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Improving the Sling Angle

Posted by Mike Parnell on Mon, Apr 23, 2012 @ 04:21 PM
  
  
  
  
  

Improving the Sling Angle

We often learn from our surroundings. By taking a page from other folk’s playbooks, we can start to appreciate new solutions to old problems.

Standard rigging generally calls for two to four slings rigged from the crane hook to the load. Either the slings connect directly to a pad eye or manufactured connector like a swivel hoist ring, or we might use the slings in choker or basket hitches. Especially when rigging with 4 slings, the angle of the slings is typically less than 90 degrees from the horizontal, which means they bear against a portion of the load. (You have heard from me numerous times in various articles about the absolute need to install sling protection when the potential exists for sling damage due to cutting or sliding friction, or load damage due to marring and scrubbing.)

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We can borrow a page from the longshore industry to help reduce the sling-to-load contact that might lead to damage or a failure. The use of a spreader frame can help orient the slings to a near 90deg. position when using a spreader frame. The sling tension is not increased since the vertical orientation takes away the “angle load factor”.  It also minimizes the potential contact points when rigging with choker or basket hitches. The pre-planned contact between sling and load is restricted to the bearing surface, not spanning across the sides or upper edges of the load.

A spreader frame is different than a spreader bar. The frame typically offerdescribe the images a 4-leg lift option, generally at 90deg. from horizontal. A spreader bar requires that the slings work at angles less than 90deg. thereby inviting upper load contact with the slings and added tension due to the <90deg. orientation.

Many spreader frames are available off-the-shelf, or can be built by qualified manufacturers who have calculations and drawings in-hand, from previous customer orders.describe the image

We can lower the risk of damaging the load and the rigging by improving the sling angle.

Happy trails to my crane and rigging friends,

Mike

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Comments

Thanks for the reminder, and good to hear from You... 
 
Hugh Byrom 
Denton, Tx
Posted @ Monday, April 23, 2012 9:54 PM by B. Hugh Byrom
HI! MIke, 
 
Thanks for the informative article on the prevention of excessive angles. Just to confirm tapping on your professional expertise. If a set of slings are used in a Double Wrap Checker hitch having an angle of 60deg, will the sling capacity calculation take into account two factors,the Sharp Bends around the square load plus the Angle in question? I have purchased your "Journeymans Reference Charts' & none of your rigging methods show Sharp Cornered loads 
 
Most of your illustrations show either Direct Top Lifts or Chocker /Basket on Cylindrical loads.Thus the question? 
 
Warm regards, 
 
Ralph 
 
PS- Say Hi to Zack
Posted @ Monday, April 23, 2012 9:56 PM by Ralph Lesslar
Ralph, 
 
Your question is wide and deep. 
 
Generally, round or square loads don’t affect the capacity of a choker hitch. Wood blocking between a wire rope sling and the load (near the corner) can round out the wrap, and minimize dog-legging. Braided wire rope slings will conform better to a corner, rather than a single part body type wire rope sling (though they are more susceptible to abrasion and torn wires). 
 
Rigging less than 60 degrees promotes slippage of the sling(s) along the load surface, if choked or basketed. Double wrapping helps create some additional grip and compression, though still rig at or above 60 degrees. 
 
The D/d ratio of the load-to-sling diameter (wire rope of alloy chain link) has an effect on basket hitch capacity, not on choker hitch capacity. 
 
When rigging 2 choker hitches at 60 degrees to a load, first multiply the single vertical rated capacity of one sling by 1.73 to arrive at the new 2-leg (60 degree) capacity, then multiply that capacity by .75 to arrive at the reduced capacity for the “choking” effect. 
 
Sling protection at the “corners” is critical. Always protect the sling from cutting or abrasion, depending on the sling material. 
 
A 60 degree angle does not totally ensure that the sling(s) won’t slide. Add friction material between the sling and load surface if there is a chance of the sling(s) sliding across the load surface. 
 
The crane hook should be over the CG, before lifting in order to avoid an out-of-level load. 
 
Mike 
Posted @ Tuesday, April 24, 2012 12:47 PM by Mike Parnell
Mike, 
 
 
 
I have a question, While lifting with a Spreader Frame with four Individual slings, do we divide load equally to all four slings, or use principle for Multi-leg slings to calculate slings load.
Posted @ Wednesday, April 25, 2012 5:51 AM by Sunjeev SHARMA
Mike, good morning; Like allways excelent your article. I'm interested to buy several reference card in spanish. Additional to products # 3011 SP, 3051 SP and 3081 SP, do you have another reference cards in SPANISH wich can help me in RIGGING and SLIGGING activities? My son Mauricio Bastos is attending a training course in Carson City, NV. and be there until next week. I should like to now if you can send him this reference cards,how much does it cost and what's the best way to pay? I'll apreciate your help.
Posted @ Wednesday, April 25, 2012 11:23 AM by Rafael Bastos Llanos
Rafael, 
 
We have the Journeyman Rigger and Rigging Inspection cards in Spanish. 
 
Carol or Karolyn will be contacting you about getting some cards to your son in Nevada, very quickly. They can process your credit card as needed for purchase, shipping and handling. 
 
Thank you for following ITI. 
 
Too bad he didn’t attend an ITI program at one of our National Training Centers. He might really benefit from our Master Rigger Course and Rigging Gear Inspector Level 1-2 program. 
 
All the best, 
 
Mike
Posted @ Tuesday, May 01, 2012 1:12 PM by Mike Parnell
Sunjeey, 
 
If the center-of-gravity is under the crane hook, and the spreader frame is uniform in dimension, the upper slings are of equal length and the lower drop slings and shackles are of equal length, then you have the very best chance of equalized loading to the sling legs.  
 
What will also help is that the spreader frame has some ability to “deflect” slightly under load. It might only be ¼” – ½” but the effect will result in slightly better equalization.  
 
Deflection is temporary, deformation is permanent. 
 
Mike
Posted @ Wednesday, May 02, 2012 5:42 PM by Mike Parnell
Good morning Guys , i am new to the site so thanks for having me. I was looking at professional rigger Volume 10 Number1, question about load B off-level pick up points and it states the answers as: 
 
 
 
TSI = 8,181 lbs. 
 
TS2 = 14,727 lbs. 
 
I might be missing something but i have tried using a little TRIG & i come up with a different answer all the time, answer being: 
 
 
 
TS1= 5,999 lbs 
 
TS2= 16,099 lbs 
 
 
 
Can any body please help me with where i might have gone wrong, remeber i have found reaction then divided by cos, sin, thank you for your help. 
 
 
 
 
 
 
 
Posted @ Thursday, June 21, 2012 1:53 AM by russell
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