Anatomy Of a U-Joint1. u-joint body (cross)
2. Trunnion (pin)
3. c-clip
4. Bearing cap
5. Needle bearings
To beat an old saying to death, it's the little things that count. Literally thousands of little components, wires, gizmos, and gadgets help power your 4x4, but it's a few of the smaller ones that really count. We know you have a few burning questions about those little cross-shaped joints in your driveshafts and axles, so here's the scoop. a u-joint (universal joint) is basically a flexible pivot point that transmits power through rotational motion between two shafts not in a straight line. The u-joint needs to be flexible to compensate for changes in driveline angle due to the constantly changing terrain under the vehicle. This theory of the u-joint has been around since ancient times, but a real working universal joint wasn't offi cially created until the 1670s by scientist and inventor robert hooke.
If you are new to automotive tech, then we need to tell you that in most cases you will find u-joints in your driveshafts at the output shafts of your transfer case and at the pinion shaft (input) of your front and rear differentials. if you have a solid front axle, then u-joints are likely used to join the inner and outer axleshafts.
Axle U-JointThis is a Spicer-style u-joint out of a Dana 44 axle, the type found on most two-piece front axles. There are also many other styles of axle joints, most of which are outdated for our off-road use.
At the end of each output and input shaft is either a yoke and/or a flange. The yokes that hold the u-joint bearing caps are sometimes referred to as ears. The bearing caps of the u-joint are pressed into the yoke (ear) and held in place with a c-clip (half circle), an internal snap ring, or a full-circle snap ring. vehicles that see heavy off-road use should use an internal or full-circle snap ring. if c-clip-style u-joints are only available for your application you can have them modified by a qualified machinist to accept fullcircle clips. Over time and under harsh conditions c-clips have a tendency to wear into irregular shapes and fall out. When the c-clip falls out eventually the u-joints bearing cap will work its way out of the yoke. if left unattended the trunnion (pin) of the u-joint will damage the shape of the yoke. This damage is very diffi cult to repair and usually requires the purchase of a new inner or outer axleshaft or costly driveshaft repairs.
Standard u-joints aren't designed to run at extreme driveshaft angles unless they are specially constructed. as a rule of thumb, the angle of a driveshaft should not exceed 22 degrees. however, some manufacturers do make quality high-angle driveshafts that operate dependably from 22 to 80 degrees. Extreme-angle driveshafts are achieved by using a double cardan constant velocity joint. This is basically a joint with two u-joints.
Custom U-Joint
This u-joint from Ox is custom manufactured and features a very unique design. The trunnions of the joint press into the body of the joint and are locked in place with circular round keys. Maintenance can be performed through the side door of the u-joint body. | 
Modified U-Joint
This an older version of a Longfield Super axle u-joint that has been heavily modified with a special heat-treating process and accepts full-circle snap rings. Notice each bearing cap has been fitted with grease fittings- remember, a well maintained and greased u-joint lasts longer than a sealed one. | 
Reengineered
This is cTM's highstrength rebuildable u-joint. The joints are rebuildable because the seals, clips, and bushings can be replaced. u-joints like this are very expensive compared to over-thecounter parts-store u-joints but, even under the harshest rockcrawling conditions, should be 10 times stronger than inferior joints if properly maintained. |
U-joint maintenance isn't too much of a factor today for the occasional off-road enthusiast. Most over-the-counter u-joints are sealed and nongreaseable and require little maintenance. keep in mind that sealed, nongreaseable u-joints can be contaminated, which will greatly reduce their lifespan. They aren't the best choice for harsh off- road conditions. greaseable u-joints will last far longer when properly maintained.
Busted Outer (Stub) AxleThis is what happens when a c-clip falls off a front axle u-joint: The trunnion moves around in the axle's yoke, causing irreparable damage like this. This is why you should keep an eye on c-clips or swap your u-joints for ones that have full circle clips.
When performing general maintenance on your vehicle, it's not a bad idea to visually and manually check your u-joints for unusual wear, play, and missing clips. Engage your vehicle's e-brake and chock the tires. Place the transmission in Neutral, crawl underneath and rotate the driveshaft back and forth, stopping quickly. also try to move the driveshaft forward and rearward. if the u-joints are worn you will be able to feel movement in the joint. a bad driveshaft u-joint can also be indicated by a clunk when accelerating or decelerating, especially while backing up. an unusual vibration while driving is also a good indicator that the u-joints should be checked.
Numerous high-strength-alloy u-joints are built for extreme off-road use. Metallurgy has come a long way and some of these aftermarket guys have turned u-joints into an extreme science. Traditionally u-joints are made with needle bearings, which offer extended lifespan under not-so-harsh conditions, but u-joints are also available with bronze alloy bushings, which seem to last longer under extremely harsh conditions.