10135 Alternative tandem axle bogie arrangements

Leading and trailing arms with inverted semi-elliptic spring suspension (Fig. 10.97) An interesting tandem axle arrangement which has been used for recovery vehicles and tractor units where the laden to unladen ratio is high is the inverted semi-elliptic spring with leading and trailing arm (Fig. 10.97). The spring and arms pivot on a central chassis member; the arm forms a right angle with its horizontal portion providing the swing arm, while the vertical upper portion is shaped to form a curved slipper block bearing against the end of the horizontal semi-elliptic leaf spring.

The upper faces of the horizontal swing arm are also curved and are in contact with a centrally mounted 'V' -shaped member which becomes effective only when the tandem axle bogie is about half laden. Initially in the unladen state, both swing arms are supported only by the full spring length; this therefore provides a relatively low spring stiffness. As the axles become loaded, the leading and

End view

Fig. 10.96 Double Inverted semi-elliptic spring

End view

Fig. 10.96 Double Inverted semi-elliptic spring

Swing arm

Vee helper

Swing arm

Vee helper

Bogie Semi Integral
Fig. 10.97 Leading and trailing arms with inverted semi-elliptic spring

trailing arms pivot and swing upward, thereby steadily pushing the central 'V' helper member into contact with the main spring leaf over a much shorter blade span. The rolling contact movement between the upper and lower faces of the swing arms and the central 'V' helper member produce a progressive stiffening of the main spring under laden conditions.

Hendrickson long equalization balance beam with single semi-elliptic springs (Fig. 10.98) This tandem suspension arrangement uses a low mounted, centrally pivoted long balance beam spanning the distance between axles and high mounted leading and trailing torque rods (Fig. 10.98). A semi-elliptic spring supports the vehicle's payload. It is anchored at the front end to a spring hanger and at the rear bears against either the outer or both inner and outer curved slipper hangers. The balance beam is attached to the spring by the 'U' bolts via its pivot mount.

The spring provides support for the vehicle's weight and transmits the accelerating or decelerat-

Slipper hangars

Slipper hangars

Balance Beam Vehicle Technology
Fig. 10.98 Hendrickson long equalization balance beam with single semi-elliptic spring

ing thrusts between the axles and chassis. The balance beam divides the vehicle's laden weight between the axles and in conjunction with the torque rods absorbs the driving and braking torque reaction. The two stage spring stiffness is controlled by the effective spring span, which in the unladen condition spans the full spring length to the outer slipper block and in the laden state is shortened as the spring deflects, so that it now touches the inner slipper block spring hanger. For some cross-country applications the outer slipper block hanger is not incorporated so that there is only a slight progressive stiffening due to the spring blade to curved slipper block rolling action as the spring deflects with increasing load. With this four point chassis frame mounting and rigid balance beam, both the springs and the chassis are protected against concentrated stress which therefore makes this layout suitable for on/off rigid six or eight wheel rigid tracks.

Pivot beam with single semi-elliptic spring (Figs 10.99 and 10.100) This kind of suspension has a single semi-elliptic spring attached at the front end directly to a spring hanger and at the rear to a pivoting beam which carries the trailing axle (Fig. 10.99).

With a conventional semi-elliptic spring suspension, the fixed and swing shackles both share half (% W) of the reaction force imposed on the chassis caused by an axle load W.

Pivot beam

Pivot beam

Fig. 10.99 Pivot beam with single semi-elliptic spring

Pivot beam Torque rod

Pivot beam Torque rod

Fig. 10.100 Pivot beam with semi-elliptic spring and torque rod

With the pivoting balance beam coupled to the tail-end of the spring, half the leading axle load (%W) reacting at the swing shackle is used to balance the load supported by the trailing axle. For the chassis laden weight to be shared equally between axles, the length of beam from the pivot to the shackle plate must be twice the trailing distance from the pivot to the axle. This means that if the load reaction at each axle is W, then with the leading axle clamped to the centre of the spring span and with a pivot beam length ratio of 2:1 the upward reaction force on the front spring hanger will be %W and that acting through the pivot 1% W, giving a total upward reaction force of %W + 1%W = 2W. In other words, the downward force at the front of the pivot beam caused by the trailing axle supported by the pivot is balanced by the upward force at the rear end of the spring caused by the load on the leading axle. Thus if the front wheel lifts as it rolls over a bump, the trailing end of the spring rises twice as much as the axle. It attempts to push the trailing axle down so that its wheels are in hard contact with the ground.

With the second axle mounted between the lower trailing arm and the upper torque rod (Fig. 10.100), most of the driving and braking torque reaction is neutralized. Only when accelerating with a single drive axle is there some weight transfer from the non-drive axle (second) to the drive axle (first).

By arranging the first axle to be underslung (Fig. 10.100) instead of overslung (Fig. 10.99), a wider spring base projected to the ground will result in greater roll resistance.

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