Differentials and Diff' Locks for Four Wheel Drives.
The differential is the sort of thing that I would love to have
invented so that it would have been named after me instead of Mr. Differential.
It was used in tricycles in the 19th century
(it still is used in some modern trikes)
and also as an adding device in mechanical analogue computers.
It is used in a car axle to allow the outer wheel to rotate more
quickly when the car turns a corner.
The outer wheel travels about 10m (30 feet) further than the inner
one in a 360 degree turn.
A 4WD has one diff' in each axle.
A full-time 4WD vehicle also has a centre-diff' in the transfer-case
because the front wheels rotate more quickly than the rear ones
when turning a corner and travelling forwards.
The Open Diff.
The heart of the differential is the diff' centre (right). It rotates, driven through the crown wheel (gear) by an input shaft and pinion gear. Internally it carries two or four planet-gears and two side-gears. These planet-gears drive the side-gears which are connected to the half-shafts which drive the wheels. When driving straight ahead, the planet-gears do not rotate relative to the diff' centre, and the centre, the side-gears, half-shafts and wheels all rotate at the same speed.
If one wheel needs to rotate more quickly, it can do so, overtaking the centre by rotating the planet-gears in the centre which makes the other wheel rotate less quickly. In fact the centre always rotates at the average speed of the wheels, C=(left+right)/2, which is why it can be used to add inputs in mechanical analogue computers. If the centre is stationary, a wheel can still rotate provided that the opposite wheel rotates in the opposite direction. For this reason it is especially important that vehicles having a transmission hand-brake be securely chocked when a rear wheel is jacked up.
The differential action of these `open differentials' is essential to stop tyres scrubbing, to reduce transmission loads, and to reduce understeer during normal cornering.
The torque carried to the wheels by the two half-shafts must be equal because they are balanced through the planet-gears. This means that if one wheel looses grip, due to slippery ground or through being in the air, the other wheel effectively losses all input torque and cannot propel the vehicle. A 2WD with an open diff' looses traction if just one driving wheel does. A full-time 4WD, with an (unlocked) open centre-diff' looses traction if any one wheel does. A vehicle with 4WD engaged (or with a centre-diff' locked) and with open axle diff's looses traction if one wheel on each axle looses traction. This can happen if the vehicle gets crossed-up by being supported on diagonally opposite wheels, or if one side is on slippery ground, for example.
Coefficients of frictions from :
The differential action while essential for normal driving can leave a 4WD stuck with just two spinning wheels. The most obvious cure is a diff' lock. This is a dog-clutch of some kind that prevents the differential action when it is engaged. The most common arrangement is a dog clutch on one half-shaft, which can lock it to the diff' centre. This also locks the other half-shaft, indirectly, via the planet-gears. The dog clutch is engaged by some sort of external actuator. This system is mechanically simple. The Maxi-Drive (right) is of this type.
Many full-time 4WD vehicles use a (centre) diff' lock of this kind in the transfer-case, although we then have front and rear transfer-case output shafts in place of axle half-shafts. It is important to note that a centre-diff' lock does not lock the axle diff's.
The Roberts (now ARB) diff' lock is internal to the diff' centre.
A dog clutch locks the side-gears to the centre.
The clutch is engaged by an annular piston
driven by compressed air and is disengaged by springs.
The problem is getting the air to the piston inside the rotating centre!
The air is carried in from outside through a rotating seal,
similar to devices used to inflate tyres on the move on
some very heavy duty offroad vehicles and some military vehicles.
The result is ingenious but complex.
A disadvantage of the design is that the "warning light"
shows when the diff' is intended to be locked
not when it is actually locked.
In some ways a diff' lock is the ultimate traction device.
All wheels revolve at the same speed when engaged.
A vehicle with locks on all axles has traction if just one wheel does.
Unfortunately a diff' lock is on or off; there is no in-between.
This can make the vehicle understeer (tend to go straight ahead in corners)
or veer suddenly sideways if one side looses traction, on ice say.
It also means that all of the engine's torque, amplified by
low-range, can go through one half-shaft and wheel.
The components had better be able to stand the strain.
Some diff' locks come with strengthened half-shafts and
other components for this reason.
