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

U-joints are available with two hub styles solid and bored. Sound hubs do not have a machined hole. Bored hubs possess 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 inside the bore; and keyed, that have keyways to avoid rotation of the U-joint on the matching shaft.

Using the incorrect lube can lead to burned trunnions.
Unless normally recommended, use a superior quality E.P. (serious pressure) grease to U Joint china provider most vehicular, commercial and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding action between two flanges that happen to be fork-shaped (a yoke) and having a hole (eyes) radially through the attention that is linked by a cross. They let larger angles than adaptable couplings and are being used in applications where large misalignment must be accommodated (1 to 30 degrees).

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

Can be due to operating angles which are too big.
Can be the effect of a bent or perhaps sprung yoke.
Overloading a drive 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 quit rolling, they remain stationary and will “beat themselves” into the area of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Every time the drive shaft tries to shorten, the load will be transmitted in to the bearings and they will indicate the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that are caused by a frozen slide are usually evident on the front and back areas of the cross trunnion.
Improper torque in 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.