Truck Carriers

Truck Carriers

Truck-type crane carriers should not be confused with ordinary commercial truck chassis. A crane carrier is designed and built for no other purpose than crane service. Figure 2.13 shows a carrier for a 300-ton model. It is still clear that this chassis is quite different from that of a standard truck. The largest units may have as many as nine axles.

A variant is the so-called truck which has a telescopic boom and stabilizers mounted on a more-or-less conventional truck chassis. The mounting must meet specifications for frame stiffness, weight, and axle limitations. A boom truck, rated for up to 50 tons and reaching as high as 180 feet (55 m), has an advantage of being able to transport the loads it lifts.

Carriers must be designed for movement about the jobsite and on the highway. To accommodate those extremes, transmissions with as many as 33 forward gears are provided, and some carriers are equipped with special creeping gears as well. Road speeds vary from crane to crane but a range from 35 to 50 mi/h (56 to 80.5 km/h) is common. Jobsite ramps of 20 to 40% can be traveled without load, depending on the crane model and surface conditions. Brakes are designed to hold on the maximum travel grade.

A good crane carrier must have an extraordinarily stiff, torsion-ally rigid frame to keep from deflecting excessively under load, but this requires the addition of diaphragms, stiffeners, and reinforcements, which add to weight. For roadability, weight must be held to legal limits. Many important differences in the quality of carriers rest on the compromises made in satisfying those conflicting needs.

Carriers in the United States can be as wide as 13 ft (4 m), and the entire crane, in transit configuration, can weigh as much as 350,000 lb (160,000 kg). In many areas, highway load limitations do not permit such load weights or widths. Machines up to 12 ft (3.7 m) wide can be readily shipped on U.S. railroads, but wider loads require special routing. Often special provisions for transit must be made.