There are many types 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 Joint china U-Joints are available with two hub models solid and bored. Sound hubs don’t have a machined hole. Bored hubs possess a hole and are known as for the hole condition; round, hex, or square style. Two bored models that deviate from these prevalent shapes are splined, which have longitudinal grooves inside bore; and keyed, which have keyways to prevent rotation of the U-joint on the matching shaft.

Using the incorrect lube can cause burned trunnions.
Unless normally recommended, use a superior quality E.P. (extreme pressure) grease to program most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by by using a telescoping shaft (square shafting or splines). U-Joints function by a sliding movement between two flanges that happen to be fork-shaped (a yoke) and having a hole (eyesight) radially through the eye that is connected by a cross. They let larger angles than adaptable couplings and are being used in applications where excessive misalignment should be accommodated (1 to 30 degrees).

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

Can be caused by 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 won’t roll in the bearing cap if the yoke ears are not aligned. If the bearings quit rolling, they remain stationary and can “beat themselves” in to the area of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Each time the travel shaft tries to shorten, the load will be transmitted into the bearings and they’ll indicate the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that will be caused by a frozen slide are always evident on leading and back floors 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 isn’t accomplished, can cause a number of bearings to be starved for grease.