Pneumatics

Compressed air operated equipment is powerful, safe, reliable and economical.  There is no danger of spark or electrical burn-out.  Pneumatic tools & components can be compact, lightweight and versatile.  

To make sure you get the best out of your pneumatic components, there are a few things it is wise to remember.  Firstly, when setting up an air system, it's much better to do things properly to begin with.  This will save lots of hassles later on..  

Compressed Air

Choosing a compressor is the first task.  There are several different types and all have their strengths & weaknesses.  At this stage, you need to work out roughly what your requirements will be.  How much air will you need?  And at what pressure?  And at what location(s)?

Add together the air consumption figures for all your pneumatic tools & components.  This is a volume of flow commonly expressed in cubic feet/min (CFM) or litres/min or cubic metres/hour.  Don't forget to add a fudge factor for future expansion and allow for the duty cycle of the compressor (like all machines, the compressor isn't designed to run at full capacity 24/7).  

This will give an idea of what size compressor is needed.  Talk to suppliers of compressors about the relative merits of rotary vane, rotary screw, recip or piston type etc.  Also consider if you may be better off to have 1 large compressor or 2 (or more) smaller units.  Many larger sites use this set-up so they can manage their air system for maintenance, different shifts or varying demands.

Air Preparation

Everyone knows that the air we breathe varies in quality.  Health Authorities warn of smoke, dust, pollen, pollutants and smog which all affect our well being.  In the same way, the quality of the air we compress affects the performance and life of our pneumatic components.  Compressors actually result in higher concentrations of moisture & contaminents and higher temperatures in pressurised air.  Oil carryover is often also a problem.  It is important to ensure good quality air in any pneumatic installation in order to avoid wear, damage and inefficiency.

Good quality air should be effectively cooled, dried, filtered, regulated and lubricated to the requirements of the application(s).  This can involve the use of filters, regulators, lubricators, receivers, drains and various different types of dryers.

An air system also needs to be designed to ensure that air is delivered with sufficient flow and pressure to the point of use and with a safe means of isolation.  The size & type of pipework is important as are the fittings & routing from compressor to workspace.  Poor design can cause excessive pressure drop, too much moisture, "dirty" or contaminated air and inconvenience.

General rules-of-thumb include:

• The compressor air intake should be in a cool dry place away from dust.
• An adequately sized receiver will allow air to cool and water particles to condense so they can be drained away.  It also helps remove compressor "pulsations" and maintain a volume of air.
• Air should undergo final filtration and regulation as close as possible to each point of use.
• Pipework should slope towards lowpoints where drains should be installed.
• Fittings should be kept to a minimum and appropriately sized.  Always check the internal diameter of fittings to ensure they don't unnecessarily restrict airflow.

Pneumatic Equipment - Sizing Cylinders

One of the most commonly used components is the "air cylinder" or "pneumatic ram".  These are available in a huge range of sizes which makes them ideal for many different applications, but sizing them is sometimes tricky.  

When selecting a cylinder, the starting point is to choose a bore size that will produce enough force to do the job (at your chosen working pressure).  Always size at a conservative air pressure.  Just because your compressor can produce 120 psi (830 kPa) doesn't mean that pressure will always be available.  The compressor will probably cycle on & off within a range of pressure and the demands of other pneumatic equipment (both current & future) can result in pressure drops.  It is safer, easier and more energy efficient to operate at a lower pressure than the maximum available.

When calculating forces, always allow a "fudge factor" for losses due to friction, wear etc.  It is generally accepted that an allowance of 20-25% provides a good rule-of-thumb but in demanding applications a bigger factor may be adviseable.  Always use a larger bore size if high speeds are required.  Rapid acceleration requires greater force.  Similarly, always oversize if a smooth reliable low speed is required.  Don't forget that less force is available on the retract stroke due to the diameter of the piston rod.  If in doubt, it is always best to go up a size.

Try this link for Cylinder Force Output Charts.

Once you have selected an appropriate piston diameter, decide what length of stroke (travel) is needed, and what physical space the cylinder and it's mountings will require.

Cylinders

Other considerations for choosing an appropriate air cylinder for your application include:

• Materials of Construction.
- Barrels, piston rods, tie rods and seals can be made from many different materials.  Consider the operating environment and the possibility of rust, corrosion or physical wear & tear.  Also consider if the plant will be subject to weather or chemicals associated with wash-down.
• Cylinder Position Sensing.
- Magnetic sensing will require a magnetic piston.  External sensing may require physical mounting considerations.
• Maintenance.
- Some cylinders (especially the smaller crimped variety) are non-repairable.  Others may require special tools or knowledge for trouble-free maintenance.
• Interchangability.
- Many cylinders from different manufacturers conform to uniform international standards for dimensions and mountings. Others do not.
• Mountings.
- Allow for mountings when pricing a job as they can add up to considerable cost.  Select the correct mountings for the job.  They should provide correct support for the cylinder's movement in the application. (Consider strength & rigidity requirements and try to minimize side loads).
• Lubrication.
- Some cylinders are not designed for use without lubrication.  An operating environment may be more suited to lubricated or non-lubricated air.  In food grade applications, it may be necessary to lubricate with vegetable based oils and greases.
• Cushions.
- End-of-stroke buffers or adjustable cushions are useful in preventing wear and controlling end-of-stroke behaviour in many applications.
• Cylinder Type.
- Your application may be suited to a single acting cylinder (spring-return or spring-extend).  Alternatively, space may dictate a more compact style of cylinder, a non-rotating piston rod variety or even a rodless cylinder.  Applications may also demand a specific construction -eg. roundline is easier to keep clean vs extruded barrel which can be more suited to sensors vs round with tie rods for heavy duty strength.

Valves

Valves are the heart of any pneumatic system.  They dictate what will happen and how.  There are many considerations:

• Function.
- What will the valve be required to do?  What configuration will you need for this function? eg; 2/2, 3/2, 4/2, 5/2, 5/3 etc  Will the valve's job be more suited to a particular design type? eg; spool, poppet, needle, slide, ball, diaphragm etc.  Is it just to give a signal or is it supplying air for a purpose (powering a cylinder, running an airmotor or tool etc)  If it's just a signal, the flow required will be small.  It will be more important to consider it's physical size, how it will need to be mounted and operated. ie; push button, roller lever, solenoid or air pilot, foot pedal etc
• Environment.
- Both ambient conditions and the quality of the air.  What is the temperature range?  Is it subject to dust, contaminents or weather?  Is there exposure to chemicals, gases or wash-down?
• Required Lifetime.
- Will it need to last?  How important is repairability and reliability?  What sort of duty cycle is expected of it?
• Operating Force.
- How much physical effort is required to operate a hand valve.  Is there a danger of RSI?  What force will be exerted on plungers, rollers & levers?  What will be the power consumption of solenoid coils?  What voltage is best?
• Plumbing.
- The size & orientation of threaded ports will need to be taken into account.  Also note that thread types vary. eg; BSP, JIS, NPT etc.  The option of manifolding valves can make plumbing simpler.  Remember to choose suitable fittings & tubing/pipework.  Do they need to be protected from chemicals, weld spatter, UV light, vibration, or abrasion?
• Materials of Construction.
- Consider weight, durability, wear properties, appearance and price.  
• Safety.
- Does the valve need to give feedback?  Is it a critical application?  What will happen in the event of loss of power or air?  Is there a need for redundancy?  Does it need to be comply with any specific standards?  Is there a danger of spark, heat or ignition?  Can maintenance safely be acheived?
• Performance.
- What are the flow rates required?  What response times are needed?  How important is repeatable performance?  Is reliability of paramount concern?