Thermal influence on torque transfer of wet clutches in limited slip differential applications

Wet clutches operating under low velocity and high load are studied with the aim of obtaining reliable models for the torque transfer during boundary lubrication conditions. A friction model which takes temperature, speed and nominal pressure into account is developed and used with temperature calculations to be able to simulate behavior of a wet clutch working in boundary lubrication regime. Predicted torque and temperatures from the model agree well with experimental data.     

The Influence on Boundary Friction of the Permeability of Sintered Bronze

Components made of sintered bronze are often used in tribological systems. Examples of applications are self lubricated bearings, wet clutches and brakes and synchromesh components for manual gearboxes. The bronze material in these applications is often porous and permeable. However, the required level of permeability i.e., the ability for a fluid to flow inside the material varies widely for different applications. This implies the need to investigate if the permeability can influence the properties of a tribological system. Various studies have been performed in order to investigate the permeability of different materials but a possible relationship between permeability and boundary lubrication performance has not yet been thoroughly investigated. In this article this relationship has been investigated in a pin-on-disk apparatus for test specimens with a permeability that is well-defined. Materials with three different permeabilities were investigated together with non-permeable test specimens. The results indicate that permeability has a small influence on the friction coefficient when the tribological system is operating with good lubrication of the contact. The function of the tribological system when working under starved conditions is also investigated and a very clear relationship between permeability and the ability to cope up with starved running conditions is shown. This is explained in terms of the varying ability of the materials’ to store lubricant in pores. Non-porous test specimens were also tested as a comparison with the permeable test specimens in the investigation with starved running conditions . The results show that a material with high permeability works much better under starved running conditions than that with low permeability.     

Frictional Behavior of a Wet Clutch Using Blends of Automatic Transmission Fluid (ATF) and Biolubricant (Jatropha Oil) in a Pin-on-Disk Tester

Nowadays, biolubricants are being used to replace mineral lubricants totally or partially and it is of great importance due to interest in environmental protection. Jatropha oil is a good alternative as lubricant feedstock and combustible. Its production is cheap in comparison to other vegetable oils. In México, Jatropha curcas L. is widely distributed in more than 15 states, which makes a sustainable supply of this natural resource possible. The aim of this work was to investigate the frictional behavior of blends of automatic transmission fluid with vegetable oil tested in a pin-on-disk rig simulating the wet clutch operation. Two different friction materials were separately tested and used as pins against actual steel disks from a wet clutch. The frictional behavior results demonstrated that the mixtures presented antishudder characteristics. Thus, Jatropha oil could be potentially used as part of automatic transmission fluid (ATF) formulations to enhance their antishudder properties and increase the efficiency of the torque transferred of wet clutches with a controlled lock-up clutch system, which improves the full economy of vehicles.     

Clutch Shudder Correlated to ATF Degradation through Local Friction vs. Velocity Measurements by a Scanning Force Microscope

Shudder in continuously slipping wet clutch (CSWC) systems is an instability that manifests itself as a vibration of the automobile's drive train. Dynamometer experiments imply that the shudder is connected to degradation of the frictional properties of the clutch friction material-automatic transmission fluid (ATF) system. The poor friction characteristics are revealed in negatively sloped torque vs. slip-speed curves which can result in dynamic friction oscillation instabilities. The authors report local measurements of friction vs. velocity with the use of a scanning force microscope (SFM) on new and glazed wet clutch friction material with either new or degraded ATF acting as a lubricant. The friction material was a commonly used cellulose/phenolic resin material. Results in the very low-speed dynamic regime indicate that the fluid condition plays a critical role in determining the slope characteristics of the friction vs. velocity curve. ATF degraded in service imparts a flat or negative slope to the friction vs. velocity curve, whether on new or glazed (degraded) friction material. A positive slope is measured for fresh ATF. The SFM measurements are therefore consistent with dynamometer and vehicle experiments which measure gross properties of the friction clutch system. The ability of the SFM to reproduce the dynamometer results on a local scale implies that shudder is not due to the gross fluid flow or mechanical effects that are present in a complete torque converter. The frictional characteristics of the clutch plate materials and the ATF are paramount. This technique allows continuous monitoring of ATF fluid degradation in test fleet vehicles without ATF replenishment since only a few ccs of fluid are necessary for these tests.     

