1016 Lift axle tandem or triaxle suspension

Vehicles with tandem or tri-axles which carry a variety of loads ranging from compact and heavy to bulky but light may under-utilize the load carrying capacity of each axle, particularly an empty return journey over a relatively large proportion of the vehicle's operating time.

When a vehicle carries a full load, a multi-axle suspension is essential to meet the safety regulations, but the other aspects are improved road vibration isolation from the chassis, better road holding and adequate ride comfort.

If a conventional multi-axle suspension is operated below half its maximum load carrying capacity, the quality of road holding and ride deteriorates, suspension parts wear rapidly, and increased wheel bounce causes a rise in tyre scrub and subsequent tyre tread wear.

Conversely, reducing the number of axles and wheels in contact with the road when the payload is decreased extends tyre life, reduces rolling forward resistance of the vehicle and therefore improves fuel consumption.

10.16.1 Balance beam lift axle suspension arrangement (Figs 10.118 and 10.119) A convenient type of tandem suspension which can be adapted so that one of the axles can be simply and rapidly raised or lowered to the ground

Tandem Suspension

(a) Heavy load, both axles operational 1 (b) Light load, single axle operational

Fig. 10.118(a and b) Hydraulically operated lift axle suspension with direct acting ram

(a) Heavy load, both axles operational 1 (b) Light load, single axle operational

Fig. 10.118(a and b) Hydraulically operated lift axle suspension with direct acting ram

Multi Axle Suspension Structure

(a) Heavy load, both axles operational [ [b) Light load, single axle operational

Fig. 10.119(a and b) Hydraulically operated lift axle suspension with bell-crank lever and ram

(a) Heavy load, both axles operational [ [b) Light load, single axle operational

Fig. 10.119(a and b) Hydraulically operated lift axle suspension with bell-crank lever and ram

Multi Axle Bell Crank Suspension

fa) Heavy load, both axles operational

Fig. 10.120(a and b) Pneumatically operated lift axle suspension fa) Heavy load, both axles operational

Fig. 10.120(a and b) Pneumatically operated lift axle suspension without having to make major structural changes is the semi-elliptic spring and balance beam combination (Figs 10.118 and 10.119). Raising the rearmost of the two axles from the ground is achieved by tilting the balance beam anticlockwise so that the forward part of the balance beam appears to push down the rear end of the semi-elliptic spring. In effect, what really happens is the balance beam pivot mounting and chassis are lifted relative to the forward axle and wheels. Actuation of the balance beam tilt is obtained by a power cylinder and ram, anchored to the chassis at the cylinder end, whilst the ram-rod is connected either to a tilt lever, which is attached indirectly to the balance beam pivot, or to a bell crank lever, which relays motion to the extended forward half of the balance beam.

Balance beam suspension with tilt lever axle lift (Fig. 10.118(a and b)) With the tilt lever axle lift arrangement, applying the lift control lever introduces fluid under pressure to the power cylinder, causing the ram-rod to extend. This forces the tilt lever to pivot about its centre of rotation so that it bears down on the left hand side of the beam. Consequently the balance beam is made to take up an inclined position (Fig. 10.118(b)) which is sufficient to clear the rear road wheels off the ground. When the axle is lowered by releasing the hydraulic pressure in the power cylinder, the tilt lever returns to its upright position (Fig. 10.118(a)) and does not then interfere with the articulation of the balance beam as the axles deflect as the wheels ride over the irregularities of the road surface.

Balance beam suspension with bell-crank lever axle lift (Fig. 10.119(a and b)) An alternative lift axle arrangement uses a bell-crank lever to transmit the ram-rod force and movement to the extended front end of the balance beam. When hydraulic pressure is directed to the power cylinder, the bell-crank lever is compelled to twist about its pivot, causing the roller to push down and so roll along the face of the extended balance beam until the rear axle is fully raised (Fig. 10.119(b)). Removing the fluid pressure permits the weight of the chassis to equalize the height of both axles again and to return the ram-rod to its innermost position (Fig. 10.119(a)). Under these conditions the bell-crank lever roller is lifted clear of the face of the balance beam. This prevents the oscillating motion of the balance beam being relayed back to the ram in its cylinder.

10.16.2 Pneumatically operated lift axle suspension (Fig. 10.120(a and b)) A popular lift axle arrangement which is used in conjunction with a trailing arm air spring suspension utilizes a separate single air bellow situated at chassis level in between the chassis side-members. A yoke beam supported by the lift air bellows spans the left and right hand suspension trailing arms, and to prevent the bellows tilting as they lift, a pair of pivoting guide arms are attached to the lift yoke on either side. To raise the axle wheels above ground level, the manual air control valve is moved to the raised position; this causes compressed air to exhaust from the suspension air springs and at the same time allows pressurized air to enter the lift bellows. As the air pressure in the lift bellows increases, the bellows expand upward, and in doing so, raise both trailing arm axle and wheels until they are well above ground level (Fig. 10.120(b)). Moving the air control valve to 'release' position reverses the process. Air will then be exhausted from the lift bellows while the air springs will be charged with compressed air so that the axle takes its full share of payload (Fig. 10.120(a)).

An additional feature of this type of suspension is an overload protection where, if the tandem suspension is operating with one axle lifted and receives loads in excess of the designed capacity, the second axle will automatically lower to compensate.

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