Samson 3767, Mounting And Operating Instructions

Page: 28 / 68

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.