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9/12Our policy is one of continued research and development. We therefore reserve the right to amend,

without notice, the specifications given in this document.© Norgren, Inc. 2012

Norgren eBookSelecting Pneumatic Cylinders

for OEM Applications

Selecting Pneumatic Cylinders for OEM Applications

Cylinders today have evolved into an almost endless array of variations, alternate configurations, sizes, and special designs. This variety makes more innovative equipment designs possible, but presents a challenge to designers to specify the best cylinder for an application. This eBook presents an overview of what engineers need to know in choosing the most efficient standard — or custom — pneumatic cylin-ders for their applications.

Consider the applicationSelecting the right cylinder begins with understanding the application. Designers need to start by answering some very basic questions.

• What has to be accomplished? What is the desired end result of the operation?

• What work will the cylinder perform? Will it move or lift an object? Open or close a valve?

• What is the load? How powerful will the cylinder need to be?

• How far must it move?

• What is the required cycle time? How quickly must the cylinder stroke and recover?

• How will the motion be stopped?

• In what environment will the cylinder operate? Will it be exposed to extreme temperatures or caustic materials?

Every day pneumatic cylinders are used in industry to generate force and provide linear motion on a vast variety of OEM equipment.

They have the capability of moving product directly or indirectly, by moving components within the machine acting on the product. They do this by pushing or pulling, lifting or lowering, rotating or just clamping the necessary components.

Pneumatic cylinders are simple, cost-effective, easy-to-install devices. They can produce high force and a broad range of velocities. Their motion can be stalled without causing internal damage. Varying cylinder materials tolerate adverse conditions such as high humidity, dry and dusty environments, and repetitive washdown with high-pressure hoses. That said, there are a number of investigations that must be made to select the specific cylinder that can assure long-term success of each application, as well as the proper overall function of the machinery on which they are installed.

By Sheila Campbell, Product Manager, Actuators

Selecting Pneumatic Cylinders



Cylinder typesThe original industrial cylinder, still in use today, Figure A, consisted of a tube or barrel — the cylinder — closed by sealed end pieces to form the envelope. Inside was a sealed piston. A rod, attached to the piston, extended through a sealed opening in one of the ends. The load was connected to or contacted by the piston rod and the cylinder was mounted so it could not move. A port at one end of the cylinder allowed compressed air to act on one side of the piston, causing it (and the piston rod) to move. A port in the other end of the cylinder allowed air from the opposite side of the piston to escape — usually to atmosphere. When the roles of the two ports were reversed, the piston and rod stroked in the opposite direction.

The most common pneumatic cylinder is still the rod style pictured in Figure A. As the name implies, it usually has a rod protruding from one or both ends. Other models have multiple rods passing through the same end to prevent piston rotation. Rod-style cylinders function in two ways, double-acting and single-acting, both of which come in a variety of types, including repairable, disposable, compact, guided or bellows cylinders. There also are rodless pneumatic cylinders.

Double-acting cylinders (Figure B) use compressed air to power both the extension and retraction strokes, moving the cylinder back and forth. This arrangement makes them ideal for pushing and pulling loads within the same application. Superior speed control is possible with double-acting cylinders, achieved by controlling the rate at which air exhausts.

Single-acting cylinders (Figure C) accept compressed air on only one side of the piston. The volume on the other side of the piston is vented to atmosphere. Depending on whether it is routed to the cap end or rod end, the compressed air may extend or retract the piston rod. The most common type of single-acting cylinder is pressure-extended; then an internal spring powers the return stroke after pressure is exhausted. In other designs, gravity or an external spring returns the piston to its original position.

FIGURE A

FIGURE B

FIGURE C

Selecting Norgren Cylinders



Repairable cylinders can be disassembled to replace seals and other internal components. This procedure extends the cylinder’s life cycle. These cylinders are generally robust in design and will be used in applications requiring heavier-duty components.

Disposable or sealed-for-life cylinders (Figure D) have end caps mechanically crimped to the tube. Internal components are pre-lubed before the cylinder is assembled. While they are less expensive to manufacture, they cannot be taken apart to attempt repair without destroying the housing. These cylinders usually are used in lighter-duty applications and must be replaced as opposed to repair when they reach the end of their life cycle.

Compact-style cylinders (Figure E) are designed to fit into smaller spaces where only a short stroke is required. They are used in lighter-duty applications due to their smaller bearing surface for the rod to ride against. They are mostly found in single-acting versions, but double-acting styles also are available.

Guided cylinders (Figure F) serve applications with significant side loads or that require that the load be guided, for example, down a conveyor. A dual-rod cylinder or a guide block may be the solution.

