Differential Assembly: Description and Operation
GENERAL DESCRIPTIONThese trucks use various rear axles. The axles can be identified by the ring gear size in inches, by manufacturer (American Axle Manufacturing or Dana) and by the type of axle shaft used (semi-floating or full-floating). American Axle Manufacturing axles include the 8 1/2, 8 5/8, 9 1/2, and 10 1/2 inch ring gear axle. Dana supplies an 11 inch ring gear axle. The locking differential is supplied by Eaton.
REAR AXLE OPERATION
A basic differential has a set of four gears. Two are side gears and two are pinion gears. Some differentials have more than two pinion gears. Each side gear is splined to an axle shaft; so each axle shaft turns when its side gear rotates.
The pinion gears are mounted on a differential pinion shaft, and the gears are free to rotate on this shaft. The pinion shaft is fitted into a bore in the differential case and is at right angles to the axle shafts.
Power is transmitted through the differential as follows: the drive pinion rotates the ring gear. The gear ' being bolted to the differential case, rotates the case. The differential pinion, as it 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 because the input force on the pinion gear is equally divided between the two side gears. Therefore, the pinion gears revolve with the pinion shaft, but do not rotate around the shaft itself. The side gears, being splined to the axle shafts and in mesh with the pinion gears, rotate the axle shafts.
If a vehicle were always driven in a straight line, the ring and pinion gears would be sufficient. The axle shaft could then be solidly attached to the ring gear and both driving wheels would turn at equal speed
However, if it became necessary to turn a corner, the tires would scuff and slide because the outside wheel would travel further than the inside wheel. 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 its rear axle side gear (as the shaft is splined 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 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 uses multi-disc clutch packs and a speed sensitive engagement mechanism that locks both wheels together if one wheel should spin excessively during slow vehicle operation.
Under light loads, the clutch plates alone tend to lock axle shafts to the differential case, and therefore, locking each other. This is due primarily to the gear separating 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. Also, heavier throttle application will cause an axle speed differential, but this starts the full-lock feature of the unit.
Full locking is accomplished through the use of a heavyweight governor mechanism, cam system, and multi-disc clutch packs. The flyweights on the governor mechanism move outward 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 side gears to the case. The 100 RPM wheel-to-wheel speed allows for cornering without 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.
All 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.
DANA MODEL 80 (11 INCH RING GEAR)
The Dana 11 inch ring gear axle uses a conventional ring and pinion gear set to transmit the driving force of the engine to the rear wheels. The gear set transfers the driving force at a 90 degree angle from the propeller shaft to the axle shafts.
This axle is full-floating. The wheel hubs support the axle shaft at the wheel ends. The shaft at the wheel end is supported and splined to the hub inner-diameter. The differential supports the other splined end of the shaft.
Two tapered roller bearings support the pinion gear. A shim pack between the inner pinion bearing cup and the axle housing sets the pinion depth. A shim pack at the front of the axle housing between the bearing cone and pinion gear sets the pinion bearing preload.
The ring gear bolts to the differential case.
Two tapered rolling bearings support the differential case. Two methods are in use in order to control the differential bearing preload and drive gear to pinion gear backlash.
One method uses a 0.76 mm (0.030 inch) hardened shim and a selective outboard spacer shim. The location of the hardened shim is between each differential bearing cone and the differential case. The location of the selective outboard spacer shim is between each differential bearing cup and the housing.
The other method uses selective shims between each differential bearing cone and the differential case, with outboard spacers between each differential bearing cup and the housing.
When moving the ring gear with outboard selective shims, choose shims of different thickness in order to accommodate the change. For example: if a change requires a shim on the ring gear side that is 0.13 mm (0.005 inch) thinner, then the opposite side will require a shim that is 0.13 mm (0.005 inch) thinner.
On axles built using the other method, accomplish gear movement by moving shims from one side of the differential case to the other. If the differential preload needs to be changed, equal amounts must be added or subtracted from each side.
Spread the axle housing in order to remove the differential from the housing. When the spreader is removed, the housing sets the bearing preload. Two bearing caps hold the differential in the axle housing. A pinion seal, hub seal, and RTV between the cover and the housing seals the axle.
A part number (2) and manufacturing date (1) on the right axle tube, cover plate side, identifies all Dana axles. The model number (3) is cast on the carrier. Tags attached to the differential cover plate carry information on limited slip lubrication (4) and the axle ratio (5). The carrier does not have a drain plug.