Sep. 08, 2023
Universal Joint: Definition, Working Principle, Applications, Advantages, Disadvantages
What is universal joint?
A universal joint, also known as a U-joint, is a type of mechanical coupling that allows two shafts to be connected and transmit torque while still being able to rotate freely and move in different directions. This is achieved by using two yokes, each attached to one of the shafts, and a cross-shaped connecting piece called a cross-pin that links the two yokes together. The cross-pin is mounted in bearings in the yokes, allowing it to rotate and swivel, allowing the shafts to move and rotate relative to each other.
Universal joints are commonly used in various applications, such as drive shafts for vehicles, machinery for industrial processes, and other mechanical systems where the flexible and versatile coupling is needed. The universal joint diagram is given below.
Working Principle of Universal Joint
The working principle of a universal joint is based on using two yokes, each attached to one of the shafts to be coupled, and a cross-shaped connecting piece called a cross-pin that links the two yokes together. The cross-pin is mounted in bearings in the yokes, allowing it to rotate and swivel, allowing the shafts to move and rotate relative to each other.
When torque is applied to one of the shafts, it is transmitted through the universal joint to the other shaft, causing it to rotate. As the relative orientation of the two shafts changes, the cross-pin can rotate and swivel in the bearings in the yokes, allowing the universal joint to accommodate these changes while transmitting torque and allowing the shafts to rotate. This allows the universal joint to provide a flexible and versatile coupling between the two shafts, allowing them to move and rotate freely in different directions while transmitting torque.
Types of Universal Joints
There are several different types of universal joints, which vary in their design and application. Some common types of universal joints include
Single Cardan joint: This is the most common type of universal joint, and it consists of a single cross-shaped connecting piece that links the two yokes together. This allows the joint to accommodate changes in the relative orientation of the two shafts while transmitting torque and allowing them to rotate.
Double Cardan joint: This type of universal joint consists of two cross-shaped connecting pieces arranged in series, which link the two yokes together. This allows the joint to accommodate even larger changes in the relative orientation of the two shafts, and it is often used in applications where the shafts may be subjected to more extreme movements.
Constant velocity joint: This type of universal joint is designed to maintain a constant rotational velocity between the two shafts, even as their relative orientation changes. This is achieved by using a series of small balls or rollers that are arranged specially, allowing the joint to accommodate changes in orientation while maintaining a constant velocity.
Hooke's joint: This type of universal joint uses a series of curved arms, or levers, to link the two yokes together. This allows the joint to accommodate changes in orientation while transmitting torque and allowing the shafts to rotate.
Cross and bearing joint: This type of universal joint uses a cross-shaped connecting piece and a series of bearings to link the two yokes together. This allows the joint to accommodate changes in orientation while transmitting torque and allowing the shafts to rotate.
Applications of Universal Joints
Universal joints are commonly used in various applications due to their ability to accommodate changes in the relative orientation of the two shafts that are coupled together while transmitting torque and allowing them to rotate. Some common applications of universal joints include
In drive shafts for vehicles, such as cars, trucks, and tractors, the shaft must be able to accommodate changes in orientation as the vehicle moves over uneven terrain.
Machinery for industrial processes, such as conveyor belts, mixers, and other equipment, may require shafts that can move and rotate in different directions.
In power transmission systems, such as those used in wind turbines, the shafts may be subject to large and varying forces, requiring a flexible and versatile coupling.
In robotics, universal joints are often used to connect the different parts of a robot's body, allowing it to move and rotate freely.
In other mechanical systems where flexible and versatile coupling is needed, such as in cranes and other lifting equipment, marine propulsion systems, and various other applications.
Advantages of Universal Joints
Universal joints have several advantages over other mechanical couplings, making them useful in various applications. Some of the main advantages of universal joints include
Flexibility: Universal joints can accommodate changes in the relative orientation of the two coupled shafts, allowing them to move and rotate freely in different directions. This makes them ideal for applications where the shafts may be subject to large and varying forces or need to move and rotate in different directions.
Versatility: Universal joints can transmit torque and allow the shafts to rotate, making them useful for a wide range of applications where power transmission or rotation is needed.
Compactness: Universal joints are relatively small and compact, which makes them easy to incorporate into a wide range of mechanical systems.
Durability: Universal joints are typically designed to be robust and durable, with high-quality bearings and other components that can withstand large forces and long-term use.
Cost-effectiveness: Universal joints are generally less expensive than other types of couplings, which makes them an economical choice for many applications.