FIGURE D

FIGURE E

FIGURE F




With the addition of mechanical components, a cylinder’s linear motion can be converted to angular rotation that can exceed 360 degrees. The rack-and-pinion rotary actuator (Figures G and H) — with a rack mounted on the rod — is often used in the process industry to operate quarter-turn valves. (Vane-type air motors drive other rotary actuators.) Sizing rotary actuators requires torque information about the load to be turned.

Bellows (Figure I) are durable, single-acting, concertina-like actuators with flexible, reinforced elastomer walls and metal end plates. They extend when inflated and develop powerful strokes because of their large diameters. Their cylindrical configuration allows them to bend in any direction, making them useful where loads might vary in angle. Some application cautions: the maximum extension and compression of the bellows must be limited by external restraints. Unrestrained extension can blow off the end plate; exhaust without restraint can let the load crush the sidewalls.

The rodless cylinder (Figures J and K) has no rod extending through its end caps. Instead, an external carriage slides back and forth on the tube’s surface. The load mounts on this carriage. An internal piston is mechanically connected to the carriage through a sealed longitudinal slot in the cylinder wall. 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 back and forth. Rodless cylinders are used for long-stroke requirements and offer high moment loading capabilities for a variety of applications. This design saves space because the stroke is contained within the overall envelope of the cylinder. Most manufacturers offer a variety of carriage designs. Three of the most common carriage options are internally guided, externally guided, and precision roller guided.

FIGURE I

FIGURES G & H

FIGURES J & K




Features: A basic vocabularyCylinder type is one selection criterion. Below are some other features to consider when deciding which cylinder to use. Cylinder and feature selection will depend on the precise application of the cylinder.

Bore size refers to the inside diameter of the cylinder tube or barrel. This key dimension is directly related to the force output capabilities of the cylinder. A discussion of how to determine bore size based on force required is found on page 7.

Stroke length is the distance between full extension and full retraction of the piston rod — the distance from start to finish of its stroke.

Pressure rating — designated in pounds per square inch (psi). Most cylinders are designed to work at pressures below those available from typical plant-air systems, but some applications may require higher pressure. The pressure actually supplied to a cylinder will normally be reduced through a pressure regulator to a level that will produce the desired thrust.

Piston rod diameter is dictated by bore size and application requirements.

Port sizes and locations are also dictated by bore size, but can be adjusted for custom designs.

Envelope dimensions: The National Fluid Power Association and International Standards Organization have established standards for many cylinder dimensions, making it possible to interchange cylinders from different manufacturers if replacement is necessary. Of course there also are many models with unique dimensions — particularly if options have been added.

Mounting configuration: Mounting configuration refers to how the cylinder is attached to the adjacent equipment. The large number of standard mountings — both rigid and articulated — usually allows the cylinder to fit into specific movement criteria as called for by the application. A variety of mounting hardware also is available.

Cylinder materials: The operating environment is the major factor that affects material choice. Typical material options for pneumatic cylinders include steel, aluminum, stainless steel, and brass. Some models are constructed of a combination of these materials.




Additional optionsIn addition to the standard features, manufacturers offer a variety of optional features, including cushions and magnetic pistons for use with external reed switches for position sensing. Cylinder manufacturers also use a variety of methods to seal the openings in the cylinder barrel and end cover of a cylinder. Designers have the option to specify alternative seal materials for applications that operate in extreme high or low ambient temperatures or are exposed to caustic chemicals.

If the piston makes metal-to-metal contact with the end covers at the extreme ends of the strokes, the result will be noise and potential mechanical damage. Cushions are devices installed in cylinders to prevent such contact. Some cushion designs include an adjustment that can change the rate at which the trapped air escapes (Figure L). This allows the machine operator to adjust the rate at which the cylinder is cushioned at the end of stroke. (Non-cushioned cylinders are suitable for full-stroke working only at slow speeds that result in gentle contact at the ends of stroke. To operate non-cushioned cylinders at faster speeds, external stops with shock absorbers can be installed. These should be positioned to prevent internal contact between the piston and end covers.)

Some cylinders have integral fixed cushions. This involves a pre-engineered fixed cushion orifice that restricts the flow of exhaust air to slow the piston down at the end of stroke. This is beneficial because it prevents the cushion from being altered under field conditions: the amount of cushioning effect will not change and is repeatable.

Magnetic cylinders (Figure M) have a band of magnetic material around the circumference of the piston within a nonmagnetic cylinder barrel. The magnetic field can be imagined as a donut shape around the barrel. This shape travels with the piston as the piston rod strokes in and out. By placing reed switches on the outside of the barrel — one at each end, for example — signals will be generated each time the piston rod completes a stroke.

FIGURE L

FIGURE M




Configuring multiple cylindersMultiple cylinders can be combined to create special force and/or stroke results. By mounting two or more cylinders end to end, they can be extended and/or retracted selectively to move an attached load reliably to a number of different positions (Figure N). This is known as a multi-position arrangement.

