Axle repair guide

Drivetrain terminology, explained

Accessible definitions of the terms technicians use, so you can understand an estimate and ask the right questions.

Manufacturer specifications apply. This page explains concepts only. Torque values, clearances, preload, backlash ranges and procedures vary by axle model and must come from the vehicle or component manufacturer's service information. This directory does not publish repair specifications.

Ring and pinion gearset: The final-drive gear pair inside the differential. The smaller pinion gear, turned by the driveshaft, drives the larger ring gear. The tooth-count ratio between them is the axle ratio (for example, 3.73:1). Ring and pinion gears are matched sets and are set up together.
Gear backlash: The small amount of free movement between the ring gear and pinion teeth. Some clearance is needed for lubrication and heat expansion; too little or too much causes noise and accelerated wear. It is measured with a dial indicator, and the allowable range comes from the axle manufacturer.
Bearing preload: A controlled load applied to tapered roller bearings during assembly so they run without free play. Depending on the design it is set with shims, a crush sleeve or threaded adjusters, and is commonly checked as rotating torque. Preload values are specific to each axle and must come from the manufacturer.
Pinion depth: How far the pinion gear's face sits from the centerline of the ring gear. It is set with shims and confirmed with setup gauges and the tooth contact pattern. Incorrect depth concentrates load on the wrong part of the tooth and leads to noise and gear damage.
Tooth contact pattern: The marking left on the ring gear teeth after they are coated with gear-marking compound and rotated under resistance. Technicians read the pattern on both the drive and coast sides to confirm pinion depth and backlash before final assembly.
Differential carrier: Depending on axle design, 'carrier' means either the case that holds the side and spider gears (with the ring gear bolted to it) or the complete removable center section of the axle, sometimes called a third member.
Limited-slip differential: A differential that resists a speed difference between the two wheels, using clutch packs, cones or special gearing, so more torque reaches the wheel with traction. Some clutch-type units require a specific fluid or friction-modifier additive; follow the manufacturer's fluid specification.
Axle shaft spline: The machined teeth on the end of an axle shaft that engage the differential side gear (and, on full-floating axles, the hub drive flange). Spline count and diameter must match the mating part exactly; worn or twisted splines are a reason to replace the shaft.
Full-floating axle: The wheel hub rides on its own bearings on a spindle that is part of the axle housing, so the housing carries the vehicle's weight and the axle shaft only transmits torque. Common on many 3/4-ton and 1-ton pickups and on commercial trucks.
Semi-floating axle: The axle shaft both transmits torque and supports the vehicle's weight at the wheel end through a bearing in the axle tube. Common on passenger vehicles and many half-ton pickups. How the shaft is retained (for example C-clip or bearing-retained) affects service procedures.
Axle housing: The structural center section and tubes that enclose the differential and axle shafts and provide mounting points for suspension and brakes. A bent or cracked housing can cause misalignment, abnormal tire and bearing wear, and leaks.
Tandem drive axle: Two driven rear axles on a heavy truck, the forward-rear and rear-rear axles, which share the load and the driving torque. Common on Class 8 tractors, dump trucks and vocational trucks.
Inter-axle differential: A differential, usually built into the forward-rear axle of a tandem, that splits torque between the two drive axles and lets them turn at slightly different speeds. Many designs include a driver-controlled lock for low-traction conditions.
Power divider: Common shop term for the inter-axle differential assembly on a tandem drive axle. Power divider wear or damage is a frequent tandem-axle repair.
Wheel-end bearing: The bearings that support the hub on the axle spindle. Heavy trucks commonly use tapered roller bearings, either adjusted using a specified procedure or supplied as pre-set or unitized assemblies. Adjustment procedures come from the axle or hub manufacturer.
Hub seal: The seal at the inboard side of the hub that keeps oil or grease in and keeps water and dirt out. A leaking hub seal can contaminate brakes and lead to bearing failure.
Driveline operating angle: The angle at each U-joint between the two shafts it connects. A standard cross-type U-joint speeds up and slows down slightly within each turn when it operates at an angle; drivelines are designed so these fluctuations cancel out. Allowable angles come from the vehicle and driveline manufacturer.
Driveshaft runout: How far a spinning driveshaft tube deviates from turning perfectly true, measured with a dial indicator. Excessive runout causes vibration; limits are set by the manufacturer.
U-joint phasing: The alignment of the yokes at each end of a driveshaft. In a typical two-joint shaft the yoke ears are in the same plane so that one joint cancels the speed fluctuation of the other. An out-of-phase shaft, for example a slip yoke reinstalled one spline off, can cause vibration.
CV joint articulation: The range of angles through which a constant-velocity joint can work while transmitting smooth, constant speed. Outer joints allow larger angles for steering; inner (plunge) joints also allow the shaft length to change as the suspension moves.
Thrust angle: The direction the rear axle points compared to the vehicle's geometric centerline. A thrust angle that is out of specification can make the vehicle track sideways ('dog-track') and leave the steering wheel off-center.