Fisher 685 - Educational Services; Contact Information; Principle of Operation; Pneumatic Operation

Instruction Manual
D103626X012
685 Piston Actuator
June 2017
3
Educational Services
For information on available courses for Fisher 685 piston actuators, as well as a variety of other products, contact:
Emerson Automation Solutions
Educational Services - Registration
Phone: 1-641-754-3771 or 1-800-338-8158
E-mail: education@emerson.com
emerson.com/fishervalvetraining
Principle of Operation
685 piston actuators utilize a pneumatically controlled piston that moves inside of a cylinder to generate thrust. A seal
contained on the circumference of the piston provides a seal between the piston and the cylinder, preventing supply
pressure leakage.
From an equilibrium state, the actuator operates by reacting to a force unbalance that is created by increasing supply
pressure on one side of the piston, and decreasing it on the other. This moves the piston up or down, and results in a
repositioning of the attached control valve. Travel can be adjusted using travel limits within a valve positioner, which
limit the travel range of the actuator. The optional handwheel manual override does not have the ability to act as a
hard travel stop.
An optional handwheel manual override is capable of extending or retracting the actuator manually and can be
engaged at any position from full open to full close. This override utilizes a worm gear assembly that is attached to the
stem connector and not attached to the cylinder or piston rod. This enables the manual override to reposition the
control valve even if the actuator cylinder or piston is removed for maintenance.
Table 2. Standard Constructions
(1)
ACTUATOR
SIZE
PISTON
DIAMETER
PISTON ROD
AREA
PISTON AREA
VALVE STEM
CONNECTOR SIZE
YOKE BOSS
DIAMETER
VALVE TRAVEL
Minimum Maximum
mm (cm
2
for Area)
12 305 16 730 32 or 51 127 or 178 >203 610
14 356 32 993 32 or 51 127 or 178 >203 610
16 406 32 1297 32 or 51 127 or 178 >203 610
18 457 32 1642 32 or 51 127 or 178 >203 610
20 508 46 2027 32 or 51 127 or 178 25 610
22 559 46 2452 32 or 51 127 or 178 25 610
24 610 62 2919 32 or 51 127 or 178 25 610
26 660 62 3425 32 or 51 127 or 178 25 610
Inches (inch
2
for Area)
12 12 2.41 113 1-1/4 or 2 5H or 7 >8 24
14 14 4.91 154 1-1/4 or 2 5H or 7 >8 24
16 16 4.91 201 1-1/4 or 2 5H or 7 >8 24
18 18 4.91 254 1-1/4 or 2 5H or 7 >8 24
20 20 7.07 314 1-1/4 or 2 5H or 7 1 24
22 22 7.07 380 1-1/4 or 2 5H or 7 1 24
24 24 9.62 452 1-1/4 or 2 5H or 7 1 24
26 26 9.62 531 1-1/4 or 2 5H or 7 1 24
1. Consult your Emerson sales office or LocaL Business Partner for additional sizes.