Limited-Slip Differential
Limited-Slip Differential
A differential does not provide a high mechanical efficiency, which is desirable for the majority of mechanical components. Even if a 'low friction' differential with reduced traction over slippery surfaces is fitted, it limits acceleration of a high-power vehicle and causes excessive tyre wear. The torque reaction of the engine of such a vehicle, during acceleration, tends to lift the left-hand driving wheel off the ground. When accompanied by an uneven road surface, this causes excessive wheel spin. To minimize these drawbacks, the differential action is counteracted by artificially increasing the friction between the sun wheel and the differential cage. When this feature is included, it is called limited slip differentials.
Two basic types of limited slip differential are :
• Mechanical limited-slip differential
• Visco-differential
Fig. 26.46. Final drive assembly with limited slip differential
Mechanical Limited-slip Differential
Figure 26.46 illustrates a limited-slip differential, which is bolted to the crown wheel. A multi-disc clutch pack mounted behind each sun wheel has the inner and outer plates splined to the sun and cage respectively. The bevel gears exert an axial thrust proportional to the torque applied by the crown wheel to the differential. Under low torque transmission, the differential functions in the normal way. When transmitted torque is increased, the clutch pack is loaded, which resists motion of the sun gear so that it rotates at a different speed than that of the cage (Fig. 26.47).
These include the following:
(i) The discs are provided with an initial load through a Belleville disc-spring washer
installed between the cage and the clutch discs of each pack.
(ii) Angled cam faces are installed between the cage and the cross pins. The cage exerts driving thrust on the pin, which in turn forces the planets against the side gear ring. In a four planets system, two separate pins are flexibly linked at the centre with opposite cam faces, so that they cause two planets to act on one clutch pack and the other two to exert force in the opposite direction.
Fig. 26.47. Limited slip differential
Viscous Differential
This type combines a standard differential unit with a viscous coupling. The coupling is used in this system as a viscous control device to regulate the speed difference between the two driving wheels. As explained above the locking action on a mechanical limited-slip differential depends on input torque, whereas on the viscous type it depends on the speed difference of the driving wheels. Very little resistance is offered with a small speed difference and this increases progressively with the increase of the difference. Compared with the mechanical limited-slip differential, the viscous type provides lower tyre wear, easier steering and lower stresses in driveline components. For high-viscosity fluid, the coupling can be designed to deliver a comparatively high torque, which progressively builds up with the increase of shear rate.
The visco unit is very similar in basic construction to a multi-plate clutch. It uses a housing and hub, and a series of perforated metal plates submersed in silicon fluid (Fig. 26.48) are sandwiched between them. Also these plates are alternatively attached to the hub and housing. A fluorinated rubber heat resistant seal isolates the silicone fluid from the lubricating oil in the final drive assembly.
The torque and shear rate relationship depends on fluid viscosity, the gap between plates and plate perforation. These factors are varied while designing a coupling suiting to the application.
Prolonged slippage of the coupling causes generation of heat due to which the fluid expands and occupies some of air space. In case spacers are not used to position the plates apart, the increase in air pressure due to fluid expansion and the reduction in fluid pressure in the gaps pushes the plates together. This causes metal-to-metal contact so that the coupling temporarily departs from its viscous mode operation. During this phase the torque output rises considerably even six times as great in some designs.
Fig. 26.48. Visco-differential.
Viscous couplings can be used to control front, centre and rear differentials. Each application can be considered as different in view of its position, speed of operation and type of vehicle to which it is fitted. For example, a viscous control unit at the rear of a powerful car requires the use of'hump' as overload protection. But, viscous coupling used for a front wheel drive car should have no 'hump', because a unit with this characteristic would adversely affect the steering. The viscous control unit can be installed and connected in a differential in two ways;
i. e. shaft-to-shaft and shaft-to-cage couplings.
Shaft-to-shaft Viscous Coupling
A bevel-type differential having a viscous control unit connected between the two axle shafts is illustrated in Fig. 26.49. The hub holding and inner plates is joined to one shaft, and the housing with its outer plates is connected to the other shaft. The unit is installed at the centre of the crown wheel by moving the differential to one side. The control unit does not function during movement of the car in a straight path with no wheel slip. But, if the shaft speed becomes different, such as when one driving wheel loses adhesion or when wheel spin occurs during acceleration, the resistance offered by the unit maintains equal driving torque to both wheels. Fig. 26.49. Shaft-to-shaft viscous coupling
Shaft-to-shaft Viscous Coupling.
A bevel-type differential having a viscous control unit connected between the two axle shafts is illustrated in Fig. 26.49. The hub holding and inner plates is joined to one shaft, and the housing with its outer plates is connected to the other shaft. The unit is installed at the centre of the crown wheel by moving the differential to one side. The control unit does not function during movement of the car in a straight path with no wheel slip. But, if the shaft speed becomes different, such as when one driving wheel loses adhesion or when wheel spin occurs during acceleration, the resistance offered by the unit maintains equal driving torque to both wheels.
Shaft-to-cage Viscous Coupling
Fig. 26.50. Shaft-to-cage viscous coupling
In the layout illustrated in Fig. 26.50, the housing of the viscous control unit is integral with the differential cage and the hub is connected to one axle shaft. This arrangement is comparatively cheaper. The speed difference between the cage and axle shaft is half the difference of the road wheels. Therefore, compared with the shaft-to-shaft layout, the shaft-to-cage arrangement exhibits a torque characteristic about three times greater to have the same locking effect.
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