9.2 Fall Configurations, Line Parts & Single-Line Pull Limits
Key Takeaways
- Fall configuration (reeving) dictates the mechanical advantage of the hoisting system: 2-part line (2-fall) provides 2:1 mechanical advantage and high line speed, while 4-part line (4-fall) provides 4:1 mechanical advantage for heavy picks at half the hoisting speed.
- Single-line pull is the maximum allowable tensile pulling force developed on a single line of wire rope exiting the hoist winch drum, governed by motor torque, gearing, and a mandatory minimum 5.0 wire rope design factor.
- Total hoist mechanical capacity equals the single-line pull rating multiplied by the number of parts of line, provided the crane's structural load moment rating is not exceeded.
- Automated shifting systems (such as SM/DM or 2/4 electric shifting) allow rapid reeving transitions between 2-fall and 4-fall configurations from the cab without manually rethreading wire rope.
- Referencing the 4-part load chart column while operating the crane in a 2-part reeving configuration is a catastrophic operational violation that can overload the hoist winch and snap the hoist wire rope.
9.2 Fall Configurations, Line Parts & Single-Line Pull Limits
In tower crane operations, reeving refers to the path the hoist wire rope takes through the sheaves of the trolley, the tower head, and the traveling hook block. Governed by ASME B30.3 Section 3-1.4 and OSHA 29 CFR § 1926.1435, the number of parts of line (also called falls) supporting the hook block directly determines the mechanical advantage, maximum hoist capacity, hook line speed, and cycle efficiency of the crane.
On the NCCCO Tower Crane Operator Written Examination, candidates are tested extensively on single-line pull calculations, reeving mechanical trade-offs, automated shifting mechanics, and the critical procedure of aligning the active reeving mode with the correct column in manufacturer load charts.
1. Mechanics of 2-Part vs. 4-Part Fall Configurations
Modern top-slewing tower cranes primarily operate in either 2-Part Line (2-Fall / Single Trolley Block) or 4-Part Line (4-Fall / Dual Trolley Block).
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| 2-PART VS. 4-PART REEVING COMPARISON |
| |
| [2-PART LINE / 2-FALL] [4-PART LINE / 4-FALL] |
| (High Speed Mode) (Heavy Lift Mode) |
| |
| [TROLLEY CARRIAGE] [TROLLEY CARRIAGE] |
| +----------------+ +----------------+ |
| | (1) (2) | | (1) (2) (3) (4)| |
| +--|----------|--+ +--|---|---|---|--+ |
| | | | | | | |
| | | (Dead-End | | | | |
| Hoist Line | | Anchor) Hoist Line | | | | |
| from Winch | | from Winch | | | | |
| | +--+ | | | | | |
| | | | | | | | | |
| v v | | v v v v |
| ( SHEAVE ) | ( SHEAVE ) ( SHEAVE ) |
| +------------+---+ +-----------------------+|
| | HOOK BLOCK | | HOOK BLOCK ||
| | (Single Sheave| | (Dual Sheaves) ||
| +-------+--------+ +-----------+-----------+|
| | | |
| v v |
| [SWIVEL HOOK] [SWIVEL HOOK] |
| |
| Mechanical Adv: 2:1 Mechanical Adv: 4:1 |
| Hook Speed: 100% (Fast) Hook Speed: 50% (Slow) |
| Max Capacity: 2 x Line Pull Max Capacity: 4 x Line P|
+-----------------------------------------------------------------------------+
A. 2-Part Line (2-Fall Reeving):
- Reeving Path: Hoist wire rope spools from the main hoist winch drum, travels through guide sheaves at the mast top/counterjib, passes horizontally along the working jib to the trolley carriage, travels down and around a single sheave inside the hook block, and returns up to a dead-end wedge socket anchor on the trolley frame or boom point.
- Mechanical Advantage: $MA = 2$. Two parts of wire rope directly support the hook block and load.
- Hook Speed: Because only two parts of line are spooled for every unit of hook elevation, hook travel speed is 100% of base hoist line velocity ($V_{\text{hook}} = \frac{V_{\text{line}}}{2}$).
B. 4-Part Line (4-Fall Reeving):
- Reeving Path: The hoist rope travels from the hoist winch to the trolley carriage, down through the first sheave of a dual-sheave hook block, up around a redirecting sheave on the trolley carriage, back down through the second sheave of the hook block, and anchors at a fixed dead-end on the trolley or jib tip.
- Mechanical Advantage: $MA = 4$. Four parts of wire rope directly support the hook block and load.
- Hook Speed: The hook travels at exactly half the speed of 2-part line ($V_{\text{hook}} = \frac{V_{\text{line}}}{4}$). The hoist winch must spool 4 feet of wire rope to raise the hook block 1 foot.
