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It is difficult, if not impossible; to select any single test that will model the expected performance of any hydraulic fluid in a wide range of hydraulic pumps made by many different manufacturers. Increasing pressures often encountered in new hydraulic pump applications compounds this problem. However, some basic assessment of hydraulic fluid performance is necessary for numerous reasons such as: developing an appropriate fluid purchase policy, international standard development, fluid classification and others. Since the now-classic standard tests such as: ASTM D-2882, DIN 51389 and others are simply inadequate for this task and also since the manufacturer, Eaton Inc., no longer manufactures these pumps, it has been necessary to develop an alternative testing strategy [1, 2 and 3]. The Bosch-Rexroth Corporation has developed a high-pressure piston pump test that has been an excellent predictor of hydraulic fluid performance for many years.

There is an ongoing interest in identifying new biodegradable hydraulic fluid compositions that may be used as alternatives to mineral oil in many hydraulic applications such as mobile off-highway equipment. To date, many basestocks have been proposed including: vegetable oils, polyol esters, diesters, synthetic hydrocarbons and others. One basestock is gaining interest as alternative, biodegradable, fire-resistant hydraulic fluid; anhydrous poly(alkylene glycol) (PAG) fluids. However, the use of these fluids is not new; they are simply being rediscovered. The objective of this paper is to provide an overview of the discovery and development of anhydrous PAGs as hydraulic fluids. This discussion will include: an overview of PAG chemistry, properties and hydraulic pump performance.

In many situations, the root cause of a particular failure may be determined based on the observation of a single part, or perhaps a photograph, taken from the failed system. However, when failure analysis is conducted on the complete component, a different result often emerges. Such is the case described here for a piston pump that was operating on an anhydrous poly(alkylene glycol) - PAG at high pressure. Initial observation suggested the fluid was the root cause of the failure. However, when failure analysis was properly applied, it was learned that the root cause of the failure was due to mechanical failure caused by the motor-to-pump coupling. This analysis procedure is presented as a case history in this report.

There are numerous fluid-related performance concerns when hydraulic fluids are being qualified for use. These concerns are especially important when non-mineral oil derived hydraulic fluids are being qualified for use in unusual and critical applications. In this paper, a number of performance-related fluid deficiencies that may be encountered in use will be described.

Cavitation erosion is one of the most important causes of loss of hydraulic pump efficiency, wear and even failure. Although cavitation process is often understood, there is typically little understanding of the joint role of the hydraulic fluid, which is a component of the system, and the operation of the hydraulic pump. In this paper, an overview is provided that describes the fluid properties that most impact on cavitation damage, the cavitation process within the pump and components and the use of computer simulation to prove improved designs that minimize hydraulic cavitation when various types of fluids are used.

There is an ongoing interest in biodegradable hydraulic fluids. Biodegradable fluids are often considered to include only vegetable oils, polyol esters and diester base stocks. However, other fluid base stocks including highly refined mineral oils, poly(alpha olefins) and fire-resistant fluids such as water-glycol hydraulic fluids are also biodegradable fluid alternatives. This paper will provide an overview of the international literature on biodegradable fluids, various international testing protocol, fluid base stocks, effect of oxidative stability, material compatibility and pump performance.

Proper hydraulic pump inlet pressure conditions are critical for proper operation of hydraulic systems. This is especially true when water-containing hydraulic fluids are used as direct replacements for mineral oil or synthetic esters. In this paper, the importance of proper inlet conditions on hydraulic system performance will be reviewed. Also provided are various reservoir design recommendations that should be followed when using water-containing hydraulic fluids such as water-glycol (HFC) hydraulic fluids.

Like all industries, the fluid power industry is under increasing pressure to identify and use hydraulic fluids that offer higher performance, improved fire safety, and exhibit improved environmental and toxicological behavior. Recently a novel hydrolube composition has been developed that is capable of being used in hydraulic pumps at pressures of 34.4 MPa (5000 psi) and greater*. In addition to being a Group 1 Fire Resistant Hydraulic Fluid according to the new Factory Mutual Research Corporation's testing methodology, this high-performance hydrolube also exhibits excellent biodegradability and toxicology properties. The objective of this paper is to discuss pump performance, fire resistance, and environmental and toxicology properties of this fluid..

The importance of minimizing air entrainment and enhancing air release in hydraulic fluids has been recognized for over 40 years. These properties are dependent on the composition of hydraulic fluids, especially on the presence of contaminants which inhibit air release and additives which promote it. One objective of this paper is to present an overview of this critically important technology as it has unfolded over the last 40 years. The second objective of this paper is to integrate into this summary recent research results on the impact of additives on air release properties.

Although the selection and role of hydraulic fluids as energy transfer agents is relatively well understood, there is no consensus on the appropriate procedures to evaluate lubrication properties on a laboratory scale. Because the use of bench tests such as the Shell 4-ball has traditionally produced poor pump wear correlations, it has been necessary to develop various hydraulic pump tests for this purpose. Since hydraulic fluid lubrication is being modeled, it is necessary to view these hydraulic pump tests as tribological tests. The objective of this paper is to provide an overview of various vane, piston and gear pump tests that have been reported as tribological tests.

Because of on-board fire problems during World War II, the us Navy initiated a program to develop hydraulic fluids that were more fire-resistant than the mineral oils that were in use at that time. Water-glycol hydraulic fluids were subsequently developed and first commercialized in 1947 which offered vastly improved fire resistance relative to mineral oils. Since 1947, in addition to formulation changes, there is significantly greater understanding of the impact of these changes on pump wear performance. This paper will present a selected overview of water-glycol formulation chemistry, some of the fluid formulation issues that have been encountered and the evolutionary improvement of hydraulic pump wear performance.

A new approach is presented for the evaluation of hydraulic fluids for pump wear performance. The approach uses performance maps developed in terms of rolling and sliding velocities to establish lubrication and failure regimes for test fluids. Testing pathways within the performance map can determine the fluid attributes for wear, scuffing and traction (friction). The measurement of oil film thickness with optical interferometry is used as part of a comprehensive approach for fluid evaluation. These measurements allow the lubricated contact itself to provide the viscous film forming properties of the fluid. An “effective” pressure-viscosity coefficient is determined for a range of fluid types. Performance mapping, together with film thickness measurements, provide an insight into the fundamental chemical and physical attributes of the fluid. The new approach provides an alternative to the limited reliability of bench tests and the time consuming and expensive hydraulic pump tests.

All types of hydraulic fluids may encounter thermal excursions at some point during their lifetime in use. When this occurs, there is the potential for the formation of degradation by-products. For most hydraulic fluids, including water-glycols, these degradation by-products include various low molecular weight carboxylic acids, e.g. formic acid and acetic acid. This paper describes the potential formation of these acids and the impact of their presence on wear and corrosion of hydraulic systems.