Edwards Heavy Lifting’s Stuart Edwards takes a look at the misconception that because you’re operating with a 50t hook and 50t capacity sling, you’re ready to lift 50t. He examines the relationship between the hook and sling and how they interact with each other at the point of the load bearing surface.
The diameters of crane hooks are generally designed by the manufacturer with good contact areas to support slings. Most have a fairly large radius supporting surface at the top and a smaller rounding off diameter at the edge.
After some preliminary research, opinions vary, but the most reliable information, according to enquiries so far, the corner radius r1 as picture below must be used. The width b1, must not be used, although if you have further information relating to this, we’d love to hear from you.
How does this apply to every day lifting?
Example:
We wish to use four 52mm wire ropes on a 100-tonne rams horn hook with a 60° included angle as per the rigging diagram below. The hook cross section dimensions are as per the photos. The direct loaded WLL of the wire rope sling is 33t. What is the capacity of this rigging arrangement?
Answer:
With the correct diameter end fittings, the SWL is 57t (any two legs take the load). However, the SWL for this configuration (calculated in accordance with AS1666.2) is actually 42.75t.
Why is this so? The hook radius r1 = 36mm. To determine if the hook radius is suitable, refer to AS1666.2 (Wire rope):
“Where an endless sling or soft eye of a sling interfaces with a fitting
- the supporting surface of the fitting shall have a diameter of not less than the rope diameter; and
- where the diameter of such a supporting surface is less than 1.5 rope diameters, the sling shall be derated by 25% in accordance with Item (m) of Clause 9.”
Since the hook diameter of 72mm (2 x 36mm) is less than 1.5 rope diameters (1.5 x 52mm = 78mm), the capacity should be derated by 25% = 42.75 tonne which is not a suitable slinging arrangement for the 50t load.
Notes:
- Sling manufacturers and other standards may specify other minimum diameters and/or reduction factors.
- Some manufacturers provide hooks with a sharper radius r1 than the standard.
- Do not force eyes over hardware. Make sure that the width of hook or diameter of shackle is no bigger than 1/2 the length of the eye.
- The same concept applies to other slings such as synthetic round slings ‘soft slings’ but these have different minimum radius and/or deration factors.
Other solutions:
- Alternative hooks that have a more rounded profile
- Connect to shackles / other hardware with large enough diameter rather than directly to the hook
- Use wire rope slings with thimble eyes (may require connection as per option above)