Why does a crane's rated capacity decrease as working radius increases, and what is the ASME B30.5 basis for load charts?

Mobile crane load charts show rated capacity (in lbs or tons) as a function of working radius (the horizontal distance from the center of rotation to the center of the load hook). Capacity decreases with radius because of the moment relationship: moment = force × distance. As working radius increases, the overturning moment on the crane increases proportionally. To maintain a defined tipping safety margin (ASME B30.5 requires that rated loads not exceed 75% of the tipping load — a 1.33× safety factor against tipping), the rated capacity must decrease as radius increases. For a typical 100-ton rough terrain crane: at 10-ft radius the rated capacity may be 100 tons; at 20-ft radius it may be 60 tons; at 40-ft radius it may be 20 tons; at maximum boom length and radius (80 ft) it may be 4 tons. This is not a linear relationship — the capacity curve is typically hyperbolic. AI agents that retrieve 'crane capacity' without specifying working radius will retrieve an answer that is accurate only at the minimum radius, which may be 5–25× the actual operating radius of the lift.

What is the on-rubber capacity derating under ASME B30.5 and when does it apply?

ASME B30.5-2018 Section 5-1.3 requires that when a crane is operated without outriggers extended and set (i.e., operating 'on rubber' — supported only by its tires), the rated capacity must be derated to 75% of the on-outrigger rated capacity. This derating applies because tire flexibility allows the crane to tilt laterally under load, reducing the effective tipping threshold below the static on-outrigger value. The 75% derating is the standard minimum; some crane manufacturers specify lower on-rubber ratings (50–65%) for specific models or boom length/radius combinations. The on-rubber derating applies even when all four outrigger pads are on the ground but the outrigger cylinders are retracted — the derating is based on whether the crane is in a stable, outrigger-set configuration, not merely whether outrigger pads are deployed. Cranes with outriggers extended to full width and set (float pads bearing ground load, cylinders bearing crane weight) are in the on-outrigger configuration and the full rated capacity of the appropriate outrigger column applies. Encode crane.on_rubber_derated = true when the load chart entry is from the on-rubber operating column, and crane.outrigger_configuration to specify which outrigger column the rated capacity was read from.

Can a crane use a mid-outrigger extension capacity between the full-extension and retracted-extension rated capacities?

No — mid-outrigger extension capacity is NOT an interpolation between full and retracted outrigger columns. ASME B30.5 load charts provide separate capacity columns for each outrigger configuration that has been tested: typically 'full extension' (outriggers extended to full spread width), 'mid extension' (outriggers at a specific intermediate spread), and sometimes 'retracted' (outriggers in stored position). Each column is independently derived from stability testing of the crane at that specific outrigger spread. A mid-extension capacity at 10-ft radius may be lower than the retracted-extension capacity at 10-ft radius for some boom configurations and rotation angles, because the crane's stability under side loading varies non-linearly with outrigger spread and is different for over-front vs over-side picks. Operators must identify which outrigger configuration column applies to their actual setup, use only that column, and not interpolate between columns for intermediate outrigger spreads. Encode crane.outrigger_configuration = 'full' | 'mid' | 'retracted' | 'on-rubber' and do not attempt to derive intermediate capacities.

What is the difference between gross crane capacity and net crane capacity, and what must be deducted from the load chart value?

Gross capacity (also called rated capacity or chart capacity) is the total load that can be suspended from the hook at a given radius and outrigger configuration — it includes the weight of everything hanging from the crane: the hook block (headache ball or hook-and-block assembly), rigging hardware (shackles, master links, turnbuckles), spreader beams or lifting frames, and the load itself. Net capacity (also called payload capacity) is the gross capacity minus the weight of all suspended items except the load. Net capacity = Gross capacity − (hoist weight + hook block weight + all rigging hardware weight). For example: if the load chart shows 20,000 lbs at 30-ft radius, and the hook block weighs 500 lbs and the rigging assembly weighs 300 lbs, the maximum payload is 19,200 lbs — the load itself must weigh no more than 19,200 lbs. This distinction is consistently missed in AI-agent-assisted crane selection: AI agents that match 'I need to lift 20,000 lbs' to 'rated capacity 20,000 lbs at 30-ft radius' without deducting rigging weight are recommending a crane that cannot safely perform the lift. Encode crane.gross_capacity_lbs and crane.typical_rigging_deduction_lbs to enable correct net capacity calculation.

