Samson 3766 - Basic Setting of the Cam Disk

28 EB 8355-1 EN
Mounting on control valves
3.3.3 Basic setting of the cam
disk
The valve model used determines the basic
setting of the cam disk.
Cam disks tailored to the special characteris-
tic 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 position (open-
ing angle) does not constitute a system devi-
ation of the positioner.
Fig.8andFig.9showlinearcamdisks.
Fig.8illustratesacontrolvalveassembly
with a rotary actuator with spring-return
mechanism that opens counterclockwise. The
arrangement of the springs in the actuator
determines the fail-safe position of the valve.
Fig.9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
holes. Depending on the operating direction
of the rotary actuator (air-to-open or air-to-
close), the starting point of the cam, either
marked N (standard characteristic) or I (re-
verse characteristic), must point towards the
lever with feeler roll. When the starting point
is located on the back of the cam disk, turn
over the cam disk.
The starting point (hole) of the selected cam
must be aligned so that the fulcrum of the
cam disk and 0° position on the scale as
well as the arrow on the window are in line
with each other.
When aligning the cam disk, clip the dou-
ble-sided scale disk on the cam disk, while
making sure that the value on the scale
matches the valve's direction of rotation.
Make sure the 0° position of the scale al-
ways corresponds to CLOSED position.
Therefore, for fail-open actuators and for
springless actuators, the maximum supply
pressure needs to be applied to the actuator
before aligning the cam disk.
NOTICE
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NOTICE
!
NOTICE
!