Selection of Hydraulic Fluid

Many petroleum and synthetic fluids are available and more are being formulated. The highly technical formulations of the fluids with their various pros and cons makes the selection of such fluids difficult for those who are not thoroughly acquainted with the latest improvements and new formulations.

Generally, hydraulic fluids are chosen based on considerations of the environment of the application and chemical properties of the fluid. Physical properties such as viscosity, density, and bulk modulus are not usually basic considerations. Viscosity is very important, but usually a variety of viscosity characteristics are available in each fluid type. Bulk modulus should be large, but this requirement usually yields to the high temperature capability of the fluid. For example, the low bulk modulus of silicone fluids is more than offset by their high temperature range.

A basic judgment in fluid selection is required concerning the Are and explosion hazard posed by the application. If the environment and high temperature limit of the application are within the range of petroleum base fluids, then any number of suitable oils are available from numerous manufacturers. If the application requires a fire-resistant fluid, a choice must be made between the chemically compounded and water base synthetics. Factors to be considered are temperature range, cost, lubricity, compatibility, chemical, and handling characteristics of the fluid. Once a fluid type is selected, a number of viscosity and viscosity-temperature characteristics are usually made available, and a suitable matching must be made to the requirements of the system hardware. Consultation with representatives of hardware and fluid manufacturers is essential to ensure satisfactory compatibility and performance.

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