2. Automated Reeving Shifting Systems (SM/DM & 2/4 Shifting)
Historically, converting a tower crane between 2-part and 4-part line required crane riggers to climb onto the jib, lower the block to the ground, manually rethread heavy steel wire rope through multiple sheaves, and reinstall wedge sockets. This process took 2 to 4 hours of jobsite downtime.
Modern tower cranes incorporate automated or semi-automated trolley reeving change systems (e.g., Potain's SM/DM – Simple Moufflage / Double Moufflage, Liebherr's LiConnect, Terex's 2/4 Part Shift).
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| AUTOMATED TROLLEY / HOOK SHIFTING SEQUENCE |
| |
| [STEP 1: 4-PART COUPLED MODE] [STEP 2: AUTO UNCOUPLING] [STEP 3: 2-PART PARKED MODE] |
| |
| [MAIN TROLLEY] [MAIN TROLLEY] [MAIN TROLLEY] |
| +------------+ +------------+ +------------+ |
| | CARRIAGE A | | CARRIAGE A | | CARRIAGE A | |
| +-----+------+ +-----+------+ +-----+------+ |
| | (Coupled) | (Unlatches) | |
| +-----+------+ | | |
| | CARRIAGE B | v v |
| +------------+ [CARRIAGE B] [CARRIAGE B] |
| | (Parks at Mast) (Locked at Jib Heel) |
| v | | |
| [4-FALL HEAVY BLOCK] v v |
| (Both Sheaves Active) [2-FALL LIGHT BLOCK] [2-FALL HIGH SPEED HOOK] |
+-----------------------------------------------------------------------------+
How Automated Shifting Works:
- Dual Trolley Architecture: The trolley is split into a primary driving carriage (Carriage A) and a secondary auxiliary carriage (Carriage B). The lower hook block is similarly split into an inner core hook block and an outer companion block.
- Conversion Sequence: To shift from 4-part to 2-part line, the operator trolleys fully inward to the mast heel. An electro-mechanical or hydraulic latch locks Carriage B and the outer hook sheaves to the jib structure at the mast.
- Decoupling: Carriage A and the lightweight center hook block decouple and traverse out along the jib as a pure 2-part line system. The transition takes less than 3 minutes and is executed entirely from the operator cab console.
- LMI Sensor Integration: Modern crane Load Moment Indicators automatically detect the physical position of the shifting lock, switching the digital display and load cutoff parameters to the active 2-fall or 4-fall load chart without operator intervention.
3. Single-Line Pull Mathematics & Hoist Winch Limits
Single-Line Pull ($F_{\text{line}}$) is the maximum rated tensile pull that the hoist winch drum can safely impart to a single strand of wire rope.
+-----------------------------------------------------------------------------+
| SINGLE-LINE PULL MATHEMATICAL MODEL |
| |
| GIVEN PARAMETERS: |
| - Hoist Winch Single-Line Pull Rating ($F_line$) = 11,000 lbs |
| - Hoist Wire Rope Breaking Strength = 55,000 lbs (SF = 5.0) |
| |
| CALCULATION FOR 2-PART LINE (n = 2): |
| $Total Hoisting Capacity = 2 \times 11,000 lbs = 22,000 lbs$ |
| |
| CALCULATION FOR 4-PART LINE (n = 4): |
| $Total Hoisting Capacity = 4 \times 11,000 lbs = 44,000 lbs$ |
+-----------------------------------------------------------------------------+
Factors Governing Single-Line Pull:
- Wire Rope Design Factor: Under ASME B30.3-1.7.1, running hoist wire ropes must maintain a minimum design factor of 5.0 (Nominal Breaking Strength $\div$ Rated Line Pull $\ge 5.0$). For rotation-resistant wire ropes, many jurisdictions and OEMs require a design factor of 5.0 to 6.0.
- Drum Layering Losses: A hoist winch develops its maximum single-line pull on the first layer (bare drum) because the torque arm (drum radius $r$) is at its minimum. As wire rope layers build up on the drum (e.g., 3rd, 4th, or 5th layer on a deep high-rise hoist drum), the effective torque arm increases, reducing the available line pull by 10% to 25%.
- Motor Thermal Rating & VFD Drive: Variable Frequency Drives (VFD) regulate motor torque and line speed across different gear steps (e.g., Step 1: full torque/slow speed for heavy picks; Step 3: low torque/high speed for empty hook).
4. Operational Trade-Offs: Speed vs. Power & Sheave Efficiency
Selecting the optimal fall configuration is an essential planning task for the crane operator and lift director.
