Page 56 - Fluid Power Engineering

Hydraulic Oils and Theor etical Backgr ound 33

FIGURE 2.17 Effect of entrained free air and operating pressure on the bulk
modulus of a typical mineral-based hydraulic oil, calculated.

The nondimensional bulk modulus, B = B /B, was calculated
e
using Eq. (2.56). The calculation results are plotted in Fig. 2.17. This
figure shows that the entrained air reduces considerably the bulk
modulus of the mixture. Moreover, the entrained air results in noise,
shuddering movements, and a large rise in temperature, in addition
to the diesel effect. The diesel effect is the spontaneous combustion
of an air-gas mixture. If the mineral oil, containing air bubbles, is
rapidly compressed, the air bubbles become so hot that a spontane-
ous combustion may occur. Hence, at specific points, a large increase
in temperature and pressure occurs, which may damage the seals on
the hydraulic components. In addition, the age of the fluid will be
reduced.
Effect of Oil Compressibility; Hydraulic Capacitance
Oil compressibility makes an important contribution to the dynamic
behavior of the hydraulic control systems. The transient pressure
variations, and consequently the transient variation of flow rates,
forces, and accelerations, are highly affected by the oil compressibility.
Herein, the effect of oil compressibility on the dynamics of hydraulic
transmission lines is discussed. The effect of compressibility can be
formulated by considering a single oil lump in a hydraulic transmis-
sion line. The system is subjected only to the effect of oil compress-
ibility. The pressure in the line is P, the inlet flow rate is Q and the
1
outlet flow rate is Q (see Fig. 2.18).
2