A crane that is three degrees out of level may reduce the rated capacity by as much as 50 percent (Campbell and Dickie, 162 Garby, 336). A crane that is not level causes side loading of the boom, and thereby, reducing the rated capacity. The load chart ratings are based on the crane being perfectly level in all directions. Pads used for outriggers should be stable and rigid, a minimum of three times larger in area than the outrigger float, and be fully supported (Garby, 331 Campbell and Dickie, 206). The ground pressure of a truck crane may be higher than that of a crawler crane due to the smaller load bearing surface area of the pads.
Lifting a load over the corner produces the maximum ground bearing pressure, which makes it the most dangerous position. The lowest bearing pressure is the total weight of the machine distributed over the entire area of the drive tracks or all outrigger supports. When a crane sets on its load bearing surfaces, it exerts varying pressures depending on the operating conditions and the quadrant(s) of operation. Extreme caution should be used when performing lift operations beside buildings, especially if newly constructed with uncompacted backfill, along trenches, or in areas where water mains, sewer pipes or steam lines are buried. The ground condition must have enough stability and bearing capacity to support all loads placed on it by the crane. For such cranes, the outriggers should still be extended to the proper point and all of the tires must be clear of the ground. Some mobile cranes have load charts for mid-extended outrigger beams. If lifting on rubber, the load chart ratings apply only when tires are in accordance with the manufacturer's specifications and in good condition, and the specified tire pressure is maintained. If these two conditions are not met, then the "on rubber" capacity charts should be used. If lifting on outriggers, the load chart rating applies only when all the outrigger beams are fully extended and all tires are clear of the ground. Be sure to check the manufacturer's reeving diagrams to ensure proper reeving. On multi-part reeved systems, if the parts of the line are evenly distributed on either side of the boom centerline, the boom twist will also be minimized. On single-line reeving, when the hoist line runs along the center sheave or on the sheave beside the centerline of the boom, the boom twist is minimized.
Crane lift capacity full#
Full chart ratings may only be applied when the boom tip is symmetrically rigged. This condition causes the boom to twist, which subsequently reduces the rated capacity of the crane. Unbalanced rope reeving occurs whenever the main hoist line is reeved to one side of the boom tip. Some questions to consider include: Is the jib erected or stowed? What type of jib is used? Are the outriggers extended properly? Are the tires properly inflated? What type of boom is used? What is the boom length? Where are the boom insert locations? What counterweights are used? Are they configured according to the manufacturer's specifications? What is the wire rope size and grade? What is the configuration of the hoist reeving? Most important, operators must know the configuration of the crane, ensure the configuration is correct, and use the appropriate load chart. Load chart ratings may only be applied if the machine is rigged according to the manufacturer's specifications. Some of these factors are described below. The first step in creating a lift plan is to understand the factors affecting the strength and stability of mobile cranes. It is likely that many fatalities could have been avoided with proper mobile crane lift planning. OSHA estimates that 89 crane-related fatalities occur each year in construction work, and many of these are high-profile events that receive a lot of adverse publicity. Mobile crane incidents can cause massive production delays, devastating property damage, and loss of life.