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DEVELOPMENT OF HIGH RELIABILITY DYNAMIC SEALS

Objective


2 computer software packages were developed to model seal dynamics - one more sophisticated, and the other quicker and easier to use. Associated experimental test rig work was done for validation of the software and to provide insights into aspects of interfacial behaviour in mechanical seals. Of particular interest are seal face isotherm maps produced by thermal imaging combined with computer based signal processing.

Techniques used for testing seals included electrical testing of interfacial pressures and thicknesses; fast response thermocouples for transient interface temperature measurement; thermal imaging for mapping interface temperature distributions; optical inferometry for film thickness observation; optical endoscopy for the study of flow behaviour near the sealing contact; electrical detection of incipient of rolling bearings.

For reciprocating rubber seals, high speed thermocouples gave valuable insight into transient interface conditions. Pressure gradient variation was found to be particularly important. Findings from short experimental tests were applied in long term performance tests simulating service conditions with real seals. Water, emulsions and oils were covered.

Seal surface texture and effects of surfactants were given particular attention. Surfactant treatments were investigated for their ability to stabilise interface films. They were found to improve seal performance in rubber water steel and carbon water ferrous metal systems.

A systematic study was made of bearing seal ingress mechanisms in wet, dirty conditions such as conveyor roller duties. Flow visualisation and an accelerated testing technique proved very effective, showing the poor performance of existing seal designs. The work led to an improved dust seal concept.

The research program involved sophisticated instrumentation techniques to permit measurements and observations on the micrometre scale films responsible for the lubrication of sliding surfaces in dynamic seals. Much of this has been specially developed or adapted.
Fast response surface thermocouples in nickel/nickel chromium have been developed for transient temperature measurement in interface contacts. These have a response in the millisecond range and are used to measure the axial distribution of temperature across reciprocating rubber seals.
For measurements of the distribution of high interfacial film pressures under fluid power reciprocating seals a manganin wire transducer was used while for lower pressures inrotar mechanical seal films a piezoelectric transducer was used. A frequency modulated (FM) capacitance based probe was used for film thickness measurement.
Thermal imaging produced a valuable insight into the distribution of temperature in the interface of a mechanical seal.
Optical interferometry was used to study fluid films in reciprocating rubber contacts, to investigate film coherence, to investigate the effects of surface active additives on fluid film stability and to investigate surface texture effects.
Optical endoscopy was used to study flow behaviour in the immediate vecinity of the sealing contact for rotary seals in contaminated environments. An endoscope was positioned under the seal in a glass shaft and the behaviour of liquid and solid contaminants was recorded using a video camera.
The electrical detection of incipient failure of roller bearings was a technique to solve the problem of determining the time at which a significant amount of contaminant ingress has occurred. The contaminant causes a measurable change in the total capacitance between the balls, cages and tracks of the bearing. This is recorded continuously thus indicating when the seal can be deemed to have failed.
Fundamental effects have been related to seal perform ance not only in short term tribological experiments but also in long term performance on test rigs simulating real service conditions.

Particular attention has been given to optimizing materials for the sliding seal faces, both polymeric and ceramic.
A particular problem with water lubricated rubber seals running against metal counterfaces is the unstable character of the very thin water films. The film can often be stabilized by compounding a surfactant into the rubber. Various surfactants have been tried, most improve seal performance, the best produce a significant improvement in seal performance without undue degradation of the other physical properties of the rubber.
A conventional low duty material combination of resin impregnated carbon sliding against nickel-resist had both faces treated with a propriety boundary lubricant. When the same faces were compared before and after treatment, the treated samples showed a signicant improvement in running temperature of the seal faces.
Various mechanical seal materials were also examined including carbons with 1 of 4 different impregnants (silver, babbitt, lead and antimony, antimony) which were run against chromium plated copper. Other hard face materials used were tungsten carbide, silicon carbide and silicon carbide plus graphite.