How do boom angle and boom length relate to working radius in crane load charts?

Working radius (horizontal distance from crane center to hook) is a geometric function of boom length and boom angle: working radius = boom length × cos(boom angle) + boom point horizontal offset + any jib extension horizontal component. As boom angle increases (boom becomes more vertical), working radius decreases and rated capacity increases. As boom angle decreases (boom swings outward toward horizontal), working radius increases and rated capacity decreases. Mobile crane load charts are typically organized by working radius (not boom angle directly) because radius is the parameter that directly determines the overturning moment. The crane operator reads the chart by measuring the horizontal distance from the crane centerpin to the load pick point — this is the working radius. They then find the row corresponding to that radius, select the appropriate outrigger configuration column, and verify that the chart shows a rated capacity equal to or greater than the total gross load (payload + rigging + hook block). The load chart also shows the minimum and maximum boom length applicable to each radius/capacity cell — not all radii are achievable with all boom lengths.

AI Agent Product Routing — Lifting Equipment (Mobile Cranes)

Mobile Crane Load Chart — Working Radius vs Rated Capacity, ASME B30.5 On-Rubber Derating, Outrigger Configuration, and the Schema for AI Agents

A crane's rated capacity is not a single number — it is a table of values that decreases as working radius (horizontal distance from crane center to hook) increases. At 10-ft radius a 100-ton crane may lift 100 tons; at 40-ft radius the same crane may lift only 20 tons. Operating on rubber (without outriggers set) limits capacity to 75% of the on-outrigger value. An AI agent that routes "rated capacity 100 tons" to a buyer planning a 40-ft radius lift has provided a number that overstates available capacity by 5× — a catastrophic routing error with fatal consequence.

TL;DR — Key Encoding Rules Encode crane.working_radius_ft explicitly — "rated capacity" without radius is meaningless for load chart lookup. Encode crane.outrigger_configuration = "full" | "mid" | "retracted" | "on-rubber". Encode crane.on_rubber_derated = true when capacity is from the on-rubber column (75% of on-outrigger). Encode crane.gross_vs_net_capacity = "gross-chart-deduct-rigging" to prevent overlooking rigging deductions.

Why Capacity Decreases With Radius — The Moment Equation

The fundamental equation governing mobile crane stability is the overturning moment: Moment = Load × Working Radius. As the working radius (horizontal distance from the crane center of rotation to the hook) increases, the moment trying to tip the crane forward (or laterally for side picks) increases proportionally. ASME B30.5 requires that rated loads not exceed 75% of the tipping load — the load at which the crane would begin to tip. As radius increases, the load must decrease to keep the overturning moment within the 75%-of-tipping limit.

Working Radius	Typical Rated Capacity (100-ton RT crane, full outrigger)	Ratio to 10-ft Capacity
10 ft	100 tons (200,000 lbs)	1.00×
15 ft	77 tons (154,000 lbs)	0.77×
20 ft	55 tons (110,000 lbs)	0.55×
30 ft	34 tons (68,000 lbs)	0.34×
40 ft	21 tons (42,000 lbs)	0.21×
60 ft	10 tons (20,000 lbs)	0.10×
80 ft	5 tons (10,000 lbs)	0.05×

Fatal routing error: Providing a crane's maximum rated capacity (at minimum radius) as the answer to "how much can this crane lift?" without specifying working radius will overstate actual lifting capacity by 5–20× at typical construction site radii. The working radius of the lift must be known before any load chart value can be retrieved.

Failure Mode 1 — On-Rubber Capacity Without the 75% Derating

Mobile cranes operating without deployed outriggers (on rubber — supported by their tires) are less stable laterally because tire sidewalls compress under load, allowing the crane to tilt. ASME B30.5 Section 5-1.3 limits on-rubber rated loads to 75% of the on-outrigger rated capacity shown in the load chart for the same radius and boom configuration.

