Page 41 - Fluid Power Engineering

18 Cha pte r T w o

the viscosity is stated at a standard temperature (40°C for the ISO
specification). A hydraulic fluid referred to as VG32 has a viscosity of
32 cSt at 40°C.
It is important to keep the oil viscosity within a certain range dur-
ing the system’s operation; otherwise, the operating conditions will
change with temperature. The viscosity index (VI) of oil is a number
used in industry to indicate the effect of temperature variation on the
viscosity of the oil. A low VI signifies a relatively large change of vis-
cosity with temperature variation. On the other hand, a high VI means
relatively little change in viscosity over a wide temperature range.
The best oil is the one that maintains constant viscosity throughout
temperature changes. An example of the importance of the VI is the
need for high VI hydraulic oil for military aircraft. These systems are
exposed to a wide variation of atmospheric temperatures, ranging
from below −18°C at high altitudes to over 45°C on the ground. For
proper operation of these systems, the hydraulic fluid must have suf-
ficiently high VI to perform its functions at the extremes of the tem-
perature range.
The acceptable range of viscosity and operating temperature is
determined by the manufacturers for each component. For gear
pump, for example, the fluid temperature range is −15 to 80°C, and
the recommended viscosity range is 10 to 300 cSt.
The effect of oil pressure on the viscosity is much less than that of
temperature. The viscosity of fluids increases as its pressure increases
(see Fig. 2.3). These characteristics must be taken into account when

FIGURE 2.3 Variation of the kinematic viscosity of a typical mineral-based oil
with pressure and temperature.