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SMART SPEC: Drivelines

The driveline may be a relatively simple component compared to other parts of the drivetrain and nowhere near as pricey. Yet a failed driveline can take your truck off the road just as easily as a pro...


The driveline may be a relatively simple component compared to other parts of the drivetrain and nowhere near as pricey. Yet a failed driveline can take your truck off the road just as easily as a problem with the clutch or transmission, and new power trends are placing increased pressures on drivelines. So a driveline deserves every bit as much attention as the rest of the drivetrain components when it comes to spec’ing.

Strong tubing, weld and slip yokes, spline plugs, seals or dust caps, and weld or expansion plugs are the components included with the driveline but it’s the universal joint that’s key to its operation. It transmits power between two or more spinning members while allowing the angles between them to vary.

The trend towards high torque/lower RPM engines that has gripped the North American market for the last decade may be helping reduce fuel emissions while improving fuel economy but it is placing higher demands on drivelines. Using the “gear fast/run slow” approach, engines with cruising RPMs of 1400 to 1600 are still able to produce 350 to 450 hp. But to achieve highway speeds, lower RPM engines resort to using faster (lower numemic) axle ratios coupled with either direct or overdrive transmissions. But, as ArvinMeritor explains, while the slower driveline speeds are beneficial to u-joint life, the associated high torque transmitted through the driveline can reduce u-joint life.

Another trend placing increased pressure on drivelines is the move towards more muscle under the hood. Engines over 400 hp have been pushing fleets to spec beefier drivelines. You will need to check the model’s rated capacity to ensure it’s correct for the truck, its engine and the rigors you anticipate to place upon it. Some of the things you will need to consider include:

gross weight: It goes without saying that the higher the GVW, the bigger the size of the engine you will need, and with that comes the need for a stronger driveline.

net driveshaft torque: This is the amount of torque, measured in pounds-feet, absorbed by the driveline. The more you multiply the torque, the beefier a driveline you will need. Compare the net driveshaft torque to the ratings in the driveline manufacturer’s models to find the right model for your operation.

axle ratio: The rear axle’s drive ratio impacts driveshaft speed and, as already mentioned, amplifies the effects of torque.

length of tube: Weight becomes an issue here. The longer the driveshaft, the greater its weight. Balance can also become a problem. Check the manufacturer’s limit on tube length.

For linehaul applications, which tend to post high annual mileage, the driveline’s ability to carry the expected torque under typical conditions while providing maximum u-joint life is usually the primary consideration. In applications other than linehaul, particularly those with low annual mileage, u-joint life doesn’t need to be a primary consideration in the spec’ing process. A driveline that can handle the torque under all conditions without failure would be a more important consideration.

In these cases, the driveline is sized on the basis of torque to accelerate the vehicle as well as the maximum low gear torque, advises ArvinMeritor. Depending on the specific vocation, the importance of the two factors is adjusted to give the proper weight to each. It’s the combination of the two factors that creates the correct driveline for that application.

Similar to other drivetrain components, even the best built and most well spec’d driveline will run into problems if it’s not properly maintained. The biggest problem you will need to worry about with drivelines is the u-joint angle. When the u-joint is forced to work at an improper angle, the resulting vibration will travel through the driveline and cause increased wear and premature failure. The harsh vibration throughout the driveline can also lead to shortened u-joint life, transmission gear failure, synchronizer damage and even damage to the differential.

And it goes without saying that proper lubrication is also critical. U-joints must be periodically lubricated with the right grease to smooth the constant movement among their parts. Spec’ing a permanently lubricated u-joint design should reduce scheduled downtime.


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