CRUD! IF IT IS ONLY AN ACRONYM, WHY WON”T MY BEARING START?


I was introduced to crud a short while after I was introduced to the spherical gas bearing. (This bearing was the engineering tour de force at the literal and metaphorical center of the Autonetics G6 and G9 gyros.) I do not recall the exact circumstances of my first experience with a gas bearing, but it is very likely one which  involved a bearing which had failed to start rotating after power had been applied to the motor. What better way to inform a newly minted engineer about “black crud”. The term “Black crud” was the generic name given to the contaminating material often observed in failed gas bearings after disassembly. The term “black crud” had thus been inserted into my engineering lexicon where it has remained until this day. After I had decided to write of my experiences with “black crud”, I realized I did not know much about the words “black crud”. When I am faced with questions of that sort, I seek answers by entering the appropriate words into the Google search engine. After I entered the word “crud”, the search engine returned the usual avalanche of possibilities. The word “crud” is apparently completely generic in that it can range in meaning the noxious substances found in sewers to a more modern computer acronym. The word “black” has more specific meanings ranging in context from physics to psychology. I used the term “black crud” sans a definition for many years, but everyone seemed to know what I meant nonetheless. Thanks to the Google search engine, today I would say that “black crud” was the unknown substance found in failed gas bearings. It is the presumptive immediate cause for the failure of the bearing. The primary source of the “black crud” is usually not revealed by inspection of the in situ “black crud”. All bearing “crud” appears black due to the minute quantity of “crud” allowed by the very small dimensions of the gas bearing gaps. In my experience, staring at deposits of “black crud” will not reveal what it is, but trying to understand how it got there may yield clues as to origin. Even the use of marvelous machines of modern science such as the infrared spectrophotometer may not yield a satisfactory answer to the questions of origin being asked. Lifting a deposit of “black crud” and removing it without contamination is an art form mastered by only a few technicians. The first name of one such person was, very appropriately we thought, Merlin.

I was assigned the task of disassembling all G6 gyros that failed for any reason during acceptance test, determining the reason for the failure, and appearing monthly in person before the Air Force Minuteman Project Officer to present my findings for each failure. Going to Norton AFB every month was a big deal for us. Despite the serious nature of the assignment, I found it to be a lot of fun. Some of what I presented concerned gas bearing “no start” failures. I remember the times I presented Merlin’s infrared spectrophotometer charts, with their many bumps and dips, of samples of “black crud” taken from  failed bearings. I usually went through the litany of the various hydrocarbons that each bump represented and then delivered the punchline. I once showed them a chart made using a sample of hand cream. It was obvious the “black crud ” lifted from the failed bearing was hand cream. I was glad the Air Force did not have a policy of shooting the messenger or I would not be telling the story as they were not happy. However, most of the “black crud” was found to be the filler material added to epoxy resin in order to lower the thermal coefficient of expansion to a value closer to that of steel. This epoxy resin mixture was used several places inside the gyro and these could become sources of contamination.

The gyro is filled with a mixture of Hydrogen and Helium gases. This gas is caused to rotate at a speed exceeding 100 mph by frictional contact with the rotor. I always thought this high speed gas would be very effective in dislodging potential contaminants from their hiding places. The self-pressurizing property of the gas bearing required a constant flow of gas into the bearing thru small holes that led from the outer surfaces of the rotor into gas feed grooves that were located at the edge of the thrust pockets of the bearing. This arrangement of holes and grooves acted like a miniature vacuum cleaner. We sometimes observed a green powder deposited in the groove. The green color was from a dye mixed in with the calcium carbonate filler material.

I had exclusive use of a Farr bench inside the dust free instrument assembly area. This is where I did my gyro diagnostic disassembly work on failed gyros. The bench was located in an area of the room that had a direct line of sight to the managers desk on the other side of a large window in the wall between us. A failed gyro was a not a common event, but when it happened I would do my thing at my Farr bench. The Manager was well known to us as somewhat of a comedian, he was fun to be around and he had the knack of keeping tense situations from becoming worse. When I was at the bench, we would communicate using hand gestures. Like I said, he was fun to be around. His name was Gerry Sammons. The usual business of diagnostic teardown was routine and aften boring and a little horseplay at the higher levels of management helps.

The G6 gyro is sealed inside a welded, gas tight, magnetic shield can. Helium gas is added to the gyro gas mixture to permit the performance of a final mass spectrometer leak check of the gyro. I believe that Helium gas has the greatest diffusion rate of all the gases and it is notorious for being difficult to seal. The welded can was the absolute fix.  If a gyro is to be disassembled, the can must be opened by first milling the weld bead from the can. When the technician sat down a the bench, there was a gyro before him with the can weld bead removed. One time we removed the gyro from the can and I found a smashed red plastic protective cover of a fairly large round connector. I was taken aback by this turn of events and I replaced the gyro back into the can. I signaled Gerry he should put on his protective smock and come inside. After Gerry arrived at the bench and saw what I had found, he immediately suspected I was playing a practical joke on him. I finally convinced him I was not. We never did find out how that red protective cover came to be inside the welded can. If it was a case of attempted sabotage, it was a most clumsy one.

We had the use of a McCrone particle Atlas to aid us in the identification of anything we found in a gyro. With the aid of the Atlas we identified a hair from an Angora rabbit. But my all time favorite find was finding a Turkey Feather. That’s right, a whole Turkey Feather. These were duly reported to the Air Force Project Officer. I survived the ordeal without undue loss of blood as I recall.

I had moved on to other tasks when I became aware of a project to investigate the characteristic events that transpired as a coasting gas bearing contacted the stationary surface and stopped rotating. It was hoped that a “good” gas bearing could be identified and accepted and a “about to be bad” bearing would be set aside for disposition. It was an attempt,no less, to invent a “crud” finder machine. A noble endeavor indeed. I stayed in touch with Mike Albertson, the Supertech doing the work, as I was interested in the outcome.

The Instrument Test Laboratory had been for many years performing gyro drift tests using an apparatus which utilized a servo circuit to cause a single degree of freedom rotating table to constantly maintain one of the two gyro pickoffs at null. A servo circuit kept the other gyro pickoff at its null by constantly torquing the rotor. As the rotor drifted over time, the drift test table moved with it, thus the table angle was identical to the rotor drift angle. With suitable modifications, the drift table could be made to keep the drift table pickoff at null, ie, it was servoed to its own pickoff. The current flowing in table torquer was proportional to the force necessary to keep its pickoff at null.

The principle that was being exploited was simple. A gyro is mounted on a drift table such that the gyro spin axis is coaxial with the table axis. The drift test table is servoed to its self. If the gyro spin motor power is turned off and the rotor is allowed to coast, the drift table torquer current will be a measure of the drag torque acting on the rotor. If the coasting rotor contacts “crud” in the bearing as the rotor slowly looses lift, the event will leave a characteristic change in the table torquer current recording. It should be possible to reliably detect “soon to be bad” rotors! I do not remember if the machine was ever incorporated into the formal gyro acceptance tests, but I do remember it being used to diagnose rotor bearing problems for a long time thereafter. I remember being impressed by the sensitivity of the machine.

 

 

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