Automotive Engineer

Limited slip differentials (LSDs) are usually associated with premium and high-performance vehicles to modify the traction and make them more stable during cornering and more adventurous driving.

The systems operate by monitoring input from the brakes, wheel speed, steering wheel, throttle, yaw and lateral acceleration sensors to ensure that each wheel is receiving sufficient torque. If slippage occurs, torque is transferred to the wheel with the most traction by modulating the differential from fully open to fully locked, and anywhere in between, depending on driving conditions.

Tests by Tier One supplier Eaton, which produces electronic and electro-hydraulic LSDs, show that oversteer is easier to manage with LSDs than with brake intervention.

But, with OEMs coming under pressure to enhance vehicle efficiency, the technology is now being looked at for the improvements it offers in emissions reduction and fuel economy. These improvements can come as the LSD is used as an alternative to all-wheel drive systems, saving as much as 130kg in weight by eliminating the added component count of such systems and at the same time removing parasitic losses. The LSD can then improve fuel economy by as much as 10%.

Suppliers are also looking to introduce the technology into smaller vehicles, as far down as the B-segment. But that will pose challenges for engineers. There will need to be innovations to get the packaging to fit within transactional housing without major changes to the castings of the housing themselves.

1. Left-hand and right-hand case
Generally made from iron to keep costs to a minimum, but aluminium could be an alternative in the future as suppliers look to reduce weight. Durability is key to the component, and dynamic case fatigue testing is used to determine lifespan and strength. Torsional loads are applied to the case to determine different potential failures.

2. Cross-shaft assembly
A carryover component from other technologies and therefore simplifying the electronic LSD’s development. The cross-shaft assembly is made from standard materials, such as alloys 8620 or 4320, depending on the application requirements. Consists of one long shaft and two short shafts fixed in a central block. The arrangement allows for a more compact design, and a two-piece case design that separates at the ring gear (crown wheel) mounting flange, utilising the ring gear attachment bolts to clamp the case joint, instead of splitting the case at the cross-shaft centreline.

3. Pinion gears
The forged pinion gears are made from alloy 8620. Forging the gears rather than machine cutting them doesn’t affect the grain structure of the metal, improving strength by as much as 30%. The gears are not developed specifically for electronic LSDs but for general use in LSD applications.

4. Splined side gear
Generally made from the same material as the pinion gears, but it can be varied depending on the application. Fatigue loads can differ from vehicle to vehicle, but for each application loading and stresses on the gear set are analysed to see if it is possible to leverage existing gear sets.

5. Clutch pack
This is where torque transfer takes place in the LSD. The pack is made up of frictional material bonded to a steel core plate. Thermal characteristics are important as the pack must be able to cope with temperatures of up to 350°C. The clutch pack is designed to be a heat sink, helping to draw heat away from the friction surface.

6. Piston plate
The steel piston plate uses hydraulic pressure to compress the clutch pack. The challenge is maintaining the seal integrity and designing the plate so it doesn’t load unevenly – uneven loading leads to premature failure of the clutch pack. Body element analysis is important during component development, stopping cantilevered forces.

7. Plenum
The aluminium plenum acts as a hydraulic slip ring and is designed to make the hydraulic connection through the existing axle assembly housing. The plenum doesn’t rotate with the differential. It houses two high-pressure rotary steels with pressurised hydraulic fluid between them. Two O-rings allow the plenum to be installed into the hydraulic housing and give external access to the hydraulic connection.