HOW THINGS WORK: CV Joints [VIDEO]

In this episode of How Things Work, we’re taking you through the ins and outs of the CV joint. A remarkable mechanical device that has allowed cars and steering to evolve. Watch the video and learn more about this marvel now.

Estimated reading time: 6 minutes, 33 seconds.

If you can’t watch the video, you can read the video transcription below:
Edited for clarity and readability

Welcome to Let’s Talk Automotive. In this episode of How Things Work, we're going to be taking you through a remarkable mechanical device that we find in our vehicles called a CV joint. The CV joint has allowed designers to design amazing products in our vehicles, such as front-wheel drive vehicles, as well as the application of independent rear suspensions on our vehicles.

We're going to take you through the remarkable design of a CV joint, but before we get there, I want to have a little bit of a step back and look at the CV joint’s granddaddy, which is the universal joint. We've often seen the universal joint working in conjunction with a prop shaft on a rear-wheel-drive vehicle, particularly on a bakkie for example, and its job is to transfer torque and to transfer the torque through at least two angles. Now as a UV joint, which is also known as a cardan joint, was in fact discovered some 300 years BC by the Greeks.

However, it was Italian mathematician, Gerolamo Cardano who actually rediscovered it in the 1500s and used it in some industrial applications. Now, as we've said, the UV joint works well on prop shafts, but it does have some shortcomings. The first is, is it has a minimum working angle. And the second is that it suffers from velocity fluctuations.

Now I'm going to explain both of those.

So first of all, the maximum working angle, unfortunately, restricts the UV joint in terms of the higher the speed that the UV joint is operating at the less angle that we can have acting on the UV joint. The velocity fluctuations actually occur four times per rotation of the UV joint. And this is due to the design of the UV joint and the cross and yoke part of the UV joint. And at the end caps of the cross and yoke, you'll find that those end caps during a rotation actually have to travel either a further or shorter distance during each rotation. And this occurs every 90 degrees.

Now we can mitigate that with a process called phasing of the UV joints. So what we do is at each end of the prop shaft, we simply phase the UV joints on each end, out of phase by 90 degrees. And that tends to cancel out each other's vibrations. However, I must say that the middle of the shaft still does suffer from this velocity fluctuation. And we see this emanating in the form of vibrations, particularly at higher rotation speeds on our commercial vehicles. So it really only tends towards getting constant velocity at the end of the day. So not good enough for our front-wheel-drive drive shafts that we're going to be talking about now.

So we don't in fact, achieve true constant velocity with a universal joint. And that is why with the front-wheel-drive vehicle, we use a CV joint and that in fact, should give you a clue as to what CV stands for, which is constant velocity. So constant velocity, or CV, joint truly gives us an accurate transfer of velocity from the output shaft through to the input shaft. And those two velocities never fluctuate. So we don't get additional vibrations from our CV joint, and this is why it was so successful in front-wheel-drive cars.

So it overcomes the typical shortcomings of universal joints and the typical layout that we have with our CV joints on a prop shaft, is inboard and outboard joints. So I have in my hand here, an example of an outboard joint, which is the part that actually butts up and joins with the wheel itself. And as we can see, the way it works is that we have six ball-bearings, we have an inner and outer housing, and those ball-bearings are held in place with a cage. There are also these tracks that you can see here, that allow the ball-bearings to physically move up and down.

And it's those tracks and these two housings that actually allow the draft shaft to now move in two different angles. The first angle that it compensates for is due to the fluctuation in suspension height and the second angle is due to the steering angle that we find on the front wheels. The CV joint can cope with both of those angles simultaneously, and by the way, with a high degree of angle that is applied to it with no fuss whatsoever.

The inboard CV joint is a lot simpler compared to the outer, and in fact, it's called a plunging ball-type because all it's really going to cope with is the fluctuation in suspension height. And at the end of the day, it's just a fancy slip joint. Now, when it comes to the maintenance of CV joints, there's not much you can do to physically maintain a CV joint other than making sure that your car is serviced regularly and that the critical grease that is applied to the CV joint is replaced on time.

Now, there are in fact, two types of grease that we use with our CV joints. So there's a different grease that's used with our inboard CV joint. And that grease is designed to offer thermal protection to the joint because you can imagine there's a lot of heat transfer that occurs from the transmission to the inner CV joint. The outer CV joint, the grease that we use there is really more to promote correct lubrication of this joint so that it offers up less resistance when it is turning, as well as with the angles.

Now, as a matter of wear and tear, CV joints do start to fail. And we know that dreaded CV knocking sound that we hear, particularly when we've got a high steering angle and we’re accelerating on pull-off, we hear that knocking sound now that's caused due to the actual ball bearings, as well as the races in which they move, wearing. And when that happens, we get too much movement and we get a bit of knocking noise. Now it's at this point that you've got to be very, very careful. It's not suggested driving a vehicle with a knocking CV.

And the reason for that is that the CV itself can seize, as you can imagine. And if that occurs, then we really do put ourselves in danger of a couple of things happening. The first is that we can cause serious damage to our transmission itself. If this seizes on the wheel hub, for example, it's going to translate through to the transmission and we can cause damage there. If it shears and the whole draft shaft pulls off, well then we've created a very dangerous missile for other road users, as well as ourselves on the road. So make sure that if your CVs do start knocking that you get them sorted out.

Now, the good news is that although CVs do come as a set, so we have our outboard, our draft shaft and our inboard CV, when they’re new they are pretty much quite expensive. However, if your vehicle is out of warranty, there's absolutely nothing wrong with you getting your CV joints reconditioned and they work just as well as the new ones.

So there you have it, the amazing CV joint that has allowed us to do so much in terms of vehicle design: it's allowed us to have front-wheel drive vehicles, it's allowed us to have rear-wheel drive vehicles with independent suspension, as well as rear-wheel-drive vehicles with rear-wheel drive steering.

So that’s it on today's episode of How Things Work,  and we look forward to you joining us next time.

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