Specifications

The upward movement of the piston rod
moves the cam downward. This causes the
follower arm, riding on the cam, to lift the
spring nut, increasing pressure on the
calibration spring. This increased pressure on
the calibration spring returns the diaphragm
to its neutral position, closing the pilot valve
air ports. Without additional air pressure,
piston movement is stopped.
As signal air decreases the calibration spring
pressure moves the diaphragm up. The
upward movement of the diaphragm nioves
the pilot valve. stem up, directing air above
the piston. This forces the piston, piston rod,
and linkage lever downward. The downward
movement of the piston rod, working
through the cam and follower arm, lowers
the calibration spring socket and reduces
pressure on the calibration spring. This
decreased pressure on the calibration spring
returns the receiver’s diaphragm to a neutral
position closing the pilot valve air ports.
Cam. The standard cam from Rosemount
produces a linear relationship between input
signal and the distance the operating lever is
moved. Additional cams can be purchased
from Rosemount to produce either a squared
(2) relationship or a square root (Jx)
relationship. Custom cam shaping in the
field can produce other relationships needed.
Refer to Section II, Installation for
procedures to custom shape a cam.
Inverse Operation. On inverse acting power
positioners, the cylinder air hoses and the
cam position are reversed. This causes the
supply air to be directed to the top of the
piston when signal air pressure is increased,
and to the bottom of the piston when signal
air pressure is decreased. In this type of
installation, piston movement is inversely
related to the signal - as signal pressure
decreases, the piston raises, as signal
pressure raises, the piston lowers.
Manual Operation. The power positioner
can be controlled manually through the
manual operator handwheel. The handwheel
is connected to a sprocket and chain which
turns a worm shaft when the wheel is
rotated. This worm shaft moves a sector
gear attached to the power positioner shaft.
d.
The sector gear movement is transferred to
the linkage lever to control the position of
the device being controlled. To place the
power positioner in manual operation move
the transfer valve to the manual position.
Svstem Considerations. Prior to installation of
your Rosemount 8 x 14 Power Positioner, check
that you have all the components necessary to
make the complete system installation.
Once you have verified that you have all the
components, select mounting location. A typical
installation is illustrated in Figure l-3. Determine
where power positioner will he placed in terms of
serviceability, available power supply, ambient
temperatures, environmental considerations, and
convenience. Power positioner operating
specifications are listed in Table l-2. Become
familiar with Section II, Installation, before
installing unit.
I
AIR FLOW
f fff
SUPPLY
AIR
OPERATING
LEVER
PWWB
Figure l-3. Typical Power Positioner Installation