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HOW DIFFERENTIALS WORK
If
you've read How Car Engines Work, you understand how a car's power is
generated; and if you've read How Manual Transmissions Work, you
understand where the power goes next. This article will explain
differentials -- where the power, in most cars, makes its last stop
before spinning the wheels.
The differential has three jobs:
• To aim the engine power at the wheels
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To act as the final gear reduction in the vehicle, slowing the
rotational speed of the transmission one final time before it hits the
wheels
• To transmit the power to the wheels while allowing them
to rotate at different speeds (This is the one that earned the
differential its name.)
In this article, you'll learn why
your car needs a differential, how it works and what its shortcomings
are. We'll also look at several types of positraction, also known as
limited slip differentials.
Why You Need a Differential
Car
wheels spin at different speeds, especially when turning. You can see
from the animation below that each wheel travels a different distance
through the turn, and that the inside wheels travel a shorter distance
than the outside wheels. Since speed is equal to the distance travelled
divided by the time it takes to go that distance, the wheels that
travel a shorter distance travel at a lower speed. Also note that the front wheels travel a different distance than the rear wheels.
For
the non-driven wheels on your car -- the front wheels on a rear- wheel
drive car, the back wheels on a front-wheel drive car -- this is not an
issue. There is no connection between them, so they spin independently.
But the driven wheels are linked together so that a single engine and
transmission can turn both wheels. If your car did not have a differential,
the wheels would have to be locked together, forced to spin at the same
speed. This would make turning difficult and hard on your car: For the
car to be able to turn, one tire would have to slip. With modern tires
and concrete roads, a great deal of force is required to make a tire
slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components.
What is a Differential?
The differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed.
Part-time
four-wheel-drive systems don't have a differential between the front
and rear wheels; instead, they are locked together so that the front
and rear wheels have to turn at the same average speed. This is why
these vehicles are hard to turn on concrete when the four-wheel- drive
system is engaged.
Open Differentials
We
will start with the simplest type of differential, called an open
differential. First we'll need to explore some terminology: The image
below labels the components of an open differential.
When
a car is driving straight down the road, both drive wheels are spinning
at the same speed. The input pinion is turning the ring gear and cage,
and none of the pinions within the cage are rotating -- both side gears
are effectively locked to the cage.
Differentials and Traction
The
open differential always applies the same amount of torque to each
wheel. There are two factors that determine how much torque can be
applied to the wheels: equipment and traction. In dry conditions, when
there is plenty of traction, the amount of torque applied to the wheels
is limited by the engine and gearing; in a low traction situation, such
as when driving on ice, the amount of torque is limited to the greatest
amount that will not cause a wheel to slip under those conditions. So,
even though a car may be able to produce more torque, there needs to be
enough traction to transmit that torque to the ground. If you give the
car more gas after the wheels start to slip, the wheels will just spin
faster.
On Thin Ice
If
you've ever driven on ice, you may know of a trick that makes
acceleration easier: If you start out in second gear, or even third
gear, instead of first, because of the gearing in the transmission you
will have less torque available to the wheels. This will make it easier
to accelerate without spinning the wheels.
Now what happens if
one of the drive wheels has good traction, and the other one is on ice?
This is where the problem with open differentials comes in.
Remember
that the open differential always applies the same torque to both
wheels, and the maximum amount of torque is limited to the greatest
amount that will not make the wheels slip. It doesn't take much torque
to make a tire slip on ice. And when the wheel with good traction is
only getting the very small amount of torque that can be applied to the
wheel with less traction, your car isn't going to move very much.
Off Road
Another
time open differentials might get you into trouble is when you are
driving off-road. If you have a four-wheel drive truck, or an SUV, with
an open differential on both the front and the back, you could get
stuck. Now, remember -- as we mentioned in the previous page, the open
differential always applies the same torque to both wheels. If one of
the front tires and one of the back tires comes off the ground,
they will just spin helplessly in the air, and you won't be able to
move at all.
The solution to these problems is the limited
slip differential (LSD), sometimes called positraction. Limited slip
differentials use various mechanisms to allow normal differential
action when going around turns. When a wheel slips, they allow more
torque to be transferred to the non- slipping wheel.
