Porous Polyimide Retainer Materials: Meldin™9000 and Minapore™
In 1970, CSDL began work with Dixon Corporation on the U.S. Air Force sponsored research project. Meldin™ 9000 was the resulting porous polyimide. By 1980 period it was qualified for use on a few United States military programs as a result of the product’s successfully completing 3,000 hours of life testing at Customer’s testing facilities. In 1980, Singer-Kearfott, together with Dixon Corporation, began work on the U.S. Navy sponsored Program to develop a stronger, more easily machinable Meldin™ 9000 product that would also have tighter controls on the pore size and volume. In order to achieve these results, Dixon went back to the basic chemistry of the process and made several significant modifications in the material workup.
In 1987, the Material Development group at Dixon began an exploratory project to evaluate materials and process techniques that would lead to a commercial porous bearing product. Timken Super Precision and Dixon both felt that an industry need existed for a less expensive retainer material than the Meldin™ 9000 with performance superior to the presently employed composites, the uses to be in non-critical products and for commercial applications.
A shopping list of desirable properties was drawn up to provide for a focus for what we hoped to achieve from the materials evaluated. These characteristics included the following:
In 1992, after reviewing dozens of different possible candidate resins and varying combinations it was again determined that polyimide is the best choice. The resultant product of this effort is named Meldin™ 8100, Minapore™. It became necessary to innovate our approach to manufacturing and later the test parameters for the product in order to achieve the primary objective of the project; that of producing a lower cost alternative to the materials presently used.
Minapore™ its properties and performance
Miniature bearings have many retainer types historically available for the designer to use. Pressed stainless steel ribbon and “C” type retainers, molded plastic retainers, a category of oil reservoir types, phenolic and Meldin™ 9000. This last category was felt to need a third choice. Graphically speaking, on one end of the cost scale, we have the very mature use of phenolic retainers, which are very economical and can hold 1-4% of oil by weight. But suffering from relatively high cost causes it to be used only in the more sophisticated applications. The other parameter represented on the scale is propensity for debris generation. Phenolic retainers suffer from tendency towards particle generation, just due to the nature of the two phase material. Meldin™ 9000, a single phase, homogeneous material can be processed such that the likelihood of foreign material and particle shedding is eliminated. All bearings that run by necessity with very small amounts of lubricant in the ball race interface for torque considerations, cry out for life extension which through-porous, high oil retention materials could offer.
Dixon Industries and Timken Super Precision have jointly pursued the search for an economical through-porous plastic that would fill this gap. This paper endeavors to report the above properties of the evolved, commercially processed material, demonstrating its application to miniature bearing retainer needs and compares ring tensile strengths, oil retention ability, oil bleed rate propensity, cleanliness compared to finished phenolic, and finally data will be described that shows improvement in ball bearing performance in a typical gyro application.
Ring tensile strength
The data in Figure 1 illustrates the ring tensile strength in pounds per square inch measured using the procedure outlined in the IBWG porous polyimide military specification for Meldin™ 9000. The tensile strength results are plotted against the total pore volume in cc/gram and illustrates the (suspected) relationship of strength to percentage of holes. The salient point is that the commercial product has been tailored in terms of pore volume to give a minimum ring tensile strength of 2700 lbs per square inch corresponding to a pore volume maximum of approximately .18cc/gram. The new process for making this material was tailored to increase the strength over Meldin 9000 by shifting the total pore volume down below that normally obtained in Meldin™ 9000. The result is minimum ring tensile strength of 2700 PSI where Meldin™ 9000 is 2000 PSI. This increase was felt to be advantageous for commercial applications.
Figure 1: Ring Tensile Strength
Figure 2: Graph of Oil Retention versus Pore Volume
For the commercial product, a balance between tensile strength and oil retention was reached. 8% minimum oil retention was chosen and controls the minimum point of pore volume. Measuring and controlling oil retention in a standardized manner, one can guarantee getting pore volumes bigger than .14-.15 cc/gram, thus getting more than twice as much oil for bearing life as compared to phenolic retainers.
Interest in other types of oil has lead to preliminary data illustrated in Figure 2, that heavy oils such as Bray 815Z, are retained in the retainer to the same volumetric extent and because of their molecular weight, result in much larger oil retention percentage numbers.
Oil Bleed Rate
Figure 3. Oil Bleed Rate – Mil-L-6085 Oil
Polyimide material is inherently clean, as shown in a commonly used test. This test involves a simple procedure using a hot water ultrasonic process on a sample batch of phenolic yields, even after good cleaning techniques, still a slight amount of particles being removed from the retainer surface. In contrast, the Minapore™ retainer, after proper washing and being subjected to this test, shows absolutely no regeneration of particles due to the ultrasonic energy. This rather qualitative result has been quite typical over the years of Meldin™ 9000 and it is not surprising that the nature of the Minapore™ material is very similar. Many lots of Minapore™ have been tested with identical results.
Northrop Corporation in Norwood, Massachusetts has quantified this kind of data by passing a carrier liquid which has been exposed to the retainers through a laser particle counter. Of particular interest is the very dramatic improvement in particle sizes bigger than 15 microns, which must be fairly substantial chunks of resin or cotton breaking off the retainer.
Chemical and Thermal Stability
Thermographic analysis was used to measure the stability of Minapore™ material by Northrop. The weight loss vs. temperature in nitrogen was measured. The results indicate phenolic starting to lose weight in the 250 degree C region whereas the polyimide Minapore™ material was stable up through 640 degree C, at which time the test was stopped, and only 1% loss was recorded.
Bearing performance degradation is the measure of life in many sophisticated miniature bearing applications. Testing in a customer’s actual system and measuring parameters significant to the customer’s is the true test. Northrop Corporation of Norwood, Massachusetts has been working with Minapore™ retainers in gyros and shares data. It represents a gyro parameter, drift versus time, as an indication of gyro bearing torque perturbations. (Figure 4).Northrop engineers feel that the inherent cleanliness and extra reserve of lubricant contribute to continued accepted drift performance in Minapore™ retainers.
Figure 4: Minapore™ and Phenolic Retainer Drift
The purpose of this work was to show the basic parameters that produce a superior oil retaining material for miniature bearing retainers. These parameters, strength, oil retention, bleed rate, shedding propensity and stability have been compared to Meldin™ 9000 and phenolic.
Please note that there are size limitations to porous polyimide due to the available rod stock. Please contact our sales representatives to determine if a retainer availability.
The authors would like to thank Mike Leary of the Dixon Facility, Furon Company, Dana Caldwell of Timken Super Precision, Tony Messa, formerly of Dixon Industries, and Jim Gingrich and Mark Schwartz of the Precision Products Division of Northrop Corporation, for their contributions and cooperation.