There are many types of U-Joints, a few of which are extremely complex. The simplest category named Cardan U-Joints, happen to be either block-and-pin or bearing-and-cross types.

U-joints can be found with two hub designs solid and bored. Sound hubs don’t have a machined hole. Bored hubs include a hole and so are called for the hole form; round, hex, or square style. Two bored types that deviate from these prevalent shapes are splined, that have longitudinal grooves inside the bore; and keyed, which have keyways to prevent rotation of the U-joint on the matching shaft.

Using the wrong lube can lead to burned trunnions.
Unless normally recommended, use a high quality E.P. (severe pressure) grease to provider most vehicular, industrial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply using a telescoping shaft (square shafting or splines). U-Joints function by a sliding movement between two flanges that will be fork-formed (a yoke) and having a hole (vision) radially through the attention that is connected by a cross. They let larger angles than adaptable couplings and are used in applications where substantial misalignment should be accommodated (1 to 30 degrees).

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

Can be U Joint china caused by operating angles which are too large.
Can be caused by a bent or sprung yoke.
Overloading a travel shaft could cause yoke ears to bend. Bearings won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings stop rolling, they remain stationary and will “beat themselves” into the surface of the cross.
A “frozen” slip assembly will not allow the travel shaft to lengthen or shorten. Each time the drive shaft attempts to shorten, the strain will be transmitted into the bearings and they’ll tag the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that happen to be caused by a frozen slide are at all times evident on the front and back floors of the cross trunnion.
Improper torque on U-bolt nuts can cause brinelling.
Most companies 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.