Samson 3767 - Basic Setting of the Cam Disk

28 EB8355-2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.
NOTICE
Cam disks tailored to the special
characteristic 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 (opening angle) does not
constitute a system deviation of the
positioner.
Fig.9andFig.10showlinearcamdisks.
Fig.9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.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
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.
NOTICE
The starting point (hole) of the select-
ed cam must be aligned so that the
fulcrum of the cam disk and 0° posi-
tion 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.
NOTICE
Make sure the 0° position of the scale
always corresponds to CLOSED posi-
tion.
Therefore, for fail-open actuators and
for springless actuators, the maxi-
mum supply pressure needs to be ap-
plied to the actuator before aligning
the cam disk.