This is a challenging list, and no single hydraulic fluid possesses all of these desirable characteristics. The fluid power designer must select the fluid that comes the closest to being ideal overall for a particular application.
Hydraulic fluids must also be changed periodically, the frequency depending not only on the fluid but also on the operating conditions. Laboratory analysis is the best method for determining when a fluid should be changed. Generally speaking a fluid should be changed when its viscosity and acidity increase due to fluid breakdown or contamination. Preferably, the fluid should be changed while the system is at operating temperature. In this way, most of the impurities are in suspension and will be drained off.
Much hydraulic fluid has been discarded in the past due to the possibility that contamination existed—it costs more to test the fluid than to replace it.This situation has changed as the need to conserve hydraulic fluids has developed. Figure 2-1 shows a hydraulic fluid lest kit that provides a quick, easy method to lest hydraulic system contamination. Even small hydraulic systems may be checked.Tie lest kit maybe used on the spot to determine whether fluid quality permits continued use.Three key quality indicators can be evaluated: viscosity, water content, and foreign particle contamination level.
In this chapter we examine the physical properties of fluids dealing with the transmission of power. These properties include density, pressure, compressibility,viscosity, and viscosity index. In Chapter 12 we discuss the types of fluids used in Physical Properties of Hydraulic Fluids