Analysis Guru
Return to Flight Issue
My correspondence to Readdy described the uplock roller design as an Achilles' heel. Stress analysis of the uplock roller design became a return to flight issue during January 2004 after Readdy decided my correspondence should be forwarded to the RTF team. The RTF team sent me confirmation a few months later stating that they would be taking a closer look at my analysis again. I can't provide all the technical details supporting this issue at my website due to size limitations. However, more background with substantiation is available on my CD.
The following is a segment of my communications relevant to this issue:
Les,
Your car AC problem was an interesting example. Now, permit me to discuss an example of mechanical part functionality. About two decades ago I was supporting a major piping replacement project at a nuclear power plant since microscopic stress cracks were found in the nuclear boiler piping. I volunteered to assist in examining the insertion and extraction of probe instrumentation sensors designed for monitoring radiation levels inside the boiler of a nuclear reactor. The nuclear fuel had been removed, but it was still a hazardous assignment due to residual radiation and heat. I won’t get into how much protective gear I donned, but I had to position myself directly beneath the boiler and observe the movement of the sensors. Whenever movement occurred I would indicate by hand to another person about ten feet away who would communicate my response by a radio headset to the control room operators. The importance of the procedure was to determine whether a mechanical part behaved like a mechanical part since the capability to achieve the designed monitoring locations was just as important as the capability to achieve the designed instrumentation signals. This procedure was an opportunity not missed even though not related to the piping stress crack problem. You might read the CAIB Final Report for examples of opportunities missed.
I’ll try to relate my example to the up-lock roller. It is all well and good that inspections are effected, but I have concerns. Visual examination of deformity, damage, cracks or breaks in the hex bolt seems extremely limited without its removal or testing the torque or tightness. How do you visually determine that the hex bolt has not sheared across the diameter somewhere in the threaded engagement section? Do the 0.030” tile step & gap measurements really determine the state of the hex bolt if a shear across the diameter in the threaded engagement section only creates a microscopic break gap of 0.020”? Do the detailed landing gear functional tests impose loading typical of flight conditions experiencing vibrations, temperature extremes and the cumulative dynamic forces imposed on connecting hardware? Does the 0.376” hex x 0.781” length at end of hex bolt have test purpose potential means for applying torque to the hex bolt?
Let’s revisit that thread stress calculation. Previously I had calculated with assumption that the failure would occur at the fitting threads under full threaded engagement. I tried to show that failure, if it occurs, would probably occur there first. Note that the pitch and length of threaded engagement is important. Assume a shear occurs across the diameter of the hex bolt just below the first thread in the threaded engagement section near one of the ends of the engagement length leaving a microscopic break gap of 0.020”. This time the stress is applied to the material of the hex bolt and there is no effective full threaded engagement since there is a 0.020” microscopic discontinuity. What is the allowable load?
Calculating allowable tension load ( F ) on the assembly according to bearing stress on threaded section of hex bolt:
Ssy = 0.577Sy = (0.577)x(185 kpsi)x( 1000 psi / 1 kpsi ) = 106745 psi
N = 18 p = 1/ N = 1 /18 At = 0.203 in2 Ar = 0.189 in2
Since stress is transmitted through only one of 18 threads in the one inch length,
h = 0.625 in / 18 = 0.035 in
F = - Ssy x h x ( At - Ar ) / p = - 941 lbs
There is a significant difference between 18812 lbs and 941 lbs of allowable tension. I could do other analyses assuming the break occurs leaving more than one thread engaged and the case of less than one complete thread, but I’ll let you do the math. By the way, extremely cold temperatures make metal brittle so consideration of maximum-shear-stress theory should also be considered where Ssy = 0.50Sy. Again, I’m not using any safety factors in this calculation.
Now, what tensile loading is anticipated for each roller assembly?
What do NASA stress engineers say about these failure scenarios?
Does NASA consider the MLG roller assembly a non-critical system?
Yours truly,
Weldon K. Chafin, Jr.