Geometric Instability Considerations And Criteria for Below the
Geometric Instability Considerations And Criteria for Below the Load Lifts Using Strong-bacs and Spreader bars 02/25/2020 John Escallier, Brookhaven National Lab 1 History Two strong-bac lift designs used in bldg 902 have been unstable by design SNS Dipole girder NSLS2 90 mm Dipole Girder Both lifts were attempted (under rigorous control) despite the instability of the design. The SNS strong-bac was abandoned, slings were
used instead NSLS2 lifting apparatus was modified to allow use 02/25/2020 John Escallier, Brookhaven National Lab 2 Concerns Test lifts of hardware which is unstable by design is not a good idea. Yaw sensitive stability may mask problems Failure to spot an unstable design is too costly We do not want to be in this position The Topex/Poseidon spacecraft was inadvertently flipped during a crane operation back in 1992. The configuration was very similar to yours (birdcage effect). I would like to get you the video but I need to make sure its cleared for me to release it. If you have other means to look for it please do so, I will keep trying from my end. This video would help convince your team of what could happen.
Another suggestion is to build a small model with strings and balsa wood. This would also help explain the instability. Sergio Valdez, JPL 02/25/2020 John Escallier, Brookhaven National Lab 3 A Video of the Topex Rigging Incident is available on the Safety & Health Services Lifting Safety Program Area 02/25/2020 John Escallier, Brookhaven National Lab 4 Definitions 1 Pitch Multipole girder lift
Roll Yaw 02/25/2020 John Escallier, Brookhaven National Lab 5 Definitions 2 Margin of stability The ratio between the Center of Gravity height and the headroom (or sling height) Margin of stability = Headroom/CG height A margin of stability of 1 means the load can be rotated and it will remain where it is put Headroom A margin of stability of >1 means gravity will provide a restoring force to the load A margin of stability <1 means gravity will provide a force which will try to flip the load
Center of Gravity Height 02/25/2020 John Escallier, Brookhaven National Lab 6 Where is the center of rotation? Roll has 2 axis of rotation. Pitch Pitch has 1 1. At the axis of hook. (stable) There are THREE axis of rotation in this configuration rotation 2. Within the 1.Roll
At the mass due to hook. (stable) the strongbac The strongbac added axis rotation (may has been locked by the be stable) Yaw crossed beams 02/25/2020 John Escallier, Brookhaven National Lab 7 Translation of rotation radius To locate the payload cg you down to load can also include the weight of the lower lift beams and gray frame of your payload. These
are all part of the hoisted payload. Sergio Valdez, JPL Center of mass above the translated rotation point is unstable Center of mass below the translated rotation point is stable 02/25/2020 John Escallier, Brookhaven National Lab 8 Potential energy diagram Potential Energy As the load rolls, the The system tries for CG rises, the lowest potential Theenergy system has no potential energy, so will return system
has the impetus to move so The load CGtoisa above theofrotation of the increases. load The roll angle zero same at energy at all stays the same angles angle As the load rolls, the The load CG is exactly where the load CG rotates lowers, the The load CG is below the rotation of potential
the load energy decreases Margin of stability less than 1 Margin of stability =1 X Margin of stability greater than 1 - Roll angle + 02/25/2020 John Escallier, Brookhaven National Lab The system tries for lowest potential energy, so will rotate further away from zero 9 The first multipole test load pitched during acceleration The margin of stability was 1.17
This margin of stability was deemed insufficient cross braces were added to lock pitch rotation This did not lock the roll axis Pitch axis with cross braces Since the diagonal straps work only in tension, they will not necessarily restrain the pitch rotation. Parallelogram can still occur. Sergio Valdez, JPL
Pitch axis height without cross braces Multipole girder lift CG height 02/25/2020 John Escallier, Brookhaven National Lab 10 The second lift using a dipole was halted before lift The roll axis was still unstable by design (no bracing chains) Margin of stability = .97 Cross chains and bracing chains have been added Cross chains converts to a birdcage configuration during roll Only the dipole bracing chains guarantee stability
Multipole girder shown Assumed new roll axis radius Dipole CG translated height Roll axis radius The cross chains are not sufficient to prevent a parallelogram movement. They only work in tension. Sergio Valdez, JPL
Cross chains Dipole bracing chains (4) not used with multipole pictured here 02/25/2020 John Escallier, Brookhaven National Lab 11 90 mm Dipole stability analysis 30.6 Margin of stability = .97 Center of rotation is 1 inch BELOW CG 31.6 Drawing courtesy
of Lewis Doom 02/25/2020 John Escallier, Brookhaven National Lab 12 NSLS2 Multipole girder 30.6 Center of rotation is 4.6 inches ABOVE CG Margin of stability = 1.17 26 Drawing courtesy of Lewis Doom
02/25/2020 John Escallier, Brookhaven National Lab 13 Figure 3.3-2 Spreader Bar Stability Analysis 3.3.3 Spreader Bar Stability Analysis - If the attach points of the spreader bar cable are close to or below the center of gravity of the weight to be lifted, the following stability equation shall be verified in all appropriate planes. where B is the height of the sling assembly, A is the horizontal distance from the crane hook to the cable attach point, C is the horizontal distance from the center of gravity of the payload to cable attach point, and D is the vertical distance from the payload center of gravity to the top surface of the cable attachment as shown in Figure 3.3-2. Verify, by load analysis, that any two diagonally opposed cables can carry the full lifted load without exceeding the proof test range of the lifting assembly. The center of gravity elevation and weight limitations of the stability analysis shall be stenciled on the lifting structure hardware.
