ACTUATORS & CONTROLS
Producing Linear Output with a
Rotary Electric Actuator
BY PAUL SOUZA
Linear actuators come in a variety of types, which are typically broken
down by power source: i.e., hydraulic,
pneumatic and electric. This article will
address two different ways to achieve
linear motion using electric rotary actuators.
The many applications suited for linear output of an actuator can be broken
down further by stroke length. Short
stroke applications include diaphragm
valves and control valves. Applications
that require a long stroke typically
include sluice and weir gates. However,
there are many other special applications well suited for linear actuators
including paper machines, dampers,
decanters, skimmers and much more.
First, let’s discuss the rotary actuator to linear output using a threaded
valve stem with stem nut. Secondly,
we’ll address rotary actuators with an
integrated linear output drive. Please
note that most of the information in this
article can be used with manually operated actuators (manual multi-turn gearboxes) because we simply convert the
rotary output of the actuator to a linear
Since rotary electric actuators are a
frequent topic of discussion in Valve
Magazine, we’ll let readers refer to other
issues for information on these actuators. For purposes of this article, we
simply need to note that the output of
the actuator rotates clockwise and
Rotating a threaded stem nut within
an output drive will cause the threaded
valve stem to rise and fall depending on
the rotational direction of the actuator.
This sounds simple; but after many
more years in this industry than I care
to admit, I hear almost every week
about a “wrong rotation” actuator. This
is typically an issue when a right-hand
valve stem is attached to a valve instead
of a left-hand stem. It’s also often an
issue when a valve or gate closes in the
opposite direction (weir valve vs. gate
valve). However, with good communication between supplier and customer,
such issues are usually preventable.
Usually, a simple reprogramming or
wiring change of the actuator can fix
The stem nut also must be made of a
material that will not gall to the valve
stem. Typical stem nut materials include
aluminum, bronze, stainless or cast
steel, and sometimes plastic. Usually,
choosing a material that is dissimilar
from the valve stem material is
The stem nut should be supported
both top and bottom with a thrust-type
bearing assembly in a separate drive
housing. This separate housing will provide support for the stem nut and allow
for easy installation and removal of the
actuator if required. Additionally, if the
output drive assembly is supplied with a
grease fitting, it should be lubricated at
the time of installation and that lubrication should be retained with a high quality lubricant designed for thrust-bearing
The basics for the rotary actuators with
stem nut type output drives are:
Actuators that have an integral linear
Figure 1. Threaded stem nut and valve stem.
One of the benefits of this design is that very
long operating strokes are possible.
output drive may also be part of a rotary
actuator combination. The linear integral
output drive assembly would replace the
threaded valve stem, actuator stem nut
and associated parts shown in Figure 1.
As the actuator rotates clockwise and
counter clockwise, the linear output drive
will extend or retract. This type of output
drive is usually limited in stroke to 20
inches or less.
A threaded stub shaft is usually provided on the end of the output drive to
attach a coupling for valve attachment.
An integral valve mounting flange
would be attached to the base of the linear drive.
As stated earlier, since there are
reverse operating valve applications, the
linear output drive should also come in
a version that extends or retracts clockwise.
Options that may be available
include pedestal mounting for pivoting
applications, and spring compensation