The Equipment and Calculations Behind Mobile Crane Stability

The Equipment and Calculations Behind Mobile Crane Stability
12/21/2017 1:18:00 PM
When most people hear about a mobile crane tipping over, they assume it’s because the load was too heavy. The truth is, a loss of stability is inevitable when operators fail to follow the crane manufacturer’s guidelines regarding crane ratings. In other words, incorrect boom length or boom angle are more likely the causes of tipping accidents.
Crane and Load Leverage

Image courtesy of Crane Tech.

There are three basic components of a mobile crane: the superstructure, carrier (chassis) and boom. Each of these has a center of gravity that affects the center of gravity of the crane assembly as a whole. Keeping the crane from tipping forward or leaning too much to one side or the other is the purpose of two mechanisms: the outriggers and counterweights. Let’s look at them individually.

Outriggers use hydraulics used to lift the entire truck, tires and all, off the ground. The outriggers are made up of the beam (the leg of the outrigger), and the outrigger pad (also called the "float” or the "foot”). Even when you’re working with loads below your crane’s lifting threshold, properly extending the outriggers is crucial. Sometimes, wood floats (or mats) are lined up to create a base under the pad to help spread weight evenly over the ground surface, which can help protect the pavement under the crane.

Counterweights are placed on the back of the crane, on the cab’s underside. How much weight is needed for a particular operation depends on the weight of the load, the radius of the boom and the boom's angle. Counterweights are detached when the crane is not engaged in a lift.

Liebherr HS 895

Liebherr HS 895 with counterweights.

Terms to Know

Improving equipment stability with outriggers and counterweights comes with its own vocabulary. Here are some of the terms you might encounter on the job:

Allowable Ground Bearing Pressure –The maximum pressure allowed on the supporting surface. The number is written in pounds per square foot (psf) or Pascals (Pa).

Blocking – Also known as "dunnage,” "cribbing,” "pads” or "mats,” blocking is used to distribute loads to the ground evenly and to support lattice boom sections during assembly and disassembly.

Crane Mat – Used to reduce the ground bearing pressure from a crane’s tires, outriggers or crawler tracks.

Crane Pad – An area of compacted soil, concrete, mats or steel plates to support a mobile crane during a lift.

Crush Rating – The allowable amount of pressure that an outrigger pad is rated to carry. This will depend on the strength of the materials used.

Deflection – The bending of supporting materials as downforce is applied.

Displacement – The difference between the unloaded horizontal positions of the outrigger pad and the fully-loaded horizontal position of the outrigger pad.

Downforce – The pressure generated from the outriggers through the outrigger float and onto the outrigger pads.

Effective Bearing Area – The area under a crane mat that is effective in distributing the applied load to the underlying surface.

Ground Bearing Pressure (GBP) – The pressure that a crane exerts on the supporting surface. Written in pounds per square foot (psf) or Pascals (Pa).

Ground Bearing Capacity (GBC) – The ability of the ground/surface to support a crane and its operation.

Ground Conditions – How well the ground can support the crane and its load. Conditions include soil compaction and slope.

Ground Load Rating (GLR) – The amount of weight that can be applied to the outrigger pad based on the ability of the ground to support it.

Maximum Outrigger Reaction Force – The maximum weight that the equipment can exert through its outriggers.

Outrigger Pad/Float/Foot – These attach to the outer end of a crane’s outrigger and help disperse the load to the supporting surface. As with many crane parts, they go by many names, usually depending on where you are. You may also know them as "outrigger floats.”, "outrigger pans,” "feet” or, simply, "pads.”

Supporting Materials – Includes blocking, mats and cribbing, as defined in OSHA 1926.1402(a)(2).

Questions to Ask

It doesn’t matter how heavy your load is. You should always follow proper setup procedures and stick to your crane lift plan when the lift exceeds 75 percent of the rated capacity of the crane or requires the use of more than one crane or derrick to perform. Be sure to ask the following questions before setting up your crane:

  • What is the expected outcome?
  • Where are the potential problems and how can you avoid them?
  • Is the crane operator certified and qualified?
  • What are the ground conditions at the work site like?
  • Are there any overhead obstacles?
  • Are you using matting (floats)?
  • Are the outriggers properly extended?
  • Are the boom length, counter-weights and boom configuration correct?
  • Have you confirmed the radius and net load weight and compared them to the load chart for your crane?

Safe crane setup means understanding ground conditions and using outrigger pads correctly to provide the necessary foundation support. Never assume ground underneath your crane will support the weight and pressure under your outriggers.

If you have any questions about setting up your mobile crane, the techs at LC Crane Parts and Service are here to help. We offer crane parts plus repair and rebuild services, and diagnostic assistance for all mobile cranes:

  • Boom Trucks
  • Truck Cranes
  • Crawler Cranes
  • All-Terrain Cranes
  • Rough-Terrain Cranes
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