Page 52 - Wire Rope News & Sling Technology - August 2018
P. 52

Denny’s                                        Mission Possible?
         crane and                              Testing Roof Lugs to Lift

         rigging notes
                                                     NASA Missile Gantry

                                                                   by Dennis O’Rourke

              hen NASA needed to launch tall-  Then, using a 400-ton  crawler crane,   to provide proper support for the ma-
         Wer missiles from Cape Kennedy,    lift the 330-foot high gantry to a height   terial resisting the “pull-out” force, the
         Florida, engineers  faced a daunting   of 370 feet and insert new sections.   size of the shackle pin must not be less
         mission  –  how  to  raise  the  top  of  the   The assembly required destructive   than 10% of the eye’s diameter.
         Pad 39 missile gantry by 40 feet.  testing to verify the design (ASME   How many times have we seen this?
           The method they proposed was novel:   BTH-1-2005).  The  amount  of  stress   In this test, pin diameter was close
         fabricate hoisting roof lugs and attach   needed for the fixture eye to fail would   to 90% of the eye diameter, providing
         them to the top of the missile gantry.   be  verified.  The  “area”  (height  above   near ideal support. When shackle pins
                                            the eye, width of the eye, and thickness   are too small, they point-load the eye
                                            of the material) would determine pull-  causing  premature failure.  This  com-
                                            out breaking strength.             promises  both  the  engineer’s  objec-
                                              Two  basic  elements  of  the  design   tives as  well  as  safety. In critical  lift
                                            pointed toward failure:  pin size and   situations, we use “key-hole” pad eyes,
                                            clearance between the eye and the pin.  which provide 180° of support in the di-
                                              What did the testing teach us? First   rection of the pulling force.
                                            – and this was the objective of the en-  Coming in the next issue: we will dis-
                                            gineers – to have confidence that lifting   cuss some common errors made by com-
                                            the  working  load with  the  fabricated   mon people like you and me. WRN
                                            roof lugs would not cause a failure!
                                              Second, it confirmed the basic assem-
                                            bly methods that must be followed  to
                                            meet the design objectives. Specifically,

         1. In test stand, roof lug is attached via 30-
         ton shackle at the top and 45-ton below, off-
         set replicating  the sling  angle  when in  use
         and the weakest point.
                                                                               5. The top eye shows predictable elongation
                                                                               of the hole and shear plane failure.
                                            3.  Then the  load is increased to failure.
                                            Shackles have a 5 to 1 design factor (capacity
                                            to yield). Here we see the bending of the pin
                                            = edge stress increasing on the eye and loss
                                            of support.

                                                                               6. Lower eye of a 45-ton shackle capacity
                                                                               shows distortion (failure) but not breaking.
         2. The roof lug is load tested to its capacity,                         DENNIS J. O’ROURKE, CSP,,
         which is 75%  of the  anticipated  working                            is the Director of National Crane Services, Inc. He has over
         load – then, to 150% of capacity with                                 fifty years’ experience in the industrial, maritime, and con-
         no measurable distortions. These  are the                             struction fields working with heavy equipment and material
                                                                               handling devices. As a safety engineer, Mr. O’Rourke has de-
         normal  overload test amounts  required for   4. Roof lug hits the ground with 267,644 lb. of   veloped and/or presented over 300 safety-training programs
         certification.                     recoil force!                      for all representative elements of government and industry.
         52     Wire Rope News & Sling Technology   August 2018
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