Specifications

Force Due to Acceleration
The forces required to overcome inertia become very large
in high speed applications and are critical to valve sizing.
F
A
= Ma
V
MAX
a =
T
a
W
L
+W
P
M =
g
Force Due to External Disturbances
These forces can be generated by constant or intermittent
sources.
Force Due to Seal Friction
Most valves are used on applications which employ some sort
of motion device.These motion devices usually utilize elastomer
seals to separate the various pressure chambers.The friction
between these seals and the moving parts acts as opposing force.
Standard practice involves setting seal friction at 10% of
the maximum force available, unless absolute values are known.
FL = mWLcosØ (lb)
WL
FL
Ø
WL
FL
FL = mWL
WL = weight of load (lb)
m = coefficient of friction
FE
CONSTANT
EXTERNAL
COMPRESSION
OR TENSILE
FORCE
PRESS
INTERMITTENT
DEFORMATION
FORCE
F
E
where:
M = mass (lb - sec
2
/in)
a = acceleration (in/sec
2
)
W
P
= weight of piston (lb)
V
MAX
= maximum velocity (in/sec)
T
a
= time period for
acceleration (sec)
W
L
= weight of load
F
S
= 0.1 F
MAX
where:
F
MAX
= stall force (lb)
Dynamic Response
A valve’s dynamic response can be easily determined by measuring
the frequency at which the phase lag between the input current
and output flow reaches 90˚ (90˚ phase lag point).The frequency
response will vary with input signal amplitude, supply pressure,
and fluid temperature.Therefore, comparisons must use consis-
tent data.The recommended peak-to-peak signal amplitude is 80%
of the valve rated current. Servovalve and ServoJet
®
response will
improve somewhat with higher supply pressure, and generally
depreciate at both high and low temperatures. Direct Drive Valve
response is independent of supply pressure.
Load Resonant Frequency
Open loop control consists of a human operator monitoring the
parameter (i.e., position or speed) and varying the input of the
control valve to obtain the desired result. Closed loop control is
capable of fast, more accurate control and requires a high perfor-
mance control valve. For optimum performance, the valves 90˚
phase point should exceed the load resonant frequency by a factor
of three or more. Load resonance is determined by the overall
stiffness (K
A
), which is the combination of the hydraulic stiffness
(K
O
) and the structural stiffness (K
S
), given by:
The load resonant frequency for an equal area cylinder is given by:
NOTE:Typical bulk modulus (ß) Å 2.0 x 10
5
psi
4
0
-4
-8
-12
-16
250
225
200
175
150
125
90
75
50
25
0
5 10 20 30 50 100 200 300 500
Hz
Degrees(˚)
K
O
K
S
(lb/in)
K
A
=
K
O
+ K
S
1 K
O
ƒ
N
=
2¹ M
where:
ƒ
N
= load resonant frequency (Hz)
K
O
= hydraulic stiffness (lb/in)
where:
ß = bulk modulus of fluid used (psi)
A = working area of double ended
piston (in
2
)
X
T
= total piston stroke (in)
where:
s = actuator volumetric efficiency
X
m
= piston stroke used for
application (in)
V = total volume of fluid between
valve control ports and
the piston (in
3
)
4s ßA
K
O
=
X
T
AX
m
s =
V
TYPICAL BODE PLOT OF DYNAMIC RESPONSE
5