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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.

Multiple plate disc clutches are used extensively for shifting gears in automatic transmissions. In the active clutches that engage or disengage during a shift the automatic transmission fluid (ATF) and friction material experience large changes in pressure, P, sliding speed, v, and temperature, T. The coefficient of friction, μ, of the ATF and friction material is a function of these variables so μ = μ(P,v,T) also changes during clutch engagement. These changes in friction coefficient can lead to noise or vibration if the ATF properties and clutch friction material are improperly matched. A theoretical understanding of what causes noise, vibration and harshness (NVH) in shifting clutches is valuable for the development of an ATF suitable for a particular friction material. Here we present a theoretical model that identifies the slope, ∂μ/∂T, of the coefficient of friction with respect to temperature as a major contributor to the damping in a clutch during engagement.

The torsional natural frequencies of axles equipped with limited slip differential clutches depend on whether or not the tires and clutches are slipping since the effective inertia at each end of the axle is different for slipping and non-slipping conditions. Limited slip axle vibrations are typically analyzed for one tire slipping and the other not since that is the case for which the limited slip clutches are used. Vibrations often arise, however, during normal turning when both drive tires have good traction.

This laboratory presents a new step-automatic transmission fluid with enhanced friction durability and robustness for the Asian marketplace. This mineral-oil-based fluid also meets JASO M315-1A performance requirements on torque capacity, anti-oxidation, anti-wear, extreme pressure (EP), anti-aeration/foam control, copper corrosion and anti-rust performance. The fluid offers a JASO M349 low velocity friction apparatus (LVFA) durability lifetime of over 1100 hours. Moreover, this fluid maintains stable torque capacity during its entire LVFA durability lifetime, across the temperature range of 40 to 120 °C. Similarly, friction level changes with sliding speed are smaller than experienced by other commercial factory-fill ATFs. These critical performance features are due to a new fluid friction system approach, which may enable new types of transmission hardware or calibration.

This report focuses on an extended investigation of the durability of Dual Clutch Transmission (DCT) fluids. The performance requirements of DCT fluids differ from those of traditional step automatic transmission fluids. For that reason, key performance lab tests are discussed in this paper. Friction durability is measured with a modified version of the JASO M348 SAE#2 friction plate test. In addition, results from a vehicle chassis dynamometer test are discussed. This test involves running a 2008 Volkswagen GTI for 60,000 dynamometer miles (42,000 cycles) of severe acceleration and high speed conditions. Finally, a new DCT fluid, which performs well in these tests, offers friction stability and superior wear protection of transmission hardware, when compared to the commercial reference fluid.

This paper reports its findings in three separate parts. First, a comparative study is made among existing commercial dual clutch automatic transmission fluids (DCTFs). Significant differences in fluid torque capacity, friction material compatibility and copper corrosion performance were found among the fluids. Second, both a new vehicle chassis dynamometer durability test and a SAE#2 durability procedure are offered, specifically designed for DCTs. A 2008 VW GTI did well in the severe 60,000 mile chassis dynamometer procedure. Third, a new DCT fluid is discussed.

Currently there is no axle test aimed specifically at bearing durability in automotive hypoid axles. Existing axle tests are primarily focused on gear distress and lubricant protection of gears. In light of the new test information showing axle bearing distress, there is a need to develop a new bearing durability test for automotive and truck axle lubricants. To fulfill this need, a new bearing durability test has been developed to better assess lubricant requirements for rolling element bearing durability. Although the final test of an axle lubricant is in a driven automobile or truck, an effective screening test based on actual light duty truck conditions can be used to accelerate lubricant development to enhance bearing performance in hypoid axles. This new test simulates actual road durability tests in the lab. A specific load cycle which retains the critical road test loading conditions reduces test time and helps speed up lubricant development.

Improving vehicle fuel efficiency is a key market driver in the automotive industry. Typically lubricant chemists focus on reducing viscosity and friction to reduce parasitic energy losses in order to improve automotive fuel efficiency. However, in a transmission other factors may be more important. If an engine can operate at high torque levels the conversion of chemical energy in the fuel to mechanical energy is dramatically increased. However high torque levels in transmissions may cause NVH to occur. The proper combination of friction material and fluid can be used to address this issue. Friction in clutches is controlled by asperity friction and hydrodynamic friction. Asperity friction can be controlled with friction modifiers in the ATF. Hydrodynamic friction control is more complex because it involves the flow characteristics of friction materials and complex viscosity properties of the fluid.

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.

A critical market driver for rear axle lubricants continues to be the improved fuel efficiency, which is related to improvements in power transfer efficiency. Power transfer efficiency improvements are achieved with a reduction in the kinematic viscosity (KV) of rear axle lubricants. General Motors (GM) recently reduced the recommended viscosity grade for their rear axle lubricants from the Society of Automotive Engineers standard (SAE) 75W-90 to SAE 75W-85. This reduction in viscosity continues to require the optimization of rear axle lubricants to ensure durability. Lubricants that form thick elastohydrodynamic (EHD) films and are shear stable even when lower kinematic viscosities are required. This work depicts how a rear axle lubricant was developed and improved with the proper selection of base oil and polymer. This newly developed SAE 75W-85 rear axle fluid was incorporated as factory fill in 2019 in T1 LDPU-GMC Sierra and Chevrolet Silverado 1500 series pickup trucks.

