761 The necessity for a third differential

When four wheel drive cars or tandem drive axle bogie trucks are to be utilized, provision must be provided between drive axles to compensate for any difference in the mean speeds of each drive axle as opposed to speed differentiation between pairs of axle road wheels.

Speed difference between driving axles are influenced by the following factors:

1 Speed variation between axles when a vehicle moves on a curved track due to the slight differ

Fig. 7.25 Two speed epicyclic gear train axle

ence in rolling radius of both axles about some instantaneous centre of rotation.

2 Small road surface irregularities, causing pairs of driving wheels to locally roll into and over small dips and humps so that each pair of wheels are actually travelling at different speeds at any one moment.

3 Tyres which have different amounts of wear or different tread patterns and construction such as cross-ply and radials, high and low profile etc. and are mixed between axles so that their effective rolling radius of the wheel and tyre combination varies.

4 Uneven payload distribution will alter the effective rolling radius of a wheel and tyre so that heavily laden axles will have smaller rolling radii wheels and therefore complete more revolutions over a given distance than lightly laden axles.

5 Unequal load distribution between axles when accelerating and braking will produce a variation of wheel effective rolling radius.

6 Loss of grip between pairs of road wheels produces momentary wheel spin and hence speed differences between axles.

7.6.2 Benefits of a third differential (Fig. 7.26) Operating a third differential between front and rear wheel drive axles or rear tandem axles has certain advantages:

1 The third differential equally divides driving torque and provides speed differentiation between both final drive axles so that the relative torque and speed per axle are better able to meet the individual road wheel requirements, thereby minimizing tyre distortion and scrub.

2 Transmission torsional wind-up between axles is minimized (Fig. 7.26) since driving and reaction torques within each axle are not opposing but are permitted to equalize themselves through the third differential.

3 Odd tyres with different diameters are interchangeable without transmission wind-up.

4 Tractive effect and tyre grip is shared between four wheels so that wheel traction will be more evenly distributed. Therefore the amount of tractive effect per wheel necessary to propel a vehicle can be reduced.

5 Under slippery, snow or ice conditions, the third differential can generally be locked-out so that if one pair of wheels should lose traction, the other pair of wheels are still able to transmit traction.

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