Differences Between Rear Truck Axles

Truck axles are the Atlas of commerce, always bearing the heaviest of loads on the most humble of shoulders.

Big trucks are something of an oddity in terms of modern, road-going vehicles. In fact, the truck's official tractor-trailer nomenclature comes closer to describing its true nature, and the phrase semi-tractor helps the description even more. Heavy haulers are and probably always will be more closely akin to trains and agricultural equipment than they are to cars; a truth made evident when you look at the axles that support them.

Idler Axles

Idler or dead axles are the simplest type of axle, being essentially just a tube with a pair of bearing hubs on the ends to hold the wheels. The axle incorporates some sort of bracket to hold the air brake or disc brake machinery. There are two kinds, tag axles and pusher axles. The major difference here is in terminology; tag axles go behind the drive axles and pusher axles go ahead of the drive axles.

Drive Axles

A drive or live truck axle contains a differential in the center that takes power from the driveshaft and splits it at a 90-degree angle to the axles, which power the wheels through the hubs. The driveshaft powers a pinion gear in the differential, which turns a ring gear around the cylindrical differential carrier. Pinion gears attached to the inside of that cylinder push on side gears attached to the ends of the axles. If one wheel slows down, the pinion gears revolve around the slower-spinning side gear and route power to the faster-spinning tire.

Drop Axles

A drop axle is an idler or even a drive axle that may be dropped to the road to hold higher loads; you'll often see these axles mounted to the bottom of dump trucks as pushers ahead of the main drives. Keeping the axle off the road except when it's needed to bear extremely high loads reduces rolling resistance and increases fuel economy. Alternately, a drop axle may be mounted to straighter leaf springs that hold it a couple of inches off the road. When the truck gets a heavy enough load, the main axle springs compress, drop the frame closer to the ground and the drop axle makes contact with the road.

Tandem Drive Axles

Most drive axles have only a single input, which faces toward the transmission. A tandem drive axle uses a shaft that runs right through the top of the differential and routes power from the driveshaft through the front tandem drive axle and to a second, standard drive axle mounted behind it. When the driver hits a switch on the dashboard, a clutch on the tandem drive axle's through-shaft engages a gear, which drives the differential unit in the front tandem drive axle. So, the truck normally drives the rear axle, but the driver can choose to engage the front axle when he needs extra traction.

Locking Axles

Air lockers have been around for a long time and are probably the ultimate in traction-enhancing devices. The locking differential does just what it sounds like it does: it mechanically locks the left and right axles into a solid unit, thus disabling the differential. In times of extreme need, the driver can lock the axle-shafts in a single axle together and hit the power-split to engage the tandem drive axle. This forces all of the drive wheels to split the load evenly to propel the truck through thick mud, snow or sand.

References

• Heavy Duty Drivetrains: System and Component Applications; Charles R. Jones