Sliding contact between friction surfaces occurs in numerous torque transfer elements: torque converter clutches, shifting clutches, launch or starting clutches, limited slip differential clutches, and in the meshing of gear teeth under load. The total temperature in a friction interface is the sum of the equilibrium temperature with no sliding and a transient temperature rise, the flash temperature, caused by the work done while sliding. In a wet shifting clutch the equilibrium temperature is typically the bulk oil temperature and the flash temperature is the temperature rise during clutch engagement. The flash temperature is an important factor in the performance and durability of a clutch since it affects such things as the reactivity of the sliding surfaces and lubricant constituents (e.g., oxidation) and thermal stress in the components. Knowing how high the flash temperature becomes is valuable for the formulation of ATF, gear oil, engine oil and other lubricants.

To develop an automatic transmission fluid (ATF) that meets DEXRON® III-H specifications, the ATF must pass two critical tests, the GM oxidation test (GMOT) and the GM cycling test (GMCT), in addition to many other performance tests. The specification on the GMOT is that delta TAN (difference in total acid number compared with the fresh oil) at the end of the test does not exceed 3.25 while the specifications on GMCT are that delta TAN cannot exceed 2.0 and the 1-2 shift time must stay between 0.30 and 0.75 seconds throughout the test. For this work, we analyze oil oxidation and changes in oils' surface tension, drum and band surface degradation and deposit formation. We have found that with respect to the delta TAN limits of the DEXRON® III-H specification, the GMCT is more severe than the GMOT. The effect of base oil chemistry on oxidation in these tests has been quantified. Oil oxidation is not responsible for the GMCT 1-2 shift time increase.

This paper presents data on the effect of ATF additives on lead corrosion as measured in a simple bench test and the MERCON® ABOT. The correlation between the bench test and the ABOT test will be discussed. The effect of base oil, carboxylic acids, and oxidation products on lead corrosion will also be discussed. Two types of additives used in automatic transmission fluids can reduce lead corrosion. Each additive has shown a statistically significant linear correlation to lead loss. There is also a statistically significant detrimental interaction between the additives when both are present in the fluid simultaneously. A mechanism to explain this interaction will be presented along with analyses of the lead surfaces after ABOT testing.

Multiple plate disk clutches are used extensively for shifting gears in automatic transmissions. In a shift from one gear to another one or more clutches is engaging or disengaging. In these active clutches the automatic transmission fluid (ATF) and friction material experience large changes in pressure P, temperature T, and sliding speed v. The coefficient of friction, μ, of the ATF and friction material depends on v, P and T, and also changes during clutch engagement. Changes in μ can lead to vibration and poor shift quality if the ATF and clutch friction material are improperly selected. An in-depth theoretical understanding of the cause of vibration in shifting clutches is crucial in the development of a suitable ATF to work with a particular friction material.

Since the mid-1990's, original equipment manufacturers (OEMs) of automobiles have been implementing torque converter clutches in automatic transmissions with a continuous, controlled slip mode, in order to improve the fuel economy of their vehicles. These Continuously Slipping Torque Converter Clutches (CSTCCs) are prone to an undesirable phenomenon commonly called shudder. This phenomenon has been attributed to specific shapes or slopes in the friction coefficient versus sliding speed curve of the fluid/clutch interface. Here, a method is explained that was developed to be able to screen fluids for shudder tendency, both in fresh and used states. Also included is a description of the reason for implementing CSTCCs, some background on shudder, and supporting data showing how the test method can distinguish between fluids that have different shudder tendencies.

After new transmission lubricants are developed there is an extensive validation program where friction durability testing is performed on multiple clutch materials. Each durability test can run for long terms and the entire validation program can take much longer terms. A well designed lubricant and friction material will deliver the necessary friction control for construction equipment to operate at optimum level. A mechanistic construct has been evaluated to calculate friction durability in clutch systems based on fluid and surface tribological properties. Fluid properties include both boundary frictional and rheological effects. Surface properties include elastic modulus, surface roughness, asperity density and asperity tip radius. Using this mechanistic construct friction durability has been predicted.

A model and simulation results are presented for the torsional dynamics of a rear wheel driveline while the vehicle is coasting in a turn. The model includes the effects of road load and powertrain drag, limited slip differential clutch friction, the inertias of the vehicle, wheels, axles, differential carrier, and driveshaft, the final drive ratio, torsional stiffnesses of the axles and driveshaft, vehicle track width, and radius of the turn. The dynamics of coasting in a turn differ from powered driving due to changes in the inertia loading the driveshaft, the damping effect of the disengaged transmission, and nonlinearities in the clutch friction. Specific focus is given to vibration in the axles and driveshaft due to variations in the torque-speed slope of the clutches, which is determined by the slope of the friction coefficient ‘μ’ versus sliding speed ‘v’ in the limited slip clutches.