Engineering topics
Publications and source records attributed to Kannel, J. W..
Report describes evaluation of dry film lubricants candidate for use in rotary joints of proposed Space Station. Study included experiments and theoretical analyses focused on longevity of sputtered molybdenum disulfide films and ion-plated lead films under conditions partially simulating rolling contact.
A research study was conducted to evaluate dry film lubrication of long life space components such as the alpha and beta joints of the Space Station. The problem addressed in the report pertains to the longevity of sputtered MoS2 or ion plated lead films in a rolling contact environment. A special technique was devised for the experiments, which incorporated a coated ball cyclically loaded against a flat plate. At fixed intervals the surface of the coating was photographed at 100X magnification. By computer scanning the photographs, the rate of coating loss was determined. Experimental variables include load and surface finish of the plate. A theory was developed to analyze the state of stress between ball and flat. The stress condition in the ball apparatus was related to the state of stress under rolling contact conditions. Based on the experiments life appeared to decrease with increasing load and increasing surface roughness. An ion plated lead film gave better life than a sputtered MoS2 film. However, by keeping the interfacial shear stress at a low level, adequate coating life was achieved for either coating. For the lead film, the critical stress was about 0.19 GPa (28 ksi). The study dealt only with mechanical wear. Before a coating is selected for a critical space application, other factors such as reaction with atomic oxygen must also be considered.
Future spacecraft must be designated to operate for very long time periods in space. For example, a target goal for the Space Station is 30 years of operation. Although the actual life may be significantly less than this optimistic goal, the life will certainly be a critical issue in design. The bearings on primary components such as the alpha and beta joints must obviously be designed and lubricated with the objective of optimum performance life. In addition to these joints, there will be numerous other tribological (rubbing or rolling) interfaced that will be required to function for the life of the spacecraft. A major key to adequate performance of tribological interface is proper lubrication. Lubricants can be divided into two basic classes: solid films and liquids. Both types have been used extensively in space applications. Both have advantages and disadvantages that must be carefully considered in their selection. The purpose here is to summarize selection criteria for liquid and solid lubricants applied to long-life spacecraft.
Increases in the size, complexity, and life requirements of satellites and space vehicles have put increasing demands on the lubrication requirements for trouble-free service. Since the development costs of large systems are high, long lives with minimum maintenance are dictated. The Space Station represents the latest level of size and complexity in satellite development; it will be nearly 100 meters in major dimensions and will have a life requirement of thirty years. It will have numerous mechanisms critical to its success, some of which will be exposed to the space environment. Designing long-life lubrication systems and choosing appropriate lubricants for these systems will be necessary for their meeting the requirements and for avoiding failures with associated dependent mechanisms. The purpose of this program was to identify the various critical mechanisms and review their designs during the overall design and development stage so that problem areas could be avoided or minimized prior to the fabrication of hardware. The specific objectives were fourfold: (1) to perform a tribology survey of the Space Station for the purpose of documenting each wear point as to materials involved, environmental conditions, and operating characteristics; (2) to review each wear point (point of relative motion) as to the lubrication used and substrate materials selected in the context of its operating characteristics and the environmental conditions imposed; (3) to make recommendations for improvement in areas where the lubricant chosen and/or where the substrate (materials of the wear couple) are not considered optimum for the application; and (4) to make or recommend simulated or full scale tests in tribological areas where the state-of-the-art is being advanced, in areas where new designs are obviously being employed and a critical review would indicate that problems are a strong possibility, and/or where excessive wear, a malfunction, or excessive leakage would create fluid systems problems or contamination of exposed optical equipment.
Experiments carried out on Batelle's rolling disk apparatus to determine the traction characteristics of polymeric transfer films developed under cryogenic conditions are described. The analyses reveal that the presence of a soft layer can significantly decrease the slope of the traction curves in a bearing. The measurement of low traction indicates the presence of soft layer in the contact zone; if this thin film can be maintained, the bearing stability may be maintained.
