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Making Sure U-Joints Survive

Posted in How To: Transmission Drivetrain on July 21, 2017
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The truck took flight off a sandy rise at speed, caught air, and landed with impact. Another ground its way up a rocky canyon full of boulders. Both were placing plenty of stress on drivetrain components and the critical link we all use: u-joints. When building and running modified vehicles, good design choices and some maintenance can help ensure those joints don’t fail while in use.

When upgrading, or going bigger, with modifications consider that u-joint upgrades may be needed to handle the greater torque. Higher quality joints will last longer, but poor operating angles and other factors can quickly destroy most any joint. Whenever you try to run a u-joint at too great an angle, you can have trouble with the driveshaft vibrating or moving erratically. As a u-joint assembly rotates, the angular velocity at each of the two axes of the joint changes during rotation and a steeper driveshaft angle exaggerates this effect. At some point, this causes severe enough movement to cause accelerated joint wear and/or severe driveline vibration. Solutions can include change of drivetrain lift and angle, use of a constant-velocity (CV) joint driveshaft, or incorporation of aftermarket high-operating-angle u-joints.

Pound-for-pound, u-joints with grease zerk fittings are typically weaker than those without due to the internal holes. However, greasable u-joints can last longer when maintained. You’ll have to decide which is more important on your rig.

Higher horsepower, bigger axles, and heavier tires can all place more stress on u-joints. Here, from top to bottom, are 1310, 1330, 1350, and 1410 series u-joints. You can see their progressive size growth which also translates to greater strength. The load rating of the 1410 (1-ton) joint is nearly double that of the 1310 (1/2-ton) joint.
U-joints may have either inside or outside snap rings to keep the caps in place in the yoke. When replacing joints, ensure the snap rings are fully seated so they’ll never leave the assembly as the joint can walk sideways in the yoke and cause further trouble.
There are two common yokes on which to bolt a u-joint: those that use metal straps with bolts and those that use small u-bolts with nuts. The u-bolt style is far more durable. However, don’t over-tighten the u-bolts as they can distort the caps and cause premature wear and failure of the joint.
When a driveshaft is off, it’s easy to drop a cap off a u-joint. A good wrap of electrical or duct tape right away should keep them intact. If you do drop a cap, make absolutely sure the caps get no dirt in them or missing a single needle bearing. If not, you’ll be back replacing it soon enough when it wears out quickly.
When making suspension changes, be mindful of how far the axle can travel and what angle this can place on the driveshaft u-joints. Minor joint binding at the limits will accelerate joint wear and more severe bind can cause joint breakage such as this one. A flailing driveshaft spinning at speed is an ugly weapon to the underside of your rig.
Swapping axles, transmissions, or transfer cases? You might need to adapt a driveshaft to another yoke on one of them. If you only need to change the mechanical connection you may be able to do so with a conversion u-joint or flange. Many are available to do just that.

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