How does a universal joint work?

The invention of universal joints dates back many centuries. Even though the universal joint’s mechanism seems simple, the physics behind this mechanism are rather complicated and interesting. In most of the literature, readers who are trying to understand the physics behind universal joints are bombarded with complex mathematical relationships. In this article we will understand the working of universal joints in a simple yet logical manner.

Why universal joints are used?

Universal joints also known as Hooke’s joints are commonly used to transfer mechanical power between 2 shafts when their axes are at an angle to each other. The way power is transmitted in a rear wheel drive, vehicle might have got your attention. They use universal joints. As you can see in the Fig 1, two universal joints are required to transmit power from the engine to differential.

Fig 1 : Use of universal joint

The basic parts

The universal joint has 3 basic parts, two yokes and a cross(refer Fig 2). The yokes are connected through a cross as shown in below image. With this arrangement, the output shaft can be turned to a wide range of angles. Now, let’s consider different power transmitting scenarios.

Fig 2 : The basic parts of universal joint

The shafts connected straight

In this case, the input and output shaft are connected in a straight line(refer Fig 3). Motion is really simple. The input shaft will turn the cross, and the cross will turn the output shaft. It is clear that both the input and output shafts will turn at the same speed.

Fig 3 : Universal joint under a straight motion

The shafts are at an angle

Now, let’s see what happens if the axes are at an angle. Assume that the input shaft is moving at a constant speed.

The spin of the cross

When the shafts are at an angle the motion is quite different. To understand why, just note the behavior of the red and green ends of the cross. You can see that the green ends, which are connected to the input shaft, turn along a vertical plane, while the red ends, which are connected to the output shaft, have to move along a different plane(refer Fig 4). To make the red ends move along the inclined plane, the cross has to spin along the axis connecting the green ends. If you observe the mark on the cross, you can see this phenomenon in Fig 5. To make the cross spin concept clearer, just imagine what happens when the spin of the green axis is halted. Such a hypothetical motion is depicted in the Fig 5.

Fig 4 : The shaft connecting the green ends should rotate

Fig 5 : hypothetical movement to illustrate the need of cross spin

It is clear that, such a situation is impossible. This means without this spin, the motion of the inclined hook joint is impossible.

The effect of cross spin

The spin of the cross makes a huge difference in the speed of the output shaft. The cross has 2 kinds of motion : rotation and spin. It is clear that, when the cross is spinning as well as rotating, the velocity of the output shaft will have an added effect. You can see here that, for the first 90 degrees of the input shaft rotation, the green axis spins to its maximum angle(refer Fig 6). The forward spin aids and changes the output shaft rotation as shown in the graph. The output angle will get an added effect during this period (refer Fig 7).

Fig 6 : Forward spin of the shaft during the first 90 degrees

Fig 7 : Angular displacement of the output shaft during the first 90 degrees

But for the next 90 degrees, it should spin back to the initial zero position. The reverse spin will have an opposite effect on the output shaft rotation. So the motion of the output shaft will look as shown in the Fig 8. Just by taking a simple time differential of this displacement graph we can find out speed of the output shaft. This is shown in the following graph of Fig 9.

Fig 8 : Angular displacement of the output shaft during 180 degrees turn

Fig 9 : Variation of output shaft speed with angle

It is clear that, the output shaft has a fluctuating speed. More the angle between the shafts more will be the speed fluctuation.

Double universal joint

This means, the universal joint is not a constant velocity joint. This jerky rotation makes the universal joint useless in its original form. But you can make it a constant velocity joint by incorporating one more joint, as shown. If a constant velocity input gives fluctuating output, a fluctuating input will give a constant velocity output. Thus, the double universal joint acts as a constant velocity joint. You can see a similar arrangement in rear wheel driven automobiles (refer Fig 10). You can see that the drive shaft is fitted between two universal joints. So the speed output at the second universal joint will be same as the input.

Fig 10 : Use of double university joint in eliminating jerky motion

Constant velocity joints

The double universal joint described in the constant velocity joint. However more efficient constant velocity joint designs, they do not require an intermediate shaft have evolved over time. Some of the popular constant velocity joints are listed below.

Tracta joints

Rzeppa joints

Weiss joints

Tripod joints

Coupling

That’s all in this article. I hope you have learned how universal joints works.

Thanks for reading!

Dive into anything

The way I modeled it is that the torque of the input shaft is a vector, and then the torque on the output shaft is just the input vector projected in the direction of the output shaft. This would mean that torque output is input*cos(theta), where theta is the angle between the two shafts, and then for a double U-Joint it would be input*cos(theta)^2. Is that not how U-Joints work? I can’t seem to find anything online that mentions whether or not torque is preserved through a U-Joint.

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 distorting and 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.

How does a universal joint work?