Nonlinear system-identification of the filling phase of a wet-clutch system

The work presented illustrates how the choice of input perturbation signal and experimental design improves the derived model of a nonlinear system, in particular the dynamics of a wet-clutch system. The relationship between the applied input current signal and resulting output pressure in the filling phase of the clutch is established based on bandlimited periodic signals applied at different current operating points and signals approximating the desired filling current signal. A polynomial nonlinear state space model is estimated and validated over a range of measurements and yields better fits over a linear model, while the performance of either model depends on the perturbation signal used for model estimation.     

湿式离合器考虑纸基衬面孔隙弹性特性摩擦振动理论(二)

本文提出考虑纸基摩擦材料界面间变形和其中造成润滑油流动的摩擦振动新理论,推荐了一摩擦振动模型,采用引入的粘性液体浸透多孔体,联合假定在切向和法向两个方向的振动导出一稳态准则,和实验相比较,确认所推荐的准则可预测该稳态极限比常规的更精确。它不考虑摩擦材料的变形,只与摩擦系数与滑动速度的斜率有关。基于本准则,讨论了材料特性摩擦振动对稳态的影响。     

Thermal Resistance of a Tapered Roller Bearing

An apparatus to simulate the thermal behavior of a tapered roller bearing was constructed and used to determine tapered roller bearing thermal behavior. Bearing specimens can be loaded axially and/or radially while rotating. Shaft heating is also provided and mechanical losses measured. An infrared scanner and thermocouples allow for temperature measurement. An analytical model of the bearing and bearing/housing system was also developed and is presented. The model is capable of considering various modes of bearing operation. Experimental and theoretical resistances are compared. Typical thermal resistances from shaft to housing were found to be of the order of 1 K/W.     

Lubrication of Rolling Surfaces by a Ree-Eyring Fluid

Recent measurements of thicknesses of lubricant films between cylinders rolling at moderate speeds have agreed reasonably well with the predictions of Grubin's formula, but at high speeds and with high viscosity lubricants significant disagreements have been observed. Therefore, the elastohydrodynamic theory underlying that formula is reconstructed so to be based, not on a Newtonian model for the lubricant, but on the Ree-Eyring model which permits shear stress to develop at a lessening rate as shear rate increase. A formula comparable to Grubin's is derived, and it is shown that the changes in the formula are in the direction needed to explain the observed discrepancies between experimental data and the former theory. Exhaustive examination or use of this new formula is hampered by lack of data on the Rev-Eyring parameters, so further experimental work is recommended.     

扭簧离合器的研究

对两种扭簧离合器的结构及工作原理作了较为详细的研究，并用实例进行相关的计算和说明。     

Anti-Shudder Properties of ATFs—Investigation into Tribofilm Composition on Clutch Friction Material and Steel Surfaces and the Link to Frictional Performance

The friction response of an automatic transmission fluid (ATF) should ensure a smooth engagement process with no shudder in the process of power transmission. To study the effect of ATF chemistry on anti-shudder properties, the frictional characteristics of the baseline additive system used in ATFs with various friction modifiers and detergent combinations were assessed using a variable-speed friction test (VSFT) machine. Tests were performed on steel-versus-clutch friction material tribocouples, with the aim of studying the tribofilm formation on both friction couple (steel and friction material) surfaces and to establish the link between the lubricant-surface interactions and anti-shudder performance. An SAE No. 2 test machine was also used to investigate the friction performance of the same ATFs for comparison with the VSFT results. Similar performance rankings were obtained from those two tests. This article focuses on the posttest surface analysis and surface chemistry results of the test samples.

Energy dispersive X-ray spectroscopy (EDX) analysis, X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) were used to assess the different surface conditions and chemical tribofilm compositions on the friction material and steel counterpart after the test. The mechanisms of the different friction performance provided by additive formulations and the correlation with the composition of tribofilms are discussed. Some adsorbed functional groups on the posttest sample surface, such as OH-, C-N, etc., were suggested to have positive effects on the anti-shudder performance of ATFs.