A Comparison of Methods for Testing Rubber Materials in Automatic Transmission Fluids

<div class="htmlview paragraph">Significant changes are occurring in the development of automatic transmission fluids (ATF) which impact the selection of both additive components and lubricant base oils. In particular, the need for improved viscometrics at high and low temperatures, combined with the need for increased oxidation performance and shear stability are moving formulators towards use of higher quality API Group II (100 - 120 VI) and/or Group III base oils.</div> <div class="htmlview paragraph">Several benefits are apparent with the use of API Group III base oils in ATF. Their naturally high Viscosity Index of >120 provides opportunities to use a higher base oil viscosity compared to API Group I and in many instances Group II base oils. This reduces the amount of Viscosity Modifier in the finished ATF and improves overall shear stability. Higher kinematic viscosity also improves volatility which leads to improved oxidation resistance.</div> <div class="htmlview paragraph">Low temperature fluidity can be improved by modifying the amount and type of residual wax in the base oil. This is achieved through two main factors, namely: (i) feed selection and (ii) the type of dewaxing process, isodewaxing being preferred to conventional solvent dewaxing. Finally, the higher concentration of saturated versus aromatic molecules in an API Group II and Group III base oil provide enhanced antioxidant response.</div> <div class="htmlview paragraph">The relationship between base oil composition and ATF performance will be discussed in terms of low temperature Brookfield viscosity, shear stability and oxidation performance. Particular emphasis will be given to API Group II+ and Group III base oils produced by severe hydro-treating and isodewaxing.</div>

<div class="htmlview paragraph">One of the barriers to successful implementation of the continuous slip torque converter clutch (CSTCC) design for automatic transmissions (AT) is transmission shudder. An important factor contributing to shudder is the frictional characteristics of the automatic transmission fluid (ATF). In this paper, we report the development of a test that allows the precise measurement of the frictional characteristics of the ATF at different sliding speeds (μ-v data) using an SAE No. 2 friction machine. A conventional SAE No. 2 machine was modified, by installing a DC static drive motor, to simulate the sliding speeds, surface pressures, and fluid temperatures found in actual vehicle operating conditions. The μ-v data obtained with the machine showed good correlation with the vehicle shudder, i.e., the ATFs showing more negative gradient in the μ-v data caused more severe shudder in the vehicle. Initial shudder and anti-shudder durability evaluations were conducted using a variety of commercial and experimental ATFs. It was found that the highly friction-modified ATFs exhibited good initial anti-shudder performance. ATFs meeting major North American OEM requirements were not always acceptable for CSTCC applications.</div>

<div class="section abstract"><div class="htmlview paragraph">Reducing loss torque in automatic transmissions (ATs) is a key factor in improving fuel economy. A promising approach is to reduce the viscosity of the Automatic Transmission Fluid (ATF) so as to minimize churning loss. Aisin AW and JXTG Nippon Oil & Energy Corporation have developed a super low viscosity ATF, called “AW-2”, which has approximately 50% lower kinematic viscosity at 40 °C compared to the conventional ATF “AW-1”. It is generally understood that if the viscosity of an ATF is too low, it can have a negative impact on the fatigue life of components such as gears and bearings, and possibly lead to increased wear or seizure. AW-2 was designed to solve these problems via the application of two key technologies. The first is a high performance base oil with a low traction coefficient, which translates to low viscosity under high pressure conditions. This decreases the shear resistance between sliding surfaces under elastohydrodynamic lubrication (EHL) conditions, which contributes in improving the fatigue life of bearings and other components. The second is an ester type base oil with high polarity. It was found that the amount of ester base oil used has a major influence on fatigue life. The adsorption of esters onto metal surfaces is thought to improve lubricity in severe lubrication conditions. Durability tests were performed in a wide range of conditions, using gear and bearing components and actual transmission units, and it was confirmed that AW-2 outperforms AW-1, despite its super low viscosity. Furthermore, AW-2 reduced loss torque in the transmission by approximately 10% compared to AW-1.</div></div>

<div class="htmlview paragraph">The rheological behavior of automatic transmission fluids (ATF) ranges from moderately shear-thinning to Newtonian. However, no commercially available ATFs are known to display shear-thickening behavior. A theoretical investigation was performed to determine if any advantages could be derived from the use of shear-thickening ATF in automatic transmission components and subsystems. A series of theoretical shear-thickening, shear-thinning, and Newtonian fluids were modeled by a power law function and compared to a reference shear-thinning ATF in simplified representations of transmission components and subsystem geometries. The results indicate that a shear-thickening ATF with zero shear viscosity, infinite shear viscosity, and power of 417 mPa-s, 6.23 mPa-s, and 1.03(dimensionless), respectively, displays optimized behavior with respect to the reference shear-thinning ATF.</div>

