Definitions of Frequently Used Terms

Aftermarket Parts
Replacement parts not made by the original equipment manufacturer (OEM) but designed to function the same or better than OEM parts.

Air Shifter Assembly
A pneumatic device used in heavy-duty vehicles and industrial equipment to control gear shifting or power engagement through compressed air. Instead of relying on manual or purely mechanical linkage, the air shifter uses air pressure to move shift forks, yokes, or actuators that engage and disengage gears or auxiliary functions.

Anchor Pin
A hardened steel pin that serves as a fixed pivot or support point for the brake shoes inside a drum brake assembly, providing a solid mounting point for one end of each brake shoe.

Axle Shaft
A shaft that delivers power from the differential to the drive wheels.

Backlash
The clearance or play between mating gear teeth, especially important in differentials and gear assemblies.

Bearing
A mechanical component that reduces friction between moving parts, commonly used in transmissions and differentials.

Bearing Strap Kit
A set of components used to secure a U-joint to a yoke, providing a clamping force to hold the bearing caps in place.

Breather
(Also called a vent, axle vent, or gearcase breather) is a pressure-relief valve or vent cap installed on top of a driveline component housing to allow hot air and expanding gases to escape while preventing dirt and moisture from entering the system.

Camshaft (S-Cam)
A critical component in air brake drum systems that converts rotational input into linear motion to apply the brake shoes against the drum.
Can also refer to the engine’s valve-timing shaft.

Carrier Assembly
The housing within the differential that contains the ring gear, pinion gear, and differential case.

Center Support Bearing (Carrier Bearing)
A bearing that supports the center of a two-piece driveshaft, reducing vibration and maintaining alignment.

Clutch
A mechanical device that connects and disconnects the engine from the transmission system, allowing the driver to change gears smoothly. It controls the transfer of rotational power from the engine to the drivetrain, enabling efficient vehicle operation and stopping without stalling.

Clutch Pack
A set of friction plates and steel plates used in limited-slip differentials or automatic transmissions to engage and disengage power flow.

Core
A used or worn-out part that is returned to be rebuilt or remanufactured.

Countershaft
A shaft in a transmission that holds gears driven by the input shaft and drives the output shaft.

Cross & Bearing Kit
A service kit containing replacement U-joint crosses and needle bearings, commonly used in U-joint repairs.

CV Joint (Constant Velocity Joint)
A joint that maintains a constant rotational speed regardless of the angle, providing smooth power transmission in front-wheel-drive and independent rear suspension vehicles.

Differential
A gear assembly that allows wheels to rotate at different speeds, especially important when turning. Found in axles.

Double-Cardan Joint
A type of universal joint configuration that reduces vibration and allows for greater angular movement, commonly used in front driveshafts.

Driveline (or Drivetrain)
The collective system of components that transfer power from the transmission to the wheels, including driveshafts, axles, and differentials.

Driveshaft (Propeller Shaft)
A rotating shaft that transmits torque from the transmission or transfer case to the differential or axle.

Driveshaft Tube
The hollow cylindrical body of the driveshaft, which houses U-joints and transfers rotational force.

End Yoke
A yoke attached to the end of a driveshaft, connecting it to the transmission, differential, or other driveline components.

Final Drive
The last set of gears that multiply torque and deliver it to the wheels, typically housed in the differential.

Flange
A flat, protruding rim or collar used to mount or connect shafts and components like driveshafts or yokes.

Full-Floating Axle
An axle design where the axle shaft only transmits torque and does not support vehicle weight, enhancing durability and load capacity.

Gear Ratio
The ratio between the number of teeth on two meshing gears, determining torque and speed output.

Half-Round / Full-Round Yokes
Designations for yoke styles based on the method of securing the U-joint; half-round yokes use bearing straps or U-bolts, while full-round yokes have a circular opening for press-fit U-joints.

Input Shaft
The shaft that brings power into a transmission or transfer case from the engine.

Kingpin (King Pin)
A central pivot pin that serves as the main steering and load-bearing joint between the steering knuckle and the axle beam on the front axle of a heavy truck. It acts as the hinge point that allows the front wheels to turn left and right while supporting the vertical weight of the vehicle and maintaining alignment with the driveline and chassis.

