J. Seabra

Experimental observations of contact fatigue crack mechanisms for austempered ductile iron (ADI) discs

Abstract Collected from several twin-disc contact fatigue tests, images concerning crack development at ADI surfaces and sub-surfaces are presented. Several auxiliary surface analysis techniques were used (electronic scanning microscopy, video imaging and perfilometry). Defects found at these ADI samples are characterised and related to operating conditions. Elastohydrodinamic theory was used to evaluate surface tribological performance, involving parameters such as lubrication, roughness and general operating conditions. Results concerning a quick procedure to allow the evaluation of fatigue resistance properties among different ADI sample discs are also presented.

Wear and scuffing of austempered ductile iron gears

This paper enhances actual knowledge of the properties of austempered ductile iron (ADI) as a gear material. Results from scuffing tests performed with ADI gears on a FZG test rig are presented and discussed. Contact condition analysis is done using elasto-hydrodynamic theory along with several experimental techniques, as surface electronic scanning microscopy, metallurgical spectroscopy and lubricant ferrographic analysis.

Coefficient of friction in mixed film lubrication: Gears versus twin-discs

Abstract This work compares the friction between two contacting discs with the friction between gear teeth, using a mixed film lubrication model developed for the evaluation of contact pressure and shear stress fields [5]. Such model is used to analyse twin-disc contacts with smooth surfaces and gear teeth contacts with rough surfaces, both lubricated with an additive-free ISO VG 150 mineral oil. Experimental lubricant traction curves [4], measured in a twin-disc machine under full film elastohydrodynamic (EHD) lubrication (Λ > 2), were used to evaluate the lubricant rheological parameters, taking into account the surface roughness of the contacting discs. Experimental power loss measurements, made during gear efficiency tests performed in a Forschungsstelle für Zahnräder und Getriebebau (FZG) test rig [4], were used to evaluate the average friction coefficient between gear teeth along the meshing line, considering the surface roughness of the tooth flanks. The experimental friction coefficients between gear teeth were compared with predictions obtained using three different approaches: (a) values directly extrapolated from the experimental lubricant traction curves, (b) values calculated using a full film EHD lubrication model, considering surface roughness, and (c) values from a mixed film lubrication model. These analysis were performed at three different scales, from the roughness peaks (inside contact) to the entire gear meshing line, passing through the contact dimension. The results obtained show that the mixed film lubrication model is able to evaluate accurately the friction between gear teeth in power transmission equipment whatever the lubrication regime, boundary, mixed or full film.

Gear power loss performance of biodegradable low-toxicity ester-based oils

Environmental issues are leading to a growing interest in bio-lubricants, which can have similar or even better performance than mineral and synthetic oils. In this work, the power loss performance of two biodegradable, low toxicity, ester-based gear oils was evaluated and compared with a commercial mineral oil and an ester-based fluid containing zinc dialkyl dithio phosphate (ZDDP) additives. Power loss tests were performed on the FZG test rig, using type C carburized gears and the different lubricants under evaluation. The operating temperatures of the oil and of the FZG gearbox wall were measured for different values of the input torque and speed and lubricant samples were periodically collected and analysed by direct reading ferrography. At the end of the tests, the gear mass loss and the oil viscosity were measured, the teeth flanks were inspected, looking for typical surface failure mechanisms, and the tooth flank surface roughness was measured. An energetic model of the FZG test gearbox, which took into account the power loss mech-anisms inside the gearbox and the heat flow mechanisms from the gearbox to the surrounding environment, was used to calculate the friction losses between the gear teeth, knowing the oil, gearbox wall, and room temperatures, in steady-state conditions. Using this model, it was poss-ible to analyse the influence of lubricant formulation on the average friction coefficient between gear teeth. The results obtained showed that one of the biodegradable, low-toxicity, ester-based gear oils generated significantly lower mass loss than all the other lubricants, presenting almost no degradation, even when operating at very high temperature for long periods. These bio-lubricants generated friction coefficients between gear teeth up to 27 per cent smaller than the commercial mineral oil, promoted significant reductions of the power loss inside the FZG gearbox, increasing the overall efficiency up to 0.25 per cent.

Scuffing and lubricant film breakdown in FZG gears Part II. New PV scuffing criteria, lubricant and temperature dependent

The analysis of experimental and theoretical results, from constant temperature scuffing tests with FZG type A gears lubricated with base oils, show a good agreement between the scuffing loads and the scuffing lubricant film thickness, meaning that a close relation exists between scuffing failure and lubricant film breakdown, for a base oil tested in the FZG rig. This relation is exploited, associating the classic PV scuffing criteria with the lubricant film thickness in scuffing conditions. Correlating experimental and theoretical results, a more general PV scuffing criteria is developed which depends on the lubricant properties and lubricant temperature. The application of the new scuffing criteria to FZG-A/8.6/90 standard tests predicted the corresponding scuffing loads with excellent accuracy.

