and the valve shaft is a clamped fitting.
Some manufacturers use a split coupling
while others use a clamping collar or
setscrews. The clamping device ensures
no mechanical play between the actuator
pinion spindle and the valve shaft. Actuators with female output shafts require a
precision fit. Some assemblers use
threadlocker on these components. The
goal for all of this is to avoid lost motion
in the drive-train.
At the top of the shaft is the NAMUR
connection that engages the positioner
feedback shaft: a pilot bore to center the
connection and a slot to drive the positioner. Analog positioners have cams and
feedback springs, and the action of the
spring on the cam causes a continuous
torque on the shaft, which preloads the
positioner shaft and prevents any backlash in the connection.
Any tolerance between the spindle and
the positioner shaft is closed by the
torque, and the relationship between the
spindle and positioner shaft never
changes. Early digital positioners did not
employ biasing springs; however, today
these springs are almost universal. The
biasing spring applies a continuous
torque so that the relationship between
the positioner shaft and the actuator
spindle is fixed.
The position of the final control element (e.g., ball, disc or plug) is transmitted through a fitted, splined or pinned
joint, through a clamped coupling and
through a spring-biased coupling, allowing the valve positioner to know the exact
position. The piston-pinion connection
falls outside the connections involved
with the positioner so it has no effect on
the precision of the steady-state control
using these actuators. “Steady-state”
applies if there is lash between the rack
and pinion and if the control valve is a
butterfly valve buffeted by system turbulence, which means the valve position
could chatter. This is unlikely to occur,
but because it’s possible, it should be considered in the specification stage.
FRICTION
Another issue with rotary valve and actu-
ator systems is friction. However, the
friction of the valve/actuator package is
primarily a function of the valve, not the
actuator. A rack and pinion actuator that
has been inactive for a long period may
take 5 psi (. 34 Bar) differential to begin
moving, while a recently cycled rack and
pinion actuator has a breakout pressure
of about 1 psi (.07 Bar). Considering
these actuators typically operate with air
supplies in the range of 60 to 80 psi ( 4. 1
to 5. 5 Bar), this is a small offset.