The next
few sections will detail some of the different types of limited slip
differentials, including the clutch-type LSD, the viscous coupling,
locking differential and Torsen differential.
Clutch-type Limited Slip Differential
The clutch-type LSD is one of the most common versions of the limited slip differential.
This
type of LSD has all of the same components as an open differential, but
it adds a spring pack and a set of clutches. Some of these have a cone
clutch that is just like the synchronizers in a manual transmission.
The
spring pack pushes the side gears against the clutches, which are
attached to the cage. Both side gears spin with the cage when both
wheels are moving at the same speed, and the clutches aren't really
needed -- the only time the clutches step in is when something happens
to make one wheel spin faster than the other, as in a turn. The
clutches fight this behaviour, wanting both wheels to go the same
speed. If one wheel wants to spin faster than the other, it must first
overpower the clutch. The stiffness of the springs combined with the
friction of the clutch determines how much torque it takes to overpower
it. Getting back to the situation in which one drive wheel is on the
ice and the other one has good traction: With this limited slip
differential, even though the wheel on the ice is not able to transmit
much torque to the ground, the other wheel will still get the torque it
needs to move. The torque supplied to the wheel not on the ice is equal
to the amount of torque it takes to overpower the clutches. The result
is that you can move forward, although still not with the full power of your car.
Viscous Coupling
The
viscous coupling is often found in all-wheel-drive vehicles. It is
commonly used to link the back wheels to the front wheels so that when
one set of wheels starts to slip, torque will be transferred to the
other set.
The viscous coupling has two sets of plates inside
a sealed housing that is filled with a thick fluid, as shown in below.
One set of plates is connected to each output shaft. Under normal
conditions, both sets of plates and the viscous fluid spin at the same
speed. When one set of wheels tries to spin faster, perhaps because it
is slipping, the set of plates corresponding to those wheels spins
faster than the other. The viscous fluid, stuck between the plates,
tries to catch up with the faster disks, dragging the slower disks
along. This transfers more torque to the slower moving wheels -- the
wheels that are not slipping.
When a car is turning, the
difference in speed between the wheels is not as large as when one
wheel is slipping. The faster the plates are spinning relative to each
other, the more torque the viscous coupling transfers. The coupling
does not interfere with turns because the amount of torque transferred
during a turn is so small. However, this also highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping.
A
simple experiment with an egg will help explain the behaviour of the
viscous coupling. If you set an egg on the kitchen table, the shell and
the yolk are both stationary. If you suddenly spin the egg, the shell
will be moving at a faster speed than the yolk for a second, but the
yolk will quickly catch up. To prove that the yolk is spinning, once
you have the egg spinning quickly stop it and then let go -- the egg
will start to spin again (unless it is hard boiled). In this
experiment, we used the friction between the shell and the yolk to
apply force to the yolk, speeding it up. When we stopped the shell,
that friction -- between the still-moving yolk and the shell -- applied
force to the shell, causing it to speed up. In a viscous coupling, the
force is applied between the fluid and the sets of plates in the same way as between the yolk and the shell.
Locking and Torsen
The
locking differential is useful for serious off-road vehicles. This type
of differential has the same parts as an open differential, but adds an
electric, pneumatic or hydraulic mechanism to lock the two output
pinions together.
This
mechanism is usually activated manually by switch, and when activated,
both wheels will spin at the same speed. If one wheel ends up off the
ground, the other wheel won't know or care. Both wheels will continue
to spin at the same speed as if nothing had changed.
The Torsen differential* is a purely mechanical device; it has no electronics, clutches or viscous fluids.
The
Torsen (from Torque Sensing) works as an open differential when the
amount of torque going to each wheel is equal. As soon as one wheel
starts to lose traction, the difference in torque causes the gears in
the Torsen differential to bind together. The design of the gears in
the differential determines the torque bias ratio. For instance, if a
particular Torsen differential is designed with a 5:1 bias ratio, it is
capable of applying up to five times more torque to the wheel that has good traction.