The perfect differential would provide little or no resistance to the differential action when the difference in rotational speed between the output shafts is small, ie. when cornering. It would provide increasing resistance as the difference in speed increases, ie. under wheel spin. Several limited-slip differentials (LSD) approximate this ideal to varying degrees.
The simplest LSDs provide some friction between the side-gears and the diff' centre. This can take the form of spring-loaded plates alternately keyed to the side-gears and to the centre and pressed against each other. The plates are naturally liable to heating and to wear in heavy use and may then become ineffective. The friction also resists the differential action at even low speed differences - and may affect cornering.
Mercedes and Porsche have developed more intelligent (and expensive) systems where the limited-slip plates are pressed together under hydraulic pressure when electronic sensors detect wheel spin.
The viscous-coupling also employs plates but these are not in physical contact with each other. Instead they are in a sealed drum full of viscous fluid (silicone based). They can contra-rotate freely at low speeds, but the resistance increases very rapidly with the speed difference. The precise characteristics of the device can be controlled by the choice of viscous fluid and by drilling holes in the plates. Range-Rovers from the late 1980s-on use a viscous-coupling in the centre-diff'. (I would be interested to know if a Range-Rover parked on a very steep, loose hill is liable to creep if the rear wheels slip.) Viscous-couplings were developed by Ferguson (of the P99 F1 car and the Jensen FF 4WD sports coupe). They are also used in some front-wheel drive cars.
Almost all open diff's use bevel gears within the centre, but this is not the only possible arrangement. Some early cars (maybe Austin 7 ?) instead used pairs of plain gears to link the two side-gears. You can see that pairs of gears are needed to allow the side-gears to rotate in opposite directions relative to the centre. All of these gears are mounted within the centre. Some modern LSDs use a variation on this theme. Pairs of gears are used but these are mounted in the centre in such a way as to provide a great deal of friction if they rotate at high speed under load. Unfortunately there is little friction under no load, ie. if one wheel is actually airborne.
Yet another arrangement uses the fact that a fine-pitch worm gear
cannot be driven backwards, and that a coarse-pitch worm gear can only
be driven backwards with difficulty.
These LSDs use pairs of coarse worm gears, mounted in the centre,
to drive the side-gears and hence the wheels.
Each worm gear contra-rotates with its twin through
The Detroit locker is quite different from other differentials. The planet-gears and side-gears are replaced by what appear to be three plates. The middle plate is driven by the crown wheel. A number of cams pass through the middle plate and drive the side plates. These cams will allow the outer, faster-rotating wheel and its plate to overtake the middle plate. This is not a simple ratchet or free-wheel because, and this bit always seems like magic, the device also works correctly in reverse.
Lockers transfer torque to the inner wheel in a corner and the action is not always smooth so they affect steering characteristics and are often advised against for SWB vehicles and for "normal applications". However...... Mark Ritter fitted a Lock-Righttm locker to his 1994 Land-Rover Discovery (full-time 4WD):
On the street there is no change except for a mild clicking noise as the outside wheel unlocks so that it can travel faster in a turn. [And off road] no tire spin or drama, and much more control.
Traction control involves no modification to differentials at all. The idea is simply to apply the brake on any spinning wheel. This will slow it down and force torque to go to its opposite number which still has traction. Some trials cars, but no road cars, do have manually operated "fiddle" brakes for individual drive wheels. However, an anti-lock braking system (ABS) already has most of the necessary bits and pieces: a speed sensor for each wheel and the ability to control the braking of each wheel individually. Clearly the normal driving characteristics of the car are not affected. Many luxury cars, including late model Range Rovers use traction control in conjunction with ABS.
A few final remarks on centre (transfer-case or gearbox) diff's and full-time 4WD:
The torque-split fore and aft need not be 50:50. If you think about it, the side-gears can be of different sizes and apply different leverages to their respective output shafts. Some 4WD rally cars used 40:60 or 30:70 torque-splits.
A centre-diff' lock on a full-time 4WD is open to abuse - see the manufacturer's handbook. If it is engaged on firm surfaces it can cause transmission wind-up and rapid wear. If it is not engaged on loose surfaces the diff' can overheat and fail; it is small and does everything three or four times faster than the road wheels. It is quite strong enough if used correctly.
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