BIZOMAT 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). |
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
- What it is
- The mass of the load being lifted, including slings and hooks below the lower block.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 1000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 500 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The fast-line pull rises in direct proportion to the load.
What this means: Parts of rope supporting the load
- What it is
- The number of rope falls running between the upper and lower blocks that share the load. The fast line pulled by hand or winch counts only if it leaves the lower block.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 20. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More parts lower the ideal pull, though sheave losses grow with each extra sheave.
- Get it right
- This is also the length of rope pulled per metre of lift.
What this means: Efficiency of each sheave
- What it is
- The efficiency of one sheave and its bearing, from the block maker’s data. The tool combines it over all sheaves for the overall efficiency.
- 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 96% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the efficiency of one sheave, not the overall efficiency of the tackle.
What this means: Rope minimum breaking load
- What it is
- The minimum breaking load of the rope, from the rope certificate or manufacturer’s data.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 100000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- kN, not tonnes or kg.
What this means: Rope safety factor required
- What it is
- The safety factor you require on the rope (breaking load ÷ fast-line pull), from the lifting standard or regulation that applies.
- 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.
What this means: Fast line pulled by
- What it is
- Whether the fast line goes to a winch or powered drive, or is pulled by a person. Only hand operation is checked against the one-person pull limit.
- Units and format
- Choose one of 2 options from the list: A winch or powered drive; Hand.
- Example
- The form opens with “A winch or powered drive” selected as a worked example. Choose the option that matches your case.
What this means: Largest pull for one person
- What it is
- The largest sustained pull you allow for one person, from your manual handling assessment or guidance. Used only when the fast line is pulled by hand.
- Units and format
- Enter a number in N: newtons. The form accepts 50 N to 2000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
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