There are many types of U-Joints, some of which are incredibly complex. The simplest category called Cardan U-Joints, are either block-and-pin or bearing-and-cross types.

U-joints can be found with two hub models solid and bored. Solid hubs don’t have a machined hole. Bored hubs have got a hole and are called for the hole condition; round, hex, or square style. Two bored designs that deviate from these common shapes are splined, that have longitudinal grooves within the bore; and keyed, that have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can cause burned trunnions.
Unless otherwise recommended, use a superior quality E.P. (severe pressure) grease to service most vehicular, industrial and auxiliary travel shaft applications.
Mechanically flexible U-Joints accommodate end movement by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding motion between two flanges that happen to be fork-designed (a yoke) and having a hole (eyes) radially through the attention that is connected by a cross. They enable larger angles than versatile couplings and are being used in applications where substantial U Joint misalignment has to 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 the effect of a bent or perhaps sprung yoke.
Overloading a travel 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 remain stationary and will “beat themselves” into the surface area of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Each and every time the drive shaft tries 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 the effect of a frozen slide are always evident on the front and back floors of the cross trunnion.
Improper torque upon U-bolt nuts can cause brinelling.
Most suppliers 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.