11.1 Stabilising and overturning moments
11.1.1 Stability function of load charts
11.1.2 Counterweights
11.1.1 Stability function of load charts
The stability factors specified by AS 1418.5 Cranes, hoists and winches - Mobile cranes (non-Queensland Government link) allow for variables such as:
- dynamic factors caused by the crane motion and the load (e.g. for boom movement, application of brakes, swaying of the load); and
- wind effects on the load and boom.
AS 1418.5 Cranes, hoists and winches - Mobile cranes (non-Queensland Government link) requires that the stability factor of mobile cranes be based on 75% of tipping for stationary mode, and 66.6% for pick-and-carry mode. All mobile cranes should comply with this design requirement and the stability factor should be written on all load charts for the crane. Where second-hand cranes are imported from overseas, the crane should be stability tested to demonstrate it complies with the stability requirements of AS 1418.5 Cranes, hoists and winches - Mobile cranes (non-Queensland Government link).
When the load chart is based on 75% of tipping, the maximum capacity in the stability range of the load chart will be 75% of the suspended load that will cause the crane to overturn. In other words, the actual overturning load will be 33.3% greater than the load being lifted. Therefore, if a crane’s maximum capacity at a given radius in the stability range of the load chart is 10 tonnes, a 13.3-tonne load will cause the crane to overturn. However, it is also possible for a crane to overturn with smaller loads when operating in windy conditions or on sloping ground, or if the crane is not operated smoothly.
11.1.2 Counterweights
The crane counterweight is critical in ensuring crane stability. A counterweight that is too light for a load and boom configuration will cause the crane to overturn in the direction of the suspended load. Additionally, a crane can fall over backwards due to the effect of the counterweight in situations when:
- the counterweight is too heavy for the boom configuration;
- the crane is travelling up a slope with the boom luffed up;
- inadequate timbers are placed under the outrigger pads below the counterweight when the crane is positioned on soft ground; and
- outriggers are not extended or lowered into position.
On the majority of smaller mobile cranes, the counterweight is fixed and cannot be easily removed. However, on an increasing number of larger cranes, some of the counterweights are designed to be removed for road travel, or when smaller boom and lifting configurations are required. In this situation, it is particularly important to attach the correct type and number of counterweights to the crane for the particular lift to be undertaken.
Counterweights must be secured to the crane in the manner specified by the crane manufacturer. Where counterweights are removable, each counterweight must be clearly and permanently identified with the crane manufacturer’s name or trademark and the mass of the counterweight (preferably in tonnes).
Where the crane is fitted with a rated capacity limiter, the data input into the computer must be correct for the counterweight configuration on the crane, and related to that shown on the appropriate load chart. This also applies to the boom configuration being used on the crane.
In some unusual circumstances, additional counterweights are attached to the crane to increase its capacity. If this is done, an engineer16 is to check the complete crane design and certify that the amended design complies with AS 1418.5 Cranes, hoists and winches - Mobile cranes (non-Queensland Government link).
16. See appendix 1 for definitions.
Last updated 21 August 2008