2 Cranes are broadly classified as either fixed (permanently installed) or mobile types. Cranes are broadly classified into fixed and mobile types. See Figure 6-1. Dozens of different fixed-crane designs are available. Each type is adapted to a specific purpose, though only a few are common in industrial settings. Fixed cranes are permanently installed and are intended for long-term use. Mobile cranes can be brought to a job site temporarily for a relatively small number of lifts.
3 Large cranes are typically operated from inside a cab, which moves with the crane to maintain a good view of the load.Depending on the size and type, cranes are operated from a cab or pendant pushbutton station. A cab is a compartment or platform attached to the crane from which an operator may ride. See Figure 6-2. A pendant hangs down from the hoists of smaller industrial cranes. This pendant includes controls for the direction and speed of the various crane motions.
4 Loads must be lifted only from directly above Loads must be lifted only from directly above. Side pulling is not permitted.A lift should never be attempted if the load is beyond the capacity ratings of any of the rigging, lifting, or crane components. Also, a crane should never be used to pull a load at an angle. See Figure 6-3. If the hoist hook cannot be positioned directly over the load lift point, either the crane or the load must be relocated.
5 If a crane operator lacks a good view of the load at all times, an assistant nearby can provide crane directions through hand signals.The standard ANSI B30.5, Mobile and Locomotive Cranes includes a set of standard hand signals to indicate various crane operations. See Figure 6-4. Only an experienced and designated individual should give hand signals as the wrong signal may result in damage to materials or serious injury to individuals working near the crane. On large projects, there is often an individual specifically designated to work with the crane operator. On smaller projects, however, the signaling may be performed by a supervisor.
6 Gantry cranes consist of a hoist on a horizontal beam, which is supported by legs that roll along the floor.A gantry crane is an industrial crane composed of a bridge beam supported on legs. See Figure 6-5. The bridge beam is the high horizontal beam that holds the hoist. The legs may be fixed to the floor, but usually they include end trucks that allow the entire crane to travel back and forth on floor rails. An end truck is a roller assembly consisting of a frame, wheels, and bearings. Floor rails are small-gauge railroad rails that are recessed into, or set on top of, the floor surface. A gantry crane allows its hoist to travel in two directions, which allows it to reach any spot within a rectangular area.
7 Overhead cranes are similar to gantry cranes except the hoist and bridge beam are supported by overhead beams.The variations of overhead-crane designs have different arrangements for how the hoist travels on the bridge beam and how the bridge beam travels on the overhead beams. See Figure 6-6. A top-running hoist has a hoist that is installed in a trolley that travels along rails on top of a pair of bridge beams. In an underhung hoist, the hoist trolley travels on the upper surface of the lower flange of a single bridge girder.
8 Jib-crane booms are supported at only one end but can be configured in several different designs. A jib crane is an industrial crane that is composed of a cantilevered horizontal beam supported by a single structural leg. A cantilever is a projecting structure supported at only one end. The three basic types of jib-crane structures are wall-mounted, base-mounted, and mast. See Figure 6-7. Wall-mounted jib cranes are top-braced or cantilevered. Base-mounted jib cranes are freestanding cranes on a heavily anchored base mounting. Mast jib cranes have one structural leg (mast) mounted to both the floor and ceiling and a boom that is cantilevered, underbraced, or top-braced. A boom is a long beam that projects from the main part of a crane in order to extend the reach of the hoist.
9 Mobile cranes are crane assemblies that are mounted on a mobile platform. The platform may use a variety of forms of mobility depending on the intended application.A mobile crane is a crane that can be moved between job sites. Mobile cranes are composed of crane assemblies and vehicle platforms. Many variations are available because the type of vehicle used is relatively independent of the crane. See Figure 6-8. The most common type of mobile crane is a motor truck, which has a platform similar to a typical flatbed truck. Motor trucks are often used on commercial or residential construction job sites that are accessible by roads. For off-road use, rough terrain or crawler vehicles allow the crane to operate in areas with soft or moderately uneven ground. Cranes are also available mounted on locomotive track vehicles.
10 An outrigger increases a mobile crane’s footprint, which improves stability. Mobile cranes are designed to be small and light enough to be driven on typical roadways, but this limits their lifting capacity. A larger crane is able to lift heavier loads at a greater distance. Therefore, most mobile cranes use outriggers to increase their effective size. See Figure 6-9. An outrigger is an extendable support structure that increases a crane’s footprint in order to improve stability.
11 Telescoping-boom cranes have the ability to extend or retract the boom, providing significant versatility to its reach.A telescopic-boom crane is a crane with an extendable boom composed of nested sections. The boom sections are extendable and retractable, allowing a wide range of boom lengths. See Figure When fully retracted, the crane is easily transported on a street-legal vehicle. The boom movement and extension is typically powered by integrated hydraulic systems.
12 The boom of a lattice-boom crane is constructed of a lightweight, open structure that allows these cranes to have particularly long booms.A lattice-boom crane is a crane with a boom constructed from a gridwork of steel reinforcing members. The lattice structure provides a very strong boom that is light for its size. See Figure The boom may be composed of one or multiple sections that must be assembled onto the crane body when on site. This makes a lattice-boom crane less transportable between sites, though its vehicle platform allows it to move around while on site. For this reason, lattice-boom cranes are often deployed for long-term use at large construction sites.
13 The lift capacity of a mobile crane is determined primarily by the horizontal distance of the load from the crane.A mobile crane may be specified by its maximum lift capacity, such as a “90-ton crane.” However, because of the necessary horizontal reach for most lifts, the actual lifting capacity is likely much less. The capacity of a crane decreases the farther it must reach from its base because the moment (tipping) force increases. See Figure This distance is determined by the length and angle of the boom. The capacity at each point in the lift must be considered. For example, a certain load located 20 from the crane must be moved to a new location 40 away. Since the farther distance allows less lifting capacity, it determines the maximum allowable load weight for the entire lift.
14 Lift capacity tables are used to determine the maximum allowable load weight for a particular crane with certain lift distances.Other factors that affect the lift capacity are the amount of counterweight, the use of outriggers, and the direction of the boom in relation to the vehicle base (over the side or over the rear). Counterweights balance the moment (tipping) force caused by the load, so more counterweight increases the lifting capacity (to a point). Outriggers increase the effective footprint of the crane, improving stability. The direction of the boom also affects stability because cranes are typically not as wide as they are long. Tables of lift capacities that account for all of the applicable factors are available with the crane manufacturer’s specifications and on cards located inside the operator’s cab. See Figure 6-13.
15 The inspection of a hoist trolley should include checking the spacing on trolley wheels. The wheels must be close enough to provide a secure attachment to the beam but not so close as to impede free travel.For industrial cranes, the positioning components are the hoist trolley and end trucks. The wheels should be checked for flat spots, cracks, and breaks. The wheel bearings should be checked for proper lubrication, intact seals, and physical damage. Periodic inspections should include checking the dimension between wheel flanges. See Figure On most hoist trolleys, the dimension between the flange landings of both wheels must be between 1/8 and 1/4 greater than the beam flange width. The addition or removal of spacing washers may be necessary to obtain the proper dimension while keeping the hoist centered under the bridge beam. Some hoist trolleys are not adjustable.