What is constant velocity joint | Importance Of CV Joints

Constant-velocity (CV) joints allow the drive shaft to transmit power through a variable angle, at a constant rotational speed, without a considerable increase in friction. The constant-velocity joints are also responsible for a smooth transfer of engine power regardless of the turning angle of the steering wheel. The constant-velocity joints are mainly used in front-wheel and all-wheel drive cars. Rear-wheel drive cars with independent rear suspensions typically use constant-velocity joints at the ends of the rear-axle half-shafts. Constant-velocity joints are protected by a rubber boot known as the CV gaiter. Each constant-velocity joint includes an inner and outer joint.

How does a constant velocity joint work?

Constant velocity, or CV, joints are used to connect your vehicle’s transmission to your wheels. constant-velocity joints are part of the driveshaft and are used primarily on front-wheel drive vehicles, but they are also used in rear- and four-wheel drive vehicles. Each drive wheel has two constant-velocity joints: the inner joint connects the transmission to the axle, and the outer joint connects the axle to the wheel. The Constant-velocity joint’s name comes from its ability to move with your vehicle’s suspension in any direction (if your vehicle hits a pothole or an uneven surface) and still be able to keep the drive wheels moving at a constant velocity. The Constant-velocity joint is a very special joint: it connects two different rotating shafts. These two rotating shafts are stuck in a fixed position; so, the Constant-velocity joint must be able to move and bend to keep these two shafts connected. Constant-velocity joints are being used in newer vehicles and are taking the place of the old “U” joints. The Constant-velocity joint is able to transmit even levels of torque to the wheels continually no matter what angle it’ in. This means that no matter how many potholes you hit, or if the vehicle is turning, the Constant-velocity joint will keep the drive wheels moving at a constant velocity.

What does a constant velocity joint do？

The Constant-velocity joints are the primary components that allow drive shafts to do their job. They work in harmony to:

Improve the range of motion of your car wheels.

Transfer rotational power from the transmission to the car wheels efficiently.

To turn the car wheels at a constant speed regardless of operating angle.

1. Improve the range of motion of your car wheels.

Imagine having a rigid shaft connected to your transmission and wheels. Your wheels will move up and down together with your car suspension. If you’re going through a pothole, one of two things can happen:

If the rigid shaft is strong enough, your entire car will shift upward as one object, causing your ride to be very uncomfortable. The rigid shafts will break from the weight of your car, disconnecting the wheels from your car and you might just see your car wheels rolling away from you! We know that outer Constant-velocity joint can twists. This gives it that much larger range of motion to work with when going through potholes.

2. Transfer rotational power from the transmission to the car wheels efficiently. Ball bearings are found within the joints. Due to the round shape, ball bearings have low contact area and therefore minimal friction during motion. This allows them to roll and transfer the power without losing too much to the heat.

3. To turn the car wheels at a constant speed regardless of operating angle. The input shaft and output shaft will always rotate at the same speed regardless of the operating angle. This creates a smooth transfer of power from the input shaft to the output shaft without causing any speed fluctuations.

Types of Constant-Velocity Joints

There are many variants and types of Constant-velocity joints. Here are some of them:

1.Coupling Type: Thompson Coupling

The Thompson constant velocity joint (TCVJ), also known as a Thompson coupling, assembles two cardan joints within each other to eliminate the intermediate shaft. A control yoke is added to keep the input and output shafts aligned. The control yoke uses a spherical pantograph scissor mechanism to bisect the angle between the input and output shafts and to maintain the joints at a relative phase angle of zero. The alignment ensures constant angular velocity at all joint angles.

Eliminating the intermediate shaft and keeping the input shafts aligned in the homokinetic plane greatly reduces the induced shear stresses and vibration inherent in double cardan shafts.While the geometric configuration does not maintain constant velocity for the control yoke that aligns the cardan joints, the control yoke has minimal inertia and generates little vibration. Continuous use of a standard Thompson coupling at a straight-through, zero-degree angle will cause excessive wear and damage to the joint; a minimum offset of 2 degrees between the input and output shafts is needed to reduce control yoke wear.Modifying the input and output yokes so that they are not precisely normal to their respective shafts can alter or eliminate the "disallowed" angles.

The cardan joints within the Thompson Coupling can use roller bearings for all bearing surfaces; this reduces friction and wear when compared to the sliding joints used in Rzeppa-type Constant-velocity joints. The novel feature of the coupling is the method for geometrically constraining the pair of cardan joints within the assembly by using, for example, a spherical four bar scissors linkage (spherical pantograph) and it is the first coupling to have this combination of properties.

