Universal joints allow travel shafts to move up and down with the suspension as the shaft is moving so power can be transmitted when the drive shaft isn’t in a directly line between your transmission and travel wheels.
Rear-wheel-drive vehicles include universal joints (or U-joints) at both ends of the drive shaft. U-joints hook up to yokes that also allow travel shafts to go fore and aft as automobiles review bumps or dips in the road, which effectively shortens or lengthens the shaft.
Front-drive vehicles also use two joints, called continuous velocity (or CV) joints, but they are a diverse kind that also compensate for steering improvements.
On rear-drive vehicles, one indication of a worn U-join is a “clank” sound whenever a drive gear is involved. On front-drive cars, CV joints generally make a clicking noises when they’re worn. CV joints are covered by protective rubber footwear, and if the boots crack or are normally damaged, the CV joints will lose their lubrication and become ruined by dirt and dampness.
A U-joint is situated in both front wheel drive and rear wheel drive cars. Although they are different in design, they possess the same purpose of giving the drive coach some flexibility. That is required as all cars and trucks flex while in motion.
U-joints are found on each of the ends of the rear drive shaft, whereas CV-joints are located on front wheel drive automobiles. Each allows the travel shaft to rotate as the differential moves in relation to the rest of drive train mounted on the chassis.
The U-joint functions to save lots of wear and tear on your own vehicle’s transmission. Failing to have a universal joint replacement done when U Joint necessary can bring about substantial harm to your car in the future.
There are a 
few warning signs that U-joint or CV-joint is failing. They include: