Design And Principle Of Operation; Fail-Safe Action - Samson 3274 Mounting And Operating Instructions

3 Design and principle of oper-
ation
The electrohydraulic actuator is used to posi-
tion Series 240, 250 and 280 Control
Valves.
The actuator is fastened to the valve bonnet
with a ring nut. The actuator and plug stems
are fastened together by a stem connector.
The actuator mainly consists of the actuator
housing, motor with oil pump and cylinder
housing with piston.
To control switching and indicating compo-
nents, the actuator travel is converted into a
rotary motion by a rack-and-pinion gear
and transferred to an actuator shaft.
Principle of operation
Î Refer to Fig. 3-1 on page 3-2.
Pilot valves regulate the flow of oil to and
from the piston. The actuator is equipped
with compression springs which determine
the positioning forces. Actuator versions with
spring mechanism also serve to move valve
to the fail-safe position "actuator stem re-
tracts" or "actuator stem extends" in the
event of a supply voltage failure.
The pressure-tight actuator housing (1),
which also serves as the oil reservoir, con-
tains the cylinder housing (2), cylinder (5.1)
and piston (5.2), motor (6.1), pump (6.2)
and solenoid pilot valves (6.4).
The oil pump (6.2) driven by the motor (6.1)
feeds compressed oil to the corresponding
cylinder chamber over the check valve (6.3)
and pilot valve (6.4). The solenoid valves are
closed in the de-energized state and open
EB 8340 EN

Design and principle of operation

when a controller output signal is applied.
When an end position is reached or when
the thrust is too high due to external forces,
the motor is switched off.
Actuators with fail-safe action only have one
force-dependent switch since these actuators
are only moved in the other direction by the
force of the springs.
Depending on the version, the actuators
have no compression springs or are
equipped with one or two compression
springs (5.7 and 5.8 in Fig. 3-1). The motor
in Types 3274-11, -12, -15, -16 as well as
-21 to -23 can only move the stem in one di-
rection. The stem is moved by spring force in
the other direction.

3.1 Fail-safe action

Versions with fail-safe action have a
spring-return mechanism and an additional
safety solenoid valve which opens when the
voltage supply is interrupted, reducing the
pressure on the cylinder chamber. The spring
assembly moves the valve plug to the fail-
safe position. The direction of action (actua-
tor stem extends or retracts) depends on how
the springs are arranged in the actuator.
Testing according to DIN EN 14597
The Type 3274-21 and Type 3274-23 Actu-
ators are tested by the German technical
surveillance association (TÜV) according to
DIN EN 14597 in combination with various
SAMSON valves (registration number on re-
quest).
3-1