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| REEVING SELECTION DECISION CRITERIA |
| |
| [2-PART LINE / 2-FALL] [4-PART LINE / 4-FALL] |
| - Concrete Placement (Buckets) - Heavy Precast Concrete Panels |
| - Formwork Stripping & Flying - Structural Steel Trusses/Girders |
| - Rebar Packs & Bundle Distribution - Mechanical Chillers / Generators |
| - Light Palletized Materials - Tower Crane Climbing Operations |
| - Deep High-Rise Hook Travel - Heavy Civil Infrastructure Picks |
| |
| >>> PRIORITIZES CYCLE SPEED <<< >>> PRIORITIZES LIFT CAPACITY <<< |
+-----------------------------------------------------------------------------+
Sheave Friction & Mechanical Efficiency:
When wire rope passes over a sheave, friction in the roller bearings and internal bending resistance of the steel strands cause energy loss. While theoretical mechanical advantage for 4-part line is 4.0, real-world mechanical advantage is slightly lower ($MA_{\text{actual}} \approx 3.75 - 3.85$) due to cumulative sheave friction ($\sim 2-3%$ loss per sheave). Modern crane load charts already incorporate these sheave friction losses into published tabular ratings.
5. Comprehensive Reeving Configuration Matrix
| Operational Parameter | 2-Part Line (2-Fall) | 4-Part Line (4-Fall) |
|---|---|---|
| Mechanical Advantage ($MA$) | 2:1 (2 supporting parts) | 4:1 (4 supporting parts) |
| Maximum Hook Lifting Limit | Capped at $2 \times \text{Single Line Pull}$ (e.g., 22,000 lbs) | Capped at $4 \times \text{Single Line Pull}$ (e.g., 44,000 lbs) |
| Hook Hoisting Speed | Full Speed (100%) — $2\times$ faster than 4-part | Half Speed (50%) — Spools 4 ft rope per 1 ft lift |
| Winch Wire Rope Spooling | 200 ft of hook travel spools 400 ft of rope on drum | 200 ft of hook travel spools 800 ft of rope on drum |
| Drum Capacity Demand | Low drum storage demand; ideal for ultra-tall towers | High drum storage demand; requires wide, multi-layer drums |
| Hook Block Self-Weight | Lighter (single sheave, ~1,200–1,800 lbs) | Heavier (dual sheaves, ~2,200–3,800 lbs) |
| Primary Construction Use | High-cycle concrete, formwork, fast material logistics | Heavy structural picks, precast erection, heavy civil |
| Load Chart Column | Must read 2-Fall (2P) Column ONLY | Must read 4-Fall (4P) Column ONLY |
6. The Fatal Mismatch: Reading the Wrong Chart Column
A critical question on the NCCCO written exam addresses the consequences of operational reeving mismatches.
+-----------------------------------------------------------------------------+
| THE DANGER OF LOAD CHART REEVING MISMATCH |
| |
| [SCENARIO: CRANE IS RIGGED IN 2-PART LINE] |
| |
| Operator looks at 4-Part Load Chart Column at 50 ft: |
| - 4-Part Chart Gross Capacity = 44,000 lbs |
| |
| ACTUAL CRANE LIMIT (2-Part Line): |
| - 2-Part Hoist Line Maximum Limit = 22,000 lbs |
| |
| IF OPERATOR ATTEMPTS TO PICK A 35,000-LB LOAD: |
| 1. Structural load moment of jib is acceptable (crane won't tip). |
| 2. BUT each line of rope experiences 17,500 lbs of tension! |
| 3. Rated single line pull (11,000 lbs) is EXCEEDED by 59%! |
| 4. Wire rope design factor drops below safe limits -> WIRE ROPE PARTS! |
| 5. Hoist brake slips or winch gearbox teeth shear -> LOAD DROPS! |
+-----------------------------------------------------------------------------+
[!WARNING] Reeving Verification Protocol: Prior to making any lift exceeding the 2-part line limit, the operator must:
- Physically verify that the hook block is reeved in 4-part line (all 4 parts of line supporting the block).
- Verify that the LMI display shows the 4P icon and active 4-fall load chart table.
- Confirm that all daily pre-shift limit switch tests for 4-part trolley travel limits and hoist upper limits were performed in that configuration.
A top-slewing tower crane's hoist winch has a manufacturer-rated single-line pull of 12,500 lbs. If the crane is reeved in a 4-part line (4-fall) configuration, what is the maximum theoretical hoisting capacity of the hoist system (assuming the crane's structural load moment rating is not exceeded)?
Why do high-rise general contractors predominantly configure tower cranes in 2-part line (2-fall) rather than 4-part line during extensive concrete bucket and formwork operations?
What is the primary danger if a crane operator reeved in 2-part line references the 4-part line column on the manufacturer's load chart when planning a heavy lift at a short radius?