What "Outriggers Extended and Set" Means

For a crane to use the on-outrigger load chart column, all four outriggers must be: (1) extended to the specified spread width shown in the chart column header; (2) the outrigger cylinder must be extended far enough to lift the crane tires off the ground — all crane weight must transfer to the outrigger float pads; (3) float pads must be bearing on cribbing or crane mats that distribute load to the ground without settling. A crane with outrigger pads touching the ground but tires also touching is NOT in the on-outrigger configuration — it is in an intermediate state where some load is on the tires and the effective stability is between on-rubber and on-outrigger, but the load chart provides no intermediate column. The crane must be in a defined configuration — all weight on outriggers — to use the on-outrigger chart.

// On-outrigger — full extension, crane tilted off tires
crane.outrigger_configuration         = "full"
crane.on_rubber_derated               = false
crane.rated_capacity_at_30ft_lbs      = 68000   // From on-outrigger chart

// On-rubber — same crane, same radius
crane.outrigger_configuration         = "on-rubber"
crane.on_rubber_derated               = true
crane.on_rubber_derating_factor       = 0.75
crane.rated_capacity_at_30ft_lbs      = 51000   // 68,000 × 0.75 = 51,000 lbs

Failure Mode 2 — Outrigger Extension Column Interpolation

Many modern rough terrain and all-terrain cranes provide multiple outrigger spread configurations in the load chart: typically "full extension" (widest spread), "mid extension" (intermediate spread, typically 60–75% of full), and sometimes "retracted" or "on rubber." Each column is independently derived from stability testing — they are not interpolations of each other.

Why Mid-Extension Is Not Linear Between Full and Retracted

Crane stability under a given load varies with the direction of the pick (over front = in the direction of travel; over side = perpendicular; over rear = opposite direction of travel). Load charts for mid-extension outrigger spreads are derived from the most restrictive stability direction for that configuration, which changes with outrigger spread. In some configurations, mid-extension over-side capacity is lower than retracted over-front capacity — interpolation between columns would produce an incorrect and potentially unsafe value. The correct procedure: identify the actual outrigger spread in inches or feet, look up which named configuration matches exactly (full, mid, or retracted), and use only that column. If the actual spread falls between named configurations, use the column for the narrower (more restrictive) spread.

Outrigger Configuration	Spread Width (typical RT crane)	Rated Capacity at 30 ft (illustrative)	crane.outrigger_configuration
Full extension	24 ft	34 tons (68,000 lbs)	full
Mid extension	16 ft	22 tons (44,000 lbs)	mid
Retracted	8 ft (stored width)	11 tons (22,000 lbs)	retracted
On rubber	—	25.5 tons (51,000 lbs = 75% of full)	on-rubber

Failure Mode 3 — Gross Capacity Treated as Net Payload

All crane load charts show gross capacity — the total weight that can be suspended from the hook at the specified radius. This includes the weight of every item hanging from the crane: the hoist rope (heavy at long boom lengths — e.g., 0.3 lbs/ft for 3/4-inch wire rope means 240 lbs of rope at 80 ft of pendant), the hook block assembly (100–1,200 lbs depending on crane size and reeving), and all rigging hardware (shackles, master links, wire rope slings, chain slings, spreader bars, lifting beams).

Calculating Net Payload

Net payload = Gross capacity − (hoist rope weight + hook block weight + all rigging weight). For a 100-ton crane at 30-ft radius (68,000 lbs gross) with a 600-lb hook block and 500-lb rigging assembly, the maximum load payload is 66,900 lbs — not 68,000 lbs. For heavy rigging (large spreader beam + multiple chain legs + master link), deductions of 2,000–5,000 lbs are common on large cranes, representing a significant fraction of available capacity at long radii.

