KEVOS® MAKEOS · Mechanics
Block and Tackle Check (Pull with Sheave Friction, Rope and Operator)
What pull does this block and tackle need at the fast line once sheave friction is counted, is the rope strong enough, and can one person pull it by hand?
Check of a rope block and tackle (simple purchase with n supporting parts): ideal pull, overall efficiency of the reeving from the efficiency of each sheave, actual fast-line pull (as the hoist mechanical advantage tool with that overall efficiency), mechanical advantage, rope pulled per metre of lift, the rope safety factor on the fast line, and for hand operation the pull against your limit for one person.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load mass | kg | Mass of the load lifted. |
| Parts of rope supporting the load | Number of rope parts between the blocks that carry the load. | |
| Efficiency of each sheave | % | Efficiency of one sheave and its bearing (about 96% for roller-bearing sheaves, 90–95% plain bushes). |
| Rope minimum breaking load | kN | Minimum breaking load of the rope. |
| Rope safety factor required | Safety factor you require on the rope (from the lifting standard). | |
| Fast line pulled by | choice | Whether the fast line goes to a winch or is pulled by hand. |
| Largest pull for one person | N | Largest sustained pull you allow for one person (your value from manual handling guidance). |
Method and formulas
- Ideal pull = W ÷ n; overall efficiency η = (1 − eⁿ) ÷ [n (1 − e)] for n sheaves in the reeving.
- Actual pull = ideal pull ÷ η (as the hoist mechanical advantage tool with that efficiency); mechanical advantage = W ÷ pull.
- Rope pulled per metre of lift = n; rope safety factor = MBL ÷ pull.
- Example: 500 kg on 4 parts needs 1,301.9 N at the fast line (94.158% efficient); rope factor 15.362.
Limitations
- Static lifting at slow speed; acceleration and swinging add load.
- Fast line leaving the fixed block; other reevings change the count.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a hand-hauled block and tackle
- Choose the number of falls for a load
- Check the rope on a hoisting purchase
Who it is for
- Riggers
- Maintenance engineers
- Design engineers
What you get
- Ideal pull and overall efficiency
- Actual pull and mechanical advantage
- Rope safety factor and hand-pull check
Interpreting the result
- Each extra part lowers the pull less than the last, because each extra sheave adds friction.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 170–171, 2613–2650. Topic reference; no source tables or recommended settings are reproduced.
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Open the Block and Tackle Check (Pull with Sheave Friction, Rope and Operator) calculator
