5.2 Wire Rope Reeving, Parts of Line & Single-Line Pull Limits

Quick Answer: The lifting capacity of a crane is physically constrained not only by its boom and chassis stability, but also by the strength of its hoisting cable and winch assembly. By reeving the hoist wire rope through multiple sheaves between the boom tip and traveling block, the machine gains mechanical advantage, dividing the suspended gross load across $N$ parts of line. The required parts of line must be calculated as $N = \lceil W_{\text{gross}} / P_{\text{line}} \rceil$. Furthermore, two critical safety mandates protect this system: an operational Anti-Two-Block (A2B) system to prevent cable severing at the boom tip, and a strict requirement that at least two full wraps of wire rope remain on the hoist drum at the lowest hook elevation.

OSHA 29 CFR 1926.1412(d)(1)(vi) requires the wire rope reeving to be checked each shift against the manufacturer's specifications, and 230 CMR 6.08(2) requires operation per those specifications. Selecting the proper reeving configuration is a daily responsibility of the crane operator. Even if a crane's structural load chart indicates a capacity of 100,000 lbs, attempting that lift with insufficient parts of line will instantaneously snap the hoist wire rope or destroy the hoist winch. A comprehensive understanding of hoist mechanics, reeving geometry, and line-pull limits is indispensable for safe operation.

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Principles of Mechanical Advantage & Multi-Part Reeving

A crane hoisting system functions as a classic compound pulley machine. It consists of two primary sheave groupings:

1. Boom Point (Crown) Sheaves: Fixed sheaves mounted on the boom head pin that remain stationary relative to the boom tip.
2. Traveling (Hook) Block Sheaves: Movable sheaves housed within the lower hook block assembly that move vertically with the suspended load.

Defining "Parts of Line"

The term parts of line refers strictly to the individual strands or falls of wire rope that actively support the traveling hook block.

• The lead line (the segment running from the hoist winch drum to the boom point sheave) does not support the hook block directly and does not count as a part of line.
• The dead-end becket attachment (anchored either to the boom head for an even number of parts, or to the traveling block for an odd number of parts) terminates the system.

Mechanical Advantage & Line Tension

Under theoretical statics (neglecting friction), each part of line supporting the traveling block carries an equal fraction of the total suspended gross load ($W_{\text{gross}}$). For a system with $N$ parts of line, the tension in each line ($T_{\text{line}}$) is:

$$T_{\text{line}} = \frac{W_{\text{gross}}}{N}$$

Conversely, the total theoretical lifting capacity of the reeved block is the product of the number of lines and the maximum allowable tension:

$$\text{Capacity}_{\text{reeving}} = N \times P_{\text{line}}$$

In practical field mechanics, rotating sheaves experience minor mechanical friction and wire rope bending resistance (sheave efficiency typically ranges from 98% for anti-friction roller bearings down to 95% for bronze sleeve bushings). As a result, line tension is slightly higher on the winch side of the block, but regulatory and chart calculations rely on nominal allowable line pull.

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Single-Line Pull Limitations & Winch Mechanics

The allowable single-line pull ($P_{\text{line}}$) is the maximum safe tensile load that can be applied to a single strand of wire rope. It is published on the crane's load chart data plate and is governed by two independent mechanical limits:

1. Wire Rope Strength & Design Factor: Running hoist ropes on mobile cranes are designed with a minimum design factor of 3.5 to 1 (ASME B30.5). For rotation-resistant rope, OSHA 29 CFR 1926.1414(e) requires an operating design factor of at least 3.5 for every type, and at least 5 for Type I rope and for Types II and III used in duty-cycle or repetitive lifts, unless the stated exceptions are met. The allowable single-line pull can never exceed the nominal catalog breaking strength ($S_{\text{breaking}}$) divided by the required design factor:

$$P_{\text{line}} \le \frac{S_{\text{breaking}}}{\text{Design Factor}}$$

1. Hoist Winch Mechanical Torque: Hydraulic hoist winches utilize planetary gear reductions driven by hydraulic motors. Winch line pull is a function of hydraulic operating pressure and the effective pitch radius of the wire rope on the drum. Because the lever arm increases as rope layers accumulate on the drum, line pull decreases as drum layers fill up! Crane load charts base allowable single-line pull on the specific drum layer rating (typically the first or third layer).

