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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.

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.

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.

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.

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.