7.3 Reeving Patterns, Parts of Line & Mechanical Advantage
Key Takeaways
- Parts of line refers strictly to the number of wire rope lines directly supporting the travelling load block, excluding the dead-end line if anchored to the boom tip.
- Mechanical advantage reduces the single line pull tension required to hoist a load: Ideal Line Pull = Total Gross Load / Parts of Line, while actual tension must account for sheave friction efficiency (typically 1%–2% loss per sheave).
- The minimum required parts of line for a lift is calculated by dividing the total gross load (load + block + rigging) by the crane's certified single line pull rating: Parts of Line = Total Gross Load / Single Line Pull (rounded up to the next integer).
- Center-to-center (reverse) reeving distributes rope tension symmetrically across the sheave cluster to eliminate block twist (cabbing) and keep the traveling block level, outperforming basic lacing patterns that induce severe side tilt.
- Two-blocking occurs when the load block contacts the boom tip sheaves, causing instantaneous rope overload and catastrophic failure; anti-two-block (A2B) switches provide audio-visual warnings and automatic hydraulic cutouts.
7.3 Reeving Patterns, Parts of Line & Mechanical Advantage
In mobile and tower crane operations, the hoisting capacity of the crane is governed by the interaction between the hoist winch line pull rating, the wire rope strength, and the reeving system. Reeving is the path that the wire rope follows as it threads through the boom tip head sheaves and the traveling hook block sheaves.
Under ASME B30.5 (Mobile and Locomotive Cranes), riggers and lift planners must correctly determine the required parts of line to ensure that single line pull ratings are never exceeded, select reeving patterns that eliminate traveling block twist, and maintain functioning anti-two-block (A2B) protection systems at all times.
1. Principles of Parts of Line & Mechanical Advantage
The fundamental purpose of reeving multiple lines through a traveling hook block is to gain mechanical advantage. By dividing the suspended gross load among multiple parts of line, the tensile tension experienced by the single hoist rope spooling onto the winch drum is reduced proportionally.
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| PARTS OF LINE & MECHANICAL ADVANTAGE |
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| IDEAL MECHANICAL ADVANTAGE FORMULAS (Zero Friction): |
| |
| Total Lifting Capacity = Single Line Pull Rating x Parts of Line |
| |
| Line Tension (Ideal) = Total Gross Suspended Load / Parts of Line |
| |
| Line Speed (Hook Travel) = Winch Drum Line Speed / Parts of Line |
| |
| Drum Rope Required = Hook Travel Distance x Parts of Line |
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The Trade-off: Force vs. Speed & Rope Length
Mechanical advantage follows the conservation of energy:
- Increasing parts of line increases total lifting capacity and lowers the line pull required.
- Increasing parts of line decreases hook hoisting speed (a 4-part line moves the hook at 1/4 the winch line speed).
- Increasing parts of line requires proportionally more wire rope spooled onto the hoist drum (to lift a 4-part block 50 feet, the winch must spool in 200 feet of rope).
2. Counting Parts of Line & The Dead-End Anchor Rule
A critical competency on rigging certification exams is properly identifying the exact number of parts of line (also called falls of line). The golden rule under ASME B30.5 states:
Parts of Line Definition: The number of parts of line is strictly equal to the number of wire rope segments that directly support the traveling load block.
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| EVEN VS. ODD REEVING CONFIGURATIONS |
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| EVEN PARTS OF LINE (2, 4, 6, 8 Parts): |
| * The dead-end becket / wedge socket is anchored to the BOOM TIP HEAD. |
| * The dead-end does NOT support the hook block directly. |
| * Example: 4 parts of line = 4 rope lines traveling between boom and block. |
| |
| ODD PARTS OF LINE (1, 3, 5, 7 Parts): |
| * The dead-end becket / wedge socket is anchored to the TRAVELING HOOK BLOCK. |
| * The dead-end DOES support the hook block directly, counting as 1 part. |
| * Example: 3 parts of line = 2 lines passing around sheaves + 1 dead-end line. |
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| REEVING SCHEMATIC: 4-PART VS. 3-PART |
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| |
| [ BOOM HEAD SHEAVES ] [ BOOM HEAD SHEAVES ] |
| ( S1 ) ( S2 ) [Becket Anchor] ( S1 ) ( S2 ) |
| | | | | | |
| #1 | #3 | #4 | (Dead End) #1 | #3 | |
| Line| Line| Line| Line| Line| |
| | | | | | |
| v v | v v |
| ( B1 ) ( B2 ) <---+ ( B1 ) ( B2 ) [Becket] |
| | | | | | |
| [ TRAVELING BLOCK ] [ TRAVELING BLOCK ]<--+ |
| (#2 Dead) |
| >> 4-PART LINE (EVEN) >> 3-PART LINE (ODD) |
| Becket anchored at BOOM TIP Becket anchored at HOOK BLOCK |
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3. Step-by-Step Calculation: Required Parts of Line
To configure a crane safely for an engineered pick, the rigger must calculate the minimum required parts of line using the crane manufacturer's certified single line pull rating.
Step-by-Step Mathematical Workflow
- Determine Total Gross Load: Sum the net load payload, hook block weight, headache ball weight, slings, shackles, spreader bars, and all rigging hardware.
- Identify Maximum Single Line Pull Rating: Obtain the certified single line pull from the crane load chart (based on winch rating and rope design safety factor).
- Apply the Parts of Line Formula: Divide Total Gross Load by Single Line Pull Rating.
- Round UP to Next Whole Integer: Parts of line cannot be a fraction. Always round up to ensure single line pull is never exceeded.
