5.3 Driveshafts, Constant Velocity (CV) Axles & Differentials

Driveshafts, Constant Velocity (CV) Axles & Differentials

Driveshaft Assemblies & Universal Joint Mechanics

In rear-wheel-drive (RWD), four-wheel-drive (4WD), and all-wheel-drive (AWD) vehicles, the Driveshaft (propeller shaft) transfers rotational torque from the transmission or transfer case output shaft to the final drive differential.

Universal Joints (U-Joints) and Driveline Phasing

A standard universal joint is a Cardan (Hooke) joint consisting of a central four-trunnion cross (spider), needle roller bearings encased in four bearing cups, and synthetic rubber grease seals.

• Retaining Methods: Bearing cups are secured into driveshaft yokes using internal snap rings (C-clips inside the yoke groove), external snap rings (fitted into the bearing cup groove), or injected nylon plastic (OEM factory staked yokes).
• Driveline Phasing & Angularity: A single Cardan joint operating at an angle does not rotate at a constant velocity; it speeds up and slows down twice per revolution. To neutralize this velocity fluctuation, driveshafts use two U-joints positioned in-phase (yoke ears lined up in the exact same plane) at matching working angles. Out-of-phase driveshafts generate severe speed-dependent vibrations.
• Center Support Bearings: Two-piece driveshafts in long-wheelbase trucks utilize a center support (carrier) bearing mounted in a rubber insulator to support shaft weight and prevent whipping.

Constant Velocity (CV) Axle Assemblies

Front-wheel-drive (FWD) vehicles and independent rear suspension (IRS) drivelines require driveshafts capable of transferring smooth, non-fluctuating rotational speed across extreme articulation angles and suspension movement. This is accomplished using Constant Velocity (CV) Axle Assemblies.

Outer Fixed Joint vs. Inner Plunging Joint

A complete CV axle shaft assembly consists of two distinct joints connected by a solid or tubular axle shaft:

1. Outer CV Joint (Rzeppa Joint): A ball-and-cage fixed joint capable of operating at steep steering angles (up to 45 degrees). It contains an inner race, six precision steel balls, a spherical cage, and an outer bell housing splined to the wheel hub.
2. Inner CV Joint (Tripod / Inboard Joint): A plunging joint that slides inward and outward (telescopes) to accommodate changes in axle length caused by suspension jounce and rebound. It utilizes a three-trunnion spider with needle-bearing rollers that glide inside a slotted tulip housing.
3. Protective Boots and Clamps: Joints are sealed in specialized high-temperature molybdenum disulfide grease and encased in flexible elastomeric boots. Older designs used neoprene rubber boots, whereas modern vehicles utilize Thermoplastic Polyester Elastomer (TPE) boots, which offer vastly superior puncture and tear resistance. Boots are clamped with stainless steel crimp bands.

Cataloging & Counter Advice for CV Axles

When a boot tears, road grit contaminates the grease, destroying joint races. Parts specialists should weigh the labor and part cost of a boot kit versus a complete new or remanufactured CV axle assembly. A complete axle assembly is almost universally recommended due to reduced installation labor and guaranteed joint integrity.

Differentials & Final Drive Assemblies

The differential assembly serves three main purposes: it changes power flow direction by 90 degrees (in longitudinal engines), provides a final gear reduction to multiply torque, and allows inner and outer drive wheels to rotate at different speeds during cornering.

Ring & Pinion Hypoid Gear Sets

The drive pinion gear receives torque from the driveshaft and meshes with the larger ring gear attached to the differential case.

• Hypoid Gear Design: The axis of the drive pinion is offset below the centerline of the ring gear. Hypoid gears feature curved, sliding gear teeth that provide strong tooth contact and quiet operation. However, the sliding action generates extreme wiping pressure and heat, requiring GL-5 gear lubricant containing extreme-pressure (EP) additives (sulfur-phosphorus compounds).

Open vs. Limited-Slip (LSD) vs. Locking Differentials

• Open Differential: Allows complete wheel speed differentiation. However, torque is divided equally. If one wheel loses traction (e.g., on ice), it spins freely, and the opposite wheel receives zero effective driving force.
• Limited-Slip Differential (LSD): Uses multi-disc clutch packs, spring-loaded cones, or helical Torsen gears to transfer torque to the wheel with traction when wheel slip occurs. Clutch-type LSDs require a specialized LSD Friction Modifier Additive mixed with the gear oil to prevent clutch chatter during low-speed turns.
• Locking Differential: Mechanically locks both axle shafts together (via dog clutches or electric/air solenoids), forcing both wheels to turn at identical speeds regardless of traction.

Axle Shafts & Housing Bearings

Drive axles transfer torque from the differential side gears to the vehicle wheels.

• Semi-Floating Axles: Common in light trucks and passenger RWD vehicles. The axle shaft supports vehicle weight as well as driving torque. The inner end is secured in the differential case via a C-clip (retained by the pinion shaft), while the outer end rests directly on an axle bearing pressed into the axle housing tube.
• Full-Floating Axles: Used in heavy-duty trucks (3/4-ton and 1-ton). Wheel hubs are supported by dual tapered roller bearings riding on an axle housing spindle. The axle shaft carries driving torque only and can be removed without removing the road wheel or jacking up the vehicle weight.