A universal joint (U joint), is a coupling that connects two rigid rotating shafts in order to allow those shafts to continue to rotate at an angle. A perfect coupling would be able to spin the shafts at exactly the same speed as the angle increased, but this is not the case with a U joint. As the angle increases so does a slightly different rate of spin and vibration occurs.
Automobile U Joints
When you think of a U joint you likely think of the ones installed in the driveshaft of your family car. Your driveshaft does not connect from the transmission to the axle bell housing in a direct straight line, therefore U joints are installed on each end to allow for the difference in angle. Most U joints have a grease fitting as their spinning creates friction and heat.
When U joints begin to wear, one of the first signs a driver might notice is a strange vibration. A joint that has developed excess play can rattle at slow speeds and vibrate at highway speeds. The vibration at first might be slight and difficult to pinpoint, but it will become worse, usually within a few hundred miles.
If you suspect your U joint might be going bad, find an empty car park and turn the wheel all the way to the right or the left and slowly drive in a circle. Most of the time, if your U joint has worn to the point of being loose you might be able to hear a distinct clicking noise with each rotating of the driveshaft.
Rotate the Driveshaft by Hand
If you have steel ramps commonly used to drive onto to change your oil you can use them to elevate your car to visually inspect your driveshaft. With your car safely on the ramps, slide under your car on your back and take a firm grasp on your driveshaft with both hands and twist it one way then back while watching the U joints. They should rotate with the driveshaft with no excess play or sloppiness.
Play it Safe
If you are unsure about the condition of your U joints, take your car to a mechanic for inspection. If a worn U joint catastrophically fails at highway speeds, the driveshaft can rotate violently out of control and cause major damage to the under body of your vehicle. If the front U joint snaps, the one directly behind the transmission, it is possible for the driveshaft to penetrate the roadway or into a pot hole and flip your car end over end as proven in the television show called "Myth Busters." If you suspect your U joints are going bad, play it safe and have them inspected right away.