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

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

Using the incorrect lube can result in burned trunnions.
Unless in any other case recommended, use a superior quality E.P. (serious pressure) grease to provider most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply by using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding motion between two flanges that happen to be fork-formed (a yoke) and U Joint having a hole (eyes) radially through the eye 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 fresh, fresh grease is evident at all U-joint seals.

Can be caused by operating angles which are too big.
Can be caused by 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 aren’t aligned. If the bearings prevent 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 travel shaft attempts to shorten, the load will be transmitted in to the bearings and they will mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that are caused by a frozen slip are often evident on the front and back surfaces of the cross trunnion.
Improper torque on U-bolt nuts can cause brinelling.
Most producers 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.