On a hydraulic crane, which item prevents the extended section from retracting unexpectedly?

• A. Locking pin
• B. Flywheel
• C. Check valve
• D. Bumper

Answer: (A)

The main idea is how to keep the extended boom from moving back on its own. A locking pin provides a positive, mechanical lock by passing through aligned holes in the boom sections, securing the extended position even if hydraulic pressure drops or external forces push on the boom. That physical restraint is what prevents unintended retraction.

A flywheel stores rotational energy for drive systems, not for locking boom position. A check valve in the hydraulic circuit prevents backflow but doesn’t fix the boom in place. A bumper cushions impact at the end of the extension, not the tracking of the boom’s length.