Start-stop Cycles Research Articles

Pole tip recession studies of thin-film rigid disk head sliders II. Effects of air bearing surface and pole tip region designs and carbon coating

Pole tip recession (PTR) results in an increase in spacing between recording elements and magnetic media which is undesirable for high-density recording. The effects of air bearing surface (ABS) designs, pole tip region designs and carbon coating on the growth of PTR with increasing start-stop cycles have been investigated. A better ABS design for less PTR should reduce contact time between slider and disk during take-off and landing of the slider. Among the two-rail, tail-dragger and negative pressure sliders, the negative pressure slider has shortest contact time during take-off and landing, followed by the two-rail slider and tail-dragger slider. The growth mechanisms of PTR for two-rail slider with or without U-shaped slot consists of knocking-off protruded poles, generation of wear particles and three-body abrasive wear. The PTR reaches a saturated value after a certain start-stop cycles. However, for the tail-dragger slider with square-shaped slot, there is a fourth step of erosion which leads to no saturation of PTR and a relatively rough surface of poles. Mechanisms of PTR for negative pressure slider is similar to the two-rail slider but with a lower saturated PTR value. Carbon coating minimizes or eliminates the PTR growth through reducing static and kinetic friction during take-off and landing, providing a wear-resistant protective layer and eliminating dissimilarities of ABS, thin film structure and poles. Based on this study, we find that negative pressure slider which minimizes contact at head-disk interface and carbon coating which protects thin-film region are desirable alternatives for slider designs. Initial recession is desirable as compared to protrusion.

Effects of Blade Bending on Aerodynamic Control of Fluctuating Loads on Teetered HAWT Rotors

Active aerodynamic control, in the form of closed-loop actuation of blade-tip ailerons or all-moveable blades, is investigated as a means of increasing the structural fatigue life of HAWT rotors. The rotor considered is upwind and teetered, with two blades of diameter 29.2 m., fiberglass construction and other properties representative of modern light-weight construction. The paper begins with a review of prior work which studied the problem for an essentially rigid structure. For that and the present research, two loading conditions were invoked: exposure to a Rayleigh distribution of operating winds with vertical shear and a 15 percent superimposed spectrum of turbulence; and occasional exposure to 62 m/s hurricanes. Accounted for herein is the effect of flatwise bending flexibility on the loads spectra of root flatwise bending moment, thrust, and torque (both open loop and closed loop). Using Miner’s rule, the moments are converted to fatigue lives. With aerodynamic control, RMS flatwise moments for the flexible blade in turbulence are found to be less than 1/2 of those without control. At a fixed blade weight of 540 kg when hurricane loads are added, the aileron-controlled blade is “designed” by that limit-load condition. In contrast, the all-moveable blade can be feather controlled in the high wind so that its life is dominated by turbulent loads. Simplified fatigue analysis permits weight reductions to be estimated which yield controlled blades capable of 30 years’ operation with a safety factor of 11. The resulting weights are about 400 kg for the aileron-controlled blade, and 230 kg for the all-moveable blade. However, such light-weight rotors require attention to other design considerations, such as start-stop cycles. Apart from limit loads, the methods of analysis in this paper are linearized (locally for aerodynamic loads). It follows that the results are more likely to be meaningful in terms of comparative, rather than absolute, values of fatigue life and weight.

Estimating the viability of a tension bolt in a gas turbine

An estimate has been made of the crack growth period in an M200 x 4 mm coupling bolt made of 26KhN3M2FA steel in a gas turbine operating at 293–523 K under fewcycle loading arising from thermal expansion of the disks together with the dimensions of nonpropagating fatigue cracks under many-cycle loading due to vibration. The critical ring crack depth in a 26KhN3M2FA steel bolt is 26 mm at a stress of 320 MPa. The crack develops to a critical size from a depth of 10 mm during 30,000 start-stop cycles.

