Pulling Up the Rears

It’s not unusual to see a 750,000-mile warranty on a drive axle, and lube drains running out to 250,000 miles or even 500,000 with the right gear oil. Your rears are engineered to take a pounding. But that kind of durability demands sensible maintenance along the way and use in a job the equipment was built to handle. That last point is a key, because abuse by misapplication is a common cause of failure.

Of course, intelligent spec’ing is where long life starts. A drive axle, single or tandem, has three jobs to do, the first being to keep the wheels running parallel to each other. The second, in concert with the suspension, is to keep the load off the ground. The third, and most complex is to deal with the engine’s power output.

And when you think about that, you get an idea of just how tough a drive axle has to be. The engine spins torque off the flywheel, in some cases 2000 pound feet or more. That raw power is caught by the transmission, and in lower gears it’s multiplied by 12 times or more before being sent down the driveshaft to the axle. Then what may be 25,000 pound feet of torque travelling longitudinally are absorbed and made to turn a right angle-suddenly sent sideways by means of a big ring gear and smaller pinion gears on the end of short shafts connected on both sides to the wheels. More impressive still, that set of differential gears housed in the bulbous bit at the axle’s centre lets the wheels on either side of the truck rotate at different speeds during a turn.

In a tandem-drive setup, two axles, each with a diff unit in the middle, are connected by way of another short driveshaft between them. That little two-footer can be a problem, incidentally. If the angle is too sharp-if the rise from one of the differentials to the other is anything over three degrees-you can get torsional vibrations with the potential to do damage all the way from clutch to axle.

The forward axle also has a power divider, or inter-axle differential, to ensure that torque is shared with its sibling, though not necessarily equally. That’s where the usually standard interaxle diff lock enters the picture, which will ensure an equal power split between the two axles on slippery surfaces. There’s also a driver-controlled lock (sometimes an automatic lock is offered) that will equalize power from side to side. This prevents a spinning wheel from getting all the torque and making traction matters worse. That diff lock will be an option, not standard equipment, but you need it so don’t forget to stick it on your must-have list.

THE RIGHT RATIO

So where do you start? One easy spec’ing decision is the choice between tandem or single rears, usually determined by weight-carrying capacity. An option here is the 6×2 setup: a tandem rear in which only one axle is driven, the “dead” axle being liftable when traction is required. Benefits include lower purchase cost, lighter weight, and better fuel economy due to less friction between fewer gears. But resale value?

Choosing the ratio you need is the biggest of your rear-axle decisions, the rest being largely pre-determined by the work you do and factors like your choice of wheel size and transmission. For instance, given the same transmission top-gear ratio and wheel/tire size, a “slow” axle ratio of 4:11 will provide a road speed of, say, 62 mph at an engine speed of 1800 rpm. Change only the axle ratio, to 3.07 for example, and your road speed will climb somewhere into the high 70s.

For highway operators, the goal is to strike a balance between driveability and engine rpm at cruising speeds. Put another way, you want the ability to climb a typical highway grade without downshifting, but you also want to limit engine revs to conserve fuel. Ideally, your engine should be turning about 1550 rpm at your chosen cruise speed. That puts you near peak horsepower and torque and leaves you easy passing or hill-climbing options without shifting a gear.

In most vocational operations, the work will dictate numerically high axle ratios-providing lots of grunt at very low road speeds on a construction site or an oilfield rig move-and you’ll then choose an overdrive transmission to give you at least a little bit of highway speed.

Rear axles can be either single- or double-reduction types. You get a two-speed rear axle by adding an extra gear set in the differential-or in the hubs, at the axle ends, as they usually do in Europe. And you would do so to get an even higher, numerically, overall ratio for some severe-service applications.

Your salesperson will have an easy way to calculate all this for you, but engine and axle makers usually have simple slide rules or booklets that lay out the various options in detail. This underscores the importance of knowing up front just how your truck will be used and carefully determining your spec’ing priorities.

SPEC FOR THE JOB

More than with many other components, you must spec your rear axles for the job you want to do, including the terrain your truck will encounter-flat, hilly, on-road, off-road, etc. The kind of freight to be carried is another consideration, along with the average and maximum weights of your loads. Every axle has a structural rating that reflects the load it can bear, called the gross axle weight rating [GAWR]. That refers to the weight actually placed on the axle, while gross combination weight-GCW-is a measure of how much load the axle can pull considering the relative strength of gears and bearings and the like. Estimate on the high side here and you’ll end up with a more expensive axle, but worth it if this avoids an axle failure due to asking it to do more or carry more than it was designed for.

Other considerations include duty cycle, namely how much time the truck runs or sits loaded versus unloaded, the number of stops it makes daily, and the rolling resistance for the type of roads most commonly traveled.

Axle torque ratings (which vary by model, axle ratio, and vocation) are determined by taking the maximum gross engine torque (in pound feet) and multiplying it by the lowest transmission forward-gear ratio. If you’ve got a first-gear ratio of 15:1 and an engine that pumps out 2000 pound feet of torque, for example, your axle’s going to face 30,000 pound feet. Note that the axle’s rating is for the maximum gear-multiplied torque, not the engine’s nominal rating.

As with other major components, this is no place to try to save a penny up front. Spec light and you’ll run the risk of an axle failure. Worse, if you’ve over-stressed the axle’s design limits, your warranty will be worth zip. Spec’d intelligently, maintained properly, and used with respect, however, your drive axles will last almost forever.

Hey, they’re designed to.