These
devices are often used in high-performance all-wheel-drive vehicles.
Like the viscous coupling, they are often used to transfer power
between the front and rear wheels. In this application, the Torsen is
superior to the viscous coupling because it transfers torque to the
stable wheels before the actual slipping occurs.
However, if
one set of wheels loses traction completely, the Torsen differential
will be unable to supply any torque to the other set of wheels. The
bias ratio determines how much torque can be transferred, and five
times zero is zero.
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WHY YOUR DIFFS KEEP BREAKING#
In
many applications, 2 pin differentials are not up to the job or in
simple terms, not strong enough. They are perfect for the normal on
road going vehicle and some gentle green laning or light off road use.
Many of you reading this will know all to well that they break easily.
The reason for this is as follows.
What is known as the
differential carrier, houses the 4 gears. The gears do not move around
while the vehicle is being driven in a straight line. They only move
when the front or rear wheels are travelling at different speeds. They
rotate around to allow the wheels to rotate at different speeds, hence
the name, differential.
For normal road use, these work fine. But, when the vehicle is being driven off road, circumstances change.
For
example, if one wheel on the same axle is allowed to spin much faster
then the other maybe due to the vehicle being stuck, all of the torque
is diverted into the centre of the diff where the gears do their job.
They spin around very fast, throwing all of the oil off them which
causes the centre pin to overheat and break due to poor pin and carrier
design.
This often results in the gears being spat out of the diff pan and a few hours of repair. Not much fun in a cold muddy field.
Now,
for those of you who wish to prevent breaking your differentials but
gain no more torque, 4 pin diff will solve your problem. (Check out the
4 pin diff link). Limited slip or locker option will also prevent this
from happening.
Here are some typical examples of common breakages
| Above is a standard 2 pin differential carrier broken from shock loading. The broken pin and broken gears also pictured are common breakages for these differentials.
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| Broken sun and planet gears are prone to breaking when off-roading.
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| Modified pins will certainly help but not prevent the gears from breaking. The modification of the pin helps to keep the gears lubricated and keep cooler, thus preventing breakages.
Below is a modified pin ground on both sides.
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| Standard Land Rover pin is machined on only one side (below)
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| A flat on each side of the cross shaft will allow more oil to get in-between the cross shaft and gears. This makes the lubrication better and less heat build up to not so much friction. Land Rover 4 pin differential have flats machined on both sides.
Another problem with standard type rover ring and pinions, especially the 4.7:1 ring and pinion as fitted to Series 1, 2, 2A and 3 is ring and pinion breakages, pictured below.
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| Above is a 4.7:1 ring and pinion. Below is a 3.8:1 ring and pinion
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| Both of these have seen better days but they have broken to either shock loading or too much horse power.
The only thing that can be done to keep the ring gear in mesh with the pinion is to have your differentials pegged.
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Please see the Diff Pegging navigation link for more information.
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HOW I BUILD DIFFS#
I
though I would take the time to do a step by step guide to how I
rebuild differentials. In principal, all differentials are built the
same. However, some differentials have different torque settings and
pre loads. The correct figures can normally be found in the repair and overhaul sections of the relevant workshop manuals.
As
an example, I am using a 10 spline metric differential for this
demonstration. If you fancy having a go at rebuilding a differential, I
recommend that you make sure you have the necessary data as well as
some special tooling and a good range of pinion height shims and pre
load shims.
Before dismantling a differential, it is critical
that you stamp one of the bearing caps and pinion housing so that they
do not get mixed up.
Then, remove the roll pins and throw them away.
Remove the locking fingers and then undo the 4 bearing cap bolts.
Remove the bearing caps and the bearing adjusting nuts.
Carefully remove the differential carrier from the pinion housing.
Remove the 10 bolts holding the crown wheel onto the differential carrier starting at 12 o clock, 6 o clock and so on.
If
the crown wheel does not come off the carrier easily, the use of a
rubber mallet may be needed. Do not use a normal or brass hammer.
Remove the bearings and throw them away.
Remove both of the circlips holding in the cross shaft and throw them away.
