Torsen Limited Slip Differential

Torsen Limited Slip Differential houses a set of complicated gear system, but is the most effective of all the LSDs. This product is the trademark of the JTEKT corporation. Torsen was first patented by Gleasman in 1958.

Construction:


The components used inside torsen are unique and they provide the most brilliant way of providing differential action and also overcoming the traction difference problem.

It has a pair of specially designed gear assembly. These are designed on the basis of spur gear (Worm wheel) and worm gear assembly. This is the basic principle behind the working of a torsen. The unique feature of worm gear and worm wheel assembly is that the spinning worm gear can rotate the worm wheel, whereas the vice-versa is not possible. It means that the rotating worm wheel cannot spin the worm gear (the assembly is locked in this case).

A pair of this worm wheel and worm gear assembly is attached to the differential case. The half output shafts are connected on either side of the worm gears that drive the wheels.

The power from the transmission is transferred to the pinion and ring gear assembly which is standard in any differential. The worm wheels rotate along with the ring gear. Each end of the worm wheels is fitted with a spur gear. The spur gears are meshed so that both the worm gears rotate at the same speed and in opposite directions.

Case 1 (Vehicle moving straight):

In this case, the power from the ring gear is directly transferred to the output shafts via the differential casing. The worm wheels and worm gears are locked and rotate as a single solid unit. The worm wheels do not spin on its own axis and the worm gears do not rotate about its own axis. Equal amount of power is distributed to the wheels.

Case 2 (Vehicle taking a right turn):

In this case, the left wheel has to rotate at a higher speed compared to right wheel. The worm gear of the faster moving left axle will rotate the corresponding worm wheel about its own axis at a higher speed. Whereas, the right axle rotates in opposite direction when considering its relative motion with the left axle. Thus the right worm wheel will rotate in the opposite direction.

The spur gears which are meshed at the end of worm wheels will make sure that the worm gears rotate at the same speeds but in opposite direction. This ensures a perfect differential action.

Case 3 ( One wheel on a slippery surface and the other on a non slippery surface ):

In this case, a vehicle equipped with a conventional differential will allow majority of the power to be transferred to the slippery wheel. Therefore the vehicle will get stuck.

In a torsen LSD, the excessive speed of the slipping wheel can be used to its advantage. As the slipping wheel ( say the right wheel) starts spinning at a higher speed, this speed increase will be transferred to the right worm wheel via the right worm gear. The right worm wheel starts rotating in its own axis, therefore transferring the power to the left worm wheel as they both are connected with the help of spur gears.

We know that the unique feature of worm wheel and worm gear arrangement is that the worm wheel cannot spin the worm gear. This principle is applied in the torsen as the left worm wheel cannot spin the left worm gear and the entire mechanism gets locked. As a result, both the left and right wheels start rotating at the same speed.

To withstand the heavy vehicle load, two more pairs of worm gear and worm wheel assembly is added to the differential case.

Electronic Limited Slip Differential (eLSD)

Introducing electronics into any technology improves its accuracy. Electronic limited slip differential (eLSD) is just like a normal LSD having pressurized hydraulic clutches, fine tuned with electronics.

An eLSD employs Electronic Control Unit (ECU) to provide sufficient torque to each wheel. Wheel sensors sense the wheel speeds and provide inputs to the ECU. In the event of slipping of wheels, the ECU actuates the hydraulic clutches allowing it to transfer more torque to the wheel having more traction. An eLSD also allows for better control over a vehicle while taking a turn or changing the lane with the help of Electronic Stability Program (ESP).

An eLSD prevents a vehicle from excessive yaw. From the top view, yaw is defined as the rotation of a vehicle around its central point. Rear wheel eLSD monitors whether the rear wheels are moving in the same direction as the front wheels while turning or changing the lane. If the yaw is more than the limit, then the vehicle could go for a spin. Hence, an eLSD prevents the vehicle from spinning.

Limited Slip Differential (LSD)

What is LSD?

The limitations in differential is overcome by using Limited Slip Differential (LSD). It is a mechanical differential gear system used in automobiles to limit the relative motion between the wheels.

In a car with the conventional differential, if only one wheel is stuck on a slippery surface, then one wheel will be slipping freely over the slippery surface while the other wheel motion ceases. Limited Slip Differential overcomes it by transferring more torque to the non slipping wheel. This helps the vehicle escape a ditch easily.

LSD Construction:



One of the most commonly used technology for LSD is clutch pack based. The differential case has the same components that are used in a conventional differential. A pinion from the propeller shaft meshed to the ring gear, allowing 90 degree power transfer. The sun gears and planet gears are also meshed with each other. Planet gears rotate around the axis of the ring gear and also in its own axis. The sun gears are connected to the 2 half output shafts connected to each wheel. It has a series of friction and steel plates arranged alternately and packed together on either side between the sun gears and the differential case. There is a pre-load spring fitted between the sun gears.

LSD Working:

Friction discs are locked with the sun gears. Therefore, friction discs and side gear will always rotate together. Steel plates have external teeth and are made to fit in the grooves provided in the differential casing. Therefore, both steel plates and case rotate together.

If the clutch assembly is well pressed, then the entire clutch and case assembly will move together. Therefore, motion from the case is directly transferred to the half output shafts. The pre-load spring fitted between the sun gears will provide a side thrust and press the clutch discs together.