Typical single-line pull ratings for modern construction cranes range from 9,000 lbs to 15,000 lbs on hydraulic cranes, and up to 25,000+ lbs on heavy lattice crawler rigs.

Consequences of Exceeding Single-Line Pull

If an operator attempts to lift a load that exceeds $N \times P_{\text{line}}$, the hoist system is overloaded:

• Rope overload: each part of line carries more than its rated pull, using up the rope's design factor. Shock or repeated overloads break wires and can part the rope, dropping the load; on a lattice boom the sudden release can whip the boom backward.
• Winch and brake overload: the drum, gearbox, shafts, bearings and hoist brake are loaded beyond their rating and can be damaged.
• Hydraulic stall: a hydraulic winch reaches its relief-valve pressure and stops lifting. Never try to force the lift; add parts of line or reduce the load.

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Calculating Required Parts of Line: The Governing Formula

To determine how many parts of line must be reeved to execute a lift safely, the hoisting engineer must apply the following formula:

$$N = \left\lceil \frac{W_{\text{gross}}}{P_{\text{line}}} \right\rceil$$

where:

• $N$ = Minimum required parts of line (rounded up to the nearest whole integer).
• $W_{\text{gross}}$ = Total gross load in pounds (payload weight + hook block weight + rigging hardware + headache ball + suspended cable allowance).
• $P_{\text{line}}$ = Maximum allowable single-line pull in pounds specified on the crane rating plate.
• $\lceil \dots \rceil$ = Ceiling function (always round fractional results up to the next higher integer).

Step-by-Step Worked Calculation

Operational Scenario: A hoisting engineer on a Massachusetts jobsite must pick a structural steel assembly with a certified payload weight of 57,200 lbs using a 75-ton hydraulic mobile crane.

• Manufacturer Single-Line Pull Limit ($P_{\text{line}}$): 11,500 lbs
• Crane 4-Sheave Main Hook Block Weight: 1,450 lbs
• Below-the-Hook Rigging Gear (Slings & Shackles): 350 lbs

Step 1: Calculate Total Gross Load ($W_{\text{gross}}$) Sum all components suspended from the boom point sheaves:

$$W_{\text{gross}} = 57{,}200\text{ (payload)} + 1{,}450\text{ (block)} + 350\text{ (rigging)} = 59{,}000\text{ lbs}$$

Step 2: Apply the Parts of Line Formula

$$N = \frac{W_{\text{gross}}}{P_{\text{line}}} = \frac{59{,}000\text{ lbs}}{11{,}500\text{ lbs/part}} = 5.1304\text{ parts}$$

Step 3: Round Up to the Next Integer Because you cannot reeve a fraction of a cable, round up:

$$N = \lceil 5.1304 \rceil = 6\text{ parts of line}$$

Step 4: Operational Verification With 6 parts of line, the reeving system provides a safe gross lifting threshold of:

$$\text{Capacity}_{\text{reeving}} = 6 \times 11{,}500\text{ lbs} = 69{,}000\text{ lbs}$$

Because $59{,}000\text{ lbs} \le 69{,}000\text{ lbs}$, 6 parts of line safely and legally accommodates the load. If the operator had mistakenly reeved only 5 parts of line, the line tension would be $59{,}000 / 5 = 11{,}800\text{ lbs}$, overloading the wire rope beyond its 11,500 lb limit by 300 lbs per line!

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Parts of Line vs. Hoisting Capacity & Line Speed

Reeving involves an inherent mechanical trade-off: multiplying capacity reduces hook travel speed by the exact same ratio.

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Two-Blocking Hazards & Anti-Two-Block (A2B) Systems

Two-blocking is one of the most destructive operational errors in crane hoisting. It occurs when the lower traveling hook block (or headache ball) makes physical contact with the boom tip sheave cluster or rooster head assembly.

