Description of third differential and viscous coupling

(Fig. 7.36) The gearbox mainshaft provides the input of power to the third differential (sometimes referred to as the central differential). This shaft is splined to the planet pinion carrier (Fig. 7.36). The four planet pinions are supported on the carrier mesh on the outside with the internal teeth of the annulus ring gear, while on the inside the teeth of the planet pinions mesh with the sun gear teeth. A hollow shaft supports the sun gear. This gear transfers power to the front wheels via the offset input and output sprocket wheel chain drive. The power path is then completed by way of a pro-pellor shaft and two universal joints to the front crownwheel and pinion. Mounted on a partially tubular shaped carrier is the annulus ring gear which transfers power from the planet pinions directly to the output shaft of the transfer box unit. Here the power is conveyed to the rear axle

Description About Differential
Fig. 7.36 Third differential with viscous coupling

by a conventional propellor shaft and coupled at either end by a pair of universal joints.

Speed balance of third differential assembly with common front and rear wheel speed (Fig. 7.36) Power from the gearbox is split between the sun gear, taking the drive to the front final drive. The annulus gear conveys power to the rear axle. When the vehicle is moving in the straight ahead direction and all wheels are rotating at the same speed, the whole third differential assembly (the gearbox mainshaft attached to the planet carrier), planet pinions, sun gear and annulus ring gear will all revolve at the same speed.

Torque distribution with common front and rear wheel speed (Fig. 7.36) While rear and front pro-pellor shafts turn at the same speed, the torque split will be 66% to the rear and 34% to the front, determined by the 2:1 leverage ratio of this parti cular epicyclic gear train. This torque distribution is achieved by the ratio of the radii of the meshing teeth pitch point of both planet to annulus gear and planet to sun gear from the centre of shaft rotation. Since the distance from the planet to annulus teeth pitch point is twice that of the planet to sun teeth pitch point, the leverage applied to the rear wheel drive will be double that going to the front wheel drive.

Viscous coupling action (Fig. 7.36) Built in with the epicyclic differential is a viscous coupling resembling a multiplate clutch. It comprises two sets of mild steel disc plates; one set of plates are splined to the hollow sun gear shafts while the other plates are splined to a drum which forms an extension to the annulus ring gear. The sun gear plates are disfigured by circular holes and the annulus drum plates have radial slots. The space between adjacent plates is filled with a silicon fluid. When the front and rear road wheels are moving at slightly different speeds, the sun and annulus gears are permitted to revolve at speeds relative to the input planet carrier speed and yet still transmit power without causing any transmission wind-up.

Conversely, if the front or rear road wheels should lose traction and spin, a relatively large speed difference will be established between the sets of plates attached to the front drive (sun gear) and those fixed to the rear drive (annulus gear). Immediately the fluid film between pairs of adjacent plate faces shears, a viscous resisting torque is generated which increases with the relative plate speed. This opposing torque between plates produces a semi-lock-up reaction effect so that tractive effort will still be maintained by the good traction road wheel tyres. A speed difference will always exist between both sets of plates when slip occurs between the road wheels either at the front or rear. It is this speed variation that is essential to establish the fluid reaction torque between plates, and thus prevent the two sets of plates and gears (sun and annulus) from racing around relative to each other. Therefore power will be delivered to the axle and road wheels retaining traction even when the other axle wheels lose their road adhesion.

7.7.7 Longitudinal mounted engine with integral front final drive four wheel drive layout (Fig. 7.37) The power flow is transmitted via the engine to the five speed gearbox input primary shaft. It then transfers to the output secondary hollow shaft by way of pairs of gears, each pair combination having different number of teeth to provide the necessary range of gear ratios (Fig. 7.37). The hollow secondary shaft extends rearwards to the central differential cage. Power is then divided by the planet pinions between the left and right hand bevel sun gears. Half the power flows to the front crownwheel via the long pinion shaft passing through the centre of the secondary hollow output shaft while the other half flows from the right hand sun gear to the rear axle via the universal joints and propellor shaft.

When the vehicle is moving forward in a straight line, both the front and rear axles rotate at one common speed so that the axle pinions will revolve at the same speed as the central differential cage. Therefore the bevel gears will rotate bodily with the cage but cannot revolve relative to each other.

Steering the vehicle or moving onto a bend or curved track will immediately produce unequal turning radii for both front and rear axles which meet at some common centre (instantaneous centre). Both axles will be compelled to rotate at slightly different speeds. Due to this speed variation between front and rear axles, one of the central differential sun gears will tend to rotate faster than its cage while the other one will move correspondently slower than its cage. As a result, the sun gears will force the planet pinions to revolve on their pins and at the same time revolve bodily with the cage. This speed difference on both sides of the differential is automatically absorbed by the revolving planet pinions now being permitted to move relative to the sun gears by rolling on their toothed faces. By these means, the bevel gears enable both axles to rotate at speeds demanded by their instantaneous rolling radii at any one moment without causing torsional wind-up. If travelling over very rough, soft, wet or steep terrain, better traction may be achieved with the central differential locked-out.

