Introduction
A
differential is a device, usually, but not necessarily, employing gears, which
is connected to the outside world by three shafts, through which it transmits
torque and rotation. The gears or other components make the three shafts rotate
in such a way that a=pb+qc, where a, b,
and c are the angular velocities of the three shafts, and p and q are
constants. Often, but not always, p and q are equal, so a is proportional to
the sum (or average) of b and c.
In
automobiles and other wheeled vehicles, a differential allows the driving road
wheels to rotate at different speeds. This is necessary when the vehicle turns,
making the wheel that is travelling around the outside of the turning curve
roll farther and faster than the other. The engine is connected to the shaft
rotating at angular velocity a. The driving wheels are connected to the other
two shafts, and p and q are equal. If the engine is running at a constant
speed, the rotational speed of each driving wheel can vary, but the sum (or
average) of the two wheels' speeds cannot change. An increase in the speed of
one wheel must be balanced by an equal decrease in the speed of the other.
Types of differential
Ø Open differential
Ø LSD(limited slip
differential)
Ø Locking differential
Ø Auto LSD(limited slip
differential)
Open differential
Are
the conventional type of differential usually consist of :
simple TAIL PINION (as shown below)
A Crown gear (as shown below)
Sun & Planet gear
When
road is straight wheel moves in a straight line direction, Pinion rotates the
crown(pinion is connected by the engine by the means of propeller shaft).Crown
with its housing consist of BEVEL gears i.e sun & planet gears. When
vehicle moves straight there is no speed difference between two wheels hence
when pinion rotates the crown gear rotates and in housing bevel gear rotates
but sun gears does not rotate.(more clearly explain in video) thus providing
constant speed to both wheels, but in case of turn both wheels travel different
distance inner-one has to travel lesser distance as compared to outer,
therefore the difference in speed is compensated by bevel gears they relatively
increase speed of one side of axle &
reduce the speed of other. Assume on straight line both wheels are moving with
speed 10km/h. but on turn the wheels will move with different speed the bevel
gears increase the speed of outer wheel say 12km/h & will reduce the speed
of inner one by 8km/h.
AXLE
RATIO : Is defined as in how much no. of pinion turns , crown turns one
revolution.
= No. of turns by pinion/ No. of turns by crown
IT’S DRAWBACK
If
one get off road or enter in low traction area whole power will transmitted to
low traction area. Hence wheel will rotate there with full R.P.M but will not
come out due to low griping or friction b/w tyre surface & road surface.
This
generally happen when one wheel struck in mud, sand or watery surface.
LSD(limited slip differential)
Like
open differential drive LSD also work on same principle but in this case we
have little different assembly. LSD is manufactured to remove the drawback of
open differential (when one wheel lose traction vehicle lose traction i.e. one
wheel spin freely over low friction area because whole torque is diverted over that wheel). Therefore in this case when
such condition occurs the major power here transmitted to that wheel but some
amount of necessary torque is given to other wheel so that vehicle moves &
one wheel which has traction pulls the other wheel which has no traction.
CONSTRUCTION
Pressure
rings with V-grooves are assembled in the case of the mechanical differential.
The four
lugs
(protrusions) on the periphery of the pressure rings fit in the grooves
provided inside the differential
case
when the pressure rings are assembled, and the pressure rings can slide in axial
directions.
The
tips of the pinion shaft are chamfered to assemble the pinion shaft into the
V-grooves of the
pressure
rings, forming V-shaped cams. Similar to the pressure rings, the friction
plates and spring
plates
also have four lugs (protrusions) on the periphery and these lugs fit in the
grooves provided
inside
the differential case when the plates are assembled.
On
the internal circumference of the friction disks and spring plates there are 4
or 6 lugs (protrusions)
which
fit in the grooves provided on the outer circumference of the side gears, and
the friction
disks
and spring plates can slide in axial directions.
These
friction plates, friction disks, spring plates, spring disks are alternately
arranged and form a
clutch
as a whole.
The
spring plates and spring disks act as Belleville springs and provide initial
torque. Moreover,
thrust
washers are inserted between the side gears and differential case halves to
adjust clearance
in
the axial direction and prevent thrust from the side gears to work against the
pressure rings.
IT’S DRAWBACK
It
has one dis-advantage that is most of power is transmitted to low traction
wheel. Here most of power is wasted due to transferring of more power to low
traction wheel.
Locking differential
To
overcome the fault or dis-advantage of open-differential & Limited slip
differential (LSD), we have applied a mechanical or electronic or hydraulic
torque provider unit which lock two axle providing with same torque or divides
1 torque coming from propeller to ½ & ½ torque on each axel & from the
to wheel.
So
when one wheel loses the traction the other wheel will be having same torque as
wheel which loses the traction.
IT’S DRAWBACK
The
dis-advantage of locking differential is that at times of turning we have to
un-lock this process of dividing the torque because at times of turning if both
wheel will turn at same speed it will be difficult to turn.
AUTO LSD or BREAK ACTIVATED LSD
To
overcome limitations of open, lsd, locking differential a new type of
differential is used they are electronic operated.
They
work on basic principle of open differential but electronic operated which
result when one wheel loses its traction the break are automatically activated
to that side wheel resulting in
providing more resistance to that wheel which result in transferring most of
the power to other wheel.
In
this type breaks are selectively applied to those wheels which have least
traction & power transfer to least resistance path. Hence will take out
vehicle if it struck somewhere in low traction area.
Refer this video for interactive learning
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