TECHNICAL DATA
SERVOVALVE OPERATION
Jet Pipe
OPERATING PRINCIPLE OF THE JET PIPE SERVOVALVE
Hydraulic fluid at system pressure is fed through a filter screen
to the Jet Pipe that directs a fine stream of fluid at two
receivers. Each receiver is connected to one end of the second
stage spool. At null (no signal to the torque motor), the jet
stream impinges on each receiver equally, therefore equal
pressure is applied at each spool end. All forces on the second
stage spool are equal and it remains at the null position.
When an electrical input signal is applied to the coils of the
torque motor, an electro-magnetic force is created. This force
causes the armature and Jet Pipe assembly to rotate about the
armature pivot point, resulting in more fluid impinging on one
receiver than the other. The resulting differential pressure
between the spool’s end chamber triggers spool movement
and, in turn, uncovers second stage porting causing fluid to
flow to and from, depending on spool direction, the two valve
control ports (A and B).
The direction of spool displacement is opposite to the Jet Pipe
rotation. As the spool moves, the feedback spring generates a
force at the Jet Pipe which opposes the torque motor’s force.
The spool continues to move until the force generated by the
feedback spring equals the force produced by the torque
motor. Then the Jet Pipe position is returned to being
centered over the two receivers. A small differential pressure
usually remains across the ends of the spool to overcome
Bernoulli flow forces that tend to close the valve and feedback
spring forces. The spool displacement is proportional to the
control current in the torque motor. As the spool moves, fluid
is metered proportionally to and from the second stage control
ports (A and B). When input signals to the torque motor vary
in amplitude and polarity, the second stage spool accurately
follows the signals and meters fluid accordingly.
At first stage null, the jet is directed exactly between the two
receivers, making the pressures on both sides of the spool equal.
The force balance created by equal pressures in both end
chambers holds the spool in a stationary position. (See Figure 1a.)
As the jet pipe and armature of the torque motor rotate
around the pivot point (the result of input current), the fluid
jet is directed to one of the two receivers creating a higher
pressure in the spool end chamber connected to that receiver.
The differential pressure created across the spool moves it in
the direction opposite to the jet displacement (See Figure 1b).
Connected to the spool and jet pipe is a feedback spring
assembly, which translates spool position into a force that is
applied on the jet pipe in a proportional manner. Increased
spool displacement away from null, increases the force exerted
on the jet pipe. Forces transmitted from the spool to the jet
pipe are opposing the forces trying to turn the armature jet
pipe assembly. When the feedback spring force is equal to the
forces from the torque motor, the jet is returned to
a position exactly between the two receivers. As mentioned
before, such a position creates a pressure balance between the
end chambers; then the spool will hold its position (See Figure
1c).
Since the torque motor forces are proportional to input
current and the feedback forces are proportional to spool
position, the resulting spool position is proportional to input
current. Increasing current to the torque motor shifts the spool
from null position.
Reversing polarity of the applied current, reverses forces on
the armature and jet pipe. The hydraulic jet flow impinges on
the other receiver, creating an imbalance in spool end chamber
forces. The spool moves in an opposite direction until a first
stage force balance is achieved by the feedback spring. Jet flow
is then directed between the receivers and equal pressure
holds the spool in position.
4 Moog • Jet Pipe