Spec'ing the Fuse
by Rolf Lockwood
Chances are you paid little attention to the driveshafts and u-joints on the last new truck you bought. You probably spent hours considering its engine/clutch/transmission combination, a lot less time on the rear axles, and hardly any - maybe none at all - on the bits in between. You were pretty safe in ignoring them because truck-makers massage your spec six ways from Sunday and their computerized spec'ing programs are well able to pinpoint exactly the components required.
But you can run into trouble when you're replacing those driveshafts and u-joints, especially if you've bought a used truck and don't know all there is to know about the drivetrain. You can also find a bucketful of woe if you uprate your engine mid-stream or change jobs and find yourself hauling heavier loads over rougher terrain. In either of those cases you can end up with under-spec'd components that will be more susceptible to premature wear and maybe catastrophic failure.
So it pays to know the basic ground rules.
Think of the driveshafts and u-joints as a fuse, or a group of fuses. Your engine spits out a bunch of torque that gets multiplied in the transmission many times over in lower gears and is then sent spinning down the driveshaft to the axles. That torque load can easily be 20,000 lb ft in first gear, gradually dropping to the actual engine output or a bit less in the top cog where's there's no multiplication going on.
Like an electrical fuse blows when the power surges at a higher amperage than it was rated to withstand, you can lose a driveshaft or u-joint if it's asked to absorb more torque that it was designed for.
So when you come to spec driveshafts, start by learning how much torque is being developed through the lowest gears (see the 'Multiple Choice' transmission article in our May issue for more detail). Take that value and add 10 or 15% to get the rating you need. Don't wimp out here, because durability is important. It's like cutting wire: if it's too long, you can always trim it; if it's too short you can't stretch it.
What will the truck do? Whether yours is a local or a long-haul operation, you have a specific, definable operating environment. Mountain roads are different from the Trans-Canada. Heavy winter conditions or off-road use will strain every component. So will steeply raked loading docks. Do you really understand your total operational situation? It matters.
Changing technology means you have to re-educate yourself every so often, and we're in such a period right now. The latest emissions-limited engines are different beasts, producing more torque lower on the tach than ever before and needing to cruise at lower rpm as well. That demands a taller rear-axle ratio (lower numerically), which in turn means more torque stress on the driveshaft.
When you select your driveshaft assembly, think of it as a chance to include a planned weak link, or fuse, that prevents catastrophic destruction of major components like the engine or differentials. Some experts suggest that you spec one easily accessible segment of the driveshaft to be exactly optimal for its anticipated load while the rest of the parts allow that 10-15% margin of extra strength.
Don't forget to spec the right universal joints. They should be rated comparably with the driveshaft to prevent premature breakage. And once rolling, don't forget to lubricate them using the manufacturer's guidance as to timing and grease type.
While under-spec'ing of your driveshaft can cause havoc, so can shock loads - like revving the engine and popping the clutch, or botching a shift while going uphill under load. Luckily, you won't need any high-tech test equipment to figure out what's happened. A quick visual inspection may well show twisted driveshaft tubes or cracked/broken u-joint crosses and yokes.
In fact, such an inspection should be part of your normal preventive-maintenance routine.
FOR MORE INFORMATION
For more information on this and the general subject of powertrains, see the 'Engines & Drivetrains' and 'Lubes, Filters & Fuels' Decision Centers on our sister magazine's website at www.TodaysTrucking.com.