ACTUATORS & CONTROLS
; Have duty-cycle motor limitations
; Offer excellent partial stroke capabilities
; Are self-locking in last controlled
position
; Are limited regarding spring-gen-erated failure position
; Have integral manual override
capabilities
Compressed gas (pneumatic) actuators
(Figures 6, 7 and 8) are available in
single- or double-acting versions. Single-acting versions have springs that consume source pressure as the springs compress, thereby providing more spring
force and less air force. For rotary versions, the mechanism used to convert linear to rotary motion also impacts the
output characteristic. Actuator output
must be carefully matched to valve
requirements at all travel positions.
Typically, but not always, you will find
pneumatic actuators are:
; Offered in aluminum, cast iron,
ductile iron, steel and stainless
steel
; Employed in both on/off and control applications
; Not self-locking except in a spring
fail position
; Available with manual override
capability only as a specified
option
; Not duty-cycle limited
; Operated at pressures of 100 psig
or less but are capable of higher
maximum pressures
; Limited to 180º F (82º C) but are
also capable of temperatures of
400º F (204º C) or above with suitable seals and bushing materials
Figure 9. Hydraulic single acting actuator -
linear motion
Hydraulic actuators (Figure 9) are essen-
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tially the same as pneumatics except they
operate on liquid and see considerably
higher pressures.
Typically, but not always, you will find
hydraulic actuators:
; Are offered with linear or rotary
motion in both single or double
acting
; Are often available with an integral electro-hydraulic pressure
source
; Operate at pressures from 500 to
3,000 psig
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CONCLUSION
We used the word “typically” in this article when describing the various actuators
because there are many options and no
absolutes. The key to actuator selection
is to define your own specific needs and
then ask your suppliers to confirm their
products are capable of meeting those
needs.
Future columns will go into the various types of actuators in more detail. VM
ED HOLTGRAVER is CEO of QTRCO, Inc.
( www.qtrco.com), Tomball, TX. He is on the VMA
Board of Directors and serves as the Actuator
Lead on VMA’s Education & Training Committee.
Reach him at edholtgraver@qtrco.com.
