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Answer: There are actually two questions within your request. We will address them separately.
First question: Why does the lifting capacity of a vertical basket hitch configuration double a sling’s lifting capacity?
Answer: The tension in a sling (neglecting friction) is equal along every portion of the sling, even if that sling (or rope) wraps around a curve, such as in a basket hitch configuration. Suppose a sling is rated for 1 Ton: When used in a basket hitch configuration, both ends of the sling are able to support the lifted load. EACH END can support its rated capacity of 1 Ton because the sling never has more than 1 Ton of tension (remember, the tension in a sling is equal throughout). Adding up the net supported weight results in a capacity of 2 Tons. 2 Tons can be supported with the 1 Ton sling because the tension in the sling is still limited to 1 Ton.
Here’s another helpful way to look at it: Picture the sling as if it were the running rope on a crane. To increase the lifting capacity of the running rope a larger wire rope is not the solution, instead, the running rope is reeved through additional pulleys. When a single part line is reeved through a block to make it two parts, the wire rope can safely lift twice as much as the single part line. The net capacity of a basket hitch works in the same way.
Second question: Why does a basket hitch require both ends of the sling to be vertical to achieve its full lifting capacity (which is double the sling’s load rating)?
Answer: The sling must be vertical because any deviation from vertical causes additional tension in the sling (in keeping with basic trigonometry). The vertical component of the sling’s tension is the equal to the tension in the sling and the lifted load… but only when the sling is vertical. As it becomes angled, only a portion of the tension in the sling contributes to lifting the load. The remainder of the sling’s tension is used up in a side load that does nothing to support the vertical load. Since the lifted load isn’t getting any smaller with the changing angle, the tension in the sling must increase with the increasing angle, thereby reducing the load the sling is able to lift.
Rigging institute would like to thank Culley Parris, P.E. – Coffman Engineers, Inc., Spokane WA for his assistance in answering these questions.
Answer: When connecting two slings together, shackles are the preferred method. When connecting slings using shackles be sure to follow the following steps.
1. Prevent shackles from eccentrically loading (pulling at an angle). If this is not possible shackle rating should be reduced for the application.
2. Prevent shackles from pinching the sling between itself and the load. This can cause damage to the sling, shackle or load.
3. Prevent the eye of the shackle pin from binding into the load.
4. When multiple slings are loaded on one end of the shackle, it must be the bow end. The pin side is designed for inline loading only. Having two slings pulling at a given angle from the pin side of the shackle compromises its rating and performance.
I have redrawn your examples plus one. Your drawing of the double wrap rendered choker hitch with a shackle has the orientation of the shackle incorrect, whether by design or accident I’m not sure. However, I have added an additional drawing indicating the correct orientation of the shackle.
Answer: The two slings of equal capacity will have a rating equal to the combined vertical hitch capacity under the following conditions:
1) The slings are of exactly the same type and length.
2) The connection points of both slings are equal distance from top to bottom.
Answer: Rotation resistant wire rope is not to be used in the fabrication of slings because of its design characteristics. Rotation resistant rope breaks from the inside out due to the extreme crossing of the internal strands with the external strands. This leads to high stresses at the inside of the wire rope and subsequently the internal wires usually will break first. Internal wire breakage is difficult to detect during an inspection since very little external evidence shows up on the rope. Tight bending of the wire rope increases internal stresses and accelerates the wire breakage. The eyes would also deteriorate rapidly and any hitch would promote damage to the internal wires in the body of the sling.
Answer: There are no D/d reductions for nylon webbing. If the pin can support the load of the basket hitch it will be fine. The only concern we have with flat web slings is spatial. The webbing must be able to fully spread, allowing it to use all the material to attain its full capacity.
Answer: The WSTDA gives this hitch a vertical capacity rating. However, in my “Rigger’s Reference Handbook” and in the “Did You Know” video http://www.iandisling.com/Did_You_Know/009/DYK_009.htm I recommend using the choker hitch capacity of the sling. I have done several break tests with web slings in this configuration and found that the break without hardware meets a 5:1 from the vertical rating, but barley. In the video I show the uses of a synthetic sling savor shackle, but in my testing since that video a standard anchor shackle works the best. The curvature creates more friction and it will break before running. Once again, this break was barely at a 5:1 from vertical. The synthetic sling saver shackle hook up barley reaches a 5:1 from the choker capacity before it runs, no break. So this our reason for recommending a choker hitch rating for the adjustable hitch.
As far as the hitches used with any particular sling, I don’t believe it is a good practice to limit the hitch used based on the hitches identified on the tag. As per the latest version of ASME B30.9 a sling only needs to have one legible rating on the tag to meet criteria for use. Furthermore the vertical, choker and basket hitches normally shown on most slings are reference points to be use for maximum capacity for that sling in that particular hitch configuration as well as the starting point for calculating sling reduction or increase of capacity for double basket and choker hitches. If the hitches recognized on the sling tag where the only hitches used then a choker hitch would probably be used for a load such as bundled pipe. When in fact the safest hitches would be a double wrap choker or double wrap basket hitch for such a load.
Not to be lost in this discussion, is the fact that some sling types such as wide body web slings and Type 2 web slings are not designed to be used in a choker hitch configuration. In fact a Type 2 web sling cannot be made into a choker hitch if attempted. The key to safe use and capacity verification for riggers, when it comes to sling use, is good positive training on the subject.
Answer: Twin-Path® sling length does not change over time, although it may seem to in certain situations. The load bearing internal K-Spec® strands do not shrink at all. However the Covermax® double cover used for these slings is made from a bulk nylon material and nylon does shrink. Therefore, the cover’s shrinkage would create an inconsistent measurement of the sling length when it is not under tension. Apply an equal tension to each sling as you measure them to verify sling length. Please refer to Slingmax® Technical Bulletin #4 for measuring information: http://www.slingmax.com/bulletins.htm
In short this is what it says in the body of paragraph 2: It is important to note that Twin-Path® Extra slings have a bulked nylon double cover which protects the K-Spec® core yarn inside. Nylon shrinks when relaxed and stretches under load whereas K-Spec® does not. Therefore, roundsling covers will show some expansion and contraction over time. This is normal and does not affect sling length tolerance. Floor measuring, especially with used slings is not always accurate. To ensure optimal accuracy, we recommend measuring Twin-Path® Slings under load even if it is only 10% of rated capacity.
Answer: The sling is good and has not lost any capacity. The wire pushing out at the fitting is due to the wire sticking up when the sleeve was pulled on. That sometimes can be fixed before pressing but not after. Second the strand being pushed out at the crown of the eye is common when the flemished eye tails are too long and the sleeve pushes it out during pressing. Neither of these affects sling capacity in a negative way since it is in the eye. Any of this in the body is a problem. Prior break testing of these “high strand” FE slings proves that they will still reach the minimum required breaking strength. The body remains the weak point of the sling, not the eye where there is double the wire rope Please go to the following website, under the video tab you will find a video discussing high stranding in the eye to prove the point on capacity.