METROLOGY ? WHAT DOES IT MEAN?


I have spent some time thinking about my long ago experiences as an engineer working on the Minuteman G6B4 gyro project. I was thinking specifically about the interesting tasks that we were engaged in that paved the way for the use of the spherical gas bearing as the rotor bearing for Autonetics free rotor gyros. The bearing literally defined the Autonetics free rotor gyro. In my work, I was fortunate to have the opportunity to become familiar with much of the work done by others and the fruit of this work became the technology that made possible the successful production of gyros such as the G6B4. Recently, I undertook to explain to a collector of Cold War era inertial instruments the inner workings of the Autonetics free rotor gyro. I found it was not possible to have this discussion without explaining the hows and whys of the spherical gas bearing. As I mused over my experiences working as an engineer assigned to the G6B4 team, I was reminded of the  measuring tools used to produce the bearing assembly and the Metrology requirements that controlled and defined all measurements. It is my aim to inspire in the reader the same sense of the importance of this work as I gained during my tenure as a member of the G6B4 team.

 

The basic concept of the spherical gas bearing is that of two spinning hemispherical cavities, joined together as a unit at the equatorial plane of the cavities, and enclosing a stationary support ball which is fixed to the gyro case. However, this simplicity is the first casualty as the requirements of the gyro are added. Here is a list of such requirements:

1.) The gas bearing must not require any external pressure source.

2.) The similar kinds bearing parts must be interchangeable.

3.) The bearing must operate normally during all phases of the missile launch operations.

4.) No special start or stop procedures are allowed for the gas bearing.

5.) All measurement tool calibration procedures must be traceable to the primary standards at the United States Bureau of Standards.

6.) The bearing must operate, without maintenance, for three years.

The G6 spherical gas bearing design evolved until it became the successful bearing of the G6B4 gyro. Here is some of the story of how this was achieved.

The gas bearing must not require any external source of pressure, ie, the bearing must be self-pressurizing. This was achieved by designing three pads into the hemispheric shaped surface of the bearing; three in each rotor half, located near the axial axis of the bearing. A  feed groove was located at one end of each pad. Each groove extended beyond the pad to a small hole that led to the outside of the rotor half. These features combined to create the gas pressure increase necessary to allow the bearing to successfully operate under external load. The pressure of the gas within each pad is reduced due to the increased volume of the pad. This pressure reduction causes gas to circulate into the hole from the outside of the rotor to the inside of the bearing. This gas is forced to flow toward the pad edge opposite the groove by the flow of gas resulting from rotor rotation. The gas undergoes a pressure rise as it is compressed by the abrupt reduction in gas volume at the pad edge. It is this pressure rise which accounts  for the axial load bearing capacity of the spherical gas bearing. The radial load bearing capacity of the spherical bearing is approximately the same as the center of an ordinary journal gas bearing of comparable dimensions. Without the pads, the axial load bearing capacity of the spherical gas bearing is zero.

 

In order to accommodate the parts interchangeability requirement, it is necessary that the bearing dimensions be measured to absolute standards. This can only happen if the measuring tools are calibrated using calibration standards traceable to the primary standards of the United States Bureau of Standards. Achieving this required only that the common methods of metrology be adapted to the calibration of the specific measurement tools germane to the gas bearing parts. However, there was an important exception to this. A vital bearing dimension is the diameter of the bearing cavity. It is not possible to measure the bearing diameter during fabrication of the rotor half because only the radius of the bearing cavity can be measured. Remember that the radius of the cavity is the distance from the bearing cavity surface to the center of the cavity. The center of the cavity is a fictitious point from the metrology point of view. A custom measurement tool to make this measurement was in use but the calibration of it was not traceable to the primary standards. It is likely no precedent for a measurement  from a real surface to a fictitious point existed. The tool used a capacitor type sensor to measure distance along a radial line within the cavity. This sensor was embedded in an assembly which was rotated incrementally around the center of the cavity. This tool measured the distance from the center of the cavity to the bearing surface, ie, the radial dimension of the cavity. The tool was calibrated using a ring gauge. The calibration of the ring gauge was traceable to primary standards. I do not know how the fictitious point calibration traceability dilemma was finally resolved, but it apparently was.

Approximate bearing parameters are listed to give the reader a sense of the magnitude of the difficulties surmounted during the bearing program.

The radial gap of the bearing was about 200 microinches.

The depth of each pad was about 100 microinches.

The hemisphere of the bearing was inset into each rotor half by about 15 microinches.

 

One of the most interesting aspects of the early gas bearing work was the question of the gas bearing manufacturing methodology to be used to produce rotor halves such that the final assembled bearing would meet all the bearing requirements. Put another way: How do you align the rotor halves at final assembly so that the assembled bearing subsequently meets all gyro requirements? The G6B4 rotor half bearing andrelated surfaces are plated with electroless Nickel metal and through processes such as grinding and lapping, rotor halves ready for the final manufacturing step are produced. The final step is to assemble two of the rotor halves over a measuring device placed inside the bearing cavity. This device allows measurement of the the distance from the bearing surface of one rotor half to the bearing surface of the other. One rotor half is moved relative to the other until the measuring device indicates that the radial distances are all equal. The rotor halves are said to aligned. The outside diameters of each the the pair of rotor halves are then ground to a suitable diameter and the rotor halves then become a matched pair. At subsequent assembly of the rotor halves over a ball assembly, bearing alignment is achieved by making the outside diameters concentric.

 

I was the lead machinist on a project to build a fixture that was to be attached to a one ton force linear shaker head. This fixture was necessary to ensure that the shaker head input to the gyro undergoing “shake test” was linear, ie, the input motion was without any rotation imparted to the gyro. The data obtained from the gyro “shake tests” was used to validate a gas bearing model that had been derived from gas bearing theory. Later in my career as an engineer, I conducted G6B4 shaker tests using this fixture. The test data was used to validate a higher order bearing model. All of these data were used in an IMU error analysis, the results of which were used to predict IMU performance.

 

Any gas bearing is susceptible to degradation of performance by the accumulation of contamination within the gas bearing. The only way to avoid contamination problems is design the gyro with as few contamination sources within the gyro as possible and to assemble clean gyro parts in a clean environment. The Autonetics gas bearings of all types were designed such that no external source of gas was required. This meant that the gas internal to the gyro had to circulate continuously thru the bearing and this guaranteed that any free particulate contamination inside the gyro would end up in the bearing wherever gas flow rates were diminished. A “no-start” event was sure to follow.In the early years of gyro development and beyond, everyone seemed to have their own “secret sauce” gas bearing boundary lube. Such materials were applied to the gas bearing surfaces using methods that were as secret as the applied materials. The gyro engineers at Autonetics all had their favorite boundary lubes. The Autonetics boundary lube that finally won the prize as the most effective was a distilled version of Andy Granitelli’s STP-7 gasoline additive. All of this boundary lube work was done to minimize the chances that a gas bearing would not start the next time the next time. The quality of one’s workday experience seemed to diminish following a “no-start” event.

 

 

 

 

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