Understanding Universal Joint

Universal Joints are commonly used to transfer mechanical power between two shafts when their axes are at an angle to each other

their invention dates back many centuries

Even though the universal joints mechanism seems simple the physics behind this mechanism are rather complicated and interesting

The universal Joint has three basic parts, two yolks and a cross

Now let's consider different power transmitting scenarios

In the first case the input and output shaft are connected in a straight line in this case 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

Now let's see what happens if the axes are at an angle

Assume that the input shaft is moving at a constant speed

Here 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

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

To make the cross spin concept clearer let's have a look at this hypothetical animation where the spin of the green axis is halted

It is clear [that] without this spin the motion of the inclined hook Joint is impossible

the Spin of the cross makes a huge difference in the speed of the output shaft

It is clear that when the cross is spinning as well as rotating the velocity of the output shaft will have an added effect

For the first 90 degrees of the input Shaft rotation the green axis spins to its maximum angle

the forward Spin Aids and changes the output Shaft rotation as shown

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

Just by taking a simple time differential of this displacement graph we can find out speed of the output shaft

It is clear that the output shaft is a fluctuating speed as shown

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

We hope you have now gained a good understanding of the workings of universal joints. Thank you