There are many varieties of U-Joints, a few of which are incredibly complex. The simplest 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 currently have a hole and are called for the hole condition; round, hex, or sq . style. Two bored types that deviate from these common shapes are splined, which have longitudinal grooves in the bore; and keyed, that have keyways to avoid rotation of the U-joint on the matching shaft.
Using the wrong lube can bring about burned trunnions.
Unless in any other case recommended, use a superior quality E.P. (severe pressure) grease to service most vehicular, commercial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply utilizing a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding action between two flanges that will be fork-formed (a yoke) and having a hole (eyes) radially through the eye that is connected by a cross. They let larger angles than versatile couplings and are used in applications where great misalignment needs to be accommodated (1 to 30 degrees).
Always make sure fresh, fresh grease is evident at all four U-joint seals.
Can be due to operating angles which are too large.
Can be caused by a bent or perhaps sprung yoke.
Overloading a travel shaft can cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears are not aligned. If the bearings end rolling, they stay 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. Every time the drive shaft attempts to shorten, the load will be transmitted into the bearings and they will tag the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are caused by a frozen slide are always evident on U Joint leading and back surfaces of the cross trunnion.
Improper torque on U-bolt nuts could 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.