Evolution of tooth flank roughness during gear micropitting tests

Purpose – The purpose of this paper is to get a better understanding of roughness evolution and micropitting initiation on the tooth flank, as well as the evolution of surface topography during the test load stages in a modified DGMK short micropitting test procedure.Design/methodology/approach – A modified DGMK short micropitting test procedure was performed, using an increased number of surface observations (three times more) in order to understand the evolution of the surface during each load stage performed. Each of these surface observations consists in the evaluation of surface roughness, surface topography, visual inspection and also weigh measurements as well as lubricant analysis.Findings – This work showed that the larger modifications on surface took place in the beginning of tests, especially during load stage K3 (lowest load, considered as running‐in) and on the first period of load stage K6, that is, during the first 200,000 cycles of the test. The 3D roughness parameters (St and Sv), obtain...

Micropitting performance of mineral and biodegradable ester gear oils

Purpose – This papers aim is to present the gear micropitting performance of two industrial gear oils: a standard mineral lubricant (CM) containing a special micropitting additive package and a biodegradable ester with low toxicity additivation.Design/methodology/approach – Gear micropitting tests were performed on the FZG machine, using type C gears made of case carburized steel. Lubricant samples were collected during the tests for analyzing the wear particles generated during operation. Post‐test analysis included the visual inspection of the teeth flanks and the assessment of the micropitting area, the mass loss of the gear, the ferrometric analysis of the lubricant samples and the surface roughness measurement of the teeth flanks, below and above the pitch line.Findings – The micropitting performance of the two lubricants was very similar, confirming the advantage of using the ester lubricant (CE) as an industrial gear oil, now that it is an environmentally friendly product.Research limitations/impl...

Friction and wear in thrust ball bearings lubricated with biodegradable greases

Thrust ball bearings lubricated with several different types of grease, including three biodegradable low-toxicity greases, were tested in a modified four-ball machine in order to evaluate the bearing friction torque. At the end of each test, grease samples were obtained and analysed through ferrography, allowing the quantification and evaluation of the bearing wear. In order to compare the grease performance in terms of friction and wear, a correlation between the grease characteristics (base oil, bleed oil, thickener structure, and rheological parameters) and the experimental results was established.

Elastohydrodynamic point contacts part II: Influence of surface speeds, surface waviness and load on the contact behaviour

Abstract In the first part of this work, the formulation of elastohydrodynamic (EHD) point contact problems was presented and a numerical solution was proposed. In the second part, two cases are analysed. 1. (1) Heavily loaded EHD point contacts for smooth surface solids. 2. (2) The influence of longitudinal surface waviness on EHD point contact behaviour. Solutions, for the pressure distribution and for the geometry of the lubricant film, are determined as a function of the material properties, the lubricant properties, the load and the surface speeds and waviness.

Power Loss in FZG gears lubricated with industrial gear oils: Biodegradable Ester vs. Mineral oil

ABSTRACT Two industrial gear oils, a reference paraffinic mineral oil with a special additive package for extra protection against micropitting and a biodegradable non-toxic ester, are compared in terms of their power dissipation in gear applications [ [1] , [2] ]. The physical properties, wear properties and chemical contents of the two lubricants are characterized. The viscosity-temperature behaviors are compared to describe the feasible operation temperature range. Standard tests with the Four-Ball machine and the FZG test rig characterize the wear protection properties. Biodegradability and toxicity tests are performed in order to assess the biodegradability and toxicity of the two lubricants. Friction and wear tests have been performed with a configuration that combines rolling/sliding in a line contact simulating the working conditions on gears. The results for the ester oil presented a lower friction coefficient and operating temperature throughout tthe test in relation to mineral oil. Power loss gear tests are performed on the FZG machine using type C gears, for wide ranges of the applied torque and input speed, in order to compare the energetic performance of the two industrial gear oils [ 3 ]. The results of the power loss gear tests show that the operating temperature of the ester oil is always smaller than that of the mineral oil. Lubricant samples are collected during and at the end of the gear tests [ 4 ]. The lubricant samples are analyzed by Direct Reading Ferrography (DR3) in order to evaluate the wear particles concentration (CPUC) and the index of wear particles severity (ISUC). Both parameters indicate that the gear lubricated with the mineral oil suffered more flank tooth wear than the one lubricated with the biodegradable ester. The influence of each lubricant on the friction coefficient between the gear teeth is discussed taking into consideration the operating torque and speed and the stabilized temperature.