Page 47 - Fluid Power Engineering

24 Cha pte r T w o

FIGURE 2.9
Velocity distribution
in the spool radial
clearance.

surface of the spool is subjected to a shear stress, τ. The velocity dis-
tribution is assumed to be linear in the small radial clearance: c = 2 to
10 μm. An expression for the friction forces is deduced as follows:

du v
τμ = μ (2.25)
=
dy c
πμ DL
τ
F =π DL = v = f v (2.26)
c v
πμ DL
f = (2.27)
v
c
where F = Friction force, N
f = Friction coefficient, Ns/m
v
L = Length of spool land, m
Assignment Derive an expression for the viscous friction coefficient
if the spool is rotated by an angular speed ω under the action of a
torque (T = f ω).
ω
The effect of viscous damping may be demonstrated by studying
the motion of the spool of a directional control valve (see Fig. 2.10).
The spool moves under the action of the driving force (F), the spring

FIGURE 2.10
A spring-loaded
spool valve.