There are many types of U-Joints, some of which are extremely complex. The easiest category known as Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.
U-joints can be found with two hub variations solid and bored. Sturdy hubs don’t have a machined hole. Bored hubs own a hole and so are called for the hole shape; round, hex, or square style. Two bored models that deviate from these common shapes are splined, which have longitudinal grooves in the bore; and keyed, which have keyways to avoid rotation of the U-joint on the matching shaft.
Using the incorrect lube can bring about burned trunnions.
Unless or else recommended, use a high quality E.P. (excessive pressure) grease to support most vehicular, commercial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement simply by by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding motion between two flanges that are fork-shaped (a yoke) 
and having a hole (eyesight) radially through the attention that is linked by a cross. They enable larger angles than adaptable couplings and are being used in applications where high misalignment needs to be accommodated (1 to 30 degrees).
Always make sure fresh, fresh grease is evident for all U-joint seals.
Can be due to operating angles which are too big.
Can be the effect of a bent or perhaps sprung yoke.
Overloading a drive shaft could cause yoke ears to bend. Bearings will not 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 area of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Each and every time the drive shaft attempts to shorten, the strain will be transmitted into the bearings and they’ll indicate the cross trunnion. Unlike U Joint brinnell marks caused by torque, brinnell marks that are the effect of a frozen slide are often evident on the front and back floors of the cross trunnion.
Improper torque upon U-bolt nuts can cause brinelling.
Most makers 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.