THE AUTONETICS G9 GYRO CASE ROTATION BEARING


The G9 gyro was an Autonetics free rotor gyro with case rotation added and it was used by the Autonetics N16 inertial navigation system. The N16 was the inertial navigation system for the FB-111 strategic bomber and a Minisins variant of the N16 was used for navigation on the Los Angeles class attack submarines of the U.S. Navy. The G9 gyro was a smaller version of the Autonetics G6 gyro which is still in use today on the Minuteman III missile.

The case rotation of the G9 gyro requires the gyro to have a case rotation bearing, a slipring capsule, a pickoff resolver, case rotation drive motor and gears, and a case rotation bearing. The gyro torquer assembly does not rotate. These items are not found on the G6 gyro. Case rotation was included in the G9 gyro design because of a requirement that the bias drift rate of the G9 be much lower than that of the G6 gyro. Case rotation has the effect of time averaging the gyro bias to zero. 

The spinning G9 rotor, like any gyro rotor, will not change its angular position in inertial space unless an external torque acts on the rotor. Assuming no torques are acting on the rotor, any changes in the magnitude of the gyro pickoff signals over the case rotation cycle will be due to the null pickoff plane not being normal to the instantaneous rotational axis of the case rotation bearing. In effect, the pickoffs are being constantly rotated away from the rotor and the pickoff signal will have a signal component with a magnitude equal to twice the non-normalcy of the pickoff null plane and frequency equal to that of the case rotation frequency. Any such signal component is a source of error to the platform gimbal servomechanisms of the system. The case rotation bearing must be of highest quality available and must be carefully designed to meet all performance requirements. For instance, the gyro is required to operate normally as the gyro is subjected to accelerations substantially greater than acceleration due to gravity. The case rotation bearing must operate normally in spite of such loads on the bearing. The bearing must be designed with an axial preload built in to preclude the bearing perfomance being degraded by externally applied loads.

The G9 gyro case rotation bearing met all of the requirements from the very beginning of the testing of the gyro. The original bearings were designed and manufactured by the New Departure Bearing Co. as I vaguely recall. The bearings were manufactured so that the bearings would be automatically set to the the proper preload value when the gap between the two pieces of the inner races was closed by the assembly screws.

We did get a big surprise at the beginning of the FB-111  program. The system environmental  tests required the system turn on and operate normally after a cold soak at -65 F. Much to our chagrin, we discovered that the bearing lubricant we used froze solid at -20 F. We obviously had a problem. Actually we had several problems on our plate. We had to find a bearing lubricant that froze at a sufficiently low temperature, had excellent lubricant properties, was available, had an adequate service life and did not evaporate at 165 F. We were very lucky because we had recently gone through the selection process for an oil for the G6 gyro stop bearing. We quickly determined that what we knew as Gyro oil D was the oil that met all requirements for the G9 bearing lubricant. It was already in use for the G6 gyro stop bearing so it had a good test history. Gyro oil D is a Pennsylvania crude “bright stock” which means the oil is not refined before its use as a lubricant. It is a rare well that produces this oil. To ensure that Autonetics always had a supply, we bought a 55 gallon drum of it. We used it by the drop, so a 55 gallon drum was a lifetime supply. Good thinking on our part, but we cold not find the 55 gallon drum when we needed to replenish our supply. Never did find that drum. I seem to remember someone commenting on the high price of the replacement oil. Gyro oil D proved to be our bearing oil never the less.

We had been in production of the G9 gyro for some time and I was beginning to think that I seen everything when I received a call for help with a G9 gyro that was not failing any test but the factory test engineer was of the opinion it should be failing something, as the gyro torquer current recordings were like nothing seen before. In a normal gyro, you can see evidence of case rotation in the recordings of the magnitude of the torquer current flowing in the servo that keep the rotor pickoff angles at null. These recordings had a huge non-sinusoidal signal component that seemed to be synchronous with case rotation. I had no idea what the problem was except it was clearly related to case rotation. I found that the signal also had a component that was rotating slower than case rotation. I was not able to associate what I  was seeing with any bearing defect I could imagine so I punted. My advice to the Test Engineer was to replace the bearing and see if the problem goes away. They changed the bearing and the problem went away. We had solved a problem but I had mystery bearing on my hands. I lacked adequate equipment with which to investigate a bad bearing so I sent it back to the bearing vendor for analysis. They quickly determined that one ball of the set of balls in that bearing was larger than the others. By this time we had certified two bearing vendors and they each had their own method of adjusting the bearing preload value. One vendor set the preload by adjusting the combined height of the inner race bearing parts so that at assembly the preload is correct. All of the balls used in their bearings were of uniform size. The other vendor adjusted the preload by using ball sets of different size balls. All balls within a ball set were of uniform size. The odd bearing we had discovered in our testing had been assembled with a ball set that mistakenly included a ball larger than the others of the set. The slower than case rotating component I saw in the torquer current recording was due to the slower speed of the balls relative to the speed of the inner race. The big ball was causing the instantaneous axis of rotation of the bearing to wobble and what we observed was the servo trying to keep up with the pickoff signal. I will always remember this as the “big ball problem”.

We once had a bearing problem we brought on ourselves. I do not remember why we did so but we vacuum baked the lower assembly of the gyo with the bearings in place. Very bad decision as the bearing lubricant was almost gone after the bake was over. We ended up sending the bearings back to the vendor to be relubricated. Our experience with the bearings was good as I do not recall any problems with life or reliability. 

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