The Mechanics of Two-Blocking Failure

When the block collides with the boom tip, all upward movement of the block stops abruptly against the solid steel structure. If the winch continues pulling or if boom geometry tightens the cable, the winch exerts its full mechanical stall torque against the cable. The wire rope is instantly stretched beyond its ultimate tensile strength, parting the line within fractions of a second.

The immediate consequence is catastrophic:

• The traveling hook block (weighing hundreds or thousands of pounds) plummets to the ground.
• The suspended payload drops freely.
• Boom point sheaves can fracture or tear away, showering shrapnel on workers below.

Operational Causes of Two-Blocking

Many operators mistakenly believe two-blocking only happens when hoisting up. In reality, two-blocking is caused by three distinct crane functions:

1. Hoisting Up: Pulling the hook block upward into the boom head.
2. Extending (Telescoping) the Boom: As telescopic boom sections extend outward, the distance between the boom tip and the winch drum increases. If the operator fails to pay out hoist line simultaneously, the boom tip moves away from the winch, pulling the hook block directly into the boom head.
3. Booming Down (Lowering Boom Angle): On certain boom geometries and lattice configurations, lowering the boom increases the distance along the suspension geometry, drawing the block toward the tip sheaves.

Anti-Two-Block (A2B) Device Requirements

Under OSHA 29 CFR 1926.1416(d)(3), telescopic boom cranes manufactured after February 28, 1992 must have a device that automatically prevents two-block damage, and lattice boom cranes made after that date need a device that prevents the damage or warns the operator in time. An A2B device is a Category I operational aid: while it is being repaired (within 7 days), the temporary alternative is to clearly mark the hoist cable and use a spotter when extending the boom. 230 CMR 6.09(6)(b) requires an anti-two-block device or two-block damage-prevention feature for any personnel lift. A typical A2B system works like this:

• Mechanism: A counterweighted sensing switch is suspended from the boom tip around the hoist wire rope. The lower hook block carries a contact plate. When the block approaches within a safe threshold (typically 3 to 5 feet from the tip sheaves), it lifts the counterweight.
• Audio-Visual Alarm: Relieving tension on the counterweight trips an internal micro-switch, instantly sounding an audible buzzer and flashing a warning light in the cab.
• Control Function Lockout: The system activates hydraulic solenoid lockout valves that physically disable all crane functions that worsen the condition: hoisting up, telescoping out, and booming down (if applicable). Only safe recovery functions—lowering the hoist line, telescoping the boom in, or booming up—remain operational.

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The Drum Wrap Safety Rule (Capstan Friction Mechanics)

Under ASME B30.5, a crane operator must ensure that a minimum of two full wraps of wire rope remain on the hoist drum when the hook block is lowered to its lowest possible operational elevation (such as deep below ground in a foundation shaft or trench excavation). OSHA 29 CFR 1926.1413 applies the same two-wrap check whenever a damaged rope is cut back: the drum must still have two wraps with the load or boom in its lowest position.

The Engineering Rationale

Why does the law mandate two drum wraps rather than relying on the cable anchor pin?

• Anchor Limitation: The rope end is held in the drum by a wedge or clamp fitting. That anchor is not intended to carry the full line pull by itself; the wraps remaining on the drum keep most of the tension off it.
• The Capstan Friction Effect: When wire rope is wrapped around the curved steel surface of the winch drum, friction between the steel rope and the drum circumference snub-locks the cable. According to the capstan equation ($T_{\text{hold}} = T_{\text{load}} e^{-\mu \theta}$), the tension left at the anchor falls exponentially with the wrap angle. Two full wraps give a contact arc of $4\pi$ radians. With a steel-on-steel friction coefficient of about 0.1 to 0.15, only about 15% to 30% of the line pull reaches the anchor ($e^{-0.1 \times 4\pi} \approx 0.28$ and $e^{-0.15 \times 4\pi} \approx 0.15$). With fewer wraps, more of the pull lands on the anchor.

Letting the drum unspool to its last wraps puts the full line pull on the anchor, which can pull the rope end free and drop the load.