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| PRACTICAL PARTS OF LINE CALCULATION EXAMPLE |
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| LIFT DATA: |
| * Net Payload Weight: 54,000 lbs |
| * Hook Block & Overhaul Ball Weight: 3,200 lbs |
| * Rigging Hardware & Slings Weight: 800 lbs |
| * Crane Certified Single Line Pull: 15,000 lbs |
| |
| CALCULATION: |
| 1. Total Gross Load = 54,000 + 3,200 + 800 = 58,000 lbs |
| 2. Parts of Line = Total Gross Load / Single Line Pull |
| 3. Parts of Line = 58,000 lbs / 15,000 lbs = 3.87 parts |
| 4. Round UP: Must reeve a minimum of 4 PARTS OF LINE! |
| |
| VERIFICATION: |
| Actual Line Pull with 4 Parts = 58,000 lbs / 4 = 14,500 lbs |
| 14,500 lbs <= 15,000 lbs rated single line pull -> APPROVED AND SAFE! |
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4. Reeving Patterns: Lacing vs. Skip vs. Center-to-Center
The physical pattern used to thread wire rope through the sheaves fundamentally affects block balance, sheave wear, and the hazardous phenomenon known as block cabbing (traveling block twisting).
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| REEVING PATTERNS TECHNICAL ANALYSIS |
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| 1. LACING (SEQUENTIAL REEVING): |
| * Rope threads sequentially from outer Sheave 1 to Sheave 2, 3, 4 across block.|
| * SEVERE LIMITATION: Lead line enters one outside sheave, concentrating high |
| tension on one side while opposite side carries lowest tension. |
| * Result: Severe traveling block TILT, sheave flange rubbing, high rope wear. |
| |
| 2. SKIP REEVING (EVEN / ODD ALTERNATING): |
| * Rope skips every other sheave to alternate tension across block sides. |
| * Significantly reduces block tilt, but can still induce dynamic oscillations.|
| |
| 3. CENTER-TO-CENTER (REVERSE REEVING / CENTER LEAD): |
| * Lead line drops from center boom sheave directly to center hook block sheave|
| and routes outward symmetrically toward both outer edges. |
| * MAXIMUM STABILITY: Symmetrical tension balances load across block center! |
| * ELIMINATES BLOCK CABBING AND KEEPS TRAVELING BLOCK PERFECTLY LEVEL! |
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| Reeving Pattern | Sheave Routing Sequence | Block Balance & Leveling | Resistance to Block Cabbing (Twisting) | Primary Application |
|---|---|---|---|---|
| Lacing | Sequential 1-2-3-4 left-to-right | Poor: Causes block to hang tilted due to lateral line tension gradient | Poor: High tendency to twist at long boom lengths | Light utility hoists with 2 or 3 parts only |
| Skip Reeving | Alternates between odd and even sheaves | Good: Equalizes side-to-side forces | Moderate: Resists twist under steady vertical hoisting | Medium-capacity mobile cranes (4 to 6 parts) |
| Center-to-Center (Center Lead) | Enters center sheaves first, progresses outward symmetrically | Superior: Symmetrical load distribution keeps block perfectly horizontal | Superior: Neutralizes torsional moments; prevents block spinning | Heavy-lift mobile cranes, long lattice booms, multi-part engineered picks |
Block Cabbing Hazard: Block cabbing occurs when traveling hook blocks twist 180° or 360°, wrapping the multiple falls of wire rope around one another. This locks the hoisting mechanism, destroys wire ropes via cross-over crushing, and can cause uncontrolled load drops. Center-to-center reeving is the primary engineering control to eliminate cabbing.
5. Two-Blocking Hazards & Anti-Two-Block (A2B) Protection
Two-blocking is one of the most destructive operational hazards in crane operations. It occurs when the traveling hook block or headache ball is hoisted into physical contact with the boom tip sheave assembly.
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| TWO-BLOCK DYNAMICS & FORCES |
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| WHAT HAPPENS DURING A TWO-BLOCK: |
| 1. Hook block physically impacts boom head sheaves (clearance = 0 ft). |
| 2. Winch drum continues pulling or boom continues telescoping outward. |
| 3. Tensile force in wire rope instantly spikes from rated line pull (e.g. |
| 12,000 lbs) to ultimate breaking strength (e.g. 60,000+ lbs). |
| 4. Wire rope snaps instantly; hook block and load drop to the ground. |
| 5. Boom tip can bend or collapse backward over the crane cab. |
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Anti-Two-Block (A2B) System Mechanics (ASME B30.5 / OSHA 1926.1416)
Under federal regulations, cranes must be equipped with an operational Anti-Two-Block (A2B) Warning and Control Cutout System:
- Counterweight & Limit Switch: A weighted ring is suspended around the hoist line below the boom point, holding an electrical limit switch closed under gravity.
- Actuation: When the hook block rises to within a safe distance (typically 3 to 6 feet), it lifts the counterweight. The limit switch opens, breaking the circuit.
- Audio-Visual Alarm & Function Lockout: The system sounds an audible horn in the cab, flashes a warning light, and automatically disables (locks out) the three crane control functions that worsen a two-block condition:
- Hoist Up (winch hoisting upward)
- Telescope Out (extending boom sections forward)
- Boom Down (lowering boom angle on hydraulic cranes, which pushes boom point forward and tightens hoist line)
- Permitted Recovery Functions: The operator is allowed only to Hoist Down, Telescope In, or Boom Up to restore safe clearance.
A crane with a certified single line pull rating of 12,500 lbs must hoist a total gross load of 46,000 lbs (including hook block and rigging). What is the MINIMUM number of parts of line required?
When reeving a crane block with an ODD number of parts of line (such as 3 parts or 5 parts), where is the dead-end becket (wedge socket) anchored?
What is the primary engineering advantage of utilizing a Center-to-Center (Reverse) reeving pattern compared to a standard sequential Lacing reeving pattern?