Universal joints

Universal joints, often referred to as U-joints or Cardan joints, are critical components in mechanical power transmission systems that allow for the efficient transfer of torque and rotational motion between shafts at varying angles. In the context of a transmission or drivetrain, their primary role is to accommodate the relative movement between the engine/transmission and the drive wheels, which is caused by suspension travel, vehicle body flexing, and misalignment.

Function in a Transmission System
In vehicles with conventional rear-wheel or four-wheel drive layouts, universal joints are key elements of the propeller shaft (driveshaft).
Accommodating Movement: They connect the transmission output shaft to the differential input shaft and the differential to the drive axles, allowing for continuous power flow despite the vertical motion of the suspension system and the change in operating angles.
Power Transmission at an Angle: U-joints ensure that the rotational force generated by the engine can be delivered to the wheels even when the components are not in perfect alignment.
Vibration Management: While a single U-joint inherently creates speed fluctuations (Cardan effect), modern drivetrains typically use a pair of properly phased U-joints (a double cardan joint or two single joints with a driveshaft in between) to cancel out these fluctuations, resulting in a smooth, constant velocity output to the wheels.
Design and Components
A standard universal joint assembly consists of several robust parts designed to handle high torque and demanding conditions:
Yokes: U-shaped components attached to the ends of the driving and driven shafts.
Spider (Cross): A cross-shaped intermediate piece with four arms (trunnions) that connect the yokes.
Bearing Cups: These fit over the trunnions and house needle bearings to reduce friction and allow smooth pivoting within the yokes.
Needle Bearings: High-quality bearings are essential for smooth operation and durability under load.
Seals: Protect the internal components (bearings and lubricant) from dirt, moisture, and debris.
Common Applications
In transmission and drivetrain systems, universal joints are indispensable components:
Propeller (Drive) Shafts: Found in most rear-wheel drive, four-wheel drive, and all-wheel drive vehicles.
Axle Shafts: Used in some specialized axle designs where constant velocity (CV) joints are not the preferred solution.
Heavy Equipment Drivetrains: Employed in construction and agricultural machinery that require robust power transmission across angles and under heavy loads.
Maintenance Considerations
Wear and Tear: Due to their dynamic nature, U-joints are subject to wear. Worn U-joints can cause significant vibration, clunking sounds, or even complete failure if not replaced.
Lubrication: Proper lubrication is crucial for their longevity. Many U-joints are "sealed for life," while others have grease fittings (zerks) for regular maintenance.
Balancing: The entire driveshaft assembly, including the U-joints, must be properly balanced to prevent harmful vibrations from entering the vehicle.