When a vehicle turns round a corner, barring the driven wheels, the others (powered) will face drag on the surface, during a turn. In order to eliminate this, a group of gears called differential is used. The differential transfers torque from the engine to the wheels and allows them to spin at different speeds. The basic form of such a mechanism is called the open differential. But the open differential has a crucial limitation.
Note that in the absence of differentials, the axles will be worn out soon. When a vehicle is taking a turn, all four tires spin at different speeds because the distance they travel during a turn is different. There is unequal supply of torque to both the wheels, tire slippage happens.
The limited slip differential includes a clutch, which overpowers one wheel receiving extra power, but gets overpowered in turns. Then, there is the differential lock, which either engages or disengages the right and left wheels and one either has to manually do that or it is automatic.
But all these are corrective mechanisms laid over the basic principle of the open differential. But in 1958, a design of a differential, that could enable differential wheel spin, but still avoid lowered torque and slip was invented. It was called ‘torsen’ and it comes from the phrase ‘torque sensing’. Unlike the other differentials which bring about traction control and different wheel spin by having extra mechanisms in place, a torsen differential does this by means of its own design.
The principle of worm gears
The basic principle behind worm gears and worm wheels is that worm gears can turn worm wheels but not vice-versa. This principle is what is effectively exploited in the design of a torsen differential.
How it works
The ring gear transfers torque to both the wheels using a set of worm gears and worm wheels. There are two to three pairs of worm wheels and they lock on to the worm gear which also is paired. The right worm gear connects to the right axle while the left worm gear connects to the left axle.
Since the worm wheels by principle cannot turn the worm gear, they lock on to the worm gear and when the ring gear rotates, turn along with the worm gears. And when the vehicle is taking a turn, one of the wheels slow down down and the other rotates with a higher speed. This difference in speeds is fed back to the worm gears, and since they have the ability to turn the worm wheels, they turn the right and left wheels of each pair at differing speeds.
Rotational balance is thus accomplished, and when the turn is complete, the worm gears would stop turning the worm wheels and so the wheels lock back once again.
In conditions like snow and wet weather, since the worm wheels don’t turn the worm gears, the torque received from the ring gear is transferred to the wheels with no difference. Even if one of the wheels is in a slippery surface, there is no way it would attract more power, defy torque and cause slippage.