A telescopic crane or hydraulic crane has a boom that is composed of a series of tubes, fitted one inside the other. The largest segment, at the bottom of the boom, is called the base section, and the smallest segment, at the top, is called the boom tip.
Telescopic cranes usually have a pinned boom, full-powered boom, or a combination of the two. As its name suggests, a pinned boom means that the sections are pinned in the extended or retracted position, whilst a full-powered boom means sections extend or retract by hydraulic means.
The way a full-powered boom works is simple. All sections (besides the base) are extended and retracted by hydraulic cylinders and, as a result, increase or decrease the total length of the telescope. The boom extension cylinders on most telescopic booms have sequencing valves which allow the telescope to extend by equal amounts. To operate this, the telescopic crane will have a control lever in the cab. For cranes that don’t have sequencing valves, the operator will need to extend each section equally. If done disproportionately, then the most fully extended part of the boom could bend. This is what’s known as ‘crane boom deflection’.
When the load is placed on the telescopic boom, the load weight engages the hydraulic rams – causing the boom to stiffen up and slightly curve. Once the load is removed, the rams return to upright.