacturer, warns that the units still require some important maintenance to keep the drum turning. Mechanics should remember that gear-oil contamination and excessive operating heat can contribute to gearbox failure.

Whether generated internally or externally, excessive operating heat can distort bearing tolerances or ruin seals. "Some gearbox manufacturers use plastic bearing cages, which can be warped by high operating temperatures," says Levin.

When properly lubricated with clean oil, the main bearing's tightly machined surface and fit tolerances create a near-frictionless rotation. Extending service periods without changing the gearbox lubricant can allow contaminated oil to accelerate wear on the bearing's almost frictionless movement, creating friction and internal heat, Levin notes. Small contaminants can scour both the outer surfaces of the bearing's spherical rollers and the inner surface races. The tiny pits act like speed bumps, causing hesitations in the roller's rotation. The hesitations cause the bearings to slide rather than roll, and they create flat spots on the roller or raceway.

Mechanics should examine gearboxes externally as well, looking for concrete or mud that prevent heat radiation and raise operating temperatures, and checking for leaky seals that lower the level of gear lube, increasing friction on the gearbox's bearing cage.

Mechanics should change gear lube just after the first 100 hours on all new or rebuilt units and then every 1,000 hours of service or every 6 months, whichever comes first.