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Calculating Sling Legs

  
  
  
  

Calculating Sling Legs

Calculating Sling Legs

When our instructors conduct rigging courses, they teach the participants a quick method to calculate the minimum size of length of slings for a given load using a formula which has been included at the bottom of our rated capacity card.

Problem:  We need to install a set of slings between the crane hook and a spreader bar, which will create a 90 degree full included angle or less.

Question #1:  What size of wire rope slings do we need?

Facts:  Spreader bar length is 8’.  The weight of the load, bar and rigging gear will be 12,000 lbs.

Solution #1:  Since we want to rig the load at a 90 degree angle or less, we can look in the 90 degree column on the rated capacity card and find what size of wire rope meets or exceeds 12,000 lbs. We find that the sling legs will need to be made of at least ¾” wire rope.

Question #2:  What is the minimum length for these slings?

Facts:  A quick calculating formula has been included at the bottom of the rated capacity card.

SLING LENGTH is the minimum length of sling needed from the crane hook to the sling’s connection or contact point on the load.

LOAD WIDTH (or length) is the distance between the load’s shackle (connection) points or load edges.

MULTIPLIER is the number located at the bottom of each degree angle column, and is used in the formula according to which angle (60, 90, 120) the slings form when installed.

Sling Length = Load Width x Multiplier

Solution #2:  Since the pad eyes are 8’ apart, let 8 represent LOAD WIDTH and the MULTIPLIER is .75, as taken from the bottom of the 90 degree column.

SLING LENGTH = 8’ x .75

SLING LENGTH = 6’

Now a chance to test the reader’s skills.

What size and length of slings would you need between the hook and a 20’ spreader bar?  (Total weight of load and rigging = 25,000 lbs)

 

Angle                  Size                           Length

60                      ______         x         ______

90                      ______         x         ______

120                    ______         x         ______

(Solution is below, so don't scroll too far unless you want to see the answers).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solution:

Angle            Size            Length

60                1”                x  20’

90                1 1/8”          x  15’

120              1 ¼”            x  12’

 

Happy Rigging, Mike Parnell.  Click here to order the Journeyman Rigger's Reference Card, which is utilized in this workshop.

 

This is an article from The Professional Rigger newsletter, 1988. To download this issue of The Professional Rigger, click Calculating Sling Legs.

Tags: 

Comments

Mike, 
 
When did the rigging cards change form 60, 45 and 30, to 60, 90 and 120?
Posted @ Thursday, October 28, 2010 12:41 PM by Matt
This is an old image from the original Professional Rigger Newsletter, 1988. The angle defined is the angle at the hoist hook, not the horizontal angles. So, 60, 90, 120 in this application does still mean 60, 45, 30. ITI Rigging Cards now all use horizontal angle. Good catch!
Posted @ Thursday, October 28, 2010 12:44 PM by Zack Parnell
Chain Slings, Web and Roundslings were always based on the angle from the horizontal. 
 
The Wire Rope Sling and Wire Rope industry under the guidance of the Wire Rope Technical Board, decided to convert from the "full included angle" to the "angle from horizontal" in about 1996-99. 
 
So, we (WRRC/ITI) also changed our Rigging Cards and future workshops in the late 90's to reflect the new angle approach. 
 
The tensions in the slings obviously never changed, just the calculation method to arrive at the answer. 
Posted @ Friday, October 29, 2010 10:11 AM by Mike Parnell
good day! 
 
I can't see the solution.. Could you send to my email account the solution for this problem??? i really want to know the solution for this problem cause it will help me in my career as safety.  
 
thank you and God bless!!!!
Posted @ Sunday, September 29, 2013 5:49 AM by edmond
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