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Slings can be connected to increase their length. However, hardware must be used to make the connections between slings. ASME B30.9 states the issue this way:
B30.9 – Rigging Practices
(a) Slings shall be shortened or adjusted only by methods approved by the sling manufacturer or a qualified person.
(b) Slings shall not be shortened or lengthened by knotting, twisting, or by wire rope clips.
The type of slings will make a difference in which is the best hardware to use and where it should rest against the load being lifted. When connecting slings that will be going around a load, consider the following:
- Be sure the hardware is not binding against the load. (Fig. 1)
- Be sure the hardware used to connect the slings does not damage the slings by crushing them against the load. (Fig. 2)
- If possible, use two different lengths of slings that will allow the connection point to be in space, not against the load. (Fig. 3)
When the two shackles are attached as seen in graphic A, 6,600 lb capacity is the maximum lifting capacity of the two shackles together.
If two shackles are attached to the load as seen in graphic B, the two shackles have a maximum combined capacity of 13,200 lbs when lifting at a 90⁰ horizontal angle from their attachment points. When lifting at less than a 90⁰ angle, the combined shackle capacity must be reduced as dictated by the angle.
To determine whether your 4 slings, each rated for 6,700 lbs in a choker hitch, are sufficient to lift and move a load of 16,000 lbs the following must be verified.
1) Is the total capacity of the 4 slings in the hitch configuration to be used greater than the weight of the load to be lifted? (26,800 lbs)
2) Are the points of attachment equal distance from the center of gravity allowing for each sling to take an equal share of the load? (4,000 lbs)
3) Are all slings the same the same length within manufacturing tolerances?
4) Will the sling tension increase be less than each sling’s capacity at the angle which the slings will be loaded? (37º minimum angle)
If the answers to the 4 questions above are all “yes”, then your rigging will be sufficient to rig and move your load safely. Sling angle can be no less than 37º. Sling tension at 37º will be 6,647 lbs of tension for each sling based on a 4,000 lbs share of load.
The method of attachment for the chain sling described is a basket hitch. However, just because our rigging application has created a basket hitch does NOT necessarily mean we can increase the rating of the sling to a basket hitch capacity. With a chain sling you have two things to consider, as follows:
First: If the oblong link at the top of the sling is not rated with a capacity that is high enough to meet the basket hitch capacity, a basket hitch rating cannot be used. (Fig 1) However, if an additional oblong link or shackle of proper capacity is placed on the lifting hook and the sling hook is attached to the additional link, a basket hitch rating may be used. (Fig. 2)
Second: The D/d ratio of the chain diameter to the diameter of the shackle must be considered. If the ratio of the shackle diameter is not at least 6 times the chain diameter, the hitch capacity must be down-rated as indicated in the chart below. (Fig. 3)
Different things can occur depending on whether the lift is being performed with an overhead crane or a mobile crane. With both crane types, the load will swing uncontrollably. Depending on load size and distance the hoist rope is out of plumb, even with a tag line, the load may not be controlled safely, thereby putting the rigging and others near the activity at risk.
When Overhead Crane hoist ropes are not kept vertical to the hoist, side loading of the crane will occur. Side loading of the crane can cause the following problems:
- Uncontrollable swinging of the load.
- Side loading has a negative effect on bridge and trolley brakes.
- When the hoist line is not vertical, it can cross over to the incorrect groove on the hoist drum. This action can cause irreparable damage to the hoist rope and possibly wedge the hoist rope between the drum and drum guard.
When Mobile Crane hoist ropes are not kept vertical as the load is being lifted, undesired actions can cause crane damage and hazards to personnel, as follows:
- Uncontrollable swinging of the load.
- Side loading of the crane boom – Crane boom capacities/strength, as shown on its load chart, are designed for the load to move vertical to the boom point. When a boom is, side loaded, it is at its weakest point and may twist or even collapse due to the side loading forces.
- When the hoist line is angled away from the boom point, the crane’s capacity is less than identified on the chart as the load is being lifted until it hangs plumb.
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.
The colored plastic filament material is a manufactures marker that is wrapped around the core of the wire rope during manufacturing. When wire rope is manipulated to form a sling, the filament is easily displaced and can sometimes be exposed to the outside surface of the sling. There is no need for concern when the filament is exposed.
Yes, you can hang an electric chain hoist from a nylon sling and shackle combination as you described in your question. However, be aware that your hoist will have considerably more horizontal swinging back and forth as it is being used. Also, be sure to use the proper sling protection placed in the correct location to protect the nylon sling from cutting and abrasive action to it. Please see the diagrams below to better understand the sling protection issues you may encounter.
When connecting two endless roundsling in a straight pull it does not matter whether the pin of the shackle is up or down. One important thing is to be sure that the sling on the pin has protection to prevent cutting and is not severely bunched and has enough space to function as designed.
When connecting two slings of any type to a shackle for a 2-leg hitch the pin ALWAYS goes up. A shackle is designed so that the bow of the shackle can handle the angular loading created when more than one leg is used in the shackle. See diagram below.
The tension in a sling (neglecting friction) is equal along every portion of the sling, even if that sling wraps around a curve, such as in a basket hitch configuration as we are discussing. For example, if 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. Therefore, the rating of the shackle connecting the 2 sling needs to only be equal to the capacity of the sling. When connecting the 2 slings together be sure that the shackle is in a position that will not damage the shackle or the load. See diagram below.
Yes, you can run the body of a web sling through the sling’s eye to create a choker hitch without the use of a shackle. (see fig. to the right)
ASME B30.9-5 has one recommendation concerning the choker hitch being applied in a synthetic webbing sling: B30.9-5.10.4 Rigging Practices (m) In a choker hitch, the choke point should only be on the sling body, not on a load-bearing splice or fitting.
The Web Sling & Tiedown Association in their Recommended Standard Specification for Synthetic Web Slings, WSTDA-WS-1, define a choker hitch as follows: “A method of rigging in which the web sling is passed around the load, fed back through itself, and then attached to the handling or lifting device”.
If you chose to use a shackle to create a choker hitch in any sling type never place the shackle pin in the running part of a sling hitch. Never allow the pin to be rotated by sling action. (see figs. below)