Check
movement of the cross shaft in the casting. If there is minimal
movement the carrier can be reused. If there is quite a bit of play the
carrier should be scrapped and a replacement should be used.
Remove the 2 sun gears and 2 planet gears.
Remove the split pin from the castellated nut and throw it away.
Undo the nut and remove the washer and drive flange holding the pinion in the other hand so it does not fall on the floor.
Remove the spacer and shims.
Remove the head bearing from the pinion and throw away.
Remove
the oil seal from the pinion housing making sure not to scratch the
inside of the case, throw the oil seal away. Remove the tail bearing
and throw that away as well.
Remove the head bearing and tail
bearing tracks. This can be done with the proper land rover tool or a
suitable brass hammer and drift.
Clean and inspect all components for wear, pitting, scoring etc.
Reassembly
should not be rushed into. Take your time as you will not get another
chance to build it again if you set it up wrong and it breaks.
When all components have been cleaned and inspected make sure your rebuild surface it spotlessly clean as well as your tools.
Tools required:
Selection of sockets.
Drive flange holding wrench.
DTI Gauge
Dial indicating torque wrench
Torque wrench
Stud lock grade 2701 or equivalent
Precision marking blue
Pinion height setting block or digital gauge
Dumbell or genuine Land Rover tool
Vernier
A selection of bearing collars for pressing in and out bearing tracks
Place the differential carrier, the 2 sun gears, 2 plant gears, cross shaft and 2 new circlips on a clean work surface.
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| Smear some EP90 onto the back of the 2 sun gears.
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| | Carefully place the 2 sun gears and 2 planet gears into the differential carrier making sure the planet gears are lined up for the cross shaft to go through. Then fit the 2 new circlips and make sure they are correctly seated. |
| Place the crown wheel onto the carrier making sure the 2 mating surfaces are dry and clean. Pull the crown wheel on Square and tighten the bolts with a torque wrench. 61Nm is the correct setting for this differential.
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| | Next step is pressing the bearings onto the carrier. |
| | Make sure that the inside of the bearing is pressed on and not the cage as this will cause damage. |
| When the first bearing is pressed on, the carrier needs to be flipped over and a collar needs to be placed underneath the first bearing to prevent any pressure on the cage.
The carrier should then be checked for crown wheel run out. If the reading is acceptable, the bolts should be undone one at a time and studlocked and tightened to the correct torque setting.
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| The next step is to press the head bearing onto the pinion.
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| | Again, using a sleeve, press on the bearing making sure the cage is not pressed on. |
| | It is now time to fit the head and tail bearing tracks and pinion height shim. |
| Using the right size boss for the bearing, press in the head bearing track into place with the pinion height shim behind it.
Make sure the pinion height shim is clean and free from any burs.
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| Once the head bearing track has been pressed in, the case can be
flipped over and the tail bearing track can be press in.
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| With the pinion housing prepared and pinion it is time to check the pinion height.
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| This can be done with either of the tools above. A setting block and dumbbell, or the latest Land Rover tooling to set it up with digital gauges. As you can see from the pictures, I use both types.
If the pinion height is to high a thinner shim will be needed.
If the pinion height is to low, a thicker shim will be required.
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| Once the pinion height has been set you will need to set the pinion pre-load.
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| A wide range of pinion height and pre load shims is essential.
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| Pinion pre loads ideally need to be set with a dial indicating torque wrench
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| Once the pinion pre loads have been set the oil seal can be pressed into the pinion housing. Using the correct tool will make sure it sits at the right depth and also sit in the pinion housing square. Do not forget to insert the tail bearing.
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| | Once pressed in, the pinion can be fitted in permanently. |
| | The mud shield can be pressed onto the drive flange |
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| | With the pinion in the pinion housing, the drive flange can be fitted and tightened up to the correct torque. The split pin can then be put through the castleated nut unless you need to use a nylock nut. |
| Make sure there are no burs where the bearing caps will sit.
Use an aluminium oxide stone, carefully remove any burs. |
| Using an airline, blow away any dust. Goggles should be worn.
