1013 Tandem and triaxle bogies

A heavy goods vehicle is normally laden so that about two thirds or more of the total load is carried by the rear axle. Therefore the concentration of weight over a narrow portion of the chassis and on one axle, even between twin wheels, can be excessive.

In addition to the mechanical stresses imposed on the vehicle's suspension system, the subsoil stress distribution on the road for a single axle (Fig. 10.85(a)) is considerably greater than that for a tandem axle bogie (Fig. 10.85(b)) for similar payloads. Legislation in this country does not normally permit axle loads greater than ten tonne per axle. This weight limit prevents rapid deterioration of the road surface and at the same time spreads the majority of load widely along the chassis between two or even three rear axles.

The introduction of more than two axles per vehicle poses a major difficulty in keeping all the wheels in touch with the ground at the same time, particularly when driving over rough terrains (Fig. 10.86). This problem has been solved largely by having the suspensions of both rear axles interconnected so that if one axle rises relative to the chassis the other axle will automatically be lowered and wheel to road contact between axles will be fully maintained.

If twin rear axles are used it is with conventional half-elliptic springs supported by fixed front spring hangers and swinging rear spring shackle plates. If they are all mounted separately onto the chassis, when moving over a hump or dip in the road the front or rear axle will be lifted clear of the ground (Fig. 10.87) so that traction is lost for that particular axle and its wheels. The consequences of one or the other pairs of wheels losing contact with the road surface are that road-holding ability will be greatly reduced, large loads will suddenly be imposed on a single axle and an abnormally high amount of tyre scruffing will take place.

Bell Crank Bogie Suspension
Fig. 10.85(a and b) Road stress distribution in subsoil underneath road wheels
Fig. 10.86 Illustrating the need for tandem axle articulation
Fig. 10.87 Uncompensated twin axle suspension

To share out the vehicle's laden weight between the rear tandem axles when travelling over irregular road surfaces, two basic suspension arrangements have been developed:

1 pivoting reactive or non-reactive balance beam which interconnects adjacent first and second semi-elliptic springs via their shackle plates,

2 a central pivoting single (sometimes double) vertical semi-elliptic spring which has an axle clamped to it at either end.

10.13.1 Equalization of vehicle laden weight between axles (Figs 10.88 and 10.89) Consider a reactive balance beam tandem axle bogie rolling over a hump or dip in the road (Fig. 10.88). The balance beam will tilt so that the rear end of the first axle is lifted upwards and the front end of the second axle will be forced downward. Consequently both pairs of axle wheels will be compelled to contact the ground and equally share out the static laden weight imposed on the whole axle bogie.

The tilting of the balance beam will lift the first axle a vertical distance h/2, which is half the hump

Fig. 10.88 Payload distribution with reactive balance beam and swing shackles

Fig. 10.89 Payload distribution with single inverted semi-elliptic spring

or dip's vertical height. The second axle will fall a similar distance h/2. The net result is that the chassis with the tandem axle bogie will only alter its height relative to ground by half the amount of a single axle suspension layout (Fig. 10.88). Thus the single axle suspension will lift or lower the chassis the same amount as the axle is raised or lowered from some level datum, whereas the tandem axle bogie only changes the chassis height relative to the ground by half the hump lift or dip drop.

In contrast to the halving of the vertical lift or fall movement of the chassis with tandem axles, there are two vertical movements with a tandem axle as opposed to one for a single axle each time the vehicle travels over a bump. Thus the frequency of the chassis vertical lift or fall with tandem axles will be twice that for a single axle arrangement.

Similar results will be achieved if a central pivoting inverted transverse spring tandem axle bogie rides over a hump or dip in the road (Fig. 10.89). Initially the first axle will be raised the same distances as the hump height h, but the central pivot will only lift half the amount h/2. Conversely if the first axle goes into a dip, the second axle will be above the first axle by the height of the dip, but the chassis will only be lowered by half this vertical movement h/2. Again the frequency of lift and fall of the chassis as the tandem axles move over the irregularities in the road will be double the frequency compared to a single axle suspension.

