Mobile crane capacity: reading the chart, net load, utilisation and reeving

A load chart is the crane manufacturer’s statement of what the crane can lift in a precisely defined configuration. Using it correctly means matching that configuration exactly and accounting for everything the hook carries.

Where the numbers come from

• Stability ratings are a percentage of the tipping load. Under ASME/SAE practice in North America, ratings are 75% of tipping for crawler cranes and 85% for cranes on outriggers. EN 13000 uses its own stability criteria. Either way, the margin is for dynamic effects and small deviations, not for planned overloads.
• Structural ratings, usually at shorter radii and shaded or marked on the chart, are limited by the strength of the boom, slewing ring or other parts. Exceeding them can cause structural failure without any warning of tipping.
• Charts assume a firm, level surface (typically within 1%), outriggers extended exactly as stated, the stated counterweight, and wind within the stated limit.

Gross chart capacity and net capacity

The chart figure is the gross capacity. Deduct everything that hangs from the boom or is attached to it, as the manufacturer specifies:

• hook block or overhaul ball;
• slings, shackles, spreader beam and load cell;
• jib or extension when erected or stowed, if the chart requires it;
• load line weight, where the chart says to include it;
• any auxiliary sheave head, or the second hook block on a two-hook set-up.

Total load = load weight + all deductions.    Utilisation = total load ÷ chart capacity at the radius and configuration used × 100%

Example. Chart capacity at 18 m radius in the planned configuration is 32.0 t. Load 22.0 t; hook block 0.8 t; slings and shackles 0.4 t; spreader 0.6 t. Total = 23.8 t; utilisation = 23.8 ÷ 32.0 = 74%.

Radius and the worst case

• Radius is measured horizontally from the centre of rotation to the centre of the hook, not from the edge of the carrier.
• Booms deflect under load, so the loaded radius is greater than the unloaded radius. Use the radius at the end of the lift, or the largest radius during the lift, whichever gives the lower capacity.
• Check pick, swing and set positions. Capacity often varies with slew angle (over the front, side or rear), and the set position is frequently the critical one.

When a lift is critical

Company procedures set their own triggers, and BS 7121 classifies lifts by complexity. Typical triggers for treating a lift as critical (or complex) and requiring an engineered plan and approval are:

• utilisation above a set percentage of chart capacity, often 75–90% depending on the procedure;
• any multiple-crane lift, especially where either crane exceeds about 75% of its capacity;
• cranes travelling with load, upending or tailing;
• lifts over live plant, occupied buildings or pipe racks, or near power lines;
• personnel lifts, or loads of very high value or long lead time.

Reeving: parts of line and line pull

The hoist rope is reeved through the block so that several parts share the load. Each sheave loses a little to friction, so the lead line pulls slightly more than the ideal share. With n parts of line and sheave efficiency e (about 0.98 for roller-bearing sheaves, lower for plain bushings):

Lead line pull F = W × (1 − e) ÷ (1 − eⁿ)    (ideal, frictionless: W ÷ n)

Example. 40 t hook load on 6 parts with e = 0.98: F = 40 × 0.02 ÷ (1 − 0.98⁶) = 7.0 t, compared with the ideal 6.67 t. The lead line pull must not exceed the hoist’s permissible line pull or the rope’s rated load. If it does, the block needs more parts of line. Crane charts give the allowable load per number of parts directly. Use them in preference to your own calculation.

Before lifting

• Confirm the crane is set up exactly as the chart line used: boom length, counterweight, outriggers, radius and parts of line.
• Set the rated capacity limiter (load moment indicator) to the actual configuration, and test it.
• Check wind (Chapter 10) and ground (Chapter 8).

Reading the load chart properly

A load chart is valid only for the exact configuration it states. Before reading any number, confirm each of these matches the crane on site:

• Counterweight fitted (and any superlift or ballast tray).
• Outrigger spread: full, intermediate or retracted beams; or on tyres, or on crawlers with tracks extended.
• Working area: 360°, over the rear, over the side or over the front. Some charts are lower over the front or side.
• Boom length and mode: telescopic sections and extension sequence (the same length can be reached by different modes with different capacities), luffing jib or fixed jib and its offset.
• Deductions: the hook block, stowed or erected jib, auxiliary head, and anything else the chart notes say to deduct.
• Notes: the chart notes state the level tolerance, wind limits, whether capacities are structural or stability limited, and how to interpolate. Read them every time.

