Timothy L. Krantz

An Experimental Investigation of the Influence of the Lubricant Viscosity and Additives on Gear Wear

The influence of lubricant viscosity and additives on the average wear rate of spur gear pairs was investigated experimentally. The gear specimens of a comprehensive gear durability test program that made use of seven lubricants covering a range of viscosities were examined to measure gear tooth wear. The measured wear was related to the as-manufactured surface roughness, the elastohydrodynamic film thickness, and the experimentally determined contact fatigue lives of the same specimens. In general, the wear rate was found to be inversely proportional to the viscosity of the lubricant and to the lambda ratio (also sometimes called the specific film thickness). The data also show an exponential trend between the average wear rates and the surface fatigue lives. Lubricants with similar viscosities but differing additives and compositions had somewhat differing gear surface fatigue lives and wear rates.

Increased Surface Fatigue Lives of Spur Gears by Application of a Coating

ABSTRACT Hard coatings have potential for increasing gear surface fatigue lives. Experiments were conducted using gears both with and without a metal-containing, carbon-based coating. The gears were case-carburized AISI 9310 steel spur gears. Some gears were provided with the coating by magnetron sputtering. Lives were evaluated by accelerated life tests. For uncoated gears, all of fifteen tests resulted in fatigue failure before completing 275 million revolutions. For coated gears, eleven of the fourteen tests were suspended with no fatigue failure after 275 million revolutions. The improved life owing to the coating, approximately a six-fold increase, was a statistically significant result. Keywords: Gear, life, fatigue, pitting, coatings. INTRODUCTION The power density of a gearbox is an important consideration for many applications and is especially important for gearboxes used on aircraft. One factor that limits gearbox power density is the need to transmit power for the required number of cycles while avoiding gear surface fatigue failure (micropitting, pitting or spalling). Effective and economical methods for improving surface fatigue lives of gears are therefore highly desirable. Thin hard coatings have potential for improving gear performance. In fact, coatings are reported to have some successful applications [1-3] where product durability improvements have been achieved by the application of thin hard coatings to gears. Diamond-like carbon and related materials have the potential for a wide variety of applications that require wear protection and/or low-friction properties. Because of the widely recognized potential, the deposition methods and resulting properties of the films have been studied extensively [4-6]. Today’s deposition technology allows for the production of a great diversity of coatings, but the ability to tailor the tribological behavior of a coating for a particular application has been elusive. Aerospace gearing requirements are demanding, calling for high power density, long life, and excellent reliability. The low friction properties and high hardness of diamond-like and related coatings offer the possibility to improve the performance of aerospace gearing. Naik, et al [7] tested the adherence and toughness of two coatings using both disk-on-rod rolling-contact and gear tests, and they reported promising results. Alanou, et al [8] found that coatings could increase the scuffing load capacity of rolling and sliding disks used to simulate aerospace gearing contacts, but they also reported poor adherence for one particular substrate and coating combination. Joachim, Kurz and Glatthaar [3] reported promising results of evaluations of tungsten carbon carbide and amorphous boron carbide coatings using laboratory tests, but they also report mixed results when applying such coatings to commercial applications. The purpose of the present investigation was to compare the surface fatigue lives of coated and uncoated gears using accelerated life tests. The testing is considered as accelerated in that the contact stresses used for testing exceeds the stresses used for design of the target application (helicopter gearing). The metal-containing, carbon-based diamond-like (Me-DLC) coating selected for this study was designed specifically for the aerospace gearing applications. NASA/TM—2003-2124631

On the Performance of Thin Hard Coatings for Gearing Applications

Thin hard coatings are possible candidates for the enhancement of scuffing and micro-pitting performance of gears. They exhibit very high levels of surface hardness (typically 1000 HV) combined with a low traction coefficient (typically 0.2) against dry steel. The paper presents the results of an experimental program carried out in order to assess the scuffing performance of thin hard coatings for various combinations of substrates and surface finish under realistic engineering conditions of sliding speed, oil temperature and contact pressure. Two types of coating were assessed: diamond like carbon (DLC) coatings and boron carbide coatings. Although the overall performance improvement is promising, caution may be required when using thin hard coatings with case carburised substrates where adhesion problems may occur. Their use with nitrided steel as a substrate seems to be particularly advantageous, possibly because a higher process temperature may be used. The coatings tested also gave better performance at relatively low sliding speeds (≤ 16 m/s). At higher levels of sliding the predominant effect is one of lubrication failure, the material parameter becoming secondary.

Pitting and Bending Fatigue Evaluations of a New Case-Carburized Gear Steel

The power density of a gearbox is an important consideration for many applications and is especially important for gearboxes used on aircraft. One approach to improving power density of gearing is to improve the steel properties by design of the alloy. The alloy tested in this work was designed to be case-carburized with surface hardness of Rockwell C66 after hardening. Test gear performance was evaluated using surface fatigue tests and single-tooth bending fatigue tests. The performance of gears made from the new alloy was compared to the performance of gears made from two alloys currently used for aviation gearing. The new alloy exhibited significantly better performance in surface fatigue testing, demonstrating the value of the improved properties in the case layer. However, the alloy exhibited lesser performance in single-tooth bending fatigue testing. The fracture toughness of the tested gears was insufficient for use in aircraft applications as judged by the behavior exhibited during the single tooth bending tests. This study quantified the performance of the new alloy and has provided guidance for the design and development of next generation gear steels.

