THE ROLE OF RACK AND PINION ACTUATORS IN
ON-OFF & MODULATING
CONTROL
The double-opposed piston rack and pinion actuator is often chosen for actuating rotary valves—
suitable in both on-off and modulating
control applications.
The actuators that make up the majority
of quarter-turn applications have been
proven reliable and durable.
BY JIM CASEY
able at that time, it put both shear and bending loads on the valve shaft, so only flexible hoses for the air lines could be used. By modern standards, this was difficult to
instrument and awkward to set travel stops. Furthermore, the exposed mechanisms
invited contamination and posed injury hazards to personnel.
When major control valve companies entered the market, typical actuation strategy
was to use existing off-the-shelf (globe-valve) spring-diaphragm actuators driving a
linear-to-rotary linkage. Over the years, other configurations and actuator designs
have been developed, and many of them are still available today, including scotch yoke,
vane and electric. Each of these has its place in the market, but this article investigates
the development, limitations and design characteristics of rack and pinion actuators.
ORIGINS
While rack and pinion mechanisms themselves date
back to the dawn of gearing history, an important
development for actuators was helped along by the
Norris brothers, successful investors and fabricators
from Southern England. The brothers set out to
make a purely rotary valve actuator—without preconceived notions of how the actuator should look or
without using a stock of existing components. The
pivotal moment came when the Norris brothers realized that they could oppose two pistons in a single cylinder (Figure 1).
With two pistons, torque was doubled for a given-sized cylinder, and most importantly, the linear piston forces cancelled one another so that the output of the actuator
was purely torsional with no side-load on either the valve components or the internal
actuator bearings. Internal forces exist, including contained or balanced gear separation forces. However, this arrangement made for a more compact and efficient actuator, and canceling the side loads on the shaft allowed seals and bearings to last longer,
both in the valve and in the actuator.
Figure 2 shows the operation of a spring-return rack and pinion actuator positioned
with the valve in the full open condition with pressure applied to P2. Pressure at P2
pushes out the pistons, rotating the pinion
counterclockwise and compressing the
springs on the outside of the pistons. Most
designs can be reversed by flipping the pis-
tons. Some designs are reversible by turn-
ing them top-side down, but it is compli-
cated by the fact that one end of the pinion
may be configured to drive the valve and
the other end may have a NAMUR-compli-
Figure 1: Cutaway of a Norris-
Brothers rack and pinion actuator
Figure 2: Pneumatic diagram of a typical
rack and pinion actuator
