THE LITTLE BEARING THAT COULD – AND DID

I was a technician working in the Instrument Test Laboratory when news was received that Autonetics had been awarded the contract to develop the guidance system for the Minuteman missile. I have long felt that we were awarded the Minuteman contract because we had successfully developed the prototype G6 gyro and had proposed a derivative gyro as part of the Minuteman guidance system. The prototype G6 gyro used a spherical gas bearing to support the rotor and it was this gas bearing design that was the basis for the claim by Autonetics that the proposed system would meet the unprecedented  requirements of the Minuteman missile program. The G6 gyro was a two axes free rotor displacement gyro of a design unique to Autonetics. However, the angular displacement of the gyro was inherently limited by the proximity of the  spin motor, torque motor and rotor support shaft within the gyro. Any contact with these would have been a gyro killing event. A ball bearing was used as the means by which angular displacement of the case would be limited. An angular rotation of the gyro case resulted in the rotor contacting the ball bearing instead of the inside of the gyro case and no damage to the gyro would ensue. The stop bearing is a vital part of the gyro design but is often not described in those terms. What follows is my recollection of the free rotor gyro stop bearing story as I experienced it.

I have no idea what went on in the mind of the person who originally conceived of  the design of the Autonetics free rotor gyro, but I can imagine that among his first thoughts was of a stationary pin for the purpose of limiting the angular displacement. This seems simple enough, but if a pin had been actually tried, it would have been discovered that rotor precession resulting from rotor contact with the pin would have caused the rotor to press harder on the pin. The rotor would have locked onto the pin and things would have gone downhill from there. On the other hand, if the rotor contacts the outermost diameter of a ball bearing mounted on the gas bearing support shaft, the resulting rotor precession will cause the rotor to move away from the shaft. It is obvious that a ball bearing is the right choice for limiting angular displacement of the free rotor gyro. But will an ordinary ball bearing suffice for this application? The geometry of the surfaces of two cylinders, one inside the other, making angular contact, is such that the contact point is asymmetric to the center the length of the contacting cylinder. This means the contact load on a standard width bearing would be applied non-radially to the bearing. This is not acceptable, so how should the bearing design be modified? The obvious modification is to reduce the width of the contact area such as to make the contact point as close to the bearing centerline as practical. All stop bearings used in free rotor gyros thus have a narrow contact surface for this reason. This narrow surface also reduces drag forces acting on the rotor – always good thing.

Referring to my assertion that the high reliability spherical gas bearing was the central reason why Autonetics was awarded the Minuteman contract, it seems fair to ask about the use of a lower reliability ball bearing as an essential part of the gas bearing assembly and the possible deleterious impact it may have on gyro reliability. It seems reasonable to assume that finding an answer to this question was the reason for the massive number of stop bearing tests that were undertaken. I remember seeing a large number of test fixtures bashing stop bearings into the interior surfaces of rotating cylinders. Each fixture had provisions for counting the number of bash cycles and the severity of the bashes was varied as part of the test cycle. The vendors of the stop bearings were selected on the basis of the outcome of these tests. Bearings were tested until they disintegrated.

I do not recall any serious problems associated with the G6B4 gyro stop bearings.  However, a number of stop bearing issues did arise during my ten year tenure as the G9 gyro Responsible Engineer. This apparent disparity was due to the fact that the G9 stop bearing was smaller in size and the performance requirements for the G9 gyro were more stringent compared to the the G6B4 gyro. I do not recall that we had any stop bearing issues associated with the production of the G9 gyro used for the FB-111 strategic bomber N16 system, but there was an issue involving gyro sensitivity to the environment within the IMU housing that changed as the IMU heading changed. The gyro was apparently sensitive to changes in the magnitude and direction of air moving within the IMU housing. It may also have been sensitive to changes in the ambient magnetic field level.

The gyro that was originally used on as apart of Navy’s Minisins system was identical to to the G9 gyro used on the N16 system. It was only after system test data began to reveal performance short comings, associated with the requirements of long duration system operation, that the gyro designs diverged. It was during a subsequent gyro testing program that the stop bearing was implicated.

The G9 gyro differed  from the G6 in that the G9 used case rotation as the means to reduce gyro bias errors. The G9 gyro was of modular design. The gyro consisted of the rotor, the case rotation, the torquer, and the electronics modules. The case  rotation module contained the case rotation bearing and was the mounting surface of the gyro. The rotor module was mounted within the rotating part of the case rotation module. The torquer module was mounted on the non-rotating part of the case rotation module. A double wall septum cover enclosed the rotor which permitted the sealing of the rotor module. The double wall septum allowed the rotor torquer sleeve to protrude into the torquer module. The electronics module was comprised of a rotating part and a non-rotating part. These were joined by a slipring. The electronics module was assembled into a cavity in the rotor module. The non-rotating part was joined to the lower housing. A motor driven pinion gear drove a large diameter gear located on the rotor housing. Any biases fixed to the rotor module were time averaged to zero by continuous rotation of the rotor module. Since the gyro torquer module did not rotate, any biases due to the torquer were unaffected by case rotation.

Testing at the system level revealed a gyro sensitivity to system heading. Analysis of the data indicated that the gyro bias was being effected by the variation of system cooling air as the stable platform’s position changed relative to the IMU housing. A possible sensitivity to magnetic field variation as the heading changed was also noted. Testing of the gyro was inconclusive. It remained a mystery as to the actual sensitivity mechanisms of the gyro that would account for the system data.  An informal review of the gyro design indicated some improvements could be made with little risk and they had a good chance of improving gyro performance. As I recall these included the addition of a thin washer shaped shield, made of a nickel alloy, to an inside surface of the double septum. This acted as a magnetic field shunt to lessen any possible effects on the stop bearing by stray magnetic fields from the adjacent torquer. A simple aluminum cover over the gyro torquer wiring was lengthened to serve both as a magnetic shield and as protective cover against system cooling air impinging on the gyro. The cover material was changed from aluminum to a nickel alloy. The material used for the rotating surface of the gas bearing was electroless nickel plate. Exposure to temperatures over 400 deg. fahrenheit could drastically alter the magnetic field properties of the nickel plate. Procedures were put into place to ensure consistency of the rotor magnetic field properties. It was learned that the local magnetic field properties of nickel could be changed by changes in contact pressure exerted by the stop bearing as it contacted the rotor. The width of the contact area was increased to lessen the contact pressure. I believe we changed the material of the stop bearing to a 400 series hardenable stainless steel. None of these changes to the gyro were ever clearly proven to have been the magic bullet that cured the problems observed at the system level, but the system level problems diminished as the gyro changes were implemented, much to our relief.

From time to time we received gyros that had been failed during system testing because they would not recover properly after the rotor contacted the stop bearing. The rotor would lock onto the stop bearing and would have a large magnitude pickoff modulation signal which was at the same frequency as the nutation frequency of the rotor. In every instance, as I recall, replacement of the stop bearing fixed the problem. This was never considered a major problem and it was chalked up to stop bearings that developed a high torque condition as a result of excessive rotor contacts with the bearing. It was as though the bearings became like a low friction stationary stop pin.