What's the difference?

20 March 2008

Truck and all–terrain crane chassis are fundamentally different in two respects, structurally and intheir running gear, writes Alex Dahm, Editor of AI's sister magazine International Cranes andSpecialized Transport. Truck chassis are usually ladder frame (two–dimensional) structures, madefrom rolled steel members. The manufacturer then attaches mountings for standard–design axlesand other running gear.

The number of axles and their positions along the frame are fixed –a steering axle near the front anddrive axles at the rear, with a gap in between. One preparation for mounting heavy booms is addingaxles, called ‘tag’ axles, to increase payload capability while complying with road loading restrictions. These are usually un–driven and often steering axles. Superstructures (everything above the slew ring) for boom assemblies typically sit on a subframeattached to the ladder frame of the carrier with bolts and brackets. The distribution of loads fromsuperstructures to the axles strongly influences the location of the boom, and vice versa.

In contrast, crane chassis are more three–dimensional, box–like structures (see picture below), achieving rigidity with large, hollow–cross section, seamlessly welded plate fabrications. Also, the boomslew ring base, boxes for outrigger beams, and axle mounting points are integrally designed into thechassis shell. They are not separate assemblies bolted onto the frame of the boom superstructure, asis the case for truck–mounts.

Crane running gear is also more durable andstronger. “All components are high–capacity,”says Wolfgang Beringer, sales promoter forLiebherr. “Trucks are typically less than 50 t andhave axle loadings of 9 to 10 t. Axle loadings oncranes are 12 t.”

In summary, crane chassis designers optimisethe unit as a single structural member: “Poundfor pound, crane chassis are stiffer, stronger, andmore durable. That's why crane manufacturersdesign their own chassis,” says Mr Beringer.

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