|KYLE’S CORNER – AUTO REPAIR HELP|
by Kyle McFadden
The differential components consist of a housing unit, ring gear, pinion gear, axles, differential case or carrier, bearings, and side gears. In addition the backing plates for the mounting of brake components are attached to the differential housing. As well as mounting flanges, and retaining studs, for securing the wheels to the axle shafts. The differential housing is used as a support for the rear suspension and to house the differential components. The differential housing generally contains two parts, a cast iron differential carrier, welded to steel axle tubing. The carrier portion of the housing contains machined surfaces used to mount the various differential compenents and bearings. The axle tubes are used as suspension component mounting points, to house the axle shafts and bearings (on most vehicles), and to mount the brake backing plates. The pinion gear is used to drive the ring gear. Fastened to the pinion gear shaft is a flange that provides an attachment for the driveshaft. The pinion gear is supported by two bearings, at for and aft, that are mounted in the differential housing. The bearings are set to a specific preload when the differential is assembled. The depth in which the pinion gear protrudes into the differential housing is also preset when the differential is assembled.
The ring gear is bolted to the differential case. The differential case is placed in the differential housing, or carrier, and supported by bearings that are referred to as side bearings. The bearings are pressed onto the differential case and retained in the differential housing using bearing caps. Most differentials use shims and spacers, placed between the side bearings and the differential housing, to adjust ring and pinion gear backlash, and set the preload of the side bearings. Backlash is the amount of freeplay, between the ring gear and the pinion gear. Backlash is adjusted by moving the ring gear laterally towards the pinion gear to minimize clearance, or moving the ring gear laterally away from the pinion gear to gain clearance.
Contained in the differential case are the differential side gears and the differential pinion gears. These gears are bevel cut and placed at right angles to one another, so as to be in constant mesh. The side gears each contain a splined opening in which the splines of the axle shaft are inserted. The side gears, as their name describes, are mounted on either side of the differential case, in order to provide torque to the axle shafts. The pinion gears are placed between the side gears, facing one another, and are retained by a shaft, placed through the differential case. When the vehicle is moving, the pinion gear rotates the ring gear, which in turn rotates the differential case. The pinion gears, inside the case, are used to rotate the side gears, which in turn rotate the drive axles. During cornering, the outside wheel must travel a farther distance than the inside wheel, so the outside wheel spins faster. When this occurs, the pinion gears allow the side gears to rotate at different speeds, to prevent the tires from sliding.
The design of the differential causes torque to be applied to the wheel that is spinning the fastest. This can cause stability problems in high performance vehicles, as well as off road vehicles. Some differentials are equipped with a special differential case that allows power to be applied equally to both drive wheels. The differential case contains special clutches that allow the wheels to corner at different speeds, and still provide equal power to the driving wheels, during wheel slip events. Differential bearings and operating components are cooled and lubricated using a special gear oil. The gear oil is contained in the housing, and circulated by the rotation of the ring gear, while the vehicle is driving. Passages are cut in the differential housing to provide lubrication to the pinion bearings and the axle bearings, when non-sealed axle bearings are used. The differential multiplies torque using the smaller pinion gear, to rotate the larger ring gear. The amount of torque multiplication, is determined by the number of rotations the pinion gear completes, in order to rotate the ring gear one revolution. This in turn is determined by the number of teeth on the pinion gear, compared to the number of teeth on the ring gear. This number is expressed in a ratio, that is used to represent the torque multiplication rating of the differential. If the pinion gear contains 11 teeth, and the ring gear contains 43 teeth, the ratio would be approxiamately 3.90:1. This would mean that the pinion gear would rotate 3.9 turns, in order to rotate the ring gear one turn. A ratio of this size would provide quick acceleration, but the top speed of the vehicle would be reduced, as well as the fuel mileage. A ratio of 2.73:1, would not supply as much torque multiplication as the 3.90:1 gear ratio, but would furnish better fuel mileage and higher top speeds. Refer to an auto repair manual for exact specs for your car.
(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)