If you are transferring power in a straight line, you can use a shaft to accomplish it. Add angles and offsets though, and things get tricky. Now you need a universal joint. U-joints are used in driveshafts where you need to get power from the transfer case down to the axles as the suspension cycles. A typical U-joint consists of the center cross (body) with four trunnions, four bearing caps that install on the trunnions, needle bearings inside the caps, and grease seals. We headed to Bayshore Truck, our local Spicer dealer, to compare sizes and types of U-joints for driveline applications.
The most common U-joint used in Jeeps and 1⁄2-ton trucks is known as the Spicer 1310 series joint. They are easy to source and are retained with external snap rings. 1330 joints use a larger body than the 1310 joint but the same-size trunnions and caps. They are often found in 3⁄4-ton trucks and allow more angularity than 1310 joints but are only marginally stronger. 1350 joints are found in 1-ton trucks and use the wider body of the 1330 joint but with larger trunnions and caps. 1410 joints use a wider body but the same-size caps as a 1350 joint, making them perfect for high-horsepower, high-angle applications.
A typical U-joint consists of the center cross (body) with four trunnions and four bearing caps
Most drivelines have two opposing yokes with a single U-joint at each end, referred to as “single cardan,” where the joints are set 90 degrees apart. The joints in the transfer case end and axle end should operate on parallel planes with a single cardan joint in order to cancel out the phasing when one joint speeds up and the other slows down. This occurs because the U-joint is at an angle as it spins, as opposed to being straight up and down.
Some drivelines use a “double cardan,” or constant velocity (CV), with two joints at one end (typically the transfer case) and a single joint at the other end. The two joints split the angle, allowing for greater angularity. With this type of driveline the pinion should be pointed up at the transfer case.
There are two concerns with driveline angles. One is the operating angle during normal driving, which can cause vibration and/or the premature failure of the joint if it is too steep or out of phase. Note that with soft leaf springs the pinion angle can change several degrees when power is applied, so even driveline angles that appear to be correct when stationary might be less than ideal. As a rule of thumb, if you double the operating angle you cut the lifespan of the U-joint in half. The solution to this issue is to switch to a joint with a wider range of motion, a double cardan setup, or ideally reduce the driveline angle.
The other issue is the maximum angle at full droop, where the yoke or U-joint can bind and break. Limiting straps can solve this issue if you catch it before you hit the trail. A single limiting strap in the center of the axle will retain full articulation while restricting the total suspension droop.
Greasable or Not? Which Is Stronger?
Many U-joint applications are available both greasable and nongreasable, and which is stronger has long been debated. Proponents of nongreasable U-joints state that drilling a hole in the cross of the U-joint and tapping it for a grease zerk weakens the body of the joint. The zerk can also be ripped off by obstacles on the trail. On the other side of the argument, if you often wheel in the snow or do frequent water crossings, greasable U-joints allow you to greatly extend the service life of your joints.
In the end this debate seems to come down to how (and if) you are breaking U-joints. If the joint is yielding due to being pushed past its torque rating, a nongreasable U-joint might be a better choice. If you have joints that are burning up needle bearings or running dry or getting contaminated, get greaseable U-joints. Just purchasing greaseable U-joints is not enough though; they should be serviced after every trail ride.