Page 42 - Fluid Power Engineering

Hydraulic Oils and Theor etical Backgr ound 19

planning hydraulic systems which operate at wide pressure ranges.
However, when operating at pressure levels within 300 bar, the effect
of pressure on the oil viscosity is usually negligible.
Effect of Viscosity on Hydraulic System Operation
The oil viscosity influences the function of hydraulic power systems
as it introduces resistance to fluid flow and to the motion of bodies
moving in the fluid. Herein, the following effects are studied:
• Hydraulic losses in transmission lines
• Resistance to fluid flow in narrow conduits
• Viscous friction forces and damping effect

Hydraulic Losses in Transmission Lines, Hydraulic Resistance In hy-
draulic transmission lines, the flow may be laminar or turbulent
depending on the ratio of the inertia forces to the viscous friction
forces. This ratio is evaluated by the Reynolds number, Re. For
laminar flow (see Fig. 2.4), the pressure losses in the line are calcu-
lated using the following relation (see App. 2B at the end of this
chapter):

ρ
ΔP =λ L v 2 (2.4)
D 2
v = 4Q/πD 2 (2.5)

λ= 64/Re (2.6)
=
Re = vD/νρ vD/μ (2.7)

where D = Inner pipe diameter, m
L = Pipe length, m
Re = Reynolds number
v = Mean fluid velocity, m/s
ΔP = Pressure losses in the pipe line, Pa
λ = Friction coefficient for laminar flow
ρ= Oil density, kg/m 3

FIGURE 2.4 Laminar fl ow in pipeline.