Water Well Journal

November 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/740475

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Page 56 of 90

M obile crane incidents can cause massive production delays, devastating property damage—and loss of life. In fact, the Occupational Safety and Health Ad- ministration estimates 89 crane-related fatalities occur each year in construction work. The first step to not having a serious incident is creating a lift plan and understanding the factors affecting the strength and stability of mobile cranes. Some of these factors will be described here. Machine Configuration Load chart ratings may only be applied if the machine is rigged according to the manufacturer's specifications. 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, make sure the configuration is correct, and use the appropriate load chart. Unbalanced Rope Reeving Whenever the main hoist line is reeved to one side of the boom tip, this results in unbalanced rope reeving. This causes the boom to twist, and this subsequently reduces the rated capacity of the crane. Full chart ratings may only be applied when the boom tip is symmetrically rigged. 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. 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. Be sure to check the manufac- turer's reeving diagrams to ensure proper reeving. Improper Use of Outriggers 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. If these two conditions are not met, then the "on rubber" capacity charts should be used. If lifting on rubber, the load chart ratings apply only when the tires are in accordance with the manufacturer's specifica- tions, in good condition, and specified tire pressure is main- tained. Some mobile cranes have load charts for mid-extended outrigger beams. For such cranes, the outriggers should still be extended to the proper point and all the tires must be clear of the ground. Soft Footing The ground must have enough stability and bearing capac- ity to support all loads placed on it by the crane. Extreme cau- tion 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. When a crane sits on its load bearing surfaces, it exerts varying pressures depending on the operating conditions and the quadrants of operation. The lowest bearing pressure is the total weight of the machine distributed over the entire area of the drive tracks or all outrigger supports. Lifting a load over the corner produces the maximum ground bearing pressure—which makes it the most dangerous position. 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. 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. Unlevel Crane Load chart ratings are based on the crane being perfectly level in all directions. A crane that is not level causes side loading of the boom, thus reducing the rated capacity. A crane that is 3° out of level may reduce the rated capacity by as much as 50%. This applies to "on crawlers," "on riggers," as well as "on rubber" lifting operations. The target level in the crane cab should be used for initial leveling. But for critical lifts, a carpenter's level should be used. After initial leveling, place the carpenter's level on the leveling plates provided on the crane, typically located on or under the boom foot pins, then rotate the boom 90 degrees and recheck. The levelness can be double-checked by raising the boom and lowering the load line. The line should lie in the center of SAFETY MATTERS MOBILE CRANE SAFETY Almost 90 crane-related deaths happen every year. SAFETY continues on page 54 JEROME E. SPEAR waterwelljournal.com 52 November 2016 WWJ

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