BIZOMAT MAKEOS · Mechanics
Snatch Block and Anchor Load Check (Rope Angle, Resultant, Ratings)
When my winch rope turns through a snatch block, what load does the block and its anchor take at this rope angle, and are the block, anchor and rope rated for it?
Check of a snatch (diverting) block in a rope pull: the resultant of the two rope legs on the block from the line pull and the included angle between the legs (as the resultant of concurrent forces tool), the angle factor, the direction of the resultant, and checks of the block working load, the anchor capacity and the rope’s design factor on its minimum breaking load.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Line pull in the rope | kN | Tension in the rope (the winch line pull, plus friction at other sheaves). |
| Included angle between the rope legs | ° | Angle between the incoming and outgoing rope legs at the block (0° = rope doubles back, 180° = straight through). |
| Ratings | ||
| Snatch block working load limit | kN | Working load limit marked on the snatch block. |
| Anchor capacity | kN | Capacity of the anchor point, sling and shackle holding the block. |
| Rope minimum breaking load | kN | Minimum breaking load of the rope. |
| Required rope design factor | Design factor the rope must have (from your standard). | |
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: Line pull in the rope
- What it is
- The tension in the rope passing round the snatch block, normally the winch line pull plus any friction losses at other sheaves before it.
- 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 10 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The load on the block and anchor rises in direct proportion to the line pull.
- Get it right
- Use the highest pull the winch can develop, not the load you hope to move; both legs are assumed to carry this same tension.
What this means: Included angle between the rope legs
- What it is
- The angle between the incoming and outgoing rope legs where they meet at the block: 0° when the rope doubles straight back, 180° when it passes straight through.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 180°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The resultant is 2T cos(θ ÷ 2), so it rises towards twice the line pull as the angle closes.
- Get it right
- Measure the angle between the legs themselves, not the change of direction; a small included angle means a large block load.
Ratings
What this means: Snatch block working load limit
- What it is
- The working load limit marked on the snatch block or stated on its certificate, which the resultant load on the block must not exceed.
- 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
- Use the WLL of the block, not the rope’s line pull rating; they are different numbers.
What this means: Anchor capacity
- What it is
- The capacity of whatever holds the block in place: the anchor point, the sling and the shackle, taken as the weakest of them.
- 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
- Check every item in the chain from block to structure and enter the lowest rating; an engineer should confirm a structural anchor.
What this means: Rope minimum breaking load
- What it is
- The minimum breaking load of the rope, from the rope’s certificate or the manufacturer’s data.
- 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 60 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The rope design factor is this load divided by the line pull.
- Get it right
- Use the minimum breaking load, not the working load limit, and allow for any reduction from terminations or wear per your standard.
What this means: Required rope design factor
- What it is
- The design factor the rope must have on its minimum breaking load for this use, as set by your lifting or pulling standard or regulation.
- 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.
- Get it right
- Take it from the standard that applies to your work; it differs between lifting, pulling and other duties.
Method and formulas
- Two legs of tension T at ±θ ÷ 2 about the bisector combine to R = 2T cos(θ ÷ 2) along the bisector (as the resultant of concurrent forces tool).
- Angle factor = R ÷ T.
- Rope design factor = MBL ÷ T.
- Example: at 60° the block and anchor take 17.321 kN, 1.7321 times the line pull; the rope has a design factor of 6.
Limitations
- Equal tension both sides of the block; sheave friction raises the outgoing tension slightly.
- Static pull; snatch loads from slack rope can be much higher.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Rate a snatch block and anchor for a winching job
- Show how the block load grows as the rope angle closes
- Check a rope against its design factor
Who it is for
- Riggers
- Lifting supervisors
- Maintenance supervisors
What you get
- Resultant load on the block and its direction
- Angle factor
- Rope design factor
Interpreting the result
- When the rope doubles back (small angle), the block and anchor carry nearly twice the line pull.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 153–163. Topic reference; no source tables or recommended settings are reproduced.
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Open the Snatch Block and Anchor Load Check (Rope Angle, Resultant, Ratings) calculator