![\"Wire](\"https:\/\/www.cnxjcm.com\/wp-content\/uploads\/2026\/09\/crane-wire-rope-reeving-parts-of-line-hook-blocks-lifting-capacity-inline-1.png\")
The Math Behind Line Pull and Load Capacity<\/h2>\n
The formula is simple in theory, messier in practice. Theoretical lifting capacity equals line pull multiplied by parts of line, minus efficiency losses at each sheave. Every sheave the rope wraps around costs roughly 2-3% efficiency due to friction and bending resistance \u2014 even on well-lubricated bearings.<\/p>\n
So a winch with 10,000 kg of single-line pull, reeved with 6 parts of line, doesn’t give you a clean 60,000 kg. Run that math with six sheaves in the reeving path and you’re realistically looking at 8-12% total efficiency loss, landing closer to 53,000-55,000 kg of actual capacity. This is exactly why manufacturer-published load charts always beat back-of-envelope math \u2014 they’ve already accounted for real-world friction, not theoretical numbers.<\/p>\n
A Practical Example<\/h3>\n
Say a contractor is lifting a 45-ton precast panel at 25 meters of boom length. The chart calls for 4 parts of line minimum at that radius. If the crew mistakenly reeves only 2 parts because it’s faster to rig, the winch will hit its line pull limit long before the load leaves the ground \u2014 or worse, it’ll strain past rated pull and risk rope damage. Reeving isn’t a shortcut you take for convenience; it’s dictated by the chart, full stop.<\/p>\n A hook block isn’t just a heavy hook with a swivel \u2014 it’s a precision-rated component with its own capacity limit, sheave count, and weight that factors into every lift plan. Buyers obsess over boom length and winch power but often treat the hook block as an afterthought. That’s a mistake.<\/p>\n The block’s sheave count caps how many parts of line you can physically reeve. A 2-sheave block tops out around 4 parts of line (plus a dead-end wrap); a 5-sheave block can handle up to 10-11 parts. If your lift plan calls for 6 parts of line and your crane only carries a 3-sheave block on site, you’ve got a rigging problem before you’ve even picked anything up.<\/p>\n Heavy-duty hook blocks used for max-capacity lifts can weigh 500-1,500 kg depending on size. That weight comes straight out of your net lifting capacity, which is why load charts list both gross and net figures. On a 60 ton RT crane<\/a> pulling close to its rated max, block selection isn’t a minor detail \u2014 it can shift usable capacity by a full ton or more.<\/p>\n Most operators skip straight to the capacity number and ignore the small reeving diagram printed beside it \u2014 that’s backwards. That diagram tells you the exact parts of line, boom configuration, and outrigger spread the listed capacity assumes. Change any one of those variables and the number no longer applies.<\/p>\n Reeving diagrams typically show a simplified boom tip with vertical lines representing rope strands, numbered 2, 4, 6, or 8. Cross-reference that number against your actual rigging before committing to a lift \u2014 not after the load is already off the ground.<\/p>\n More parts of line does increase capacity, but it also slows hoist speed proportionally and increases the chance of rope crossing or bird-caging on the drum if spooling isn’t managed carefully. For lighter, faster-cycle lifts \u2014 like moving steel beams around a laydown yard \u2014 over-reeving just wastes time. Match the reeving to the job, not to the crane’s theoretical maximum.<\/p>\n Wire rope diameter isn’t arbitrary \u2014 it’s matched to the drum groove, sheave groove radius, and rated breaking strength needed for the crane’s maximum reeving configuration. Swap in a slightly undersized replacement rope and you’ll accelerate groove wear on every sheave it passes through, even if it technically still fits.<\/p>\n Inspect rope at every sheave contact point, not just where it’s visible on the drum. Wire breaks concentrate at pressure points \u2014 typically where the rope bends sharpest around the hook block sheaves. API and OSHA guidelines generally call for retirement once you see 6 or more randomly distributed broken wires in one rope lay, or 3 broken wires in one strand within a single lay.<\/p>\n A crane rental company running a fleet of rough terrain units for pipeline support work found one crane’s rope wearing out nearly twice as fast as the others. The cause wasn’t the rope brand \u2014 it was a slightly warped boom-tip sheave from a prior overload event, chewing through wire strands every cycle. Reeving problems often trace back to hardware wear, not the rope itself.<\/p>\n Capacity doesn’t just depend on parts of line \u2014 it’s a moving target across boom length and radius, and reeving requirements shift accordingly. A crane might need only 2 parts of line at short boom and tight radius for a given load, but require 4 or 6 parts once that same load moves to full boom extension.<\/p>\n This is one of the biggest reasons crane selection guides emphasize matching tonnage to the actual working radius, not just the nameplate rating. Our earlier piece on choosing the right crane tonnage without overpaying<\/a> covers this exact trap \u2014 buyers chase a bigger nameplate number without realizing reeving and radius will govern real-world capacity anyway.<\/p>\n Rough terrain cranes and truck cranes generally use similar reeving principles, but jobsite conditions push the two toward different practical setups. RT cranes working uneven ground and shorter cycle times more often run lighter reeving configurations for speed and maneuverability, especially on units like a 75 ton rough terrain crane<\/a> handling frequent pick-and-carry work.<\/p>\n Truck cranes doing highway-adjacent or plant-yard lifts, on the other hand, often reeve heavier for sustained near-capacity picks since travel speed between lifts matters less. If you’re still deciding between the two crane types for your application, our comparison of rough terrain vs all terrain cranes<\/a> digs into the broader tradeoffs beyond just reeving.<\/p>\n Your crane’s load moment indicator (LMI) only calculates capacity correctly if it knows the actual reeving configuration \u2014 most modern systems require the operator to manually input parts of line before starting a lift. Get this wrong, and the LMI will either falsely restrict a legitimate lift or, worse, give a false green light on an overloaded configuration.<\/p>\n![\"Crane](\"https:\/\/www.cnxjcm.com\/wp-content\/uploads\/2026\/09\/crane-wire-rope-reeving-parts-of-line-hook-blocks-lifting-capacity-inline-2.png\")
Hook Blocks: The Overlooked Half of the Equation<\/h2>\n
Block Weight Eats Into Net Capacity<\/h3>\n
![\"Heavy-duty](\"https:\/\/www.cnxjcm.com\/wp-content\/uploads\/2026\/09\/crane-wire-rope-reeving-parts-of-line-hook-blocks-lifting-capacity-inline-3.png\")
Reading a Reeving Diagram on a Load Chart<\/h2>\n
Common Mistake: Assuming More Parts Is Always Safer<\/h3>\n
Rope Diameter, Wear, and When to Re-Reeve<\/h2>\n
Real-World Scenario<\/h3>\n
![\"Worn](\"https:\/\/www.cnxjcm.com\/wp-content\/uploads\/2026\/09\/crane-wire-rope-reeving-parts-of-line-hook-blocks-lifting-capacity-inline-4.png\")
Matching Reeving to Boom Length and Radius<\/h2>\n
Reeving Differences: RT Cranes vs Truck Cranes<\/h2>\n
Safety Systems That Depend on Correct Reeving<\/h2>\n