National Commission for the Certification of Crane Operators (NCCCO) Mobile Crane Practice Exam

Unless the manufacturer states otherwise, what crane capacity chart is appropriate when a crane is set up with 3 of its 4 outriggers extended and the fourth is retracted?

1. On-outrigger chart

2. On-rubber chart

3. On-outrigger chart minus 20%

4. On-rubber chart plus 20%

The correct answer is: On-rubber chart

When a crane is set up with three outriggers extended and one retracted, the appropriate crane capacity chart to refer to is the on-rubber chart. This chart is specifically designed for situations where the crane is operating without full support from its outriggers. The reason for this is that the crane's ability to lift safely and effectively is significantly affected by how it is stabilized. In this case, with one outrigger retracted, the crane is not fully stabilized and is essentially operating in a configuration similar to that of having its tires on the ground (or "on-rubber"). The on-rubber chart accounts for that diminished stability by providing load limits that reflect the reduced capacity of the crane in such a stance. It is crucial for safety and adherence to manufacturer specifications that operators utilize the correct chart based on the crane's setup to avoid overloading and potential accidents. Using the on-outrigger or any adjustments to those charts would not accurately represent the crane's lifting capacities given its current configuration. Therefore, the on-rubber chart is the most appropriate reference for safe operation in this scenario.