Section 5.5: Differentials, Final Drives & Gear Setup

Section 5.5: Differentials, Final Drives & Gear Setup

Differential Fundamentals and Types

The differential allows the drive wheels to rotate at different speeds when the vehicle is turning, while continuously transmitting torque. When cornering, the outer wheel travels a longer path than the inner wheel and must rotate faster.

• Open Differentials: Divide torque equally between both wheels. If one wheel loses traction (e.g., on ice), it spins freely, and torque delivered to the opposite wheel drops to zero, preventing vehicle movement.
• Limited-Slip Differentials (LSD): Use mechanical clutch packs, helical gears, or viscous couplings to transfer torque to the wheel with traction. In a clutch-type LSD, spring-loaded clutch discs are compressed behind the side gears. When one wheel slips, friction between the plates transfers torque to the gripping wheel. These require friction modifiers added to the gear oil to prevent clutch chatter.
• Locking Differentials: Mechanically lock the axle shafts together, forcing both wheels to turn at the same speed regardless of traction. These can be manual, pneumatic (air-lockers), or electromagnetic (e-lockers), and are commonly used in off-road and heavy-duty applications.

Final Drives and Hypoid Gears

A hypoid gearset is the standard design for rear-wheel drive final drives. The pinion gear centerline is offset below the centerline of the ring gear. This offset lowers the vehicle's driveshaft tunnel to increase passenger cabin space. However, the hypoid design creates a combination of rolling and high-friction sliding action between the teeth. To prevent metal-to-metal contact and gear scoring under high load, hypoid gears require Extreme Pressure (EP) lubricants, typically designated as GL-5.

Critical Gear Setup Procedures

When rebuilding or replacing a differential gearset, four critical parameters must be measured and adjusted in a specific sequence: pinion depth, pinion bearing preload, ring gear backlash, and carrier bearing preload.

1. Pinion Depth

Pinion depth determines how far the pinion gear extends into the carrier housing, controlling where the pinion teeth contact the ring gear teeth from face (top) to flank (bottom).

• Measurement: Measured using a specialized pinion depth gauge or by subtracting shim measurements.
• Adjustment: Shims are added or removed behind the inner pinion bearing cup or head. Adding shims moves the pinion closer to the ring gear centerline (deeper). Removing shims moves it further away.

2. Pinion Bearing Preload

Pinion bearing preload is the structural tension placed on the tapered roller bearings supporting the pinion shaft to prevent axial and radial deflection under load.

• Measurement: Measured by rotating the pinion shaft using a dial-type or beam-type torque wrench. It is recorded as rotating torque (typically 10 to 30 inch-pounds or 1.1 to 3.4 Newton-meters), not breakaway torque.
• Adjustment: Adjusted by tightening the pinion nut to crush a collapsible spacer (crush sleeve) or by changing shims between the bearings. If a crush sleeve is over-tightened, the preload will exceed specifications, and a new crush sleeve must be installed; backing off the nut to reduce preload is not permitted, as it relaxes the tension on the assembly.

3. Ring Gear Backlash

Backlash is the clearance or "play" between the teeth of the ring gear and the pinion gear, preventing the gears from binding as they heat up and expand.

• Measurement: Mount a dial indicator to the differential housing. Place the indicator plunger perpendicular to the drive side of a ring gear tooth, close to the outer heel. Hold the pinion shaft still and rock the ring gear back and forth. The movement on the dial is the Total Indicator Reading (TIR). Typical backlash specs range from 0.005 to 0.010 inches (0.13 to 0.25 mm).
• Adjustment: Adjusted by moving the ring gear laterally. Moving the ring gear closer to the pinion decreases backlash; moving it away increases backlash. This is achieved by changing side shims or adjusting threaded adjuster nuts behind the carrier bearings.

4. Carrier Bearing Preload

Carrier bearing preload keeps the differential case tightly located in the housing to prevent case deflection under high load, which would alter backlash and ruin the gear contact pattern. It is set by adding extra shims (typically 0.004 inches per side beyond zero play) or tightening carrier adjuster nuts past the zero-clearance point.

Reading Gear Tooth Contact Patterns

After adjusting backlash to specification, apply gear marking compound (a non-drying yellow or white paste) to several teeth on both the drive side (convex face) and the coast side (concave face) of the ring gear. Rotate the ring gear through the pinion several times in both directions while applying resistance to the ring gear (using a pry bar) to load the teeth.

Interpret the resulting patterns:

• Ideal Pattern: A centered oval located midway between the toe (inner edge) and heel (outer edge), and centered between the face and flank.
• High/Face Pattern: The contact area is too close to the top of the tooth (face). This indicates the pinion is too far out. Remedy: Add shims behind the pinion head to drive it deeper, then readjust backlash.
• Low/Flank Pattern: The contact area is too close to the bottom of the tooth (flank). This indicates the pinion is too deep. Remedy: Remove shims behind the pinion head to pull it out, then readjust backlash.
• Toe Pattern: The contact is concentrated on the inner edge. Remedy: Move the ring gear away from the pinion (increase backlash).
• Heel Pattern: The contact is concentrated on the outer edge. Remedy: Move the ring gear closer to the pinion (decrease backlash).

Diagnostic Troubleshooting

• Pinion Bearing Whine: A high-pitched whine that occurs specifically during acceleration indicates a worn inner pinion bearing, as torque drives the pinion into the housing. A whine on deceleration indicates a worn outer pinion bearing, as the pinion is pulled forward.
• Carrier Bearing Noise: A deep rumble or growl that is constant and varies directly with vehicle road speed (not engine speed).
• Side Gear Noise: A clucking or clicking sound heard only while turning. When driving straight, side gears do not rotate relative to the differential case.