Disadvantages of Universal Joints
While universal joints have many advantages, they also have disadvantages. Some of the main disadvantages of universal joints include
Limited range of motion: Universal joints can only accommodate a limited range of changes in the relative orientation of the two coupled shafts. This means they may not be suitable for applications where the shafts may be subject to large or extreme movements.
Loss of power: Due to how universal joints are designed, some power is lost as the torque is transmitted through the joint. This means that the shafts may not rotate as efficiently as they would if coupled using a different type of coupling.
Noise and vibration: Universal joints can sometimes produce noise and vibration as the shafts rotate, which may be undesirable in some applications.
Maintenance: Universal joints require regular maintenance and lubrication to function properly and to maintain their durability and performance. This can add to the overall cost and complexity of using universal joints in some applications.
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Axle & Driveshaft · BlueStar Inspections
Drive shafts and universal joints are present on most rear-wheel drive and four-wheel drive vehicles. A universal joint (U-joint) is a mechanical joint used for connecting shafts that rotate and are aligned at different angles to each other. The U-joint compensates for the misalignment between the shafts in any direction. U-joints can be used when the alignment between the shafts is about 30 degrees or less and ideally as close to alignment as possible. Universal joints are used on the front axle shafts of some vehicles, as well as front and rear drive shafts. They are a connection point allowing the smooth delivery of torque.
Constant velocity (CV) joints are a different kind of connecting joint, similar to U-joints. CV-joints also compensate for steering changes, and are found on front-wheel drive and all-wheel drive vehicles, and on many four-wheel drive vehicles.
The drive shaft and U-joints connect the transmission to the rear drive axle on most rear-wheel drive vehicles. Many four-wheel drive vehicles also use drive shafts with U-joints, with one drive shaft between the transfer case and rear drive axle, and a second drive shaft between the transfer case and the front drive axle. A drive shaft is also known as a propeller shaft. Some four-wheel drive vehicles also use U-joints to connect front axle shafts to the front wheels, and these U-joints allow torque to be provided to the front wheels when they are turning.
Drive shafts and U-joints provide an efficient and durable means of transferring engine torque to the drive axles. U-joints allow drive shafts to move up and down with the suspension while the shaft is rotating so power can be transmitted even when a drive shaft is not at the same angle as its connection point. Some drive shafts also have a slip joint, a yoke that slides onto the transmission output shaft splines or drive shaft splines, allowing the drive shaft to make minor length changes as the vehicle suspension height changes. U-joints and slip joints function to save wear and tear on your vehicle's transmission and gear boxes.U-joints typically look like a + sign, or a cross, and are made of high strength hardened steel. They have four steel bearing cups with tiny needle bearings placed around the wall of the cup. These cups press onto four circular steel bearing surfaces, which the needle bearings contact, and contain special lubricant. There is a seal on each cup to prevent lubricant from escaping. The cups rotate on each bearing surface and allow variable movement based on the angle. The cups are retained into each arm of a yoke, and there are two arms on each yoke. A yoke is present at the transmission, at the gear box, and on the drive shaft. Yokes are also present on axles with a U-joint on the main axle shaft and the shorter stub shaft axle at the wheel.U-joints used on most modern vehicles are lubed for life from the factory and do not require periodic lubrication. Even if a vehicle's U-joints cannot be lubricated, they should be inspected at every oil change. SUVs and pick-up trucks sometimes have lubrication fittings on drive shaft slip joints. These should be lubricated when the vehicle is serviced. Replacement U-joints can also come with lubrication fittings, so it is important to inspect all of a vehicle's U-joints for grease fittings even if grease fittings were not originally equipped on the vehicle.
Symptoms of a bad U-joint include a squeaking sound while moving at slow speeds or when accelerating from a stop, a clunking noise when shifting into any gear, a shuddering or vibrating sensation when accelerating or driving, clicking noise near the front of the vehicle when turning, or evidence of rust coming out of the grease seals. If your vehicle shows any of these symptoms, have it inspected as soon as possible by an ASE certified technician. Neglecting the warning signs of a bad U-joint may cause the drive shaft to separate from the vehicle which may leave you stranded, result in more expensive repairs, and possibly damage the vehicle.
The most common cause of U-joint failure is the lack of or loss of lubrication. If you are considering buying a used vehicle that is equipped with a drive shaft (front and/or rear) with U-joints, have an ASE certified technician inspect the drive shaft for damage, and inspect the U-joints, including front end U-joints present on four-wheel drive vehicles, for looseness, noises, broken grease seals, and damage. The vehicle should also be test driven, and if equipped with four-wheel drive, it should be engaged to listen for any unusual noises.
