There are many varieties of U-Joints, a few of which are incredibly complex. The simplest category named Cardan U-Joints, will be either block-and-pin or bearing-and-cross types.

U-joints can be found with two hub variations solid and bored. Sturdy hubs do not have a machined hole. Bored hubs possess a hole and are named for the hole condition; round, hex, or sq . style. Two bored types that deviate from these prevalent shapes are splined, which have longitudinal grooves inside bore; and keyed, which have keyways to prevent rotation of the U-joint on the matching shaft.

Using the incorrect lube can bring about burned trunnions.
Unless otherwise recommended, use a superior quality E.P. (intense pressure) grease to support most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by utilizing a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding action between two flanges that are fork-formed (a yoke) and having a hole (eye) radially through the attention that is linked by a cross. They allow larger angles than versatile couplings and are being used in applications where high misalignment should be accommodated (1 to 30 degrees).

Always make sure new, fresh grease is evident by all U-joint seals.

Can be caused by operating angles which are too big.
Can be caused by a bent or perhaps sprung yoke.
Overloading a travel shaft could cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings quit rolling, they continue to be stationary and can “beat themselves” in to the surface of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Each time the drive shaft attempts to shorten, the load will be transmitted in to the bearings and they’ll tag the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that will be caused by a frozen slide are usually evident on the front and back surfaces of the cross trunnion.
Improper torque in U-bolt nuts can cause brinelling.
Most manufacturers U Joint publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging is not accomplished, can cause a number of bearings to be starved for grease.