Analysis of Cracked Gas Turbine Blades

Results from an analysis of cracked first-stage blades (or buckets) from two General Electric MS7001E industrial/electric utility gas turbines are presented. Numerous cracks were observed along the leading-edge and midchord regions of the pressure and suction surfaces. In one unit cracks were found after 874 start-stop cycles, which included 218 trips from load and 11,000 service hours. Buckets in the sister engine were examined after 1800 cycles, which included 218 trips from load and 24,000 service hours. In both cases, cracks initiated in the platinum aluminide coating and propagated into the IN-738LC base metal. For the 11,000-hour bucket, 20-mil (0.5-mm) deep cracks were observed, and for the 24,000-hour bucket, the leading edge cracks had grown to the leading edge cooling hole, a distance of 0.2 in. (5 mm). The number of cycles to crack initiation was in good agreement with thermal mechanical fatigue (TMF) predictions from the REMLIF computer program, which is part of the Electric Power Research Institute’s (EPRI) Life Management System. The cracking was greatly accelerated by the large number of trips experienced. The extensive crack propagation that occurred is thought to have been strongly assisted by oxygen and sulfur penetration along the grain boundaries. The coating on the leading edge degraded from the original platinum-aluminide plus beta phases to a gamma prime phase after 24,000 hours of service, but it was still protective except where it was cracked. Where the coating was cracked, environmental attack of the interdiffusion zone and base metal occurred, resulting in spallation of the coating and preferential grain boundary attack. Operating and maintenance considerations for optimizing bucket life in demanding cyclic duty environments are also discussed.

Head flying characteristics on canasite disks

The isotropic texture on canasite surface is generated intrinsically by polishing due to the differential hardness of crystalline and amorphous phases. Consequently, excellent frictional and gliding properties are achieved on lubricated canasite disks at remarkably low roughness. Lower disturbances and fewer impacts of head flying over a canasite disk are observed by using an acoustic emission sensor. The contact-start-stop testing of canasite disks shows a pattern without continuous increase in friction. Another feature of canasite disks is the lower take-off speed during start-stop cycles. This translates to a lower power consumption and less wear for the disk drive.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Tribological composition optimization of chromium-carbide-based solid lubricant coatings for foil gas bearings at temperatures to 650 °C

In this paper a test program is described to determine the tribologically optimum composition of chromium-carbide-based solid lubricant coatings using a foil gas bearing test apparatus. The coatings contain a wear-resistant chromium carbide “base stock” with the lubricant additives silver and BaF 2-CaF 2 eutectic. The optimum amounts of each constituent were determined in a previous study using a pin-on-disk test apparatus. The coating composition optimization is repeated in this study for air-lubricated foil gas bearings from 25 to 650 °C. The various coating compositions were prepared by powder blending, then plasma sprayed onto Inconel 718 test journals and diamond ground to the desired coating thickness and surface finish. The journals were operated against preoxidized Ni-Cr alloy foils. The test bearings were subjected to repeated start-stop cycles under a bearing unit of 14 kPa (2 lbf in -2). Sliding contact between the coated journal and the smooth foil occurs during bearing start-up before lift-off or hydrodynamic lubrication by the air film is achieved and during bearing coast-down. The bearings were tested for 9000 start-stop cycles or until specimen wear reached a predetermined failure level. In general, the addition of silver and eutectic to the chromium carbide “base stock” significantly reduced foil wear and increased journal coating wear. The optimum coating composition, PS212 (70 wt.% metal-bonded Cr 3C 2, 15 wt.% Ag, 15 wt.% BaF 2-CaF 2 eutectic), reduced foil wear by a factor of 2 and gave rise to coating wear well within acceptable limits.

Raman spectroscopy of disk coatings in a working magnetic disk drive

Raman spectroscopy is a very sensitive indicator of the structure and history of hard carbon films which have been recently used as wear-resistant coatings in thin film magnetic disk drives. In this first application of Raman spectroscopy to a working disk drive we obtain a sensitivity of 0.5 nm in the thickness of a carbon layer with respect to spatial variations. To within this sensitivity, no wear is detected in the slider track after 43000 start-stop cycles. The effect of laser heating is observed on the carbon overlayer while monitoring the disk temperature through the Raman spectrum.

動圧気体軸受面へのスパッタリング (第2報)

MoS2 dry lubricant film was deposited onto the surface of self-acting gas bearings by sputtering technique to improve the frictional properties for start-stop conditions. The sputtered MoS2 films (0.5-3 μm) were deposited on highly-finished metal surfaces. The low-speed sliding test shows that MoS2 films have a low coefficient of friction. To evaluate their lubricating properties for start and stop conditions, MoS2 films were deposited on spiral grooved thrust gas bearings (flatness of bearing surface 1 μm). Bearing coated with MoS2 film of more than 2 μm thickness could bear over 250 start-stop cycles. The frictional coefficient μ (calculated value from measured torque) of the MoS2 film coated bearings for start and stop conditions reached μ=0.3. Although the TiN film reported by the 1st report served mainly for surface hardening and prevention of surface damage, but the MoS2 film reported here was useful for improving the frictional property for start and stop conditions, particularly to decrease the starting torque. For practical use, TiN and MoS2 films were deposited onto the surface of self-acting gas bearings of a centrifugal helium compressor which operates in temperature about 80 K. These films showed the good friction and wear properties in low temperature operation.

