There are many varieties of U-Joints, some of which are incredibly complex. The easiest category known as Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.

U Joint U-Joints are available with two hub designs solid and bored. Stable hubs do not have a machined hole. Bored hubs have got a hole and so are called for the hole shape; round, hex, or square style. Two bored designs 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 result in burned trunnions.
Unless otherwise recommended, use a superior quality E.P. (serious pressure) grease to service 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 movement between two flanges that will be fork-shaped (a yoke) and having a hole (eye) radially through the attention that is connected by a cross. They let larger angles than versatile couplings and are being used in applications where huge misalignment should be accommodated (1 to 30 degrees).

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

Can be due to 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 quit rolling, they stay 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 and every time the travel shaft tries to shorten, the load will be transmitted in to the bearings and they’ll mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be the effect of a frozen slide are constantly evident on the front and back surfaces of the cross trunnion.
Improper torque on U-bolt nuts can cause brinelling.
Most manufacturers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging is not accomplished, can cause one or more bearings to be starved for grease.