Fisher 3570 Instruction Manual page 16

3570 Positioners
September 2015
6. Rotate the cable spool to obtain the correct initial range spring extension. Each full revolution of the spool extends
the range spring 50.8 mm (2 inches) [6.4 mm (1/4‐inch) for 1/8 revolution]. If the initial range spring extension is
not specified, calculate it using one of the equations given below. Round off the amount of extension (e) obtained
from the equation to the next higher 6.4 mm (1/4 inch).
T
e =
1.87 (P )
n
T
(optional high pressure bellows)
e =
1.25 (P )
n
where:
e = initial range spring extension required in mm
T = actuator travel in mm
P = input signal span in bar (for example, 0.8 bar for a 0.2 to 1 bar input signal range)
n
or where:
e = initial range spring extension required in inches
T = actuator travel in inches
P = input signal span in psi (for example, 12 psi for a 3 to 15 psig input signal range)
n
7. If necessary, move the actuator cable ball to the spool slot nearer the access opening. With the actuator piston rod
fully retracted and the range spring at the correct initial extension, attach the cable strap to the actuator feedback
arm. Use the set of cable strap holes that is closest to the tapped holes in the feedback arm.
8. Refer to the positioner adjustment procedures.
16
(standard bellows)
Instruction Manual
D200137X012