An axle is a load-bearing assembly that connects two wheels together on opposite sides. There are several types of axles used on cars and light trucks. The most common is the transaxle, which encases an automatic or manual transmission and a differential. Another popular type of axle is the drive axle, sometimes referred to as a live axle, which contains the differential. The least popular type of axle, which is fading quickly from use, is the non-driving straight axle. This axle is basically a straight beam that connects the wheels together, but has no differential. The straight axle was once used commonly at the rear of front-wheel-drive vehicles and on the front of rear-wheel-drive pick-ups. With the growing popularity of independent suspension systems, the straight axle is rapidly becoming obsolete.
Since axles vary in complexity, so do their purposes. A simple straight axle helps to support the weight of the vehicle and serves as an attachment point for the wheels. A drive axle does all of this, along with providing the torque transfer capabilities of a differential. A transaxle serves all of the same purposes as a drive axle, but also includes the gear-changing capabilities of a transmission.
Axle maintenance is related to the complexity of the axle. A straight axle requires periodic cleaning and repacking of wheel bearings, usually every two years or 40,000 kilometres. Drive axles should have the axle lube level checked with every oil change. On most front-wheel-drive vehicles, the differential is part of the manual or automatic transaxle, and therefore does not require a separate differential lube check. If you have a rear-wheel-drive or four-wheel-drive vehicle, check the owner's manual to find out the recommended interval for differential lube changes.
Positive-lock differentials may require a different lube or a lube additive. Many manufacturers claim their differentials to be "lubed for life", unless it has been submerged in water. If you have a pickup or SUV, this may happen quite often, especially if you pull a boat. If this is the case, have the axle lube changed at least once a year.
In cold climates, you may want to consider changing the standard axle lube to a synthetic type. Synthetic lubes flow easier in cold weather, improving lubrication. With a 4x4, this is doubly the case because of the two axles. Always use a lubricant that meets or exceeds the manufacturer's lube specifications. When cared for properly, differentials provide many thousands of kilometres of trouble-free operation.
When a differential problem does occur, symptoms may include: a high-pitch sound when accelerating or decelerating, clunking when accelerating or when shifting between reverse and drive, or a howling sound. Differential problems should be checked out as soon as possible by a qualified service professional.