FZG Test Rig Research Articles

A New System for Testing Gears Under Variable Torque and Speed

In many applications, the transmitted power by a gearbox is fluctuating strongly. For example, in wind energy converters the rotor torque depends on the wind velocity and therefore varyies over a wide range, whereas the rotor speed is constant. In automotive applications, both torque and speed vary according to the driving conditions. It is evident, that in order to test such gearboxes, test rigs are needed that enable torque and speed to vary during the test according to the testing requirements and simulate the actual loading conditions as accurately as possible. The first part of this paper is a review of the many known mechanical devices developed to load gears. Their advantages and disadvantages are commented and presented. The second part describes a novel system designed and built by the authors (patent pending), which makes it possible to apply the test torque and speed according to a given load pattern during the test of the gears. It can further be implemented in close power loop test rigs (known also as back-to-back test rigs) such as the FZG test rig. The gearboxes of the test rig may be similar enabling thus the measurements of the power loss and efficiency of the gear pairs. It can also be used in the research of fatigue related surface failures, such as pittings and micro-pittings. The system consists mainly of a self-locking planetary Wolfrom gear train. Its sun gear is connected to a high starting torque electric motor, while its two ring gears are connected to the shafts ends of the two gearboxes. The operation of the motor is controlled by a computer. The software compares the actually applied torque, which is measured by a contactless torque meter, with the required torque, which has been already prescribed. Keywords: Back-to-back gear test rig, gear testing machine, FZG test rig, Ryder gear machine, &, IAE test machine

Power loss performance of high pressure nitrided steel gears

Environmental issues are leading to continuous search for more efficient solutions; in this work the influence of high pressure nitrided steel gears in combination with a bio-lubricant was analysed. So, power loss tests were performed on the FZG test rig, using carburized (C) and high pressure nitrided (N) steel gears and two different lubricants. The operating temperatures of the oil and of the FZG gearbox wall were measured for different values of the input torque and speed. At the end of the tests, the gear mass loss was measured, and the tooth flanks were inspected, looking for typical surface failure mechanisms. Using an energetic model of the FZG test gearbox it was possible to quantify the influence of lubricant formulation and gear materials on the average friction coefficient between gear teeth. The tests results showed that for the conditions tested the high pressure nitrided steel gear had lower dependence of lubricant type than a carburized steel gear and when a mineral lubricant was used the nitrided gear also promoted lower stabilization temperatures. The nitrided steel gear showed higher wear with both lubricants tested.

Micropitting of carburized gears lubricated with biodegradable low-toxicity 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 biodegradability and toxicity characteristics of ester-based gear oils are presented and their physical, chemical, and wear properties are compared with those of a commercial mineral oil. In an earlier work, power loss tests were performed on the FZG test rig, using carburized gears and several different lubricants. The operating temperatures of the oil and of the FZG gearbox wall were measured for different values of the input torque and speed. An energetic model of the FZG test gearbox was developed, taking into account the power loss mechanisms inside the gearbox and the heat flow mechanisms from the gearbox to the surrounding environment. Using this model it was possible to quantify the influence of lubricant formulation on the average friction coefficient between gear teeth. Gear micropitting tests were performed on the FZG test rig, using carburized gears and different oils. Post-test analysis included the mass loss measurement of the gear, the ferrometric analysis of lubricant samples collected during the tests, and the visual inspection of the gears. The tooth flanks were inspected using surface topography measurements to assess the number and depth of micropits. In comparison with the gear lubricated with mineral oil, the gears lubricated with ester-based oil showed significantly higher mass loss (3—4 times higher), smaller number of micropits, and less deep micropits. If carefully formulated, biodegradable low-toxicity ester-based gear oils, beside their environmental advantages, enhance gear performance when compared with the mineral lubricant providing (a) lower friction coefficients and lower operating temperatures of gearboxes and (b) lower number of micro-pits (with depths greater than 4 μm). One of the ester-based gear oils passed the FVA gear micropitting criterion.

Assessing the energy efficiency of gear oils via the FZG test machine

Energy efficient lubricants are becoming increasingly popular. This is due to a global increase in environmental awareness combined with the potential of reducing operating costs. A new test method of evaluating the energy efficiency of gear oils has been described in this report. The method involves measuring the power required by an FZG test rig to run while using a particular test lubricant. For each oil that was being evaluated, the rig was run for 10 minutes at a load stage of 10. Six extreme pressure (EP) industrial gear oils of mineral base were tested. The difference in power requirements between the best and the worst performing oils was 2.77 and 3.24 kW, respectively. This equates to a 14.6% reduction in power, a significant amount if considered in relation to a high powered industrial machine. The oils of superior performance were noticed to run at reduced temperatures. They were also more expensive than the other products of lesser performance.

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.

Global and local analysis of gear scuffing tests using a mixed film lubrication model

Gear tests were performed in a FZG test rig in order to evaluate the influence of the operating conditions (torque, speed and oil bath temperature), gear geometry and base oil viscosity on gear scuffing. A mixed film lubrication model was used to evaluate the normal pressures and shear stresses in several points along the gear meshing line, for each load stage and for all the gear scuffing tests performed. The gear scuffing results were analyzed using two different approaches: one considering global gear parameters defined at the meshing line scale and another based on local parameters at the roughness asperity scale, determined using the mixed film lubrication model. The analysis at the roughness asperity level was used to complete the scuffing study performed with global gear contact parameters, explaining the occurrence of scuffing during ‘running-in’, justifying the zones in teeth flanks where the first scuffing marks appear and supplying indicators for low scuffing resistance at high oil bath temperatures.