Development of Surface Coatings for Air-Lubricated, Compliant Journal Bearings to 650°C

Surface coatings for an air-lubricated, compliant journal for an automotive gas turbine engine were tested to find those capable of withstanding temperatures of either 540°C (1000°F) or 650°C (1200°F). Also, the coatings have to be capable of surviving the start-stop sliding contact cycles prior to rotor lift-off and at touchdown. Selected coating combinations were tested in start-stop tests at 14 kPa (2 psi) loading for 2000 cycles at room and maximum temperatures. Specific coating recommendations are: Cdo and graphite on foil versus chrome carbide on journal up to 370°C (700°F); NASA PS-120 (Tribaloy 400, silver, and CaF2) on journal versus uncoated foil up to 540°C (1000°F); and chemcially adherent Cr2O3 on journal and foil up to 650°C (1200°F). The chemically adherent Cr2O3 coating system was further tested successfully at 35 kPa (5 psi) loading for 2000 start-stop cycles.

Development of a hybrid flywheel/battery drive system for electric vehicle applications

Today's electric vehicles are severely limited in multistop and go driving range and accelerating capability by the lead acid battery's inability to handle high power peaks while maintaining maximum energy storage capability. A hybrid flywheel/battery system can be used to isolate the battery from the accelerating power peaks, and should recover a substantial part of the braking energy. This paper describes the development of a small, high speed, lightweight flywheel/ ac synchronous motor alternator sealed energy storage package coupled into the battery and dc drive motor system through a simple rectifier/inverter power circuit. This system stores just enough energy in the rotor of the machine for one start-stop cycle. Provision is made to add a flywheel to store energy for several cycles, or enough energy for climbing or descending long grades. The fields of the two machines are electronically controlled to achieve optimum performance and effective energy utilization.

Gas-Lubricated Foil Bearings for High Speed Turboalternator—Construction and Performance

Foil bearings were designed and fabricated to replace pivoted-shoe journal bearings in a Brayton cycle turboalternator, within space limitations and constraints imposed by the existing machine. The foil bearings were integrated into a unified assembly with the rotor, housing, seals, and gimbal-mounted thrust bearing, without changes and modifications of machine components other than the journal bearings. The gas-lubricated foil bearings, which require no external pressure-source, furnished a stable support for a 21.9 pound rotor in the vertical attitude at speeds to 43,200 rpm. Excellent wipe-wear characteristics permitted well over 1000 start-stop cycles, without deterioration of performance in the entire speed range. The paper reviews salient aspects of design, fabrication, and performance. An account is given of rotor dynamics during starting, stopping, and traversing the region of resonances. The state of journal and foil surfaces is examined following intensive start-stop cycling and high-speed runs over extended periods of time.

Hydrostatic Bearings for Cryogenic Rocket Engine Turbopumps

This paper summarizes an analytic and experimental investigation of hydrostatic bearings for use in high-pressure cryogenic rocket engine turbopumps. A hydrostatic bearing analysis was developed to predict bearing steady-state and time-dependent (stability) performance. The analysis accounts for the effects of turbulence, inertia, and compressibility in the fluid film. Bearing design charts were generated from the analysis to permit rapid prediction of bearing performance and to aid in the selection of optimum bearing configuration. Liquid hydrogen and liquid oxygen turbopump conceptual designs were prepared and the designs were used to establish hydrostatic bearing size and performance requirements. The turbopump designs included provision for operation when bearing supply pressure is insufficient to support the rotor. Two bearing designs, one LH2 and one LO2, were fabricated and tested in a test rig. Test bearing designs met turbopump requirements and included the turbopump start-stop rotor support provision. Experimental data for both test programs agreed well with steady-state predictions and the bearings were stable under all conditions. Both test bearing designs were in excellent mechanical condition after testing, which included 10 simulated turbopump start-stop cycles.