This revision removes the incorrect cosine reduction of tackle force and unsupported universal lifting, boiler and operating claims. It separates exact ideal load-fall identities from boom-tip reach and rise, without producing a crane capacity or lift plan.
Ideal Derrick Reeving and Boom Geometry Explorer
Enter declared line tension, supporting parts, boom length and boom angle from horizontal. The worksheet reports ideal hook force, rope take-up and boom-tip geometry; none is a rated capacity.
Equation Used
- All supporting rope parts are parallel and carry the same declared tension.
- Hook-block weight, rope weight, sheave friction and bending loss are excluded.
- Boom angle alpha is measured upward from horizontal and boom length is pin-to-tip length.
- Reach and rise are measured from the boom heel pin; mast/platform elevation is not added.
- Animation phase prescribes connected drum and traveling-block motion and does not predict speed or acceleration.
- Rope and block ratings, drum layers, fleet angle, brakes, engine/boiler output, boom and mast strength, topping-lift and guy forces, stability, ground support, wind, dynamics, rigging, inspection, load control and safe operation are excluded.
Removes the incorrect boom-angle cosine from ideal tackle force and separates reeving identities from boom-tip geometry.
A hoist drum and traveling block form the load path
A portable derrick places a boom and hoisting drum on a movable platform. The drum pays in or out the lead line, which is reeved through a fixed block at the boom tip and a traveling block carrying the hook. A separate topping lift supports the boom and controls its angle.
The animation connects the power-unit topology, drum, lead line, supporting parts and traveling hook block. It does not simulate steam generation, brakes, load dynamics or structural reactions.
Use this worksheet only for ideal reeving and tip geometry
The results can audit a declared count of supporting parts and calculate the boom tip's pin-relative position. They cannot determine a permissible load, choose rigging or plan a lift.
OSHA guidance requires boom-angle indication and use of the applicable load chart because capacity changes with boom length, radius, direction and machine configuration.
Do not apply boom-angle cosine to tackle force
With n parallel parts supporting the traveling block at common tension T, their vertical ideal forces add. Raising the hook by one metre shortens every supporting part by one metre, so the drum takes up n metres of line.
Boom angle changes the tip position and the derrick's structural reactions, not the ideal force multiplication of a vertical load fall.
A transparent declared example
At T=4448 N with four supporting parts, ideal hook force is 17.79 kN and four metres of rope are taken up per metre of hook rise. For a 9 m boom at 60° from horizontal, tip reach is 4.50 m and pin-relative rise is 7.79 m.
These are ideal ledger quantities, not a rated load.
A real derrick needs its load chart and complete configuration
Actual line pull and motion depend on sheave friction, rope bending, drum layers, drive torque, brakes, hook-block weight and dynamics. Permissible lifting further depends on rope and block ratings, boom/mast/topping-lift/guy strength, platform stability, ground support, radius, direction, wind and inspection condition.
The former universal tonnage, boiler pressure, component dimensions, guy arrangement, historical deployments, wear limits, failure diagnoses and operating instructions are removed because they were not sourced to one documented machine and configuration.
Derrick ledger questions
Why does boom angle not change ideal hook force?
The vertical load fall still has n parallel supporting parts. Boom angle changes tip geometry and structural reactions.
Does nT equal rated capacity?
No. It is an ideal rope-force sum only.
Why is rope take-up n times hook rise?
Each of the n supporting parts shortens by the hook displacement.
Does the model calculate topping-lift or guy forces?
No.
Portable derrick references
- DOE-RL-92-36: parts-of-line mechanical advantage and line-pull relation.
- US Navy Block and Tackle: fixed/traveling blocks, fall terminology and ideal mechanical advantage.
- US Patent 890,158: portable platform, projecting boom and rope drum.
- OSHA 29 CFR 1926.1401: boom suspension and controlled load-lowering definitions.
- OSHA crane guidance: boom-angle indication and load-chart use.
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