Search Results

This paper provides an overview of driveline fluids, in particular automatic transmission fluids (ATFs), and is intended to be a general reference for those working with such fluids. Included are an introduction to driveline fluids, highlighting what sets them apart from other lubricants, a history of ATF development, a description of key physical ATF properties and a comparison of ATF fluid specifications. Also included are descriptions of the chemical composition of such fluids and the commonly used basestocks. A section is included on how to evaluate used driveline oils, describing common test methods and some comments on interpreting the test results. Finally the future direction of driveline fluid development is discussed. A glossary of terms is included at the end.

Friction plate degradation and/or friction plate glazing has often been related to the loss of friction control in automatic transmissions. However, in JASO SAE No.2 and LVFA tests, friction material glazing has been found to not be a sufficient condition for the loss of anti-shudder performance or a reduction in torque capacity durability. Therefore, changes in automatic transmission fluid properties rather than changes to the friction surfaces would be expected to play a dominant role in controlling anti-shudder performance and torque capacity. Earlier theoretical studies have proposed that friction in wet clutches is a combination of boundary and hydrodynamic friction. Therefore, changes in these properties should control anti-shudder durability and torque capacity. In this paper, we confirm that boundary and thin-film friction contribute to friction measured in JASO SAE No.2 and LVFA tests.

A model of an automotive powertrain equipped with a slip-controlled torque converter clutch (TCC) is presented that incorporates the clutch control system and the friction-related properties of the automatic transmission fluid (ATF) and clutch friction material. Prior research has established that stability of a slip-controlled TCC is enhanced by maintaining a positive slope of the coefficient of friction, μ, with respect to sliding speed, v. The model presented here agrees with this result, but suggests that it is neither a necessary nor sufficient condition guaranteeing stability. The model indicates that other factors affecting stability at the equilibrium sliding speed include the magnitude of μ, the engine speed, the engine torque-speed slope, the ATF pressure, and the time constants of the clutch control system. This model will aid in the development of future wet clutch systems with improved friction stability performance.

Extended gear fatigue pitting life is an essential performance requirement for today's gear oils in automotive driveline applications. One of the important industrial standard tests used to evaluate fully formulated oil's ability to extend gear pitting fatigue life is the FZG pitting test. To understand the fatigue pitting behavior in these gears we have conducted surface analyses on the FZG gears to determine fatigue modes. We have found that micro-pitting is the major fatigue mode and pitting/spalling is mostly initiated by micro-pitting in the FZG test. To help further understand how pitting and micro-pitting relate to gear oil properties and gear surface morphology, we have also carried out a statistical analysis correlating fatigue pitting life with four major physical parameters: boundary friction coefficient, oil film thickness, oil corrosiveness, and surface roughness of the gear tooth.

In automatic transmission technology development the degradation of paper friction plates has often been considered a major failure mechanism by which transmissions lose their anti-shudder characteristics. One of the most common degradation processes for paper friction plates is known as glazing. In this study, we focus on the relationship between friction plate glazing and anti-shudder durability in the Japanese Automobile Standards Organization (JASO) low velocity friction apparatus (LVFA) rig test following the procedure M349-98. We also investigate the impact of used friction plates and used oil on torque capacity durability as measured by an SAE No. 2 machine following the JASO procedure M348-95. We find that friction plate glazing has no correlation with anti-shudder durability. A completely glazed plate can have long anti-shudder durability but a barely glazed plate can have short anti-shudder durability.

An automatic transmission planetary gear fatigue test is used to screen lubricant performance of various automatic transmission fluids. The key use of this test is to assess the ability of a lubricant to extend or limit planetary gear system fatigue life. We study the fatigue behavior in this test and find the major failure modes are tooth macropitting, and macropitting-related tooth fracture of the sun and planetary gears (short and long pinion gears). Micropitting appears to be responsible for these gear failure modes. Macropitting is also seen on the shafts and needle rollers of the bearings. Gear tooth fracture appears to have originated from the surface as a secondary failure mode following macropitting. Bearing macropitting is initiated by geometric stress concentration. Bending fatigue failure on the sun and planetary gears also occurs but it is not a micropitting-initiated failure mode.

In order to prevent shudder in automatic transmissions, friction must decrease as the sliding speed between the friction plates in clutches decreases. Theoretical studies have shown that friction in wet clutches is a combination of boundary friction and the friction due to flow of fluid through the friction materials (thin-film friction). Therefore, these physical properties of oils should control the anti-shudder performance of automatic transmission fluids. Recently, we demonstrated that boundary and thin-film friction contribute to friction measured at low speeds in JASO SAE No.2 and LVFA tests. Two different friction materials are used in these tests and the relative effect of thin-film friction on low speed friction is greater in the JASO SAE No. 2 test than in the JASO LVFA test.

Rheological properties of automatic transmission fluids (ATFs) are typically characterized by their kinematic (ASTM D 445) and Brookfield (ASTM D 2983) viscosities. However, ATFs contain polymeric viscosity modifiers, which often result in non-Newtonian fluid behavior as the polymers align and stretch under the shear stresses experienced in automatic transmissions. Therefore, the standard rheological tests, which are normally run under low shear stresses, may not adequately characterize an ATF's flow properties under the operating conditions of the automatic transmission. This study was designed to characterize the rheological properties of ATFs containing different amounts of viscosity modifiers, different base oil types and different levels of permanent shear stability under the shear and temperature conditions which exist in automatic transmissions.