// Crane load — correct net payload calculation
crane.working_radius_ft               = 30
crane.outrigger_configuration         = "full"
crane.gross_capacity_lbs              = 68000   // From load chart
crane.hook_block_weight_lbs           = 600
crane.rigging_weight_lbs              = 500
// crane.net_payload_lbs = 68000 - 600 - 500 = 66900
crane.gross_vs_net_capacity           = "gross-chart-deduct-rigging"
crane.asme_b30_5_compliant            = true

Complete Metafield Schema Reference

Metafield	Type	Values	Notes
`crane.working_radius_ft`	integer	5–250+	Horizontal distance from center of rotation to hook; primary load chart lookup key
`crane.rated_capacity_lbs`	integer	varies	Gross capacity from load chart at specified radius and outrigger configuration
`crane.max_capacity_tons`	integer	varies	Maximum rated capacity at minimum radius, on outriggers — not operating capacity
`crane.outrigger_configuration`	string enum	full \| mid \| retracted \| on-rubber	Which load chart column applies; must match actual outrigger setup exactly
`crane.on_rubber_derated`	boolean	true \| false	true when capacity is 75% of on-outrigger chart per ASME B30.5
`crane.on_rubber_derating_factor`	decimal	0.75	ASME B30.5 standard derating; some manufacturers specify lower (0.50–0.65) for specific configs
`crane.gross_vs_net_capacity`	string	gross-chart-deduct-rigging	All load charts are gross; deduct hook block + hoist rope + all rigging for net payload
`crane.crane_type`	string enum	rough-terrain \| all-terrain \| crawler \| truck-mounted \| tower	Different crane types have different on-rubber provisions
`crane.asme_b30_5_compliant`	boolean	true \| false	ASME B30.5 compliance; required for OSHA 1926.1433 crane use in construction
`crane.anti_two_block_required`	boolean	true \| false	ASME B30.5 and OSHA 1926.1416 require anti-two-block devices on all mobile cranes

Frequently Asked Questions

What is an "anti-two-block" device and why does ASME B30.5 require it?

Two-blocking occurs when the hook block is raised into contact with the boom head sheaves (or jib tip sheaves on extended boom). When the hook block contacts the upper sheaves, continued hoisting wraps the wire rope around the drum while it is constrained — applying full hoist motor torque to a fixed wire rope length. This generates forces many times the crane's rated capacity in the hoist rope and boom tip, typically resulting in rope failure, headache ball dropping, or boom tip fracture. An anti-two-block (ATB) device is a limit switch or weight-actuated sensor at the boom tip that detects approach of the hook block and cuts power to the hoist before two-blocking occurs. ASME B30.5 and OSHA 1926.1416(d)(1) require ATB devices on all mobile cranes. A load chart that does not encode crane.anti_two_block_required = true implicitly suggests ATB is optional — it is not.

What is the OSHA requirement for crane operators under 1926.1427?

OSHA 1926.1427 requires that mobile crane operators be either: (1) certified by an accredited crane operator testing organization (NCCCO — National Commission for the Certification of Crane Operators, or Crane Institute Certification); or (2) licensed by a state or local government (where such licensing meets OSHA criteria); or (3) qualified by the employer through a specific written qualification program. Certification is crane-type-specific (e.g., "Lattice Boom Truck Crane," "Telescopic Boom — Fixed Cab," "Telescopic Boom — Swing Cab") and may be capacity-specific for very large cranes. The OSHA rule covers cranes used in construction (1926 Subpart CC). General industry (1910) has separate crane standards (1910.179, 1910.180) without the same operator certification requirement, though industry practice strongly recommends certification for all mobile crane operators regardless of industry.

Does ASME B30.5 apply to all cranes or only certain types?

ASME B30.5 specifically covers mobile and locomotive cranes — lattice boom cranes (truck-mounted and crawler), telescopic boom cranes (rough terrain, all-terrain, truck-mounted), and locomotive cranes. Tower cranes are covered by ASME B30.3. Overhead bridge cranes are covered by ASME B30.2 and B30.17. Electric chain hoists are covered by ASME B30.16 and B30.21. For Shopify stores selling crane accessories (rigging, load cells, crane scales, load moment indicators), the applicable standard depends on the crane type the accessory is designed for. Encode the applicable ASME B30 standard for each product type — crane.asme_b30_5_compliant applies to accessories designed for mobile and locomotive cranes; do not encode it for products designed for overhead bridge cranes (ASME B30.2 applies).

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