Based on simple principles, universal joints allow power to be transmitted along two unaligned shafts, vastly improving the versatility of a powertrain. In modern vehicles with complex suspension and large ranges of motion, this versatility is a must-have feature, especially for off-road equipment and 4WDs. For all their simplicity, universal joints are a critical power transmission component. To help you understand the role they play in your drivetrain, we are going to explore universal joints in more detail below.
What is a Universal Joint?
A Universal Joint (often referred to as a U-Joint or Uni Joint) is a special type of pivoting joint that allows rotational power to be transmitted between two shafts that are imperfectly aligned. Universal joints are commonly found in rear-wheel drive cars where the driven axle moves independently of the gearbox. The universal joint allows the shaft to move along with the suspension while transmitting power when there isn’t a straight line between the engine and axle.
A universal joint consists of an X-shaped pivot point based on the design principles of gimbals. The universal joint connects two separate shafts and allows them to pivot and move independently, at relative angles of up to 45 degrees. Typical universal joints provide two degrees of freedom, meaning the two shafts are free to rotate along two axes simultaneously (commonly the X and Y axes). In more complex applications, the universal joint may include two separate X-shaped pivots to allow the shafts to operate at much steeper angles, up to 90 degrees.
How Does It Actually Work?
Picture two shafts connected by the cross-shaped joint. When the engine turns one shaft, the cross passes that rotation through to the other. But because of the angle between them, it does so through a pivoting motion rather than a straight push. Each arm of the cross sits inside a bearing, so the whole assembly can flex as the angle changes (as happens constantly when your suspension moves) while still keeping the power flowing.
A single U-joint on its own doesn’t spin both shafts at exactly the same speed throughout each rotation. When the joint is working at an angle, the output shaft actually speeds up and slows down slightly, twice per revolution. You can’t feel it at low angles, but as the angle increases, that variation becomes noticeable as a vibration through the driveshaft.
The solution is to use two U-joints, one at each end of the prop shaft. The second joint introduces the same speed variation in reverse, effectively cancelling the first one out. The result is smooth, consistent rotation by the time it reaches the diff. This is why prop shaft installation isn’t just a matter of bolting things back on. The orientation of the joints relative to each other has to be correct, or you’ll end up with vibration no matter how good the parts are.
Universal Joints vs. CV Joints: What's the Difference?
A question that comes up often, particularly for 4WD owners and fleet managers: should I be looking at universal joints or CV joints for my application?
CV joints solve the velocity fluctuation problem inherent in single U-joints by using a different mechanism, typically a set of precision-ground steel balls running in curved grooves inside a spherical housing. The geometry of the design ensures that power is always transmitted at a perfectly constant velocity, regardless of the operating angle. That’s why CV joints became the standard for front-wheel drive vehicles and the front axles of AWD systems, where smooth power delivery through a steered, articulating wheel is critical.
The short version is if you’re driving a front-wheel drive car, you’ve got CV joints. If you’re running a rear-wheel drive vehicle, 4WD, heavy truck, or industrial equipment, universal joints are almost certainly part of the picture.
Where are Universal Joints Used?
Universal joints predate modern engineering by centuries. For as long as humans have been producing power – such as through wind or water mills – universal joints have been used to transmit power. These days, universal joints are found in some of the world’s most common machinery types, including:
- Vehicles. Universal joints are commonly found in the prop shaft of rear-wheel and four-wheel drive cars. The prop shaft runs from the gearbox at the front of passenger vehicles back to the rear axle. Placing a universal joint at either end of the shaft allows power to be transmitted without bending or snapping the shaft as the rear axle moves on its suspension.
- Heavy Trucks and Road Transport. Long-wheelbase trucks often run two-piece prop shafts with a centre bearing, requiring three or more U-joints to manage the driveline geometry across the full length of the vehicle. These joints operate under high torque loads and high cycle counts, making quality and correct specification critical.
- Marine. Passenger vehicles and trucks aren’t the only vehicles that use prop shafts. Boats and other marine craft commonly use propeller shafts to transfer power from the engine to the propeller where the two components are out of plane.
- Machinery. Mining, manufacturing, agriculture and a wide variety of materials handling technologies all require universal joints to transfer power from their motors to the final consumer. Universal joints designed for machinery and manufacturing applications are typically far more robust than those used in vehicles, and are often expected to stand up to harder use and higher-powered equipment.
- Agriculture and Materials Handling. Tractors, harvesters, and other implements use universal joints extensively in PTO (power take-off) shafts and other drivetrain components. Forklifts, conveyor systems, and other materials handling equipment also rely on U-joints to route power efficiently around the constraints of the machine’s design.
Greaseable vs. Sealed Universal Joints
One practical choice when selecting or replacing a U-joint is whether to go with a greaseable or sealed (non-greaseable) design.
Greaseable U-joints allow fresh lubricant to be pumped into the needle bearings throughout the joint’s service life. They require regular attention but offer a significant advantage in environments where mud, water, sand, or dust can contaminate the joint because regular greasing also flushes out contaminants. For off-road vehicles, mining equipment, and agricultural machinery, a greaseable joint that’s regularly maintained can outlast a sealed unit in the same conditions.
Sealed U-joints come pre-lubricated and are designed to be maintenance-free for their service life. Without hollow grease channels running through the trunnions, they can actually be stronger than equivalent greaseable units. Most modern OEM prop shaft U-joints are sealed. They’re the better choice when regular maintenance access isn’t practical, but they can’t be serviced if contamination occurs. Once they wear, they need to be replaced.
Signs of a Failing Universal Joint
Universal joints are a critical component in the drivetrain of all types of vehicles and other equipment. Over time, the bushes and bearings in the joint will wear out with repeated use, requiring the joint to be replaced.
There are a few common signs that a universal joint is about to fail or needs to be replaced:
- Excess vibrations when accelerating and decelerating
- Clunking noises when changing gears
- Loud banging or clanking during normal use
- Rust staining or visible wear around the bearing caps
A broken universal joint won’t just prevent your vehicle from moving. If it fails while driving, the broken joint can cause serious damage to the prop shaft, underside of your car and other driveline components. It’s important to regularly inspect universal joints as part of your vehicle’s maintenance schedule, especially if the vehicle is regularly used in dusty, sandy or salty environments that can accelerate the joint’s wear and tear.
Need Drivetrain Maintenance and Servicing? Speak to Driveline Services Today!
Universal joints are a critical component in the drivetrains of thousands of vehicles, boats and other equipment. To avoid causing any unnecessary damage, universal joints should regularly be inspected and serviced by an experienced technician. The team at Driveline Services are Australia’s underbody and power transmission specialists!
We work with everything from 4WDs and passenger vehicles to heavy trucks, mining machinery and industrial equipment. We produce and import more than 20,000 high-quality parts, including universal joints that can keep your machinery turning and your fleet on the road. Speak to us today to arrange drivetrain and underbody inspection and servicing by the experienced team at Driveline Services!
