Instruction Manual
D101728X012
3710 and 3720 Positioners
September 2017
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determined by the location or rise of the cam which is attached to the feedback shaft. When the two opposing forces
are equal or at a steady state, the summing beam holds the spool in a neutral position. At steady state, a small flow of
air passes from supply through both outputs of the spool valve to the actuator, holding the actuator at a constant
position. At the same time, another small flow of air exhausts out each end of the spool valve.
When the input pressure is increased to the diaphragm of the input module, the diaphragm strokes down, increasing
the effective force from the input module and compressing the range spring. The summing beam moves the spool
down in the spool body, opening output port B to supply air to the left side of the actuator. At the same time, output
port A of the spool valve opens, allowing the right side of the actuator to vent to atmosphere.
The piston in the actuator moves to the right, rotating the feedback shaft and cam counterclockwise. This rotation
causes the feedback lever to rotate clockwise, increasing the compression on the range spring. These rotations
continue until the additional force from the spring balances with the input module force on the summing beam. When
the forces are equal, the summing beam returns to its steady state or neutral position and the actuator is held at a new
position.
Maintenance
Parts are subject to normal wear and must be inspected and replaced as necessary. The frequency of inspection and
replacement depends upon the severity of service conditions. The following procedure describes disassembly and
reassembly of the positioner. When inspection or repairs are required, disassemble only those parts necessary to
accomplish the task. When reassembly is complete, adjust the positioner as described in the Calibration section of this
manual.
WARNING
Avoid personal injury or equipment damage from sudden release of process fluid. Before performing any positioner
maintenance:
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Wear protective eyewear, gloves and clothing whenever possible when performing any maintenance operation.
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Do not remove the actuator from the valve while the valve is still pressurized.
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Disconnect any operating lines providing air pressure, electric power, or a control signal to the actuator and
accessories. Be sure the actuator cannot suddenly open or close the valve.
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Use bypass valves or completely shut off the process to isolate the valve from process pressure. Relieve process pressure
on both sides of the valve.
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Vent the actuator loading pressure.
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Use lock‐out procedures to be sure that the above measures stay in effect while you work on the equipment.
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Check with your process or safety engineer for any additional measures that must be taken to protect against process
media.
Positioner Maintenance
Replacing the Standard or Beacon Indicator
1. Remove the positioner cover assembly (key 7).
2. Remove the four machine screws (key 8) that hold the indicator scale (key 14) to the positioner cover. Remove any
other external indicator parts such as the pointer (key 13).
Note
The standard, low‐profile indicator uses a retaining ring (e‐clip) (key 12) inside the positioner cover. The retaining ring holds the
indicator. After removing the external scale, remove the internal retaining ring to free the indicator.
