What is Physical Properties of Hydraulic Fluid

Physical Properties

The physical properties of a hydraulic fluid determine how well it transmits power. The chemical and some physical properties, determine how long the fluid will function before it must be replaced. You must understand all of the properties that affect the performance and service life of a hydraulic fluid when selecting a fluid. Consideration must be given to both the properties of available hydraulic fluids and to the specifications and recommendations of the equipment manufacturer.

Viscosity 

Fig. 2-2. A Saybolt univer Viscometer

The most important physical property a hydraulic fluid must have is good VISCOSITY. The viscosity of a fluid describes its thickness or resistance to flow, and is a measure of the fluid’s internal friction. The viscosity must be proper, and it must remain stable over the entire operating temperature range of the system.

Viscosity also affects a fluid’s ability to lubricate two contacting surfaces. A high viscosity fluid prevents metal-to-metal contact between moving surfaces under load better than a low viscosity fluid. However, a high viscosity fluid is not necessarily a good lubricant, because it may not flow or adhere to the contacting surfaces. Flow requirements are discussed later in this lesson.

There are several ways of specifying fluid viscosity. The most common is the Saybolt Universal Second, usually abbreviated SUS or SSU. The SSU of hydraulic fluid is determinate by first heating 60 cubic centimeters of fluid to a uniform temperature in a Saybolt Viscometer shown in Fig. 2-2. The fluid is then permitted to run out through a small tube in the bottom of the viscometer. The length of time (in seconds0 that it takes for the fluid to run out is its viscosity in SSU. Fr example, fluid which takes 215 seconds to run out has a viscosity of 215 SSU. This viscosity is higher than the viscosity of a fluid that has a viscosity of 150 SSU (150 seconds). The higher the number, the higher viscosity. The viscosity of hydraulic fluids is usually measured at both 380C (1000F) and 990C (2100F). Most hydraulic fluids operate at temperatures within this range. 

Table 2.1 - Viscosity Grade Number

The recommended American Society for Testing Materials (ASTM) standard viscosity grades are given in Table 2-1. They provide fluid suppliers, fluid users, and equipment manufacturers with a uniform basis for selecting and specifying the viscosity of industrial hydraulic fluids. Each ASTM viscosity grade is numbered according to its NOMINAL (as close to exact as possible) fluid viscosity at 380C (1000F) expressed in Saybolt Universal Seconds (or SSU). The viscosity range of each ASTM grade is ±10%of the nominal viscosity.

ASTM grades of 150,215 and 315 are those most commonly specified by pump and equipment manufacturers. These grades are readily available from hydraulic fluid suppliers. Grades 150 and 215 are often recommended for service (ambient or surrounding air) temperatures below 50C (400F) and equipment operating (or fluid) temperatures below 1800F. Grade 215 and 315 are often recommended for service temperatures above 400F and equipment operating temperatures above 820C (1800F). Grades 645, 700 and 1000 are also specified to some extent. However, viscosities lower than 45 SSU and higher than 4000 SSU should not be specified, except for special applications.