PHASE 9

3.1.2 THE FUNCTION OF A DIFFERENTIAL:

• To aim the engine power at the wheels
• To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
• To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)

3.1.3 TYPES OF DIFFERENTIALS

§  Conventional or Open
§  Limited Slip
§  Automatic Locking
§  Manual Locking

i. OPEN DIFFERENTIALS:

Use two side gears inside the differential case. Each gear is splined to accept an axle shaft. These side gears are in turn driven by a set of spider gears. The spider gears, also inside the differential case, ride on a shaft which is pinned into the differential case and through which all the power is transmitted. The case is driven by the ring gear which is bolted fast to the case. The conventional differential is fitted as standard equipment on most vehicles.

On paved roads this system is very successful, giving predictable handling, even tire wear and requiring very little maintenance. However, in off road situations where traction surfaces vary greatly, this type of differential has a major limitation. When one wheel has greater traction than the other, all the power will be directed to the wheel with the least traction. For example, if one wheel is in the air and the other wheel is still on a hard surface, then all the power will be transferred to the wheel in the air. No power will go to the one on the ground and the vehicle will not move.

     

Fig. 3.1.1 WORKING OF A DIFFERENTIAL UNIT

ii. LIMITED SLIPS:

 (LSD's) come in a variety of designs. Most use friction plates, cones and/or gears to reduce slippage between each of the tires. These units have a dual power path from the differential case to the axle shafts. Some power is transmitted through the spider gears to the side gears in the conventional manner. The remainder is transmitted by friction between the differential case and the clutch plates and the side gears. A certain amount of "clutch preload" is built into the unit in a static condition.

Then, as load is applied to the differential, the separation forces between the spider gears and the side gears increases this clutch loading. This increase in friction provides for a good positive power flow from the case directly to the side gears. When traction is available to both wheels, the power going to the differential causes the plates to bind tightly together, giving even power to both wheels.

However, in a situation where there is little or no traction available to either one wheel or the other, the amount of power that can be transmitted to the other wheel which has traction is dependent on the friction or "preload" in the clutch plates. High levels of "clutch preload" will result in good torque transfer but some chattering of the clutches during cornering may occur. Lower levels of preload results in minimal chatter but reduced levels of torque transfer to the wheel with traction. Because LSD's restrict true differential action, tire wear is accelerated. Changes in vehicle handling may also occur, particularly in short wheelbase vehicles. Wear rates on limited slip differentials are generally higher than on other types due to the reliance on friction to reduce wheel slippage. Also, special lubricants may be required to minimize rough and noisy operation. Despite their limitations, LSD's are popular as original equipment options as well as an aftermarket replace because:

1) Some traction improvement off road is provided

2) Vehicle handling idiosyncrasies are not excessive

3) Installation is simple

4) Cost is reasonable