Limited slip differential
From WOI Encyclopedia Italia
A Limited Slip Differential (LSD) is a modified or derived type of differential gear arrangement that allows for some difference in rotational velocity of the output shafts, but does not allow the difference in speed to increase beyond a preset amount. In an automobile, such limited slip differentials are sometimes used in place of a standard differential, where they convey certain dynamic advantages, at the expense of greater complexity.
The main advantage of a limited slip differential is found by considering the case of a standard differential where one wheel has no contact with the ground at all. In such a case, the contacting wheel will remain stationary, and the non-contacting wheel will rotate at twice its intended velocity – the torque transmitted will be zero and the vehicle will remain stationary. In everyday use on typical roads, such a situation is very unlikely, and so a normal differential suffices. For more demanding use however, such as driving off-road, or for high performance vehicles, such a state of affairs is undesirable, and the LSD can be employed to deal with it. By limiting the velocity difference between a pair of driven wheels, useful torque can be transmitted as long as there is some friction available on at least one of the wheels.
Contents |
Types
Two main types of LSD have been generally used – mechanical (geared or clutch-based) and fluid based (viscous). The latter is gaining ground especially in modern all-wheel drive vehicles, and generally requires less maintenance than the mechanical type.
Mechanical
In the mechanical clutch type, a mechanism, such as a centrifugal weighted rotor, detects differential wheel velocity and applies friction to the clutch mechanism which links the two shafts together. As the differential wheel velocity increases, more friction is applied. This mechanism forms a negative feedback loop which limits the slip to a preset degree. In some designs, the clutch is self-actuating and oftentimes small multi-plate clutches are used. Because the slip-limiting action (increasing friction) occurs quite rapidly, this method can create unsettling dynamic effects for the vehicle as a whole.
In this case, the use of the word mechanical implies that the limited slip differential is engaged or not due to interaction between two (or more) mechanical parts. This category includes clutch and helical limited slip differentials. For road racing, many prefer a helical limited slip differential, because it does not lock the two output shafts to spin at the same rate, but rather biases torque to the wheel with more grip by up to 80%.
Clutch limited slip differentials use a center cam that moves within a casing as the torque changes. The casing is made up of two symmetrical left and right segments. However, the cuts in the casing making the notches for the cam to slide in are not. That determines 1, 1.5, or 2-way LSD. As the cam slides in the notch, it pushes the casing outward, engaging a series of clutch discs--some attached to the casing, some to the output shafts. When engaged, both output shafts will rotate at the speed of the casing, making both axles and subsequently both wheels, rotate at the same speed.
A 1-way notch is cut like an upside down triangle. While the cam can push backward against the tapered edges, expanding the casing, it cannot push forward against the flat surface. Therefore under acceleration torque (cam rotating backwards) it will lock, and under deceleration torque, when the cam is forced to rotate forward due to forces from braking, engine braking, etc.. it will just contact a flat "wall" and the casing will not expand.
A 1.5-way notch is like an upside down triangle with a half triangle on top of it. During acceleration it will expand the casing at one rate, and during deceleration, it will still expand the casing, but due to the cuts' higher angles, it will require more force to move the casing apart. Therefore, only during Very hard braking will it have enough force pushing it forward to expand the casing.
A 2-way notch is shaped like a diamond. It requires almost the same amount of acceleration or deceleration to force the casing apart. Usually, the top cuts are slightly more dramatic, forcing the 2-way to require slightly more deceleration force to push the cam to expand the casing.
The more the casing expands, the more clutches contact each other, hence the more the output shafts get locked into the same rotation. Some manufacturers produce adjustable clutch limited slip differentials whereby you may set a breakaway torque level. Resultantly, the clutch discs are moved closer together or further apart to dictate the SOFT, MED, or HARD setting. The closer the clutch plates are to each other, the more readily the output shafts--thus the wheels--will spin in sync.
Geared, torque-sensitive mechanical limited slip differentials utilize planetary gears to "sense" torque on one shaft. The most famous version is the Torsen differential invented by Vernon Gleasman in 1958, then sold to Gleason Corporation, who started marketing it in 1982. Geared LSDs are less prone to wear than the clutch type, but some have found their torque distribution characteristics to be less than ideal.
Viscous
The viscous type is generally simpler, and relies on the properties of a dilatant fluid – that is, one which thickens when subject to shear. Silicone-based oils are often used. Here, a chamber of fluid rotates with the normal motion of the output shafts, but a differential motion causes paddles or vanes to move through the fluid. The greater the speed of the vanes, the more resistance the fluid will put up to oppose this motion. In contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, so for the average driver is generally much easier to cope with.
Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. However, they are less prone to breakdown as long as the fluid is changed regularly.
Factory Names
In the 1950's and 1960's many manufacturers began to apply brand names to their LSD units. The most famous of these was Chevrolet's "Positraction". Since then, Positraction (often shortened to "positrac" or merely "posi") has become a genericized trademark for LSD's.
Other factory names for LSD's include
Pontiac: Safe-T-Track
Ford: Equa-Lock and Trac-Lok
American Motors Corporation: Twin-Grip
Mopar: Sure Grip
Ferrari: E-Diff
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