Automobile Front Wheel Drive Fig
Fig. 7.37 Longitudinally mounted engine with integral front final drive four wheel drive system
Front Wheel Drive Fig
Fig. 7.38 Longitudinally mounted engine with independent front final drive four wheel drive system

7.7.8 Longitudinal mounted engine with independent front axle four wheel drive layout

Epicyclic gear central differential (Fig. 7.38) A popular four wheel drive arrangement for a front longitudinally mounted engine has a transfer box behind its five speed gearbox. This incorporates a viscous coupling and an epicyclic gear train to split the drive torque, 34% to the front and 66% to the rear (Fig. 7.38). A chain drives a forward facing drive shaft which provides power to the front differential mounted beside the engine sump. The input drive from the gearbox mainshaft directly drives the planet carrier and pinions. Power is diverted to the front axle through the sun gear and then flows to the hollow output shaft to the chain sprockets. Output to the rear wheels is taken from the annulus ring gear and carrier which transmits power directly to the rear axle. To minimize wheel spin between the rear road wheels a combined differential and viscous coupling is incorporated in the rear axle housing.

Bevel gear central differential (Fig. 7.38) In some cases vehicles may have a weight distribution or a cross-counting application which may find 50/50 torque split between front and rear wheel drives more suitable than the 34/66 front to rear torque split. To meet these requirements a conventional central (third) bevel gear differential may be preferred, see insert in Fig. 7.38. Again a transfer box is used behind the gearbox to house the offset central differential and transfer gears. The transfer gear train transmits the drive from the gearbox mainshaft to the central differential cage. Power then passes to the spider cross-pins which support the bevel planet pinions. Here the torque is distributed equally between the front and rear bevel sun gears, these being connected indirectly through universal joints and propellor shafts to their respective axles. When the vehicle is moving along a straight path, the planet pinions do not rotate but just revolve bodily with the cage assembly.

Immediately the vehicle is manoeuvred or is negotiating a bend, the planet pinions commence rotating on their own pins and thereby absorb speed differences between the two axles by permitting them not only to turn with the cage but also to roll round the bevel sun gear teeth at the same differential. However, they are linked together by bevel gearing which permits them independently to vary their speeds without torsional wind-up and tyre scuffing.

7.7.9 Transversely mounted engine with four wheel drive layout (Fig. 7.39) One method of providing four wheel drive to a front transversely mounted engine is shown in Fig. 7.39. A 50/50 torque split is provided by an epicyclic twin planet pinion gear train using the annulus ring gear as the input. The drive to the front axle is taken from the central sun gears which is attached to the front differential cage, while the rear axle is driven by the twin planet pinions and the crownwheel, which forms the planet carrier. Twin planet pinions are used to make the sun gear rotate in the same direction of rotation as that of the annulus gear. A viscous coupling is incorporated in the front axle differential to provide a measure of wheel spin control.

Power from the gearbox is transferred to the annulus ring gear by a pinion and wheel, the ring

Fig. 7.39 Transversely mounted engine four wheel drive system

gear having external teeth to mesh with the input pinion from the gearbox and internal teeth to drive the twin planet gears. Rotation of the annulus ring gear drives the outer and inner planet pinions and subsequently rotates the planet carrier (crownwheel in this case). The front crownwheel and pinion redirect the drive at right angles to impart motion to the propellor shaft. Simultaneously the inner planet pinion meshes with the central sun gear so that it also relays motion to the front differential cage.

7.7.10 Rear mounted engine four wheel drive layout (Fig. 7.40)

This arrangement has an integral rear engine and axle with the horizontal opposed four cylinder engine mounted longitudinally to the rear of the drive shafts and with the gearbox forward of the drive shafts (Fig. 7.40). Power to the rear axle is taken directly from the gearbox secondary output shaft to the crownwheel and pinion through 90° to the wheel hubs. Similarly power to the front axle is taken from the front end of the gearbox secondary output shaft to the front axle assembly comprised of the crownwheel and pinion differential and viscous coupling.

The viscous relative speed-sensitive fluid coupling has two independent perforated and slotted sets of steel discs. One set is attached via a splined shaft to a stub shaft driven by the propellor shaft from the gearbox, the other to the bevel pinion shank of the front final drive. The construction of the multi-interleaf discs is similar to a multiplate clutch but there is no engagement or release mechanism. Discs always remain equidistant from each other and power transmission is only by the silicon fluid which stiffens and produces a very positive fluid drag between plates. The sensitivity and effectiveness of the transference of torque is dependent upon the diameter and number of plates (in this case 59 plates), size of perforated holes and slots, surface roughness of the plates as well as temperature and generated pressure of fluid.

The drive to the front axle passes through the viscous coupling so that when both front and rear axle speeds are similar no power is transmitted to the front axle. Inevitably, in practice small differences in wheel speeds between front and back due to variations of effective wheel radii (caused by uneven load distribution, different tyre profiles, wear and cornering speeds) will provide a small degree of continuous drive to the front axle. The degree of speed sensitivity is such that it takes only one eighth of a turn in speed rotational difference between each end of the coupling for the fluid to commence to stiffen. Only when there is a loss of grip through the rear wheels so that they begin to slip does the mid-viscous coupling tend to lock-up to provide positive additional drive to the front wheels. A mechanical differential lock can be incorporated in the front or rear axles for travelling over really rough ground.

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  • eglantine labingi
    Which gear in vehicle not revolve when moving on a straight path?
    3 years ago
  • Tero
    What is the meaning of third differential in automobile?
    3 years ago
  • grossman
    When vehicle moving through a straight path. differential unit permits?
    3 years ago
  • medhanie
    When will the differential gear revolve on their shaft?
    3 years ago

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