Load Charts and Capacity Determination
Load charts are configuration-specific. Before reading a capacity, identify boom length, jib or extension, counterweight, track or outrigger configuration, working area, and parts of line. Radius is measured from the center of rotation to the load's center of gravity, with the load hanging free; boom deflection under load increases radius, so the loaded radius may exceed the geometric radius. The range diagram helps convert boom length and angle to radius.
Read the notes. Charts typically assume level support, static loads, no wind, proper assembly, and the specified reeving. Deductions matter: hook block, slings, shackles, jib, auxiliary boom head, stowed attachments, and any other suspended equipment reduce the allowable load. Gross capacity minus deductions equals the net load you may pick. Structural limits govern heavy loads at short radius; stability limits govern long radius and long boom. Line pull and parts of line must also be checked; more parts of line increase capacity only up to the chart's structural limit.
Traps: using the chart for the wrong boom length; reading radius from the boom foot; ignoring chart notes; deducting rigging after comparing to capacity; using a shorter-boom chart when the actual boom falls between charted lengths; and assuming a jib capacity equals main boom capacity.
Worked example: 80 ft boom at 30 ft radius, chart capacity 30,000 lb. Deduct hook block 1,000 lb and slings 500 lb: net allowable load 28,500 lb. Check reeving: suspended load (load plus hook block) 29,500 lb; if permitted line pull is 15,000 lb, two parts give 30,000 lb-adequate but tight, so verify the chart's parts-of-line requirement and hook block rating. Rule: parts of line equals total suspended load divided by permitted line pull, rounded up, then confirmed against the chart.