05-03-2013, 10:22 AM
The Norgren Guide to SpecifyingPneumatic Actuators
The Norgren Guide.pdf (Size: 1.29 MB / Downloads: 232)
INTRODUCTION
Pneumatic actuators, of which cylinders are the most common,
are the devices providing power and movement to automated
systems, machines and processes. A pneumatic cylinder is a
simple, low cost, easy to install device that is ideal for
producing powerful linear movement over a wide range of
velocities, and can be stalled without causing internal damage.
Adverse conditions can be easily tolerated such as high
humidity, dry and dusty environments and repetitive
cleandown with high pressure hoses.
The diameter or bore of a cylinder determines the maximum
force that it can exert and the stroke determines the maximum
linear movement that it can produce. Cylinders are designed to
work at different maximum pressures up to 16 bar. The
pressure actually supplied to a cylinder will normally be
reduced through a pressure regulator to control the thrust to a
suitable level. As an example of cylinder power, a 40mm bore
cylinder working at 6 bar could easily lift an 80kg man.
The basic construction of a typical double acting single rod
cylinder is shown in the cut away section (Figure 1), where the
component parts can be identified.
SINGLE ACTING CYLINDERS
Single acting cylinders use compressed air for a power stroke
in one direction only. The return stroke is effected by a
mechanical spring located inside the cylinder. For single
acting cylinders with no spring, some external force acting on
the piston rod causes its return. Most applications require a
single acting cylinder with the spring pushing the piston and
rod to the instroked position. For other applications sprung
outstroked versions can be selected. Figure 2 shows both
types of single acting cylinder.
DOUBLE ACTING CYLINDERS
Double acting cylinders use compressed air to power both the
outstroke and instroke. This makes them ideal for pushing and
pulling within the same application. Superior speed control is
possible with a double acting cylinder, achieved by controlling
the exhausting back pressure.
Non cushioned cylinders will make metal to metal contact
between the piston and end covers at the extreme ends of
stroke. They are suitable for full stroke working only at slow
speeds which result in gentle contact at the ends of stroke
(Figure 4). For faster speed, external stops with shock
absorption are required. These should be positioned to prevent
internal contact between the piston and end covers.
RODLESS CYLINDERS
For some applications it is desirable to contain the movement
produced by a cylinder within the same overall length taken up
by the cylinder body. For example, action across a conveyor
belt, or for vertical lifting in spaces with confined headroom.
The novel design of a rodless cylinder is ideal in these
circumstances. The object to be moved is attached to a
carriage running on the side of the cylinder barrel. A slot, the
full length of the barrel, allows the carriage to be connected to
the piston. Long sealing strips on the inside and outside of the
cylinder tube prevent loss of air and ingress of dust. The slot is
unsealed only between the lip seals on the piston as it moves
backwards and forwards (Figure 8). Direction and speed
control is by the same techniques as applied to conventional
cylinders.
CLAMPING CYLINDERS
For use in confined spaces where only a short stroke is
required these cylinders have a small axial overall dimension
for their bore size. They are mostly used in single acting
versions (Figure 12), but are also available as double acting
through-rod styles (Figure 13). They are usually used in light
duty applications.
USABLE THRUST
When selecting a cylinder size and suitable operating pressure,
an estimation must be made of the actual thrust required. This
is then taken as a percentage of the theoretical thrust of a
suitably sized cylinder. The percentage chosen will depend on
whether the thrust is required at the end of movement as in a
clamping application or during movement such as when lifting
a load.
CLAMPING APPLICATIONS
In a clamping application the force is developed as the cylinder
stops. This is when the pressure differential across the piston
reaches a maximum. The only losses from the theoretical
thrust will be those caused by friction. These can be assumed
to be acting even after the piston has stopped. As a general
rule, make an allowance of 10% for friction. This may be more
for very small bore cylinders and less for very large ones.
If the cylinder is operating vertically up or down the mass of
any clamping plates will diminish or augment the clamping
force.
DYNAMIC APPLICATIONS
The actual thrust and speed from a moving cylinder are
determined by friction and the rate at which air can flow in and
out of the cylinder’s ports. The thrust or pull developed is
divided into two components. One for moving the load, the
other for creating a back pressure to help expel the air on the
exhausting side of the piston.
For a lightly loaded cylinder, most of the thrust is used to expel
the back pressure and will result in a moderately fast speed.
This is self limiting however as the faster the speed, the less
will be the pressure differential across the piston. This is due
to the increasing resistance through the ports, tubing, fittings
and valve as the rate of flow increases.