The tandem (or multi-power) arrangement also joins two cylinders end to end, but in this case with a shared central end cover and a common piston rod (Figure O). The combination then can double the pull and nearly double the thrust of a single cylinder of the same bore size. The tandem arrangement is suitable as an alternative to a larger bore cylinder when there is plenty of space available for length but restricted width and height.

Force is important…As mentioned earlier, understanding the application is critical to selecting the right cylinder. Knowing what the cylinder must do allows the designer to calculate how much force is needed, which, in turn, determines bore size. In general, for vertical and high friction applications, the force required is two times the load to be moved. Besides the force required to accomplish the task, in some cases additional force to compensate for friction may need to be calculated.

A designer who knows the force required and the air pressure available can solve the following equations to determine the diameters of the cylinder bore and the piston. A cylinder’s push force on extension or pull force on retraction is calculated by multiplying the effective area of the piston by the working pressure. The effective area for push force is the full area of the cylinder bore. The effective area for pull force is the full-bore area reduced by the cross-sectional area of the piston rod.

The theoretical push force is, F=π(D2/4)PWhere: F is force in pounds

D is cylinder bore in inches

P is pressure in pounds per square inch

The theoretical pull force is, F=π(D2/4-d2/4)PWhere: F is force in pounds

D is cylinder bore in inches

d is piston rod diameter in inches

P is pressure in pounds per square inch

Calculating the forces of single-acting cylinders with a spring is more complicated. The spring force opposing the push or pull will increase as the stroke progresses. In practice, manufacturers’ catalogs often list push and pull values for both double-acting and single-acting cylinders.

When estimating the relative force of cylinders with different bore sizes, remember that thrust increases with the square of the diameter. In other words, if you double the bore, you will quadruple the thrust.

FIGURE N

FIGURE O




…And so is stroking speed Speed impacts productivity, longevity and controlla-bility. The stroking speed of a pneumatic cylinder can be calculated from

s = 28.8 q /AWhere s is speed in inches per second

q is air flow in standard cubic feet per min-

ute

A is piston area in square inches

Other factors — external to the cylinder — that will affect speed in an application are:

• port size

• inlet and exhaust flow through the

control valve

• air pressure

• diameter and length of the hoses, and

• the load against which the cylinder is

working.

With any fixed combination of valve, cylinder, pressure, and load, it is usually necessary to have adjustable control over the cylinder speed. Flow controls at the cylinder ports can provide this control and tune speed to the application.

For the majority of applications, the best control-lability results from uni-directional flow regulators that are installed to restrict flow out of the cylinder and allow free flow in. The regulator fitted to the rod-end port controls the extension speed, and the one fitted to the cap-end port controls the retraction speed.

What about air consumption?Calculating a cylinder’s air consumption may be need-ed most often for fast-cycling production equipment. Obviously, enough supply air must be available to meet the application’s requirements. There are two parts to the air consumption of a cylinder. One is the vol-ume displaced by the piston; the other is the unswept volume, such as cavities in the end covers, the cylin-der ports, connecting tubing, and valve cavities. The unswept part is likely to be a small percentage and will vary with individual installations.

Engineering custom solu-tionsIn today’s competitive environment, many

times turning to a catalog and picking out stan-

dard components are not enough to give OEM

designers the breakthrough performance they

require. Recognizing this need, Norgren, Inc.,

created its EngineeringAdvantageapproach to

custom design, creating solutions “off-the-menu”

and working with customers to identify motion

control requirements within the larger context of

the pneumatic application.

Sometimes the solution involves combin-

ing standard or modified components in a new

configuration. Sometimes the solution involves

inventing something new. Norgren defines this

approach as “custom” because it is designed for

a specific customer’s application. So when does

Engineering Advantage offer the most benefits?

• Whenanexactsolutionisnot

availableinthecatalog

• Whentherequiredcombinationof

componentsdoesn’texist

• Whenanapplicationrequires

particularlyhighefficiency

• Whencomplexmotioncontrol

isneeded

• Whenequipmentdesignconstraints

dictatespecializedconfigurations

The company’s vision is to create competi-

tive advantage for its customers. That means the

Norgren engineering staff exploits the full poten-

tial of motion and fluid control technologies for

their customers’ benefit. Norgren’s goals are

improved machine performance, reliability, and

uptime— as well as lower cost of ownership,

including end-user operating costs.




As in many design situations, it’s best to connect the pneumatic equipment to a system with enough capacity to supply it with sufficient air during a “worst case” scenario. Otherwise there can be air starvation at a critical time and performance will suffer.

A couple of cautions A very important general design consideration is to keep the cylinder thrust as close as possible to the cen-terline of the piston rod and free from misalignment or side thrust. Cylinders are normally intended to push and pull without excessive side load. Off-center thrust or side loads can substantially reduce the service life expected from the rod bearing and seals. Off-center thrust and side loading can be caused by improper mounting, cylinder deflection under load, machine frame deflection, and rod bending or sagging — as well as by poor design of the machine.

