Mobile Cranes. Mobile cranes are available in a variety of
configurations wheel-mounted or track-mounted (crawler); solid telescoping boom
or fixed length lattice boom; and crane operator stations on the chassis, on the
upperworks, or both. These cranes are standard models of established
manufacturers. They are designed for highway travel/transportation and of
necessity are optimized within the highway weight and size restrictions. As a
direct result of these restrictions, the stability of mobile cranes is poor
compared to that of portal cranes.
General Description. Wheel-mounted cranes (as shown in figure 10)
utilize an automotive style of chassis with a standard driver's cab for road
travel. Crane controls may be located in the same fixed cab or a separate
operator's station on the rotating upperworks. On crawler cranes (as shown in
figure 11) a single cab is used for driving and crane operation. Mobile cranes
are equipped with hydraulically extendable outriggers near the corners of the
chassis, which must be deployed to stabilize the crane when making a lift. On
some models, the outriggers may not have the capability of relieving all of the
weight from the wheels. A counterweight is always installed on the upperworks,
which rotates on a moment carrying bearing or on rollers/roller path with hook
rollers to take up the overturning moment.
Telescoping booms are luffed and extended by means of hydraulic
cylinders. Lattice booms are luffed by means of a wire rope hoist and sheaves at
the top of the upperworks. The design of lattice booms permits disassembly of the
boom sections to shorten their length for road travel or to increase the length
for longer reach when required.
The crane functions on mobile cranes are generally powered by hydraulics
either motors or cylinders. Some models use a combination of hydraulic and
mechanical drives. The reeving systems are single reeved and arranged for easy
conversion to any of several configurations for the desired combination of hoist
Distinctive Features. The main virtue of mobile cranes is their ability
to travel to the work site and position themselves in the most advantageous
location for the task. Their foremost weakness is limited stability, even when
set on outriggers, and inability to travel with any significant load on the hook.
Operators' understanding of the site/ground conditions and the reference to the
proper chart for crane stability limits for each particular upperworks orientation
and boom length are critical to safe operation.
Industry Standards. There are no comprehensive industry standards for
the design of mobile cranes. ASME B30.5, Mobile and Locomotive Cranes, focuses
primarily on the safety aspects of the design and operation of these cranes. SAE
J1028, Mobile Crane Working Area Definitions, provides in graphic form the
limiting position of any load for operation within the major working quadrants.
SAE J1289, Mobile Crane Stability Ratings, provides the methodology for
calculating the tipping load of a mobile crane.
Gantry and Semi-Gantry Cranes. These cranes are similar to the bridge
cranes in many respects. But unlike the OET and underrunning cranes, where the
lifting height is established by the elevation of the runway rails, gantry cranes
travel on ground-level rails and obtain their lifting height by means of tall