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

On what principle is crane stability based?

1. Torque principle

2. Velocity principle

3. Lever principle

4. Gravity principle

The correct answer is: Lever principle

Crane stability is fundamentally based on the lever principle, which involves the use of a fulcrum—specifically, the crane’s support points must provide adequate balance against the loads being lifted. In essence, the crane acts like a lever, with the load on one side and the counterbalancing force on the other. The weight of the load, along with the position of the center of gravity, creates a moment (or torque) around the pivot point. To maintain stability, it is crucial that the center of gravity of the system lies within the base of support provided by the crane's outriggers or tracks. If the load exceeds the crane's capacity or is positioned too far from the base of support, the lever effect can shift the center of gravity outside this base, leading to a potential tipping hazard. This understanding is vital for operators to ensure that loads are managed safely, confirming that the crane remains stable throughout all phases of operation. The other principles listed, such as torque, gravity, and velocity, relate to the broader mechanics of lifting but do not specifically address the stability principle in the context of crane operation as effectively as the lever principle does.