Constant velocity (CV) joints are components that are often used in vehicles that have front wheel drive, though they are also common in rear wheel drive vehicles with independent suspensions. The term "constant velocity" refers to the fact that the rotational speed of these joints is not affected by vertical or horizontal movement, which is not the case with universal joints. This makes CV joints well suited to transferring power from a transaxle to the wheels of a vehicle, since that situation usually involves both vertical movement due to the vehicle suspension and horizontal movement when the wheels are turned. A CV joint axle, which is often known as a half shaft, typically consists of an axle with a constant velocity joint at each end.
Some vehicles still use universal joints in their drive axles, though the CV joint axle is far more common. In vehicles where a transverse engine and transmission are located on the same end as the drive wheels, CV joint axles are typically used. These half shafts are also found in many vehicles that have both independent rear suspension and four wheel drive due to the horizontal movement of the tires in relationship to the rear differential. Universal joints are more commonly found in vehicles that do not have independent rear suspensions and in drive shafts due to the limited range of movement in those situations.
In most cases, a CV joint axle consists of a shaft that is joined to two constant velocity joints via splined couplings. When a CV joint axle is constructed in this manner, the joints will often be held in place with circlips. It is also common for a CV joint axle to connect to a transmission or differential with a splined coupling and circlip as well. Some applications have CV joint axles that connect to the transmission or differential via a female splined coupling instead, in which case a roll pin is often used to hold them in place.
Each CV joint is typically covered by a malleable boot component that keeps dirt and grit out of the delicate inner components. These boots can also keep a lubricating materials inside the joints from escaping. If this CV boot breaks due to age or external damage, it must be replaced before either the lubricating grease escapes or contaminants contact the bearings, or the joint can be ruined. Each CV joint can usually be removed for cleaning and lubrication, but if the bearings are damaged the entire unit usually needs to be replaced.