What is a CV joint or constant velocity joint?

There are several types but a constant velocity joint or CV joint consists of ball bearings or rollers with needle bearings that allow for near constant velocity throughout the parts allowed range of motion.

 Who needs CV joints? 

You would be surprised.  If you drive something made in the last 20 years odds are there are CV joints that drive your vehicle.  But that is only the most common use of CV joints.  There may be some in the steering that actually turn the steering column in your vehicle.  Additionally there are CV joints in equipment and manufacturing.  There are many factories that use CV joints running their manufacturing equipment.  This brings another question WHY?  If you think about your car when you are driving the CV joints deliver the engines power to the road smoothly whether you are driving straight or turning.  Also the modern AWD car and 4X4 truck uses these in the front axle to smoothly put power to the road.  CV joints do the same thing as universal joints but deliver power in a more even and efficient way.  In doing so CV joints eliminate the inherent flaw in universal joints of changing acceleration.  This is the CV joints best feature.  Since a universal or u-joint accelerates and decelerates twice per revolution with the speed change increasing with angle the universal joint is limited to use in application where uniform and even power delivery is not critical.  The constant velocity of a CV joint is what makes for efficient automobile drivetrains.  Taking this to a larger scale CV joints in the industrial realm make for more efficient shafting in stationary equipment.  In mechanical drives CV shafting connects drive and driven ends, performing multitudes of tasks with improved performance over universal joint shafting in many instances. 

1. CV joints are more efficient in the terms of even power delivery
2. less associated maintenance
3. in many cases need less power to operate the shafting. 

There are CV joints for high speed use as well.  These are used in engine dynamometers and as driveshafts in rolling stock applications. 

Limitations:  CV jointed shafts are not perfect, there are limitations:

1. Allowable angle decreases as the RPM increases.
2. Friction generated heat increases as angle increases. 

The CV shaft is a behind the scene piece of manufacturing and mobility, but it is used in manufacturing many things you use everyday....from rolling the toilet paper you buy to making the box in which your last package was delivered...from moving a hybrid electric car to moving a military vehicle...from testing the engine on some Sunday racing stock car to testing the engine on a open wheeled racer...from grinding ore for the aluminum industry to pumping gas in the oil industry...from tube mills to paper mills....from labeling machines to canning machines... from one inch 25mm CV joints to ten inch 254mm or larger.

CV shaft with center slip spline for paper product production.  This shaft is a replacement for the original.