3660 and 3661 Positioners
Instruction Manual
Form 5265
March 2006
23
A
PILOT
SHAFT
(KEY 19A)
FEEDBACK LEVER
ASSEMBLY (KEY 19)
FEEDBACK SHAFT
LOCATION
38B0195-B / DOC
Figure 20. Pilot Shaft Setting
14. Set the input signal to the maximum value.
15. Readjust the travel (span) adjustment to achieve
correct actuator travel.
Note
When a travel (span) adjustment is
made, there will be a zero shift.
16. Repeat steps 11 through 15 as necessary to
achieve correct actuator travel.
17. Install the cover (key 19T) on the feedback lever
assembly (key 19) with cover screw (key 19U).
18. Install the positioner cover (key 21) and secure
with the machine screws (key 24). Make sure the
Fisher logo reads correctly and the vent is pointing
downward.
Split-Range Operation
Type 3660 and 3661 positioners can be used for
split-range operation with the instrument input signal
from a single controller or another instrument split
between two or three control valves. Tables 6 and 7
show some typical split ranges for the positioners.
To change from a full range to a split range, change
the range spring (key 30, figure 24 or 25) to the
appropriate spring shown in the tables. Complete the
Changing Range Spring portion of the Maintenance
section. Refer to tables 6 and 7 for valve stem travel
available with split range operation.
Type 3660 Bypass Operation
Type 3660 positioners may be supplied with a
bypass assembly.
CAUTION
Do not use bypass operation when the
positioner is reverse acting or is in
split-range operation. In these cases,
bypassing the positioner sends the
input signal directly to the actuator.
Such a change will affect the desired
operation and possibly upset the
system. Use bypass operation only
when the instrument signal range is
the same as the positioner output
range required for normal actuator
operation.
Labels on the bypass body assembly (key 41, figure
23), and a pointer on the bypass lever (key 42 in
figure 23) indicate if the input signal from the
instrument goes to the positioner or directly to the
control valve actuator.
With the pointer of the bypass lever over the word
POSITIONER, the instrument pressure goes to the
positioner and the output pressure of the positioner
goes to the actuator.
With the pointer of the bypass lever over the word
BYPASS, the instrument pressure goes directly to
the actuator.
Note
A difference between the input signal
pressure and the positioner output
pressure could cause a transient bump
in the controlled system when the
bypass lever is moved to BYPASS.
With a reverse-acting or split-range positioner, the
bypass lever may be secured in the POSITIONER
position so that bypass cannot be used. To lock the
bypass lever in the POSITIONER position, shut off
the instrument and supply pressure to the positioner.
Then, move the bypass lever (key 42 in figure 23) so
the pointer is over the word POSITIONER. Align the
hole in the pointer with the hole in the body
assembly and thread the plastic wire tie (key 79 in
figure 23) through both holes to secure the bypass
lever.
Principle of Operation
Refer to figure 21 for the operational schematic.
The instrument pressure acts on the input module,
which controls the flapper-nozzle system of the
