Valve Magazine - Spring 2010

ant drive for accessories. There is a pis-ton-effect on most pinion designs in which one end (usually the bottom) must be large enough for the pinion to be inserted into the case and the other end is customarily smaller (illustrated in Figure 3). Pressure in the actuator, acting against different upper and lower areas, causes an axial load on the pinion. In this case, an inverted actuator would have a greater chance of pinion blowout; if the actuator is installed conventionally, the pinion would be secondarily retained by the valve stem.

This cutaway differs from the Norris Brothers’ design, which uses rods for guiding the piston. Almost all rack and pinion actuators produced since the original design use thrust pads to absorb the side load on the skirts. Since the faces of the rack and pinion teeth have slope, a small component of the force exerted by the pistons acts to lift the racks off the pinion. The thrust pad keeps the racks properly meshed with the pinion. These thrust pads are commonly molded of an engineering resin with a large portion of dry lubricant such as 25% MoS2, which helps reduce friction. Another notable difference between the designs is that the actuator pictured in Figure 2 uses concentric springs while the Norris Brothers design uses multiple parallel springs.

Releasing the pressure at P2 enables the springs to push the pistons together, rotating the pinion clockwise and closing the valve. Springs are optional—used to ensure fail-safe operation on loss of air pressure—and they are selected so the torque generated during opening, using supply air pressure at P2, matches the torque when the springs close the valve. Without springs, the actuator may be operated as double-acting by applying pressure at P1 to close the valve.

The actuator shown in Figure 2 has an
outward travel stop, also called a stop-
bolt, in the right endcap and an inner
travel stop in the left endcap. Other actu-
ator designs may apply the travel stops
against the pinion. There are advantages
and disadvantages to each of the loca-
tions for the stops. The full-travel stops
shown in the endcaps in Figure 2 may be
adjusted to stop the valve at any point in
its travel, in either the opening or closing
direction. These place a load on the cen-
terline of the piston that is transmitted
back through the rack to the pinion,
causing an asymmetric load on the actua-
tor bearings when the assembly rotates
far enough to hit the stop. Stop-bolts can
also be arranged to engage the pinion
directly, by placing them in the side of the
actuator housing where they engage a
shoulder on the pinion gear itself. These
arrangements generally offer adjustabili-
ty over a much shorter range; typically,
on the order of plus or minus 6 degrees
and place significant bending loads on
the stop bolts at the extremes of adjust-
ment. In this arrangement, asymmetrical
loading on the pinion bearings still occurs
when the stop-bolts are engaged, but the
force is not transmitted to the rack(s).

Figure 3: Cutaway of a typical actuator illustrating the travel stops and pinion design

to be subtracted: a preload multiplied by a constant and the stroke of the actuator. The torque curve will be a straight line, sloping downward.

A typical ball valve requires the highest torque to open from its closed position due to the process differential pressure exerted against a closed valve and a “ set-torque” that occurs as the resilient seat material conforms to small irregularities in the surface of the ball. As the ball is opened, the run torque drops to about 60% of the break torque, out to full-open position. About 80% of the break torque is required to re-seat the valve. For a fail-

VMA’S ONLINE PRODUCT FINDERS

The quick and easy way
to locate:
• More than 90 different
product types and
services
• More than 120 manu-
facturers, suppliers
and service companies
Visit the VALVE,
ACTUATOR & CONTROLS
PRODUCT FINDER to
locate U.S. and Canadian
companies that manufac-
ture valves, actuators and
controls, and choose
from 40 different valve
and actuator types. Search by product category, end-user industry or company.

Visit the SUPPLIERS & SERVICES PRODUCT FINDER to locate companies that offer a wide array of products and services used by valve and actuator manufacturers, from castings to gaskets, to valve testing facilities and instrumentation, to flowmeters and diaphragms… and dozens of other product types. Search by product type, end-user industry or company.

Find the Product Finders on VMA.org. Just look for the link on the navigation bar on left side of the home page, and choose which of the two Product Finders you want to search.