Many charts mark the boundary between structurally limited and stability limited capacities with a line or shading. Above it (short radius), the boom or other structure governs; below it (long radius), tipping governs. A structural failure gives no warning from the crane tilting. Do not rely on feeling the crane “go light”.

Worked example: utilisation

At the planned maximum radius of 18 m, the chart (for this configuration and working area) gives 24.5 t. Utilisation = 19.4 ÷ 24.5 = 79%. Many company procedures call this a critical lift or require extra controls above a set percentage (often 75% to 85%), and would also look again at the radius: a 1 m increase in radius at this point on the chart could easily cost 1.5 t of capacity.

The range diagram: boom angle, tip height and clearance

The range diagram links radius, boom length and boom angle. For a boom of length L whose foot pin is a horizontal distance p from the slew centre and a height f above ground:

cos α = (R − p) ÷ L    tip height ≈ L × sin α + f   (R = radius measured from the slew centre)

Example. L = 40 m, p = 1.5 m, f = 2.5 m, R = 18 m: cos α = 16.5 ÷ 40 = 0.4125, so α = 65.6°; tip height = 40 × 0.911 + 2.5 = 38.9 m. Subtract the hook block, rigging and load height to see how high the load can actually go.

Clearance. The perpendicular distance from the boom centreline to a point at radius r and height y is |(r − p) × sin α − (y − f) × cos α|. For an obstruction at r = 10 m, y = 12 m: |8.5 × 0.911 − 9.5 × 0.4125| = 3.82 m. For one at r = 14 m, y = 20 m: |12.5 × 0.911 − 17.5 × 0.4125| = 4.17 m. Take off half the boom width (and allow for deflection) to get the real clearance. Most lift plans require a minimum clearance, often 0.5–1.0 m, between any part of the crane, load and rigging and the nearest obstruction.

Line pull and parts of line

The hoist rope runs from the winch over the boom head sheaves and down and up through the hook block. With n parts of line the load is shared between n falls, but each sheave loses a little to friction, so the lead line carries more than W ÷ n. With a sheave efficiency e (often taken as 0.98 for roller bearings and 0.96 for plain bushings):

Lead line pull = W ÷ (1 + e + e² + … + eⁿ⁻¹) = W × (1 − e) ÷ (1 − eⁿ)

The lead line pull must not exceed the winch’s rated line pull or the rope’s permitted working load. ASME B30.5 requires a design factor of at least 3.5 on running ropes. For 4 parts, the rope needs a minimum breaking force of at least 10.31 × 3.5 = 36.1 t. Charts usually state the parts of line needed for each capacity; never lift a load that needs more parts than are reeved, even if the chart number at that radius is high enough.

Out of level, side load and dynamic effects

• Level. Charts assume the crane is level within the tolerance stated (commonly 1%, about 0.6°). Out of level, the load swings out, the radius grows, and the boom takes a side load. On long booms a small error can cut capacity sharply. Level the crane on its outriggers or mats and check the level again after the first heavy lift, as mats settle.
• Side load. Booms are designed for only a small side load (one common design assumption is 2% of the rated load at the tip). Slewing fast, dragging a load sideways or wind on a large load can exceed that.
• Dynamics. Charts include an allowance for normal smooth operation. Snatching a load, sudden stops, pick-and-carry over rough ground and lifting out of water (suction, added mass, wave motion) need further allowances. Write them into the plan.

Common mistakes

• Reading the chart for the wrong counterweight, outrigger setting or working area.
• Measuring radius from the outrigger or the cab instead of the slew centre, or measuring it before the load is lifted (the boom deflects and the radius increases under load).
• Forgetting the hook block, jib or rigging deductions.
• Interpolating where the chart notes forbid it; use the next larger radius or longer boom.
• Planning to the exact chart figure with no allowance for radius growth, wind or a heavier load than expected.

Self-check questions

General guidance only. It does not replace the manufacturer's data, the applicable standards or the judgement of a competent person.

References

Technical references consulted for this chapter. The text and worked examples are original to LiftPlan Pro.

• Rigging Handbook, 2nd edition (Bechtel Equipment Operations, 2002).
• Ronald G. Garby, IPT’s Crane and Rigging Training Manual: Mobile, EOT and Tower Cranes (IPT Publishing and Training).
• ASME B30.5 Mobile and Locomotive Cranes; BS EN 13000 Mobile cranes; BS 7121 Code of practice for safe use of cranes.
• J. Keith Anderson, Rigging Engineering Calculations (Industrial Training International).

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