How Constant Velocity (CV) Joints Work

How Constant Velocity (CV) Joints Work


 

If you have ever looked under a Jeep vehicle you have seen the drive shafts and the cross like devices that connect them to the axles or the transfer cases.  These cross like objects are the universal joints, or commonly referred to as simple i-joints.  This style of connection works for many applications as they are simple and allow for the drive shaft to operate at different angles than the part they are turning, or from what is turning them. However, they do have their shortcomings when it comes to operating angles and their ability to do their job quietly.  Ordinary Cardan type universal joints cause a change in speed between the driving and driven shafts whenever the joint operates at an angle. As the operating angle of the joint increases, the speed of the driven shaft varies more and more during each revolution of the shafts. If you were to draw this out it would look as if the joints were moving on two different oval paths rather than round paths that would be expected.  The greater the operating angle, the greater the variation in speed of the driven shaft and the greater the vibration it produces.  In Jeeps the use of a Double Cardian style drive shaft often corrects
the issue of driveline vibrations but that is by only halving the angles at one end of the shaft.  In new models where vibration must be eliminated for a smoother ride and more comfortable driving experience other styles of true constant velocity joints are being used.  This is explained in greater detail in the Driveline Geometry 101 article.

To get a shaft to transmit power without providing vibration, and to function at a high angle and provide the same speed on the driven and driving shafts a true Constant Velocity joint would have to be used.  These are used in many applications such as in front or rear independent suspensions or even some solid axle applications with full time four-wheel drive applications.  The CV's can operate at greater angles for greater time without any issues, however normally they have to be submerged in a type of special high temperature grease and surrounded by a special boot.  Many CV's fail due to contamination from water and other impurities due to damaged boots. 

When a CV is used in a drive shaft application they are often sealed inside of a metal casing in applications where abuse may be seen.  In newer vehicle the use of drive shaft CV's are becoming more common as customer expectations are growing and their tolerance for vibrations and driving distractions is shrinking.  A true CV is a ball and cub design. The most common type of outboard CV joint is the "Rzeppa" style. A Dana engineer named Alfred H. Rzeppa invented this type of joint in 1920. His design allowed power to be transmitted through six spherical balls located between an inner and outer race. In this design, the balls are held in position by small windows in a cage assembly that fits between the inner and outer races. The design of the joint is such that the position of the balls always bisects (cuts in half) the operating angle of the joint. The design works like a bevel gear; but instead of gear teeth transmitting the torque across the joint, the balls push against their respective tracks in the inner and outer housings. A downside to this is though that if the joint is overstressed the balls can be pushed
out of their tracks and stretch and crack the housing causing a failure.  This design is very
common and used in many different variants such as Birfield and Marfield joints in many import 4x4 vehicles.  There are other designs that involve different configurations of cages and balls but all are effectively the same.