There are many types of U-Joints, a few of which are very complex. The easiest category named Cardan U-Joints, are either block-and-pin or bearing-and-cross types.

U-joints are available with two hub types solid and bored. Sturdy hubs do not have a machined hole. Bored hubs own a hole and are called for the hole shape; round, hex, or square style. Two bored types that deviate from these prevalent shapes are splined, which have longitudinal grooves in the bore; and keyed, that have keyways to U Joint prevent rotation of the U-joint on the matching shaft.

Using the incorrect lube can cause burned trunnions.
Unless or else recommended, use a high quality E.P. (serious pressure) grease to program most vehicular, commercial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-designed (a yoke) and having a hole (eyesight) radially through the attention that is connected by a cross. They allow larger angles than versatile couplings and are used in applications where large misalignment must be accommodated (1 to 30 degrees).

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

Can be caused by operating angles which are too large.
Can be caused by a bent or sprung yoke.
Overloading a drive shaft can cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings prevent rolling, they continue to be stationary and can “beat themselves” into the surface area of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Each time the drive shaft attempts to shorten, the strain will be transmitted into the bearings and they will tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that are the effect of a frozen slide are always evident on the front and back surfaces of the cross trunnion.
Improper torque in U-bolt nuts can cause brinelling.
Most makers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging isn’t accomplished, can cause one or more bearings to be starved for grease.