A numerical model of cage motion (CAGEDYN) was used to analyze the stability of bearing cages in the Space Shuttle main engine (SSME) high pressure oxygen turbopump (HPOTP). The stability of existing bearing geometries, as well as perturbations of these geometries, was analyzed for various operating conditions. Results of the analyses show that some combinations of operating parameters, exacerbated by the sparse lubrication that exist in the HPOTP bearings, can cause unstable cage oscillations. Frequencies of cage oscillations were predicted by the CAGEDYN numerical model by Fourier analysis of predicted cage motions. Under conditions that cause unstable cage motion, high frequency oscillations were predicted that could cause premature cage failures.
Report presents evaluation of bronze-filled polytetrafluoroethylene (PTFE), known as Salox M, as cage material for ball bearings in high-pressure turbopumps for liquid oxygen. Material evaluated as potentially longer-lived replacement for glass-filled PTFE, known as Armalon. Cage transfers PTFE to balls to form solid lubricant film. However, glass fibers in glass-filled material tend to interfere with transfer. Two cage-design concepts developed; one involves metal-reinforced cage of bronze-filled PTFE; other calls for bronze-filled PTFE inserts in metal structure.
A technique has been developed for modeling solid films in rolling element bearings. For a given bearing geometry an effective viscosity can be calculated for a solid film as a function of the film shear modulus and ball-race friction coefficient. The calculated effective viscosity can subsequently be used as an input to a numerical model of cage motion and stability. Results from a sample calculation of effective viscosity and prediction of cage stability for a turbopump bearing are presented for films of molybdenum disulfide (MoS2) and polytetrafluoroethylene ()PTFE).
The objective of the task was to evaluate the suitability of a bronze-filled polytetra fluoroethylene (Salox M) as the cage material in Space Shuttle Main Engine (SSME) high pressure oxygen turbo pump (HPOTP) bearings. The role of the cage pocket material will be to provide a transferred lubricating interface at the ball-race contact region. A series of experiments was conducted which involved block-on-ring tests (the block was the polytetrafluoroethylene (PTFE) -filled material and the ring was through-hardened 440C steel) and high speed traction tests of two 440C disks with one disk rubbed with a PTFE block to generate a transfer film. Measurements included post test visual observations of the condition of the 440C, wear rate measurements of the blocks, and traction measurements between the disks. It was observed that both Salox M and glass-filled PTFE (Armalon) transferred PTFE to 440C at cryogenic temperatures. Bronze is also transferred to uncoated 440C from the Salox M. At room temperature no PTFE transfer was observed in the high speed disk tests due to severe frictional heating, although bronze transfer still occurred with the bronze-PTFE Salox M material. Since the bearing will operate at cryogenic temperature, transfer films are very probable. Salox M wore slightly, although probably tolerably, more than glass-filled PTFE against 440C. However, Salox M is clearly less abrasive to 440C than is the glass-filled material. When the surface layer of PTFE is depleted from the glass-filled material, the glass fibers tend to seriously abrade the steel. This problem does not occur with Salox M. The surface roughness studies indicate that smooth balls are quite reasonable for transfer films. No significant difference in wear rates of the candidate cage materials was observed when the 440C surface finish was increased from 0.025 micron to 0.1 micron cla. At higher levels of roughness, the wear rate increased. Two surface coatings (MoS2 and TiN) were tested. The surface coatings tended to reduce the Salox M wear rate and reduce bronze transfer. PTFE transfer at cryogenic temperatures still occurred with the surface coatings. Two cage design concepts are presented utilizing Salox M material. One design involves a metal-reinforced Salox M cage, while the second design uses Salox M inserts in a metal structure.
Some of the advances in tribology that have been associated with aerospace mechanisms are discussed. The needs of aerospace have been the dominant forces leading to improvements in understanding and applying tribology technology. In the past two decades improvements in understanding bearing torque, elastohydrodynamic lubrication, lubricant distribution, cage stability, and transfer film lubricants have been made. It is anticipated that further developments will be made in response to future aerospace requirements.
