Geometric Instability Considerations and Criteria for Below the Load Lifts Using Strong-bacs and Spreader bars

1. Geometric InstabilityConsiderations And Criteria forBelow the Load LiftsUsingStrong-bacs and Spreader bars John Escallier, Brookhaven National Lab

2. 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 John Escallier, Brookhaven National Lab

3. 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 it’s 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 John Escallier, Brookhaven National Lab

4. A Video of the Topex Rigging Incident is available on the Safety & Health ServicesLifting Safety Program Area John Escallier, Brookhaven National Lab

5. Definitions 1 Pitch Multipole girder lift Roll Yaw John Escallier, Brookhaven National Lab

6. 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 • 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 Headroom Center of Gravity Height John Escallier, Brookhaven National Lab

7. Where is the center of rotation? Pitch Roll has 2 axis of rotation. Pitch has 1 axis of rotation 1. At the hook. (stable) There are THREE axis of rotationin this configuration 2. Within the mass due to the strongbac rotation (may be stable) Roll 1. At the hook. (stable) The strongbac added axis has been locked by the crossed beams Yaw John Escallier, Brookhaven National Lab

8. Translation of rotation radiusdown to load To locate the payload cg you 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 John Escallier, Brookhaven National Lab

9. Potential energy diagram Potential Energy As the load rolls, the CG rises, the potential energy increases. The system tries for lowest potential energy, so will return to a roll angle of zero Margin of stability less than 1 The load CG is above the rotation of the load The system has no impetus to move so stays at the same angle Margin of stability =1 The load CG is exactly where the load rotates The system has the same energy at all angles X As the load rolls, the CG lowers, the potential energy decreases Margin of stability greater than 1 The load CG is below the rotation of the load The system tries for lowest potential energy, so will rotate further away from zero - Roll angle + John Escallier, Brookhaven National Lab

10. Since the diagonal straps work only in tension, they will not necessarily restrain the pitch rotation. Parallelogram can still occur. Sergio Valdez, JPL • 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 height without cross braces Pitch axis with cross braces Multipole girder lift CG height John Escallier, Brookhaven National Lab

11. 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 The cross chains are not sufficient to prevent a parallelogram movement. They only work in tension. Sergio Valdez, JPL Multipole girder shown Roll axis radius Dipole CG translated height Assumed new roll axis radius Cross chains Dipole bracing chains (4) not used with multipole pictured here John Escallier, Brookhaven National Lab

12. 90 mm Dipole stability analysis 30.6 Center of rotation is 1 inch BELOW CG Margin of stability = .97 31.6 Drawing courtesy of Lewis Doom John Escallier, Brookhaven National Lab

13. 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 John Escallier, Brookhaven National Lab

14. Figure 3.3-2 Spreader Bar Stability Analysis Picture and text From JPL Standard for system safety(rev D) John Escallier, Brookhaven National Lab

15. 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 John Escallier, Brookhaven National Lab

16. 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 John Escallier, Brookhaven National Lab

17. 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) John Escallier, Brookhaven National Lab

18. 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 John Escallier, Brookhaven National Lab

19. Other SuggestionsSergio 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). John Escallier, Brookhaven National Lab

20. Yaw stability dependency(incremental rotation) Roll center A 90 degree Yaw swaps the rotation centers Pitch center With CG above the hook, This can be metastable Roll center Pitch center John Escallier, Brookhaven National Lab

21. Metastable example Equal forces on two contact points No rotation Perfect alignment Translated CG Margin of stability <1 Roll axis at Hook Strong-Bac Plate Hook section External forces (Ropes, Acceleration) Force now at one point Rotation will occur Causes movement of CG horizontally John Escallier, Brookhaven National Lab

22. Potential energy diagramMetastable example Margin of stability slightly >1 L R Margin of stability <1 When the roll angle is sufficient that the CG is at the plane of the flange surface, there will be one line contact and the system energy function slope becomes negative. While the CG is within the width of the two point flange contact, the system will continue to gain potential energy as the roll angle increases R L zero - Roll angle + John Escallier, Brookhaven National Lab

23. Backup materials John Escallier, Brookhaven National Lab

24. Ellipse generation John Escallier, Brookhaven National Lab

25. Trammel of Archimedes John Escallier, Brookhaven National Lab

26. From www.noble.com.au John Escallier, Brookhaven National Lab

27. Lift types Symmetric Bird-Cage Spreader Bar Lifts Symmetric Parallel Spreader Bar Lifts Symmetric Umbrella Spreader Bar Lifts From JPL Standard for system safety(rev D) John Escallier, Brookhaven National Lab

28. Thanks John Escallier, Brookhaven National Lab

29. 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 John Escallier, Brookhaven National Lab