Differential Assembly: Description and Operation

Rear Axle Description
General Description
These trucks use various rear axles. You can identify the axles by the ring size in inches and the type of axle shaft used (semi-floating or full-floating). Axles include the 8 1/2 inch, the 9 1/2 inch and the 10 1/2 inch ring gear axle.


Rear Axle Operation
A basic differential has a set of 4 gears. Two are side gears and 2 are pinion gears. Some differentials have more than 3 pinion gears. Each side gear splines to an axle shaft. Each axle shaft turns when the axle's side gear rotates.

The pinion gears are mounted on a differential pinion shaft. The gears are free to rotate on this shaft. The pinion shaft fits into a bore in the differential case. The pinion shaft is at right angles to the axle shafts.

Power is transmitted through the differential in the following manner:
^ The drive pinion rotates the ring gear.
^ The gear, being bolted to the differential case, rotates the case.
^ The differential pinion, as the differential pinion rotates with the case, forces the pinion gears against the side gears.
^ When both wheels have equal traction, the pinion gears do not rotate on the pinion shaft. The gears do not rotate because the input force on the pinion gear is equally divided between the 2 side gears.
^ The pinion gears revolve with the pinion shaft but do not rotate around the shaft.
^ The side gears, splining to the axle shafts and in mesh with the pinion gears, rotate the axle shafts.

If a vehicle is always driven in a straight line, the ring and the pinion gears are sufficient. The axle shaft could then solidly attach to the ring gear. Both driving wheels would turn at an equal speed.

However, if it becomes necessary to turn a corner, the tires would scuff and slide because the outside wheel would travel further than the inside wheel. In order to prevent tire scuffing and sliding, the differential allows the axle shafts to rotate at different speeds.

When the vehicle turns a corner, the inner wheel turns slower than the out wheel and slows the rear axle side gear (as the shaft splines to the side gear). The rear axle pinion gears will roll around the slowed rear axle side gear, driving the rear axle side gear and the wheel faster.

Locking Rear Axle
The locking rear differential allows for normal differential function as indicated in the standard rear axle description. Additionally, the locking rear differential used multi-disc clutch packs and a speed sensitive engagement mechanism that locks both wheels together if 1 wheel should spin excessively during slow vehicle operation.

Under light roads, the clutch plates alone tend to lock the axle shafts to the differential case, and therefore, locking each other. This is due primarily to the gear separating the load developed on the right clutch pack. This induced clutch torque capacity resists motion between the side gear and the rear axle differential case. The differential allows the wheels to turn at different speeds while the axle shafts continue to transmit the driving force. Heavier throttle application will cause an axle speed differential. This action starts the full-lock feature of the unit.

You can accomplish full-locking through the use of a heavyweight governor mechanism, a cam system and a multi-disc pack. The flyweights on the governor mechanism move outward in order to engage a latching bracket whenever the wheel-to-wheel speed varies by approximately 100 RPM or more. This action retards a cam which, in turn, compresses the multi-disc clutch packs locking both the side gears to the case. The 100 RPM wheel-to-wheel speed allows for cornering with differential lockup.

At vehicle speeds above approximately 32 km/h (20 mph), the latching bracket overcomes a spring preload and swings away from the flyweights. At this vehicle speed, or greater, the differential is designed not to lock since added traction is generally not needed.

The axle parts of vehicles equipped with the locking rear axle are interchangeable with those equipped with the conventional rear axle, except for the case assembly.