Your U-joint and you

| January 03, 2006

Technician Ernie Fry at Associated Truck Parts in Gilbertsville, Pa., uses a special tool to press out the u-joint cross, so the parts will fit tight when reinstalled.

Experienced Class 8 tow truck drivers arriving on the side of the intestate to pull in a broken down big rig will tell you one cause never surprises them. A broken U-joint.

“You do actually have to grease the joint,” says David Goode, owner of Goode Towing and Recovery based in Killeen, Texas, “and if you do it will last. If you don’t it won’t.”

The driveshaft typically consists of two U-joint assemblies linked by a thin steel tube. The joint at one end has a sliding, splined section called a slip-yoke, so the shaft’s length can change when the truck goes over bumps. If the shaft is long enough, there is also a center bearing that helps support it. This is typical of the shaft connecting the rear of the transmission to the forward drive axle.

The tube between the U-joints actually proves to be the weak point in the drivetrain under many circumstances, bending severely and saving the U-joints, transmission and axle parts in the process, when severe overstrain occurs. A typical example of conditions that produce overstrain is a very heavy vehicle crawling out of a muddy spot and developing a hop.

Popping the clutch or releasing the clutch rapidly with too much engine rpm are also common causes of driveshaft failure. Still another is spinning on ice and having the tires catch on a dry spot.

If properly assembled, balanced and installed, driveshafts still need regular greasing and, because of the way too much torque or even normal wear can put them out of balance, regular inspections. Even under favorable operating conditions, U-joints still need to be disassembled and the needle bearings and caps replaced when they wear out.

We visited Associated Truck Parts in Gilbertsville, Pa., to find out about doing driveshaft repairs the right way. This heavy-duty specialist is a dealer for ArvinMeritor and several other major vendors of driveshaft parts. Technician Ernie Fry uses an array of expensive special tools and sophisticated equipment, along with many years’ experience, to artfully turn out driveshaft assemblies that are properly balanced and phased. (Phasing refers to having the two U-joints at the proper angle to one another, which is the only way to minimize vibration.)

Fry spends most of his time either rebuilding worn or damaged U-joints, or replacing bent driveshaft tubes. Tubes may also be replaced to change a driveshaft’s length when customers are modifying vehicle suspension systems to improve weight distribution, or for other reasons.

Rebuilding U-joints
This is a lengthy process. We can only hit the high points here, showing the kinds of special tools and care a good shop gives this work.

  1. It’s necessary to pull parts apart without damaging them. Fry always removes the zerk (grease) fittings first. Otherwise, they can be damaged during disassembly.

  2. The bearing caps should fit tight in the yoke, and the cap’s two parts, which are spot-welded together, must not rotate in relation to one another. If caps are loose in the yoke, so turning the cross rotates the cap, the yoke has probably been mechanically overstressed. If a new cap has play, the yoke should be replaced. Fry then uses a special tool to force the bearing caps out of the yoke.
  3. Next, he uses a special tool to press out the cross. This kind of tooling makes his job easier and also preserves surfaces so parts will fit tight when reinstalled.
  4. Once the yoke is disassembled, Fry uses a wire brush to clean out rust. Edges roughened during removal have to be filed smooth so the cap won’t be damaged when it is replaced.
  5. Burrs have to be removed from the cap seating surface so the cap will seat flat when it is replaced.
  6. The new needles, assembled inside the caps, must be thoroughly packed with grease.
  7. As the cross is assembled into the yoke, Fry makes sure the two grease fittings will line up (so when you go to grease later, the job is easier). This is a tricky business because you have to slide the parts together skillfully and in the right order so the needle bearings won’t fall out during assembly.
  8. This particular design uses Loctite, factory applied to a knurled bolt that is renewed every time to keep the torque in spite of vibration. Many other designs use locking straps that must be bent against the bolthead’s flats after assembly to hold them tight.
  9. The cap bolts are brought down evenly (in several stages) and then torqued to 40-40 pounds-feet. The torque wrench clicks when torque is correct.
  10. Of course, the other end of the U-joint assembly must be similarly assembled to the opposite trunnions on the cross.
  11. The U-joints for this shaft were later welded onto the new tube, and then the assembly was balanced, as described above. Even when there is no damage to the tube, and new crosses, needle bearings and caps are being installed to renew all bearing surfaces, the final step is a careful balancing.

Replacing a broken or bent driveshaft
This driveshaft tube (below) failed after many years of hard service. There may have been a tiny defect in the weld at one end that allowed water in, eventually resulting in rust. The rust weakened the tube and, eventually, it gave up. This case serves as a reminder of the value of carefully inspecting driveshafts. Fry says the only sign of trouble would probably have been a tiny rust spot or fleck of missing paint at the welded seam where the tube was put together out of a flat metal sheet.

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