(a) Unequal driving torque distribution

Fig. 10.90 (a and b) Reactive balance beam tandem axle suspension

(a) Unequal driving torque distribution

Fig. 10.90 (a and b) Reactive balance beam tandem axle suspension

10.13.2 Reactive balance beam tandem axle bogie suspension (Fig. 10.90(a and b)) Suspension arrangements of this type distribute the laden weight equally between the two axles due to the swing action of the balance beam (Fig. 10.90 (a and b)). The balance beam tilts according to the reaction load under each axle so that, within the chassis to ground height variation limitations, it constantly adjusts the relative lift or fall of each axle to suit the contour of the road.

Unfortunately the driving and braking torques produce unequal reaction through the spring linkage. Therefore under these conditions the vehicle's load will not be evenly distributed between axles.

Consider the situation when tractive effort is applied at the wheels when driving away from a standstill (Fig. 10.90(a)). Under these conditions the driving axle torque TD produces an equal but opposite torque reaction TR which tends to make the axle casing rotate in the opposite direction to that of the axle shaft and wheel. Subsequently the front spring ends of both axles tend to be lifted by force F, and the rear spring ends are pulled downwards by force F. Hence the overall reaction at each spring to chassis anchor point causes the balance beam to tilt anticlockwise and so lift the chassis away from the first axle, whereas the second axle is drawn towards the chassis. This results in the contact reaction between wheel and ground for the first axle to be far greater than for the second axle. In fact the second axle may even lose complete contact with the road.

Fig. 10.91 Reactive balance beam with slipper contact blocks and torque arms tandem axle suspension

Bell Crank Bogie Suspension

Fig. 10.92 Tandem wide spread reactive bell crank lever taper leaf spring

Fig. 10.91 Reactive balance beam with slipper contact blocks and torque arms tandem axle suspension

Fig. 10.92 Tandem wide spread reactive bell crank lever taper leaf spring

Conversely if the brakes are applied (Fig. 10.90(b)), the retarding but still rotating wheels will tend to drag the drum or disc brake assembly round with the axle casing TR. The rotation of the axle casing in the same direction of rotation as the wheels means that the front spring ends of both axles will be pulled downward by force F. The corresponding rear spring ends will be lifted upward by the reaction force F. Thus in contrast to the driving torque directional reaction, the braking torque TB will tilt the balance beam clockwise so that the second axle and wheel will tend to move away from the chassis, thereby coming firmly into contact with the road surface. The first axle and wheel will move further towards the chassis so that very little grip between the tyre and road occurs. In practice the upward lift of the first wheel and axle will cause the tyres to move in a series of hops and rebounds which will result in heavily loading the second axle, reducing the overall braking effectiveness and causing the first axle tyres to be subjected to excessive scuffing.

A reactive balance beam tandem axle bogie suspension using tapered leaf springs and torque arms to transmit the driving and braking forces and torques is shown in Fig. 10.91. With this layout driving and braking torque reactions will cause similar unequal load distribution.

To enable a wide spread axle to be used on trailers, the conventional reactive balance beam interconnecting spring linkage has been modified so that laden vehicle weight can still be shared equally between axles. Thus instead of the central balance beam (Fig. 10.90) there are now two bell crank levers pivoting back to back on chassis spring hangers with a central tie rod (Fig. 10.92).

In operation, if the front wheel rolls over an obstacle its supporting spring will deflect and apply an upward thrust against the bell crank lever slipper. Accordingly, a clockwise turning moment will be applied to the pivoting lever. This movement is then conveyed to the rear bell crank lever via the tie rod, also making it rotate clockwise. Consequently the rear front end of the spring will be lowered, thus permitting the rear wheels to keep firmly in contact with the road while the chassis remains approximately horizontal.

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