Austempered ductile iron (ADI) gears: Power loss, pitting and micropitting

This work is a systematic experimental evaluation of austempered ductile iron (ADI) as a gear material. The evaluation of this ADI as a gear material concerns two main aspects: power loss and contact fatigue resistance (pitting and micropitting). All gear tests were performed on the FZG test rig. For the micropitting and power loss evaluation two gear lubricants were considered. The results show that ADI is a material to be considered by gear designers, once the energetic behavior of ADI gears is similar to carburizing gears, being although slightly worst at high input power. The pitting life is infinite for contact pressures below 1.2 GPa. The best wear behavior of ADI is attained if combined with ester biodegradable lubricant, both on power loss and micropitting tests, so the best overall behavior of ADI gears is obtained with biodegradable ester lubricant.

Friction coefficient in FZG gears lubricated with industrial gear oils: Biodegradable ester vs. mineral oil

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, were characterized in terms of their physical properties, wear properties and chemical contents and compared in terms of their power dissipation in gear applications [Höhn BR, Michaelis K, Döbereiner R. Load carrying capacity properties of fast biodegradable gear lubricants. J STLE Lubr Eng 1999; Höhn BR, Michaelis K, Doleschel A. Frictional behavior of synthetic gear lubricants. Tribology research: from model experiment to industrial problem. Elsevier 2001; Martins R, Seabra J, Seyfert Ch, Luther R, Igartua A, Brito A. Power Loss in FZG gears lubricated with industrial gear oils: biodegradable ester vs. mineral oil. Proceedings of the 31th Leeds-Lyon symposium on tribology. Elsevier; to be published; Weck M, Hurasky-Schonwerth O, Bugiel Ch. Service behaviour of PVD-coated gearing lubricated with biodegradable synthetic ester oils. VDI-Berichte Nr.1665 2002.]. The viscosity–temperature behaviors are compared to describe the feasible operating temperature range. Standard tests with the Four-Ball machine and the FZG test rig [Winter H, Michaelis K. FZG gear test rig—desciption and possibilities. In: Coordinate European Council second international symposium on the performance evaluation of automotive fuels and lubricants; 1985.] characterize the wear protection properties. Biodegradability and toxicity tests are performed in order to assess the biodegradability and toxicity of the two lubricants. Power loss gear tests are performed on the FZG test rig 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. Lubricant samples are collected during and at the end of the gear tests [Hunt TM. Handbook of wear debris analysis and particle detection in liquids. UK: Elsevier Science; 1993.] and are analyzed by Direct Reading Ferrography (DR3) in order to evaluate and compare the wear particles concentration indexes of both lubricants. An energetic model of the FZG test gearbox is developed, integrating the mechanisms of power dissipation and heat evacuation, in order to determine its operating equilibrium temperature. An optimization routine allows the evaluation of the friction coefficient between the gear teeth for each lubricant tested, correlating experimental and model results. For each lubricant and for the operating conditions considered, a correction expression is presented in order to adjust the friction coefficient proposed by Höhn et al. [Höhn BR, Michaelis K, Vollmer T. Thermal rating of gear drives: balance between power loss and heat dissipation. AGMA Technical Paper; October 1996. pp 12. ISBN: 1-55589-675-8.] to the friction coefficient exhibited by these lubricants. 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 operating temperature.

Analysis of micropitting of prototype surface fatigue test gears

AbstractThe early stages of micropitting were examined on experimental gears designed for surface fatigue studies by the Helical Gear Rating Committee of the American Gear Manufacturers Association. Optical profilometry, scanning electron microscopy, and metallographic examinations were conducted on the first prototype gearset tested. The gearset was tested in an FZG test rig. To evaluate tooth bending fatigue strength, the gears were loaded to load stage 12. A tooth broke from bending fatigue in 0.88 h (the expected time). This short runtime allowed for the examination of micropitting at an early stage of development. The distribution and morphology of micropitting over the tooth face and associated relationships with surface micro‐ and macrogeometry features and traction have been examined. Micropitting was primarily associated with finish geometry. A few pits were associated with sulphide inclusions and one pit was possibly initiated by a small non‐metallic inclusion. Metallographic examinations revealed asperity‐related lines of dark etching alterations caused by high asperity contact and traction stresses.

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.

TESTING PROCEDURES FOR GEAR LUBRICANTS WITH THE FZG TEST RIG

The FZG test rig has become, within the past twenty years, a very important method for evaluating and testing different types of gear lubricants such as greases, mineral and synthetic oils, hydraulic fluids, ATF‐fluids, and hypoid oils. The test can evaluate scuffing and wear resistance in gear drives. There are about 100 FZG machines working in laboratories all over the world. These have a centre distance of 91·5 mm and the standard test A/8.3/90, standardized in DIN 51 354 is recommended as a basic testing procedure for gear oils, eg in the AGMA‐Standards and by the Coordinating European Council (CEC). However, there seems to be some confusion concerning different test procedures which can be carried out on this rig. To help to clear the air, the author deals, in this article, with the additional procedures that can be carried out.