<div class="htmlview paragraph">In recent years, progress has been made in reducing the viscosities of manual transmission fluids (MTFs) and automatic transmission fluids (ATFs). Lower viscosities of MTFs and ATFs are expected to improve the fuel economy of automobiles by reducing the viscous resistance. Examples of low-viscosity ATFs already commercially available include Toyota Auto Fluid WS and ZF Friedrichshafen AG's ZNF 13014. This paper first reports methods for measuring the torque transmission efficiency in manual and automatic transmissions. We explain a simple rig test that we developed using an IAE gear test machine, and we describe oil temperature increase tests and torque measurement tests using actual transmissions and fuel economy tests using actual vehicles. Next, we describe the effects of lower viscosities on the torque transfer efficiency as measured with these measurement methods. Because lower viscosities are likely to worsen the wear resistance, extreme-pressure performance, and fatigue resistance of MTFs and ATFs, we studied the effect of the base oil composition and the molecular structure of PMA-based viscosity index improvers (VIIs) on the oil film thickness. We were able to determine by three methods that the film thickness of low viscosity lubricant could be made greater than that of conventional lubricant. The oil film was made thicker in the first method by increasing the molecular weight distribution of the base oil, in the second method by reducing the molecular weight of the PMA-based VIIs, and in the third method by changing the alkyl groups of the polymethacrylate-based (PMA-based) VIIs. The results of our study showed that our optimized lower-viscosity MTF, when evaluated in terms of its wear prevention, extreme-pressure properties, and fatigue prevention, has equal or better performance in each area in comparison with MTFs with conventional viscosities.</div>

<div class="htmlview paragraph">Friction modifiers have been around for many years. Originally, the application was for limited slip gear oils, automatic transmission fluids and multipurpose tractor fluids.</div> <div class="htmlview paragraph">Since fuel economy became an international issue, initially to reduce crude oil consumption, friction modifiers have been introduced into automotive crankcase lubricants as well. The current emphasis is to improve the fuel efficiency through the engine lubricant and to reduce emissions to the environment.</div> <div class="htmlview paragraph">This paper describes the chemistry of the various organic friction modifiers as well as the non-organic types. A basic understanding is given on the mechanisms how and why these products work as friction modifiers and what is known about structure - activity relationships.</div> <div class="htmlview paragraph">Definitions are given about the various lubrication regimes as well as figures to what extend these regimes are present in current engine tests measuring fuel economy. In addition, it is described which type of friction modifiers is most active in the various lubrication regimes.</div> <div class="htmlview paragraph">Factors influencing the friction reducing properties, like base fluid characteristics and competing additives, are covered and appropriate tests to measure frictional properties are described. Finally, some recent work regarding the effect of some friction modifiers on frictional levels, determined with a pin-on-ring tribometer, is shown.</div>

<div class="htmlview paragraph">An additive package for the automatic transmission fluid (ATF) used in slip lockup control automatic transmissions has been developed that satisfies all the requirements of the DEXRON<sup>®</sup> III, MERCON<sup>®</sup> and JASO M315 specifications. The influence of the base oil and viscosity index improver that are combined with this additive package was also investigated with respect to ATF performance.</div>

<div class="htmlview paragraph">Past experience indicates that the method for evaluating the foaming tendencies of automatic transmission fluids, shown in the “Specification for Automatic Transmission Fluid, Type A, Suffix A,” is adequate for the qualification of fluids for transmission operation. Its usefulness, however, for evaluating field failures and for monitoring the shelf-life stability of the foam inhibitor is severely limited by the fact that the procedure requires a sample size of at least 6 qt.</div> <div class="htmlview paragraph">The purpose of this paper is to present a new test procedure for the evaluation of foaming tendencies that utilizes a small sample size (500 ml) and shows good correlation with the results obtained during qualification.</div> <div class="htmlview paragraph">The paper also presents the results of studies which were necessary for the development of the proposed test procedure. These studies were concerned with the mechanism of foam formation and collapse, with the conditions affecting the equilibrium between the rates of formation and collapse, with air entrainment and retention in the fluid, and with the effect of siloxane additives on the foaming characteristics of the fluids.</div>

