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
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.