2.1 Lifting Principles
There are four basic lifting principles that govern a
crane's mobility and safety during lifting operations:
- Center of Gravity The center of gravity
of any object is the point in the object where its weight can be
assumed to be concentrated or, stated in another way, it is the point
in the object around which its weight is evenly distributed. The
location of the center of gravity of a mobile crane depends primarily
on the weight and location of its heaviest components (boom, carrier,
upperworks and counterweight).
- Leverage Cranes use the principle of
leverage to lift loads. Rotation of the upperworks (cab, boom,
counterweight, load) changes the location of the crane's center of
gravity, its leverage point or fulcrum.
As the upperworks rotates, the leverage of a mobile crane
fluctuates. This rotation causes the crane's center of gravity to change and
causes the distance between the crane's center of gravity and its tipping axis
to also change. Stability can be effected by the fluctuating leverage the
crane exerts on the load as it swings. The crane's rated capacity is therefore
altered in the load chart to compensate for those changes in leverage.
Provided the ground is capable of supporting the load, a
crane can be made more stable by moving the tipping axis further away from its
center of gravity. The extra stability gained by moving the tipping axis can
then be used to carry larger/heavier loads.
INCREASED STABILITY = MORE LOAD
- Stability Is the relationship of the load
weight, angle of the boom and its radius (distance from the cranes
center of rotation to the center of load) to the center of gravity of
the load. The stability of a crane could also be effected by the
support on which the crane is resting. A crane's load rating is
generally developed for operations under ideal conditions, i.e., a
level firm surface. Unlevel surfaces or soft ground therefore must be
avoided. In areas where soft ground poses a support problem for
stability, mats and or blocking should be used to distribute a crane's
load and maintain a level stable condition.
In addition to overturning (stability failure), cranes can
fail structurally if overloaded enough. Structural failure may occur before a
stability failure. In other words, a mobile crane's structure may fail long
before it tips. As loads are added beyond its rated capacity, a crane may fail
structurally before there is any sign of tipping. Structural failure is not
limited to total fracture; it includes all permanent damage such as
overstressing, bending and twisting of any of the components. When a crane is
overstressed, the damage may not be apparent. Nevertheless, a structural
failure has occurred and overstressed components are then subject to
catastrophic failure at some future time.
- Structural Integrity The crane's main frame, crawler
track and/or outrigger supports, boom sections, and attachments are
all considered part of the structural integrity of lifting. in
addition, all wire ropes, including stationary supports or attachment
points, help determine lifting capacity and are part of the overall
structural integrity of a crane's lifting capacity. The following
elements may also affect structural integrity:
- The load chart capacity in relationship to stability;
- The boom angle limitations which affect stability and
capacity; and
- The knowledge of the length of boom and radius in determining
capacity.
Stability failures are foreseeable, but in structural
failure it is almost impossible to predict what component will fail at any
given time. No matter what the cause, if the crane is overloaded, structural
failure can occur.
2.2 Operational Considerations
Cranes are carefully designed, tested, and manufactured for
safe operation. When used properly they can provide safe reliable service to
lift or move loads. Because cranes have the ability to lift heavy loads to
great heights, they also have an increased potential for catastrophic
accidents if safe operating practices are not followed.
Crane operators and personnel working with cranes need to
be knowledgeable of basic crane capacities, limitations, and specific job site
restrictions, such as location of overhead electric power lines, unstable
soil, or high wind conditions. Personnel working around crane operations also
need to be aware of hoisting activities or any job restrictions imposed by
crane operations, and ensure job site coordination of cranes. Crane inspectors
therefore should become aware of these issues and, prior to starting an
inspection, take time to observe the overall crane operations with respect to
load capacity, site coordination, and any job site restrictions in effect.
Rough Terrain (R/T) 45-Ton Crane
(Hydraulic Crane)
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150-Ton Crawler Lattice Boom
Friction Crane |
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