2. Sliding Type: Tripod Joint

These joints are used at the inboard end of car driveshafts. The joints were developed by Michel Orain, of Glaenzer Spicer of Poissy, France. This joint has a three-pointed yoke attached to the shaft, which has barrel-shaped roller bearings on the ends. These fit into a cup with three matching grooves, attached to the differential. Since there is only significant movement in one axis, this simple arrangement works well. These also allow an axial 'plunge' movement of the shaft, so that engine rocking and other effects do not preload the bearings.

A typical Tripod joint has up to 50 mm of plunge travel, and 26 degrees of angular articulation. The tripod joint does not have as much angular range as many of the other joint types, but tends to be lower in cost and more efficient. Due to this it is typically used in rear wheel drive vehicle configurations or on the inboard side of front wheel drive vehicles where the required range of motion is lower.

3.Fixed Cup Type: Rzeppa Joint

A Rzeppa joint consists of a spherical inner shell with 6 grooves in it and a similar enveloping outer shell. Each groove guides one ball. The input shaft fits in the centre of a large, steel, star-shaped "gear" that nests inside a circular cage. The cage is spherical but with ends open, and it typically has six openings around the perimeter. This cage and gear fit into a grooved cup that has a splined and threaded shaft attached to it. Six large steel balls sit inside the cup grooves and fit into the cage openings, nestled in the grooves of the star gear.

The output shaft on the cup then runs through the wheel bearing and is secured by the axle nut. This joint can accommodate the large changes of angle when the front wheels are turned by the steering system; typical Rzeppa joints allow 45°–48° of articulation, while some can give 54°. At the "outboard" end of the driveshaft a slightly different unit is used. The end of the driveshaft is splined and fits into the outer "joint". It is typically held in place by a circlip.

How Do Check Constant Velocity Joint?

Your constant velocity joint boots stop dirt and water from getting into your constant velocity joints, and keep the lubricating grease from getting out, but they can and do fail. So how do you check if they’re in good health?

Under each CV boot is, one at the transaxle end and one at the wheel, is a constant velocity joint. Without the boot, these grease-laden joints would become thick with dirt and filth and lose their ability to flex and rotate. If you hear a clicking noise that is especially pronounced when tuning a corner, chances are a Constant-velocity joint has already gone bad. In extreme cases, if left to its own devices, the Constant-velocity joint can fall apart scattering ball bearings and pieces all over the road.

1.You’re not going to get decent access to the Constant-velocity joints while the car is on the ground, so get it jacked up and supported on jack stands. Once up, take the front wheels off, this should give you decent access to the drive axles and Constant-velocity joints.

2.The inner fender and front suspension are probably going to be thick with road dirt and filth. You can’t properly inspect anything in that, so give it a wash. Don't worry too much about getting the brakes wet, but try not to get soap on the discs.

3.For the outer Constant-velocity joint boots, turn the steering wheel full lock. This will give you access to get your head behind the wheel hub where you’ll be able to better see the Constant-velocity joint.

4.The Constant-velocity joint boots are held in place by circular clips, either metal or plastic. Check these first. If they’re present, correct and tight that’s good. If they’re missing or just a zip tie, you may well have dirt within the CV boot, or a lack of grease. Replace a missing tie asap, but not before pulling the Constant-velocity joint back from the wide end to check that there’s nothing in there but grease.

5.At the inner, transaxle end, check for any signs of grease leaking out. Again, check the ties and that they’re holding the CV boot tight and in place.

6. The CV boots are rubber, and rubber should be pliable. If they are dry and inflexible, they don't have many miles left on them. Get in there and pull them, twist them, push the folds apart and look into the crevices. Look for any signs of cracking or cracked rubber, look for splits and cracks. Often times you won’t see these with just a casual glance - you need to get your hands on the boots.

7. Put the transmission in neutral and spin the brake disc. Often times you will be able to hear a Constant-velocity joint that is starting to go bad if you listen closely. Between engine and road noise the quiet clicking of a joint that has stated to fail is easy to miss when driving, but much more obvious from under the car with the wheel off.

8.Clean, road test, repeat - Finally, if you can’t see any cracks or leaks, but you want to be sure, give everything a good clean and then snap a picture with your phone. Do a test drive, making sure to stay on clean roads and parking lots, and then have another look. On the test drive, be sure to do lots of tight turns to really flex the Constant-velocity joints. If there are any grease spots when you look again, you know your CV boots are weeping.