<div class="htmlview paragraph">Modern vehicles are expected to operate satisfactorily at relatively low ambient temperatures, often without auxillary heating equipment. At these cold temperatures, the lubricant plays a key role in determining how easily the vehicle can be started, and how quickly all vital moving parts are oiled. A lubricant which is too thick under these conditions may result in poor operability, or in some cases catastrophic equipment failure due to oil starvation.</div> <div class="htmlview paragraph">Extensive testing has demonstrated the low temperature performance advantages of SAE 0W-30 or 5W-30 engine oils versus more common SAE 10W-30 and 15W-40 viscosities, in both light and heavy duty engines. Automatic transmission fluids with enhanced low temperature properties have shown faster start-up flow and in some cases, more rapid shift times. Hydraulic fluids with improved cold flow performance allow more rapid equipment warm-up and faster response at low temperatures.</div> <div class="htmlview paragraph">Modern multigrade lubricants such as these also provide good performance at more normal operating temperatures, even under high load conditions.</div>

<div class="section abstract"> <div class="htmlview paragraph">This SAE Standard covers hose intended for use with automatic transmission cooling system applications. Type A hoses are intended for original equipment or replacement applications while Type B hoses are intended for aftermarket auxiliary cooler applications only. The reference fluid for tests requiring the use of automatic transmission fluid (ATF) shall be Dexron III / Mercon 5 or equivalent ATF that is agreed to by hose manufacturer and customer.</div> </div>

<div class="htmlview paragraph">The temperatures of engine oil, automatic transmission fluid, and differential lubricant were measured under a variety of steady-state conditions in 1969, 1971, 1972, and 1973 model year cars. Increases in engine speed and stroke and coolant temperature caused significant increases in engine oil temperatures. Engine load and coolant temperature increases appeared to be the principal causes of automatic transmission fluid temperature increases. Differential lubricant temperatures increased as engine speed and load increased, but speed was the more influential variable. The effect of ambient temperature was negligible in all cases over the narrow range of ambient temperatures investigated (65-85°F).</div> <div class="htmlview paragraph">Analysis of the data for year-to-year trends showed that only engine oil temperatures increased from 1969 to 1973. This trend, and several observations of engine oil temperatures greater than 300°F, suggest that improved resistance of engine oils to oxidative thickening may be required.</div>

<div class="htmlview paragraph">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. Tooth macropitting and the related tooth fracture appear to occur independently from bearing macropitting. As expected, the directions of micro- and macro-crack propagations in mating gears go in opposite directions.</div>

The location of the electric motor (EM) inside the transmission in an electric vehicle requires the compatibility of the automatic transmission fluids (ATFs) with the materials of the EM and the transmission. This work studies the compatibility of four conventional ATFs with three elastomers: fluoroelastomer (FKM), ethylene-propylene-diene monomer (EPDM), and vinyl-methyl silicone rubber (silicone). Changes in volume, hardness, tensile strength, and elongation at break of the elastomers after aging in the ATFs were measured, and additional Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermogravimetric and derivative thermogravimetric (TGA and DTGA) tests were performed. The four ATFs showed high or medium compatibility with FKM and silicone, and low compatibility with EPDM. This low compatibility was related to changes in the composition and crystalline structure of the elastomer. The non-compatibility of the EPDM with the oils from Group III was also proven.

The various gear types and automotive transmissions in drive trains are described. The need for automotive differential axle gears is demonstrated. SAE gear lubricant classifications are related to performance specifications. Automatic transmission fluids, ATFs, are described together with agricultural and off-highway fluids. Generic formulations of driveline fluids are discussed in terms of their tribology and performance and also the contribution of various additive classes to that performance. The main functions of manual gearbox, automatic transmission and axle lubricants are described. General trends and emerging technologies in drive train components are discussed in terms of the requirements placed on the lubricant, evolution in transmission technologies and relative market penetration.

<div class="htmlview paragraph">An automatic transmission fluid has been developed which can meet the needs of transmissions designed for either friction modified or non-friction modified fluids. Approved under the DEXR0N<span class="xref">*</span>-II and M2C138-CJ specifications, it also meets all the requirements of the M2C33-F (Type F) specification except for the friction modifier content. Since use of a friction modified fluid in Type F applications would represent a radical departure from current recommended practice, data, both pro and con, on the concept are presented.</div>