Rear axle and differential – Lesson 5 | Manual Transmission and Drivetrain
Table of Contents
Manual Transmission and Drivetrain : Lesson 5 – Rear axle and differential
General
Objectives
Upon completion of this lesson, you will be able to:
! Explain the purpose and function of a rear axle and differential.
! Describe a rear axle and differential and identify the types.
! Identify the components of a rear axle and differential.
! Explain the theory and operation of a rear axle and differential.
At a glance
Rear axle and differential
Rear axle assembly
On rear-wheel drive vehicles, torque is transmitted from the transmission through a driveshaft to the rear axle and differential assembly. The rear axle serves many purposes. The housing serves as a mounting point for suspension components and supports the vehicle. Inside the housing is the ring and pinion gear set and differential assembly. The ring and pinion gear set and differential assembly transfer power from the driveshaft to the rear wheels via the axle shafts.
The ring and pinion gear set allows the torque to change direction 90 degrees from the driveshaft to the axle shafts. The ring and pinion gear set also provides a gear reduction since the pinion gear is much smaller than the ring gear. The relationship between the number of teeth on the ring and pinion gear is known as the gear ratio.
At a glance
Rear axle and differential (continued)
Differential assembly
The rear axle shafts must be able to turn at different speeds to compensate for the fact that the wheel on the outside of a turn must travel a greater distance and therefore, faster than the wheel on the inside of the turn. The rear axle assembly contains a differential.
The differential is a set of gears that transmits torque from the driveshaft to the rear axle shafts individually, as required. The result is the freedom of each axle and wheel to rotate at the correct speed independently of the other axle. There are basically two types of differentials: conventional (open/non-locking), and limited slip (locking).
Types
Types of rear axle assemblies
Semi-floating axle assembly
- Bearing
- Housing
- Axle shaft
There are three basic types of rear axle assemblies, depending on how the axle shafts and wheels are supported:
! Semi-floating
! Full-floating
! Independent
The semi-floating rear axle uses shafts that are typically held in the housing by C-clips. These C-clips are positioned in a slot on the splined inboard end. The C-clips also fit into a machined recess in the differential side gears within the differential case. A semi-floating axle shaft rides on one straight roller bearing at the outboard end. The semi-floating axle shaft supports the weight of the vehicle, as well as providing driving torque.
Types
Types of rear axle assemblies (continued)
Full-floating axle assemblies
- Axle shaft 4 Inner bearing
- Hub 5 Outer bearing
- Axle housing
The full-floating rear axle provides increased load carrying capability. The hub is supported, or “floats”, on the axle spindle, on two opposing tapered roller bearings. All of the vehicle’s rear weight is placed on the axle housing, and none on the axle shaft. The axle shaft merely drives the wheel. The hub is retained on the spindle by a ratcheting nut that is tabbed to a slot in the spindle.
Types
Independent rear suspension (IRS) rear axle
- Differential assembly
- Halfshafts
- Constant velocity (CV) joints
The third type of rear axle used is the independent rear suspension (IRS) rear axle. This axle is similar to the other types, except that a separate suspension system and not the axle assembly carries all of the vehicle weight. No axle tubes are used. Instead of axle shafts, halfshafts are used like driveshafts to connect the axle housing to the drive wheels.
Constant velocity (CV) joints at both ends of the halfshafts are capable of operating at varying angles, and provide a way for the shaft length to change. The changing of the shaft length allows for wheel suspension and dynamic movement. The inboard CV joint stub shaft is held in the differential side gear by a circlip. The outboard shaft is pressed into the hub and secured with a rear axle wheel hub retainer.
Components
Rear axle differential components
Limited slip differentials
The typical limited slip rear axle assembly consists of basically the same components as the conventional axle assembly, with the exception of some clutches and springs added to the differential assembly.
Components
Limited slip unit
- Differential side gear
- Differential clutch spring
- Differential clutch pack
- Differential case
- Pinion gear
Rear axle/differential powerflow
The drive pinion, which engages the ring gear, receives engine power through the transmission and driveshaft. The drive pinion drives the ring gear, which is bolted to the differential case outer flange. The differential case then rotates. The drive pinion and ring gear multiply torque, and reduce speed according to the rear axle ratio. As the case turns, its internal gears are driven. The drive pinion and ring gear change the direction of powerflow from the driveshaft to the axle shafts. The differential case has two openings to allow for lubricant flow and repairability.
Differential assembly
- Ring gear
- Driveshaft
- Drive pinion
- Axle shafts
- Side gears
- Pinion gears
Rear axle/differential powerflow (continued)
Conventional differentials
During straight-ahead driving, each wheel turns at the same speed. The differential side and pinion gears turn with the case, with no movement between the teeth of the pinion and side gears. The complete differential assembly rotates as a unit.
Differential assembly straight ahead driving
- Inside axle – 100 rpm
- Drive pinion
- Pinion gears
- Outside axle – 100 rpm
- Side gears
- Ring gear
Operation
During a turn, the outside axle shaft must rotate faster than the inside axle shaft. In this situation, the pinion gears “walk” forward on the side gear of the inside (slower) axle shaft, increasing the speed of the side gear on the outside (faster) axle shaft. As the pinion gears “walk” around the slower side gear, they drive the faster side gear at greater speed. The sharper the turn, the greater the speed difference.
Differential assembly during left turn
- Inside axle – 90 rpm
- Drive pinion
- Pinion gears
- Outside axle – 110 rpm
- Side gears
- Ring gear
Rear axle/differential powerflow (continued)
Limited slip differentials
Typical limited slip differential
- Differential case
- Differential clutch pack shim
- Differential clutch pack
- Differential pinion gear
- Preload spring
- Differential side gear
- Differential pinion shaft
There are many names for limited slip differentials; Traction-Lok, Trac-Lok and Power-Lok. The conventional or “open” differential can be a disadvantage in a limited traction situation. When a vehicle is stuck in the snow, one drive wheel spins and the other stays stationary. Increasing torque to the spinning wheel will not increase torque to the stationary wheel.
The limited slip differential is designed to increases the driving torque to the wheel with the greatest amount of traction. Friction ia built into the system by adding a series of friction plates between the differential side gears and the differential case. Traditional differential action will only occur when enough torque is supplied to overcome the friction. If one drive wheel has no traction, the other wheel will always receive some torque.
Operation
Limited slip operation
Within the differential case, the clutch plates are stacked on each side gear hub. A preload spring is located between the side gears. This spring applies preload force to the clutch packs by pushing against the side gears. As the clutch plates wear, the preload spring separates the pinion and side gears, which can increase total axle lash. Axle lash may be felt when shifting from forward and reverse.
The side gear thrust loads create additional force. These loads, from separating forces of the side and pinion gears, are caused by torque in the drivetrain. The steel plates are stacked in pairs between the friction plates, and are splined to the side gear hub.
The side gear, in turn, is internally splined to the rear axle shafts. The friction plates thus engage notches in the differential case. The friction created by the plates creates torque capacity, which tries to prevent the side gears from rotating relative to the case. Available torque is a function of preload and added thrust load. In low traction, applying the brake and torquing the drivetrain, and then slowly releasing the brake and driving away can increase torque. This increases thrust load on the side gears.
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