BIZOMAT MAKEOS · Mechanics
Wire Rope Reeving: Line Pull and Safety Factor
What pull is on the hoist rope with this reeving, and is the rope’s safety factor enough?
Line pull at the drum for a load reeved over several parts of line with sheave friction, the rope safety factor against its minimum breaking load, and the sheave-to-rope diameter ratio.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load mass (including hook block) | kg | Mass lifted including the hook block and slings. |
| Parts of line supporting the load | parts | Number of rope falls between the upper and lower blocks. |
| Efficiency per sheave | % | Efficiency of each sheave (about 98% for rolling bearings, 96% for plain bushes). |
| Rope minimum breaking load | kN | Minimum breaking load from the rope certificate. |
| Required rope safety factor | Rope design factor required by your crane or hoist code (often 5 or more). | |
| Sheave pitch diameter | mm | Pitch diameter of the smallest sheave. |
| Rope diameter | mm | Nominal rope diameter. |
| Required minimum D/d | Minimum sheave-to-rope ratio from the code or rope maker. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Load mass (including hook block)
- What it is
- The total mass hanging from the rope: the load plus hook block, slings and lifting gear.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 100000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5000 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Line pull rises in direct proportion; safety factor falls.
- Get it right
- Include the hook block and rigging, not just the load.
What this means: Parts of line supporting the load
- What it is
- The number of rope falls running between the upper and lower blocks that share the load.
- Units and format
- Enter a number in parts: parts. The form accepts 1 parts to 40 parts. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 parts as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More parts of line lower the line pull at the drum.
- Get it right
- Count the falls supporting the lower block, not the number of sheaves.
What this means: Efficiency per sheave
- What it is
- The share of rope tension passed on over each sheave after bearing and rope bending friction, from the sheave or crane maker.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 50% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 98% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower sheave efficiency raises the line pull.
- Get it right
- Type the percentage itself; plain bushes lose more than rolling bearings.
What this means: Rope minimum breaking load
- What it is
- The minimum breaking load of the rope, from its test certificate or the rope maker’s catalogue for that construction and grade.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 120 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Safety factor rises in direct proportion.
- Get it right
- Use the certified minimum breaking load, not a calculated or nominal strength.
What this means: Required rope safety factor
- What it is
- The minimum rope safety factor required by the crane or hoist code, or the equipment maker, for this duty.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line.
- Get it right
- Take it from the applicable code or a competent person, not from the example.
What this means: Sheave pitch diameter
- What it is
- The diameter of the smallest sheave in the reeving, measured to the centre of the rope where it sits in the groove.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger sheave raises the D/d ratio.
- Get it right
- Use pitch diameter at the rope centre, not the flange or groove bottom diameter.
What this means: Rope diameter
- What it is
- The nominal diameter of the wire rope, from its certificate or catalogue.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 14 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger rope lowers the D/d ratio for the same sheave.
- Get it right
- Measure across the crowns of the strands, not across the flats.
What this means: Required minimum D/d
- What it is
- The smallest ratio of sheave pitch diameter to rope diameter allowed by the code or rope maker for this duty.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 100. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line for D/d.
- Get it right
- Take it from the applicable code or rope maker.
Method and formulas
- Line pull F = W (1 − η) ÷ (1 − ηⁿ), with W = m g.
- Reeving efficiency = W ÷ (n F).
- Safety factor = MBL ÷ F; D/d = sheave ÷ rope diameter.
- Example: 5000 kg on 4 parts gives 12.632 kN line pull and a safety factor of 9.4995.
Limitations
- Hoisting at steady speed; acceleration, dynamic and wind loads must be added per the code.
- Lead and equaliser sheaves outside the block are not included.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check hoist rope and drum pull for a block
- Choose the number of parts of line
- Check sheave size against the rope
Who it is for
- Lifting engineers
- Maintenance engineers
- Crane and hoist designers
What you get
- Line pull at the drum
- Rope safety factor
- Reeving efficiency
- Sheave D/d ratio
Interpreting the result
- Lifting equipment must meet its design code and be certified by a competent person.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2613–2650. Topic reference; no source tables or recommended settings are reproduced.
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