The sun and planet gear assembly is a bevel gear assembly. In a bevel gear system, axial forces are also induced apart from tangential forces. The axial force tries to push the sun gears against meshing with the planet gears. In the differential casing, a small allowance is provided to the sun gears for axial movement. Therefore, during high torque transfer, there is a huge amount of axial force acting on the sun gears that push it outwards towards the clutch pack. This axial thrust presses the clutch discs against the wall of the casing.

Vehicle with one wheel on a slippery surface:

Lets consider the case where one wheel is on slippery surface and the other is on a non slippery surface. Due to higher traction on the non slippery surface, the torque transferred towards the non slipping wheel will be higher. As a result, more axial thrust is created on the non slippery side and the clutch assembly on that side is locked. So power from the transmission is transmitted directly towards the non slippery surface via the clutch pack assembly.

On the slippery side, the axial force is not enough to lock the clutch pack. Therefore, power flow towards the slippery side is limited. As a result, the vehicle will overcome the traction difference problem.

While taking a turn:

While taking a turn, the LSD acts as a normal differential. In this case, the clutch pack won't come to use as the axial force developed will be less. As a result, the left and right wheels will turn at different speeds just like the conventional differential.

Differential in automobiles

What is a differential?

Differential is a mechanical device used in rear axles and it's primary function is to vary the power delivered to the rear wheels when the car is negotiating a turn.

Differentials also acts as a final gear reduction, to slow down the rotational speed of the transmission before it is delivered to the wheels. Another function of a differential is to transfer power at right angle from the propeller shaft to the wheels.

Why is differential used?

When a car is negotiating a curve, the inner wheel has to cover a smaller distance compared to the outer wheel. Therefore, the inner wheel must rotate at a lower speed compared to the outer wheel.

Without a differential, both the driven wheels would rotate at the same speed. If a solid shaft is used without differential, then wheels will have to slip to accomplish the turn. Differential is not required on non-driven wheels i.e. if a car is a front wheel drive, then differential would be installed at the front axle and will not be required at the rear axle.

History of differential:

There is no clear fact about the invention of differential. Some researchers day that it was first used in China way back in 30 B.C. However, Onésiphore Pecqueur, A French mechanical engineer patented the modern day differential gear in the year 1827.

In 1874, Aveling and Porter, a British agricultural engine and steam roller manufacturer listed a 2 ton crane engine with a rear differential to permit tight cornering without disconnecting both the rear wheels.

In 1876, James Starley who was known as the father of bicycle industry invented a chain drive differential for bicycles. Interestingly, Karl Benz used this invention on his automobiles, as we can see in Benz patented Motorwagen of 1885. Power was transmitted by means of two roller chains to the rear axle.

Working of a deferential:

Differential is an integral part of a driven axle. Wheels receive power from the engine via driveshaft. The main function of a differential is to allow the wheels to rotate at different rpm while receiving power from the transmission.



The ingenious mechanism of the differential allows wheels to rotate at different rpm, while transferring power to both wheels. Lets learn a differential's construction in a step by step manner:

• Power from the transmission is transferred to a ring gear via a pinion gear.


• The crown wheel is attached to a differential cage, which contains the 'sun' and 'planet' gears.


• There are 2 sun gears which rotate in the same axis of the ring gear. There are 2 planet gears which rotate in a axis perpendicular to the ring gear.


• The planet gears are attached to the ring gear and meshed with the sun gears.


• Each wheel is connected to the sun gears independently with the help of half axle shafts. The drive from the sun gear is transferred to the wheels

Case 1 (Vehicle moving in a straight direction):

Consider a vehicle moving in a straight direction. At this point, the planet gears rotate along with the ring gear but does not rotate along it's own axis. Therefore, both the sun gears rotate at the same speed, so does the wheels. In other words, the entire differential cage unit will rotate as a single solid unit.

Case 2 (Vehicle taking a right turn):

Consider a case where vehicle is taking a right turn. In this case, the planet gears play a pivotal role. The planet gears not only rotates along with the ring gear, but also rotates about it's axis. The left wheels has to rotate faster than the right wheel.

The effective combined rotation of the 2 sun gears should be equal to the planet gears i.e. peripheral velocity of the sun gears should be equal to the planet gears.

Another simple way to understand the mechanism is that the speed of the left wheel is equal to the sum of the rotational speed and the spinning speed of the planet gears. Whereas, the speed of the right wheel is the difference between the rotational speed and the spinning speed of the planet gears. This helps in making the left wheel rotate faster than the right wheel.

It is the vice-versa when a vehicle is taking a left turn. The planet gears spin in the opposite direction.

Speed reduction at the differential:

This is also one of the functions of a differential, where the speed from the transmission is reduced by having a larger ring gear compared to the pinion gear. This set-up increases the gear ratio, thus reducing the speed. This is also known as final gear ratio. If the final gear ratio is 1:5, that means the ring gear has 5 times more teeth than that of pinion gear.

Disadvantage of a standard differential:

The main disadvantage of a standard or an open differential is that it is not effective on surfaces offering different traction. Consider a situation where one wheel is moving on a slippery track (water) and the other wheel is on a rough  surface. In this case, the standard differential will send the majority of the power to the slippery wheel and hence the vehicle is stuck.  To overcome this problem, Limited Slip Differentials (LSD) are used.