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| | Carefully lower the differential carrier into the pinion housing and place the bearing adjusting nuts in place making sure they are not cross threaded. |
| | Put the 2 bolts through the bearing cap and lower it into place. |
| | Tighten all 4 bolts by hand (a good hand tight) |
| | Using either the proper tool or a home made version below, it is time to set up the backlash and bearing preload. |
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| Unwind the carrier bearing nut as illustrated but not completely out.
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| Wind the tool in one serration at a time until all of the backlash has been removed. Check on 3 points of the crown wheel.
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| Using a DTI gauge, wind the carrier bearing adjusting nut in until it reaches the back of the bearing. Then, one serration at a time, introduce backlash until you reach a minimum of 4 thou and a maximum of 7 thou. When this figure has been reached, check the crown wheel backlash in 3 different places.
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| Using loctite 2701 or equivalent. Remove one bolt at a time from the bearing caps and coat the thread in studlock. Be careful not to over or under apply.
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| | When all 4 bolts have been tightened by hand, tighten the bolts to the correct torque. |
| Nearly done, fit the locking fingers and new roll pins.
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| | It’s nearly finished, time for some engineers marking blue for a final mesh check |
| The diff is stamped with a serial number for reference
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| | Cleaned and polished, ready for sale |
| | Boxed up and ready to send. Purpose built heavy duty boxes to ensure your differential arrives as safely as possible and intact. |
| Some of the special tools used building differentials
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| A selection of bearing drifts and oil seal inserters are essential
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HOW TO PAY#
We accept the following methods of payment:
Cash, Cheque, Postal order, Direct banking transfer, IBAN and BACS
We also accept the following credit and debit cards
A
receipt and invoice is provided with every transaction Once ordered, I
aim to get your differential to you as quickly as possible.
An estimated date will be given to you on the telephone.
You
are more than welcome to collect your differential from our workshops,
or, if required, from my home after 18:00 weekdays until 22:00 and
weekends by prior appointment.
We use Fed Ex couriers for all
of our U.K based deliveries. Fed Ex provide us with a tracking number
for every consignment enabling us to track your item every step of the
way.
Prices start at £13.69 per box, discount for 2 boxes or
more. An overnight or a two to three day service is available together
with timed deliveries including Saturdays for the more urgent requests.
All differentials are packed in purpose built, heavy duty
designed boxes (see below) to ensure your differential gets to you
safely. The price for carriage covers most of mainland United Kingdom.
If you live in some parts of Scottish Highlands, Offshore Islands,
Northern Ireland, Southern Ireland, Isle of Man, Isle of Wight, Scilly
Isles, Channel Islands etc please ask for a quote first.
INTERNATIONAL SHIPMENTS
We
use UPS or Interlink for all of our international shipments, with full
tracking service you will always be able to see where your parcel is.
SMALLER ITEMS
For smaller and lightweight items, I can post via the Royal mail by 1st or 2nd class recorded or special delivery.
Purpose
built heavy duty boxes to ensure your differential arrives as safely as
possible and intact INTERNATIONAL PAYMENTS BY IBAN ONLY
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WARRANTY#
All
reconditioned differentials come with 12 months unlimited mileage
warranty on parts. This applies to road going vehicles. If a
differential is used off road and is broken then the warranty is void.
For lockers, limited slips, 4 pins etc, please refer to the manufactures warranty or full details can be obtained by request.
If
a fault develops on a reconditioned differential within a 12 month
period from the purchase date then please do not hesitate to contact me
where I assure you the problem will be dealt with as soon as possible.
In some cases, a return to base of the unit may be required at the cost to the customer.
For differentials and associated parts such as KAM, ARB, DETROIT etc, manufactures warranty applies.
CUSTOMER SUPPORT#
What
to expect from Myself and Crown differentials. As you may have already
read, I am always at the end of my phone. If there is a problem, if you
need technical support, please phone me. You can call the workshop
number from 10am to 6pm and out of hours including weekends up until
10pm any night on my mobile.
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PRICES#
Due to the vast quantity of products we supply, we have not completed a price list to post on our website. Until this becomes available, please call us for all details.
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