Limited Slip Differentials


Limited Slip Differentials

As mentioned earlier in this website, driveline torque is evenly divided between the two rear drive axles by the differential. As long as the tires grip the road providing a resistance to turning, the drive train forces the vehicle forward. When one tire encounters a slippery spot on the road, it loses traction, resistance to rotation drops, and the wheel begins to spin. Because resistance has dropped, the torque value delivered to both drive wheels also drops. The wheel with good traction is no longer driven. If the vehicle is stationary in this situation, only the wheel over the slippery spot rotates. When this is occurring, the differential case is driving the differential pinion gears around the stationary side gear.

This situation places stress on the differential gears. When the wheel spins because of traction loss, the speed of some of the differential gears increases greatly while others remain idle. The amount of heat developed increases rapidly, lube film breaks down, metal-to-metal contact occurs, and the parts are damaged. If spinout is allowed to continue long enough, the axle could break down completely.

Other failures can occur during spinout. If a spinning wheel is subjected to grabbing on a firm surface, a shock impact occurs. If the shock is severe enough, broken gear teeth or shaft fracturing can result. A fatigue failure can start if the shock is only great enough to damage the gear or shaft material.

To overcome these problems, differential manufacturers have developed the limited slip differential. While the limited slip differentials are manufactured under such names as sure-grip, no spin, positraction, or equal-lock, there are really only two types: the clutch pack and the brake cone.

Clutch Packs

The clutch pack limited slip differential uses two sets of clutch plates and friction discs to prevent normal differential action. The friction discs are steel plates with an abrasive coating on both sides. The discs are splined internally to mate with external splines on the hub of the side gears. Steel plates without friction linings are placed between the friction discs. These plates have tangs that fit into the grooves in the differential case.

When installed in the differential, the discs are connected to the side gears and the plates are locked to the case. There is one set of clutch plates between each side gear and the differential case.

Pressure is kept on the clutch packs by springs. Either an S-shaped spring or coil springs are placed between the side gears. Those differentials using coil springs also have a set of spring retainers against which the springs bear. The springs (or spring) keep pressure against the side gear and the clutch pack.

As long as the clutch friction discs maintain their grip on the steel plates, the differential side gears are locked to the differential case. The case and drive axles rotates at the same speed which prevents one set of wheels from spinning on a slippery pavement or spinning momentarily faster when operating on uneven roads.

The clutch plates are designed to slip when a predetermined torque value is reached. This enables the car or truck to negotiate turns in a normal manner. The high torque caused by the wheels on the outside radius of a turn or curve rotating faster than the gear support case and differential causes the plates in the clutch pack to slip, allowing the side gear to increase in speed and the pinions to walk around the side gear in nirmal differential fashion.

Brake Cones

The most common limited slip differential in late model vehicles employs two cone-shaped friction components to lock the side gears to the differential case. The cones are located between the side gears and the case and are splined to the side gear hubs. The exterior surface of the cones is coated with a friction material that grabs the inside surface of the case. The friction surface on the cones has a coarse spiral thread to provide a passage for lubricant. Four to six coil springs mounted in thrust plates between the side gears maintain a preload on the cones. The operation of the cone system is essentially the same as the clutch-plate system. The cones are forced against the carrier and squeezed or compressed. At this point, the cone rotates with the carrier and locks up both axles.

Some vehicles have an antiskid sensor (part of the braking system) attached to the differential carrier for extra nonskid protection during braking.

More Drive Axles And Differentials

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