Experiments were performed to determine the effect of cyclic loading on bearing cage strength. A long term working tensile load of approximately 1300 N (300 lbs) was found to be the likely maximum. Higher loads caused a decrease in cage tensile strength after the 125,000 cycle testing period. Poisson's ratio in compression was found to be highly dependent upon the direction of the fiberglass plies. At room temperature the value was 0.15 with the plies and 0.68 across the plies. At -196 C (-321 F), the value with the plies was 0.20. The results of the analyses conducted have again demonstrated the critical need for improved lubrication in the high pressure oxygen turbopump bearings. Lubricant films with low shear strength and low friction coefficients promote cage stability and decrease ball/cage forces during marginal operating conditions. The analysis of the effect of combined bearing loads on ball/cage loads has identified a radial load of 3600 N (800 lbs) as the maximum for the current clearance of the balls and cage pockets. Liquid oxygen impinging on the cage in the direction of rotation was found to enhance cage stability.
Bearings supporting the rotor in the High Pressure Oxygen Turbopump (HPOTP) were examined. The No. 1 bearings from both units were in good condition and had successfully completed 5000 seconds of operation. The No. 2 bearings, which were in service the same length of time, were significantly degraded in the form of ball wear, race pitting, and damage from high axial loads. The No. 3 and 4 bearings were in generally acceptable condition. The general conclusion from the examinations was that improved cooling on the No. 2 bearings and further improvements in controlling axial and radial load would likely result in the HPOTP meeting the qualification test results.
Some aerospace rocket engine applications require that bearings operate at high speeds with cryogenic rocket fuel as the only liquid lubricant. Analyses and field data on these bearings indicate that life would be limited to a few minutes if they operated unlubricated. The extent to which cryogenic fluids are capable of generating elastohydrodynamic films is, therefore, an area of concern and is investigated in this paper. Results from experimental and analytical work at the authors' laboratories indicate sufficient films are not generated to lubricate the bearing reliably. This conclusion is supported by experimental data and analysis which show liquid nitrogen, the cryogenic fluid tested, to be isoviscous.
Problems concerning the bearings in the high pressure oxygen turbopumps (HPOTP) were investigated. The tasks involved: failure analyses, bearing dynamics calculations, lubrication studies, wear studies, and analyses of thermal transients. The radial load system on MSFC's bearing and seal tester used to study components for the HPOTP in liquid oxygen (LOX) is analyzed and the wear behavior of AISI 440C steel with polytetrafluoroethylene (PTFE) lubrication is studied.
The reusable design of the Space Shuttle requires a target life of 7.5 hours for the turbopumps of the Space Shuttle main engine (SSME). This large increase from the few hundred seconds required in single-use rockets has caused various problems with the bearings of the turbopumps. The berings of the high pressure oxygen turbopump (HPOTP) were of particular concern because of wear, spalling, and cage failures at service time well below the required 7.5 hours. Lubrication and wear data were developed for the bearings. Since the HPOTP bearings operate in liquid oxygen, conventional liquid lubricants cannot be applied. Therefore, solid lubricant coatings and lubricant transfer from the polytetrafluorethylene (FTFE) cage were the primary lubrication approaches for the bearings. Measurements were made using liquid nitrogen in a rolling disk machine to determine whether usable elastohydrodynamic films could be generated to assist in the bearing lubrication.
Report presents results of study of bearing distress resulting from malfunction of spring-preloading arrangements. Study examined effect of thermal growth on wear depth of bearing. Report considers bearing-failure modes, relationships between growth and wear, maximum stresses as function of load, and effect of thermal growth on spring-load deflections.
The low service life of the high pressure oxygen turbopump (HPOTP) bearings used in the space shuttle main engine was examined by use of the Battelle "BASDAP' bearing computer stability model. The dynamic instability of the bearing cage resulted in excessive wear and eventual failure of the unit. By maintaining a cage/race clearance of no more than 0.25 millimeters (0.010 inches), ball/pocket clearance of no less than 0.54 millimeters (0.025 inches), dynamic balancing of the cages, and maintaining adequate lubricant films between the balls and races, cage instability and subsequent bearing degradation can be reduced.
A computer model has been developed for evaluating the subsurface stresses incurred within rolling/sliding (elastic) contacts. The model involves first defining the stress tensor at any point (x, y, or z) beneath the surface in terms of the surface stresses. The stress tensors are analyzed to determine the maximum shear stresses and stress reversals. As a result of computations with the model, several observations were made. For example, the maximum reversing shear stresses are on the plane of the orthogonal shear stress. Further, the magnitude of these stresses is not altered by friction. However, under very high friction (typical of dry contact) surface stresses can dominate over subsurface stresses.