How It Works: U-Joints
The u-joint plays a critical role in the drivetrain. They are one of those parts that are simple in their task and therefore often taken for granted. Whether in the front axle or on a driveshaft, the u-joint’s capabilities need to be understood.
Doing The Job
U-joints or universal joints join spinning components together while allowing them to move up and down in coordination with the vehicle’s suspension. The cross-shaped units join driveshafts to transmissions/transfer cases, differentials, and other sections of driveshafts. They also connect two-piece front axles.
Types Of U-Joints
There are two types of u-joints; sealed greaseable and non-greaseable. There is a long-standing debate over which style of u-joint is stronger. But independent lab testing done by various entities have proven neither style has a clear, real world advantage. You will see a four-digit number accompanying many u-joint descriptions. The numbers, 1310, 1350, 1410, 1550, and others, refer to the series, or size, of the u-joint which is determined by its dimensions.
Lubrication Importance
Putting all the specs aside, lubrication shortcomings are the most common cause of u-joint failure. Not keeping up with general maintenance in a greaseable unit or contamination of the lubricant in a sealed, non-greaseable joint. Other culprits include plain old wear and improper installation or poor build quality, i.e. the use of poor-quality grease or using too little grease, in a non-greaseable u-joint.
Torque Multiplication
When talking about strength and durability of front axle u-joints one must remember there is a torque multiplication factor at work here. It comes into play when the front driveline experiences angles and at a maximum angle of 40 degrees 30% more torque can be projected onto the u-joint. The multiplication is nominal up to 15 degrees and then the curve progresses from there. This means that a u-joint that is working near its yield point can deform if you’re crawling or bogging and putting a max load on the driveline via angularity. The workaround is to keep your wheels as straight as possible during these high load situations.
Ring Clips FTW
If a u-joint is being pushed to failure there are ways to increase its survivability envelope or at the very least prolong its life momentarily. It’s all about rings and clips. Using full circle snap rings in place of the traditional c-clips will resist distortion of the joint as the yoke ears begin to thrust the caps out of place. The c-clips usually used to secure the caps cannot resist the side forces very well but the full circle clips will do a better job and therefore help delay u-joint failure. The full circle clips must be installed at assembly and OE axles may require some clearancing in the yoke to allow fitment. Many aftermarket axles have grooving large enough to accommodate full circle clips.
Weak Links
There are two primary failure points in conventional OE-spec u-joints – the cross and the caps. The failure scenario plays out at the caps. Typically, in OE-style axles the yokes will be stressed to distortion and as they spread outward the caps are also affected. The caps move and eventually destroy the needle bearings and often become dislodged from the yoke and fall off. If the caps somehow survive the cross itself can suffer the effects of the stress by distortingand failing.
Yukon Super Joints
Yukon Super Joints offer an upgrade in strength and durability over factory u-joints and excel in off-road applications. Designed for Dana 60, Dana 44, Dana 30, and GM 8.5” differentials, Yukon Super Joints feature a 4340 chromoly cross and the caps are made from 4140 steel. The caps are formed from slightly softer steel to isolate the wear to the caps not the cross because it’s a lot more convenient and less expensive to replace the caps. A diamond-like coating offers outstanding wear resistance on the trunnions and corrosive resistance for the entire cross. The end of each trunnion is home to a reservoir for the grease. You’ll notice the trunnion is larger in circumference compared to conventional u-joints. This is the result of eliminating the needle bearings entirely, replacing them with a bushing setup. These joints were never designed for daily drivers. They require fanatical maintenance, greasing each of the eight caps after each long trail run. We do not recommend installing them in a vehicle that does not have locking hubs… this is NOT one of those products you can ‘get by’ with on the street. If they lose their lubrication properties the joints can gall and lock up the vehicle’s steering. They are intended for competition vehicles or dedicated off-roaders that strictly run the trails.
Along with the Super Joint(s) and caps you’ll receive an installation manual, sticker, grease gun, high-quality anti-seize grease, and associated small parts like O-rings, zerk fittings, and a set of full-circle snap ring clips.
Yokes and u-joints are critical links in the drivetrain’s chain of performance. Ensuring your yokes and u-joints are strong enough and properly maintained will keep you and your ride running smoothly for years to come.
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