Investigation of Low-Cycle Bending Fatigue of AISI 9310 Steel Spur Gears

An investigation of the low-cycle bending fatigue of spur gears made from AISI 9310 gear steel was completed. Tests were conducted using the single-tooth bending method to achieve crack initiation and propagation. Tests were conducted on spur gears in a fatigue test machine using a dedicated gear test fixture. Test loads were applied at the highest point of single tooth contact. Gear bending stresses for a given testing load were calculated using a linear-elastic finite element model. Test data were accumulated from 1/4 cycle to several thousand cycles depending on the test stress level. The relationship of stress and cycles for crack initiation was found to be semi-logarithmic. The relationship of stress and cycles for crack propagation was found to be linear. For the range of loads investigated, the crack propagation phase is related to the level of load being applied. Very high loads have comparable crack initiation and propagation times whereas lower loads can have a much smaller number of cycles for crack propagation cycles as compared to crack initiation.

Feasibility Study of Vapor-Mist Phase Reaction Lubrication using a Thioether Liquid

Aerospace drive systems are required to survive a loss-of-lubrication test for qualification. In many cases emergency lubrication systems need to be designed and utilized to permit the drive system to pass this difficult requirement. The weight of emergency systems can adversely affect the mission capabilities of the aircraft. The possibility to reduce the emergency system weight using vapor-mist phase lubrication (VMPL) technology has been considered by NASA and the Army Research Laboratory (ARL). Phosphate esters have been the lubricant of choice in most VMPL studies primarily because they do provide adequate lubrication for short periods of time. However, during the lubrication process, the phosphate esters react continuously with the surface iron in gears and bearings, resulting in excessive wear. To minimize this problem an alternative non-phosphate liquid, a thioether, was used to mist phase lubricate a spur gearbox rig operating at 10,000 rpm under highly loaded conditions. After 21 million shaft revolutions of operation the gears exhibited only minor wear.

Wear of Spur Gears Having a Dithering Motion and Lubricated With a Perfluorinated Polyether Grease

Abstract Gear contact surface wear is one of the important failure modes for gear systems. Dedicated experiments are required to enable precise evaluations of gear wear for a particular application. The application of interest for this study required evaluation of wear of gears lubricated with a grade 2 perfluorinated polyether grease and having a dithering (rotation reversal) motion. Experiments were conducted using spur gears made from AISI 9310 steel. Wear was measured using a profilometer at test intervals encompassing 10,000 to 80,000 cycles of dithering motion. The test load level was 1.1 GPa maximum Hertz contact stress at the pitch-line. The trend of total wear as a function of test cycles was linear, and the wear depth rate was approximately 1.2 nm maximum wear depth per gear dithering cycle. The observed wear rate was about 600 times greater than the wear rate for the same gears operated at high speed and lubricated with oil. Introduction Gear contact surface wear is one of the important failure modes in gear systems. Wear and the associated material loss can lead to structural failure (gear tooth fracture). Wear can also lead to changes in vibration and noise behavior (ref. 1 to 3). In addition, wear can change the patterns of gear contact such that the altered load distributions and contact stresses will accelerate the occurrence of other failure modes such as pitting and scoring (ref. 4). Gear wear debris can also be detrimental to the performance of bearings or other components of a drive system (ref. 5). The study of wear is becoming one of the emerging areas of gear research. A number of recent wear modeling efforts (refs. 6 to 10) form a solid foundation for studying gear wear. The common thread to these studies is that all use the well-known Archard’s wear model (ref. 11) in conjunction with a gear contact model and relative sliding calculations. Archard’s wear equation can be expressed for a local point on one of the contacting gear surfaces as

Efficiency study comparing two helicopter planetary reduction stages

Abstract : A study was conducted to compare the efficiency of two helicopter transmission planetary reduction stages. Experimental measurements and analytical predictions were made. The analytical predicted and experiments verified that one planetary stage was a more efficient design due to the type of planet bearing used in the stage. The effects of torque, speed, lubricant type, and lubricant temperature on planetary efficiency are discussed. Keywords: Gears, Transmission, Efficiency, Power loss.

Grease Degradation in Critical Helicopter Drivetrain Bearings

An investigation of critical aviation bearings lubricated with MIL-PRF-81322 grease was conducted to derive an understanding of the mechanisms of grease degradation and the loss of lubrication over time. Chemical analysis was performed on grease samples from fielded bearings and compared to fresh grease and samples taken from bearings run for extended times in a laboratory environment. Size exclusion chromatography and Fourier transform infrared spectroscopy were used to investigate the condition of the grease, and evidence of additive depletion, oil evaporation, and thickener degradation were seen, consistent with results reported by other authors. Given the relatively light loading conditions experienced by the test bearings, they were able to continue operating at high temperature despite having most of the original oil depleted from the grease.Copyright