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Limited Slip Differential
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.
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.
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 LSD 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. Kaaz LSD http://www.kaazusa.com/introductionToLSD/introduction_To_LSD.htm article source: wikipedia.org |
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