The effects of the surface finish of mechanical seals has been investigated. Surface roughness effects for impregnated carbons run against plated counterfaces have been studied to determine the effect of roughness variation on leak rate.
The effects of details of the sealing face microgeometry on the tribology and performance of reciprocating seals was given particular attention leading to a better understanding of design requirements for improved performance. A sinusoidal profile on the sliding surface of a reciprocating rubber seal can significantly reduce the sliding friction level.
In the case of reciprocating rubber seals the effects of the fine detail of the seal surface face axial profile shows up in the measured interface film pressure profile. Interpretation of such information has lead to a more rational basis for seal design.
Data from high speed thermcouples provide insight into the lubrication conditions at any point in time and by integrating the temperature profile they provide a direct measure of the thermal load on the seal. This approach avoids the problems associated with measuring the friction of a single seal.
IT IS VERY IMPORTANT TO BE ABLE TO DETECT CRITICAL DEFECTS IN ADVANCED CERAMICS I.E. DEFECTS WHICH ARE LIKELY TO BRING ON RUPTURE OF A PIECE IN SERVICE.
AVAILABLE NON-DESTRUCTIVE TESTING METHODS ARE NOT REALLY ADAPTED FOR ADVANCED CERAMICS QUALITY CONTROL. SO OTHER TECHNIQUES ARE TO BE DEVELOPED AND TESTED.
DEFECTS IN ADVANCED CERAMICS ARE OFTEN VERY SMALL (SOME MICRONS TO A FEW HUNDRED MICRONS). MOREOVER, THEIR SITUATION IN REAL PIECES WITH COMPLEX SHAPES DOES NOT PERMIT AN EASY CONTROL WITH CLASSICAL NON-DESTRUCTIVE EVLUATION SYSTEMS.
SOPHISTICATED TECHNIQUES LIKE HIGH-FREQUENCY ULTRASONICS AND X-RAY METHODS EXIST BUT ARE NOT APPLICABLE IN INDUSTRY, BECAUSE OF THEIR PRICE AND TEST DURATION.

ONE SOLUTION COULD BE MICROWAVE TECHNIQUES, WITH FREQUENCIES HIGHER THAN 25-30 GHZ
THE FIRST MAIN OBJECTIVE OF THIS PROGRAMME IS TO VERIFY THE FEASIBILITY OF THE NON-DESTRUCTIVE TESTING OF ADVANCED CERAMICS BY MICROWAVE TECHNIQUES.
SAMPLES WITH AND WITHOUT ARTIFICIAL DEFECTS ARE TO BE PREPARED BY TWO DIFFERENT PRODUCERS, WITH DIFFERENT PRODUCTS NATURE AND FABRICATION PROCESS. THE SAMPLES ARE TO BE FIRST CHARACTERIZED BY CLASSICAL MEANS OF PHYSICAL, MECHANICAL AND MICROSTRUCTURAL CHARACTERIZATION, AND BY X-RAY TOMOGRAPHY AND HIGH FREQUENCY ULTRASONICS.

AFTER THAT, MATERIALS DIELECTRIC PROPERTIES ARE TO BE MEASURED BY DIFFERENT MICROWAVE MEANS.
AFTER COMPARISON OF CLASSICAL AND DIELECTRIC CHARACTERIZATIONS, THE SECOND MAIN OBJECTIVE OF THIS PROGRAMME IS TO DESIGN, BUILD AND TEST AN EXPERIMENTAL NON-DESTRUCTIVE EVALUATION (NDE) MICROWAVE DEVICE ADAPTED FOR MANUFACTURED ADVANCED CERAMIC PARTS. THIS DEVICE MUST BE ABLE TO DETECT AND DETERMINE CRITICAL DEFECTS SITUATED IN CRITICAL ZONES OF REAL CERAMIC PARTS WITH A SIZE OF AIMED DEFECTS OF ABOUT ONE OR TWO HUNDRED MICRONS.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Compagnies Générale d'Automatisme CGA
Address
12 Rue De Baume
75008 Paris
France

Participants (2)

British Hydromechanics Research GroupLtd.
United Kingdom
Address
Cranfield
MK43 0AJ Cranfield - Bedfordshire
Martin Merkel GmbH & Co KG
Germany
Address
Sanitasstraße 17-21
21107 Hamburg