Limited-Slip Differential (LSD)
A differential that limits the difference in wheel speed between two wheels, improving traction by distributing torque to the wheel with more grip.

Locking Differential
A differential that can be mechanically locked so both wheels turn at the same speed, often used in off-road or heavy-duty applications.

Marine Gearbox
A specialized transmission system used in boats and ships to transfer power from the engine to the propeller efficiently. It adjusts engine speed and torque to match propulsion requirements, providing forward, neutral, and reverse functions while ensuring smooth and reliable operation in marine environments.

Mounting Brackets
Hardware used to secure driveline components like center bearings or differential assemblies to the vehicle frame.

Output Shaft
The shaft that transmits power out of a transmission or transfer case to the next driveline component.

Overdrive
A gear configuration in a transmission that allows the output shaft to turn faster than the input shaft, improving fuel efficiency at high speeds.

Pin Shoe Anchor
The pivot point or mounting pin that holds the brake shoes in place at the anchor end of a drum brake assembly.

Pinion
A small gear that works together with a larger ring gear (or crown gear) inside the differential to transfer power from the driveshaft to the axle shafts, which ultimately turn the wheels, converting the rotational torque from the driveshaft into motion that can move the truck forward or backward.

Pinion Yoke
A yoke that connects the driveshaft to the pinion gear of the differential, transmitting torque to the axle shafts.

Planetary Gear Set
A set of gears commonly used in automatic transmissions, consisting of a sun gear, planet gears, and a ring gear.

Power Take-Off (PTO)
A device that transfers mechanical power from a vehicle’s engine to auxiliary equipment like hydraulic pumps or winches.

Rebuild
The process of disassembling, cleaning, inspecting, and replacing worn or damaged parts to restore a component to good working condition.

Remanufactured
A component that has been completely disassembled, inspected, re-machined, reassembled with new or restored parts, and tested to meet OEM specifications.

Ring and Pinion
The main gear set in a differential that transmits power from the driveshaft to the wheels.

Seals and Gaskets
Components used to prevent fluid leaks and contamination in assemblies like transmissions and differentials.

Semi-Floating Axle
An axle design where the axle shaft supports both torque transmission and vehicle weight, commonly found in lighter-duty applications.

Shift Fork
A component in a manual transmission that moves gears into engagement when shifting.

Slip Yoke
A driveline component that allows the driveshaft to slide in and out of the transmission or transfer case, accommodating changes in length due to suspension movement.

Splines
Grooved shafts that fit into matching grooves in other parts to transfer torque while allowing axial movement.

Spool
A solid component that replaces the differential gears, locking both wheels on an axle together for maximum traction, commonly used in off-road or racing applications.

Stub Shaft
A short shaft used to connect components within the driveline, often found in configurations requiring a center support bearing.

Synchronizer (Synchro)
A component in manual transmissions that allows smooth gear engagement by matching gear speeds before shifting.

Tapered Roller Bearing
A bearing that can handle both radial and axial loads, often used in transmissions and wheel hubs.

Tie Rod
A mechanical linkage that connects the steering mechanism (usually the steering knuckle or arm) to the wheels, ensuring both wheels turn together when the driver steers.

Torque
The twisting force transmitted through driveline components, measured in pound-feet (lb-ft) or Newton-meters (Nm).

Torque Converter
A fluid coupling in automatic transmissions that transfers rotating power from the engine to the transmission, allowing the vehicle to come to a stop without stalling.

Transfer Case
A gearbox that splits power from the transmission and sends it to both the front and rear axles in four-wheel or all-wheel drive systems.

Transmission (Gearbox)
A system of gears that adjusts engine power to control vehicle speed and torque delivery to the driveshaft.

Trunnion
A pivoting or rotating support component that allows connected parts—such as drive shafts, suspension arms, or axles—to articulate and transfer load while maintaining alignment.

U-Bolt
A U-shaped bolt used to attach the U-joint to the yoke, commonly found in driveline assemblies.

Universal Joint (U-Joint)
A flexible coupling that allows the driveshaft to transmit torque and power while accommodating changes in angle.

Yoke
A forked component that connects the driveshaft to the transmission, differential, or PTO.