2.3.3 Default setting of the cam
disk
The valve model used determines the default
setting of the cam disk.
NOTICE
Cam disks tailored to the special character
-
istic of a valve cause the valve to open in a
non-linear or non-equal percentage way.
The visible difference between the set point
(4 to 20 mA) and the actual value (opening
angle) does not constitute a system deviation
of the positioner.
Figs. 9 and 10 show linear cam disks.
Fig. 9 illustrates a control valve assembly
with a spring-loaded rotary actuator that
opens counterclockwise. The arrangement of
the springs in the actuator determines the
fail-safe position of the valve.
Fig. 10 shows how to adjust the cam disk
when a double-acting springless rotary actu-
ator is used. The direction of rotation, either
counterclockwise or clockwise, depends on
the actuator and valve model used. The cam
disk must be set when the valve is closed.
Use the turnboard (7) to adjust the operating
direction of the positioner, i.e. whether the
valve opens or closes when the reference
variable increases (direct >> or reverse <>).
Each cam disk carries two cam sections
whose starting points are indicated by small
bores. Depending on the operating direction
of the rotary actuator – signal pressure
opens or closes the valve – the starting point
of the cam, either marked N (standard char
-
acteristic) or I (reverse characteristic), must
point towards the cam follower roll. When
the starting point is located on the back of
the cam disk, turn over the cam disk.
NOTICE
The starting point (bore) of the selected cam
section must be aligned with the fulcrum of
the cam disk, the 0° position of the scale,
and the arrow symbol on the inspection
glass.
When aligning the cam disk, the double-
sided scale disk must be clipped on the cam
disk such that the value on the scale corre
-
sponds to the control valve's direction of ro-
tation.
NOTICE
Make sure the 0° position of the scale al-
ways corresponds to CLOSED position.
For actuators with fail-safe position "Valve
OPEN" and for springless actuators, it is
therefore necessary to apply the maximum
supply pressure to the actuator before align-
ing the cam disk.
22 EB 8355-1 EN
Attachment to control valve
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