KEVOS® 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). | |
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
