What is an Automotive Limited Slip Differential ?


The LSD is a mechanism that limits the differential when one of the wheels begins to slip and generates an appropriate drive force in the other wheel to allow the vehicle to drive smoothly. There are various kinds of LSD.

1. Viscous coupling LSD

A viscous coupling is a kind of fluid clutch which transmits torque by means of the viscous resistance to oil. It uses this viscous resistance to limit differential slip. The viscous-coupling LSD is used as the differential limiting mechanism in the center differential of 4WD vehicles, and some viscous coupling LSDs are used in FF and FR model differentials.

2. Helical gear type torque-sensing LSD

The limited slip is accomplished primarily by the friction that is generated between the planet gear’s tooth tips and the differential case’s inner wall, and the friction that is generated between the side gear end face and the thrust washer. The principle of limited slip enables the resultant reaction force F1 (which is created by the meshing reaction of the planet gear and the side gear and the meshing reaction of the planet gears themselves) to push the planet gear in the direction of the differential case in proportion to the input torque. Due to the reaction force F1, the friction force F1 (which is generated between the tooth tip of the planet gear and inner wall of the differential case) acts in the direction to stop the planet gear’s rotation. (2/4)

3. Torque-sensing LSD

The differential limiting force is generated by both the tooth flank friction between the side gears and worm wheels and the thrust friction between the differential case, thrust washers and side gears. In this type of torque-sensing LSD, the differential limiting force changes greatly and vary rapidly in accordance with the torque applied to it. Accordingly if the accelerator pedal is released during cornering, the differential operates smoothly just like a normal differential. However, in cases where higher torque is applied, more differential limiting force is delivered.

4. Multiple-disc type

A barrel-shaped compression spring is fitted between the left and right side gears to keep the thrust washers pressed up against the clutch plate via the retainers and side gears. Therefore, the friction generated between the clutch plate and thrust washer limits the differential. HINT: The special LSD oil is used for multiple-disc type LSDs.

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