۱۳۸۶ بهمن ۲۴, چهارشنبه

Cranes

Francis Hardy, Project CoordinatorConstruction Safety Association of Ontario
A crane is a machine with a boom, primarily designed to raise and lower heavy loads. There are two basic crane types: mobile and stationary. Mobile cranes can be mounted on motor vehicles, boats or railroad cars. Stationary cranes can be of a tower type or mounted on overhead rails. Most cranes today are power driven, though some still operate manually. Their capacity, depending on the type and size, ranges from a few kilograms to hundreds of tons. Cranes are also used for pile driving, dredging, digging, demolition and personnel work platforms. Generally, a crane’s capacity is greater when the load is closer to its mast (center of rotation) and less when the load is further away from its mast. Crane hazards

Accidents involving cranes are usually costly and spectacular. Injuries and fatalities involve not only workers, but sometimes innocent bystanders. Hazards exist in all facets of crane operation, including assembly, dismantling, travel and servicing. Some of the most common hazards involving cranes are:
Electrical hazards. Overhead powerline contact and arcing of electrical current through the air can occur if the machine or hoist line is close enough to the powerline. When powerline contact occurs, the danger is not just limited to the operator of the hoist, but extends to all personnel in the immediate vicinity. Twenty three percent of crane fatalities in the United States, for example, in 1988–1989 involved powerline contact. Aside from injury to humans, electrical current can cause structural damage to the crane.
Structural failure and overloading. Structural failure occurs when a crane or its rigging components are overloaded. When a crane is overloaded, the crane and its rigging components are subject to structural stresses that may cause irreversible damage. Swinging or sudden dropping of the load, using defective components, hoisting a load beyond capacity, dragging a load and side-loading a boom can cause overloading.
Instability failure. Instability failure is more common with mobile cranes than stationary ones. When a crane moves a load, swings its boom and moves beyond its stability range, the crane has a tendency to topple. Ground conditions can also cause instability failure. When a crane is not leveled, its stability is reduced when the boom is oriented in certain directions. When a crane is positioned on ground that cannot bear its weight, the ground can give way, causing the crane to topple. Cranes have also been known to tip when traveling on poorly compacted ramps on construction sites.
Material falling or slipping. Material can fall or slip if not properly secured. Falling material can injure workers in the vicinity or cause property damage. Undesired movement of material can pinch or crush workers involved in the rigging process.
Improper servicing, assembling and dismantling procedures. Poor access, lack of fall protection and poor practices have injured and killed workers when servicing, assembling and dismantling cranes. This problem is most common with mobile cranes where service is performed in the field and there is lack of access equipment. Many cranes, particularly older models, do not provide handrails or steps to facilitate getting to some sections of the crane. Servicing around the boom and top of the cab is dangerous when workers walk on the boom without fall-arrest equipment. On lattice-boom cranes, incorrect loading and unloading as well as assembly and disassembly of the boom has caused sections to fall onto the workers. The boom sections were either not properly supported during these operations, or the rigging of the lines to support the boom was improper.
Hazard to the helper or oiler. A very hazardous pitch point is created as the upper portion of a crane rotates past the stationary lower section during normal operations. All helpers working around the crane should stay clear of the deck of the crane during operation.
Physical, chemical and stress hazards to the crane operator. When the cab is not insulated, the operator can be subjected to excessive noise, causing loss of hearing. Seats that are not properly designed can cause back pain. Lack of adjustment to the seat height and tilt can result in poor visibility from operating positions. Poor cab design also contributes to poor visibility. Exhaust from gasoline or diesel engines on cranes contains fumes that are hazardous in confined areas. There is also concern over the effect of whole-body vibration from the engine, particularly in older cranes. Time constraints or fatigue can also play a part in crane accidents.
Control Measures

Safe operation of a crane is the responsibility of all parties involved. Crane manufacturers are responsible for designing and manufacturing cranes that are stable and structurally sound. Cranes must be rated properly so that there are enough safeguards to prevent accidents caused by overloading and instability. Instruments such as load-limiting devices and angle and boom length indicators aid operators in the safe operation of a crane. (Powerline sensory devices have proved to be unreliable.) Every crane should have a reliable, efficient, automatic safe- load indicator. In addition, crane manufacturers must make accommodations in the design that facilitate safe access for servicing and safe operation. Hazards can be reduced by clear design of control panels, providing a chart at the operator’s fingertips that specifies load configurations, handrails, non-glare windows, windows that extend to the cab floor, comfortable seats and both noise and thermal insulation. In some climates, heated and air-conditioned cabs contribute to the worker’s comfort and reduce fatigue. Crane owners are responsible for keeping their machines in good condition by ensuring regular inspection and proper maintenance and employing competent operators. Crane owners must be knowledgeable so that they can recommend the best machine for a particular job. A crane assigned to a project should have the capacity to handle the heaviest load it must carry. The crane should be fully inspected by a competent person before being assigned to a project, and then daily and periodically (as suggested by the manufacturer), with a maintenance record kept. Ventilation should be provided to remove or dilute engine exhaust from cranes working in enclosed areas. Hearing protection, when necessary, should be provided. Site supervisors must plan ahead. With proper planning operating near overhead powerlines can be avoided. When work must be done near high-voltage power lines, clearance requirements should be followed (see table 93.6 [CCE06TE]). When working near powerlines cannot be avoided, the line should either be de-energized or insulated.

* Meters have been converted from recommendations in feet. Source: ASME 1994.

Signalers should be used to aid the operator near the limit of approach around powerlines. The ground, including access in and around the site, must have the ability to bear the weight of the crane and the load it is lifting. If possible, the crane operating area should be roped off to prevent injuries from overhead lifting. A signaler must be used when the operator cannot see the load clearly. The crane operator and the signaler must be trained and competent in hand signals and other aspects of the job. Proper rigging attachments must be supplied so that riggers can secure the load from falling or slipping. The rigging crew must be trained in the attachment and dismantling of loads. Good communication is vital in safe crane operations. The operator must carefully follow the manufacturer’s recommended procedures when assembling and disassembling the boom before operating the crane. All safety features and warning devices should be in working order and should not be disconnected. The crane must be leveled and be operated according to the crane load chart. Outriggers must be fully extended or set according to manufacturers’ recommendations. Overloading can be prevented by the operator’s knowing the weight to be lifted in advance and by using load-limiting devices as well as other indicators. The operator should always use sound craning practices. All loads must be fully secured before they are lifted. Movement with a load must be slow; the boom should never be extended or lowered so that it compromises the stability of the crane. Cranes should not be operated when visibility is poor or when the wind can cause the operator to lose control of the load.

Standards and Legislation

There are numerous written standards or guidelines for recommended manufacturing and operating practices. Some are based on design principles, some on performance. Subjects covered in these standards include methods of testing various safety devices; design, construction and characteristics of the cranes; inspection, testing, maintenance and operation procedures; recommended equipment and control layout. These standards form the basis of government and company health and safety regulations and operator training.

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