184.108.40.206 Margin of Stability - The 1.5 margin of stability may be reduced to 1.2 with a waiver. Picture and text From JPL Standard for system safety(rev D) 02/25/2020 John Escallier, Brookhaven National Lab 14 Recommendations Adopt the JPL/NASA lifting criteria for margin of stability of 1.5 This eliminates CG height based stability issues It prevents position/rotation dependent instabilities Excerpt from NASA Lessons Learned Lesson Number: 1089 Lesson Date: 1992-05-29 Just prior to the final crane move, the T/V Fixture Assembly was lifted by the crane and
a "rocking test" was performed on the assembly by the test team to assess its stability. The T/V Fixture Assembly appeared to be stable at that time because the lifting clevis friction was not exceeded during the rocking test. Never rely on a test lift/rocking to determine lift stability 02/25/2020 John Escallier, Brookhaven National Lab 15 Recommendations Always require an engineering analysis of critical lifts prior to any lift tests Any configuration which is out of the ordinary requires a thorough analysis Sometimes a different lifting configuration can have unknown geometric consequences. Unknown geometric consequences are rarely a good thing
02/25/2020 John Escallier, Brookhaven National Lab 16 Recommendations Adopt the waiver requirement for the 1.2 margin of safety This margin allowed multipole pitch at an uncomfortable level, waivers allow a level of control Adopt the stenciling of CG elevation and weight limitations of the stability analysis on the hardware Continue the test lift policy even for stable designs (errors do occur) 02/25/2020 John Escallier, Brookhaven National Lab
17 Recommendations Educate the riggers and engineers via training, procedures, and models. Make the information available within the SBMS system Explain in detail: Why an under the CG strong-bac (spreader) lift is different What causes the instability Teach the translation of rotation method How to determine stability CG height vs rotation height Detail the differences between parallel, birdcage, and umbrella lifts Birdcage being the most challenging 02/25/2020 John Escallier, Brookhaven National Lab 18
Other Suggestions Sergio Valdez, JPL Do not move at fast speeds on the crane. Orient the load such that it moves along the more stable axis. Do not step the crane by doing bumps on the crane pendant. It may excite the swinging. Use tag lines at all times to help stabilize any swinging. Lower the CG by adding ballast. Modify lower lift beams with vertical posts to raise the cable attach point. Make cross braces such that they can take compression as well (i.e. tubes or channel). 02/25/2020 John Escallier, Brookhaven National Lab 19 Yaw stability dependency (incremental rotation) Roll center
Pitch center 02/25/2020 A 90 degree Yaw swaps the rotation centers With CG above the hook, This can be metastable John Escallier, Brookhaven National Lab Pitch center Roll center 20
Metastable example Equal forces on two contact points No rotation Perfect alignment Roll axis at Hook Translated CG Margin of stability <1 Strong-Bac Plate External forces (Ropes, Acceleration) Hook section Force now at one point Rotation will occur Causes movement of CG
horizontally 02/25/2020 John Escallier, Brookhaven National Lab 21 Potential energy diagram Metastable example R L Whenthe theCGroll While is angle is sufficient that the CG within width
of is at thethe plane of the the two surface, point flange flange there contact, will be the onesystem line will continue contact andto thegain potential energy as system energy the roll angle
function slope increases becomes negative. 02/25/2020 Margin of stability slightly >1 Margin of stability <1 R zero L - Roll angle + John Escallier, Brookhaven National Lab 22 Backup materials
02/25/2020 John Escallier, Brookhaven National Lab 23 Ellipse generation 02/25/2020 John Escallier, Brookhaven National Lab 24 Trammel of Archimedes 02/25/2020 John Escallier, Brookhaven National Lab 25
From www.noble.com.au 02/25/2020 John Escallier, Brookhaven National Lab 26 Lift types Symmetric Parallel Spreader Bar Lifts Symmetric Bird-Cage Spreader Bar Lifts Symmetric Umbrella Spreader Bar Lifts From JPL Standard for system safety(rev D) 02/25/2020
John Escallier, Brookhaven National Lab 27 Thanks 02/25/2020 John Escallier, Brookhaven National Lab 28 Thanks to the people who have critiqued this presentation and provided recommendations Jet Propulsion Laboratory Sergio Valdez, owner of JPL document ES501492, Safety Requirements for Mechanical Support Equipment for JPL Critical Items Equipment BNL Scott Buda
Walter Czekaj 02/25/2020 John Escallier, Brookhaven National Lab 29
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