Sometimes a cylinder’s bearing load can be reduced with the addition of an optional internal stop tube. A stop tube is a spacer placed between the piston and the rod end head. It increases the distance between the piston bearing and piston rod bearing when the rod is fully extended. For additional information about rod diameter and stop tube sizing, click here.

This configuration is also helpful in applications calling for cylinders with very long strokes. If there is a compressive axial load applied to the piston rod, care must be taken to ensure that the system parameters of length, diameter and load are within the safety limits to prevent the exposed rod from buckling. In some instances, the nature of the application and the mounting style of the cylinder will allow greater stroke lengths, while in others the safe stroke length should be less.

Most pneumatic cylinders are assembled with a coating of grease on the bore of the barrel and the seals for service with non-lubricated air. If the com-pressed air supply is clean and dry, the grease will give the seals a long life without adding oil through an air

line lubricator. However, contaminated air will gradu-ally compromise the original grease lubricant and shorten the life of the seals. A lubricated air supply will extend the life of the cylinder, but it will also wash out the original lubrication, so once lubricated air is introduced, it must always be used, and the lubricator should be regularly checked and maintained.

Putting it togetherPneumatic cylinders are the workhorses of industry, moving and positioning products or machine compo-nents that act on products. Pneumatic cylinders are simple and versatile components that have evolved over time to make more innovative equipment designs possible. They are also part of an increasingly com-plex mix of products in today’s industrial world. The development of cylinders with feedback capabilities and the use of programmable logic controls are just two examples. While a cylinder’s performance basics can be calculated, many external factors will affect its actual performance.

Putting together an actuator package that will meet the requirements of an OEM application can be daunting and time-consuming. Collaborating with a company that has pneumatic control experience plus a full product line can shorten the time to solution. Norgren, Inc, with more than 85 years of motion and fluid control innovation in a range of industries, understands the design of standard and custom pneumatic cylinders — inside and out. The company delivers world-class, innovative products and services that set its customers apart from their competition. n

Note: Line drawings in figures B, C, H, K, L and M are the standard National Fluid Power Association symbols for the depicted components.




Examples of Engineering Advantage solutions

BaghouseCylinderNorgren recently worked with a power genera-

tion plant on an air pollution control system.

Air pollution control equipment that uses engi-

neered-fabric tubes, envelopes, or cartridges to

capture, separate, or filter dust is known in the

trade as a “baghouse.”

A power generation plant needed to rou-

tinely close off chambers in its baghouse. The

plant wanted to do this with a pneumatic-cylin-

der package that:

•providedimmediate,repeatableresponse

•couldbeadaptedtoeachapplicationat

thefacility,and

• integratedalltherequiredpneumatic

andelectroniccomponents.

Norgren combined a standard NFPA-

interchangeable cylinder with a directional valve,

air filter, pressure regulator, and electrical wiring

with junction box into the baghouse cylinder

assembly. The unit was factory-assembled and

tested. The customer only needed to run control

wires directly to the terminal block — saving time

by eliminating the need to wire up all individual

components.

A mechanical lockout feature allows the cyl-

inder to be used to isolate a particular chamber

for system or component maintenance within

the baghouse. With the valve and electronics

mounted directly to the cylinder, response time

is faster and air consumption is reduced. In addi-

tion, the space-saving combination leaves room

for other components in the air pollution control

system.

Other recent examples of collaboration with

customers to take their requirements from con-

cept to production include:

Unit-AirassemblyThis cylinder/valve combination integrates a

valve and base to operate the cylinder with a

faster

response time. The space-saving design sim-

plifies installation and maintenance, as well as

reducing air consumption. This package can be

used in various industrial applications including

packaging, material handling, printing, paper con-

verting and more.

PositionfeedbackcylinderPneumatic cylinder completely integrated with

LVDT transducer (internal or external) or LRT

linear potentiometer provides continuous indica-

tion of piston position. It can be used on sorting

systems, conveyors, and diverters. The unit is

manufactured to accept multiple brands of feed-

back devices.

ReciprocatingairmotorThis cylinder/valve combination has stroke-signal

valves that serve as external pilots. It is used to

pump media such as lubricants, fiberglass, foams,

resins, hot melt glue, liquid polymers, waste

gases, and other liquids. The cylinder’s quick

response time assists dispensing. The self-con-

tained package simplifies installation.

Unit-Air assembly

Position feedback cylinder

Reciprocating air motor

Baghouse Cylinder

This article was previously published in Maching Design.

Selecting Pneumatic Cylinders for OEM Applicationscdn.thomasnet.com/ccp/01298792/48305.pdf ·...

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