Machine Design #19: Using Pneumatic Cylinders in a Clean Room — From Air Supply to Exhaust Treatment

In automated machine design, the pneumatic cylinder is one of the most familiar mechanisms.

Push out, pull back. Clamp, stop, lift, lower. Simple structure, easy to buy, easy to control, and not too expensive.

But if the machine runs in a clean room, the way you look at it changes completely.

An ordinary pneumatic cylinder can work very reliably in a normal workshop. But in a clean room it can become a particle source if you do not choose the right type, do not treat the supply air, do not treat the exhaust air, and do not account for the overall layout of the machine.

Put simply, the issue is not just:

Choose a clean cylinder or a non-clean one?

You have to look at the whole system:

Is the supply air clean? Does the inside of the cylinder generate dust? Where does the exhaust air go? Are the valve, tubing, fittings and speed controller suitable for a clean room? Is the moving assembly positioned correctly relative to the downflow? Should you use a pneumatic cylinder or switch to an electric actuator?

This article records the points to watch when using an air cylinder in an automated machine for a clean room.

1. First, understand which clean-room class is required

When designing a machine for a clean room, the first thing is not to choose components right away.

The first thing is to confirm:

Which cleanliness class does the clean room require?

The standard commonly used to classify cleanliness is ISO 14644-1. It classifies a clean room by the amount of particles in a given volume of air, from ISO Class 1 to ISO Class 9.

The smaller the class number, the cleaner the requirement.

For example:

Semiconductors, wafers, precision electronic components: usually a very high requirement.
Precision equipment, pharmaceuticals, cosmetics, food: the requirement can differ by product.
A simple clean booth in a factory: can have a lower requirement.

The important point for the machine designer is: the cleanliness has to be maintained while the machine is running.

A stationary machine is clean, but if while running it generates dust, exhausts dirty air, and makes particles settle on the product, it is still a failing design.

So when you receive a clean-room machine request, ask clearly:

What ISO Class is the clean room?
Measured in which state: at-rest or operation?
Where is the product located in the airflow?
Which dust sources are allowed in the clean area?
Can exhaust air be released directly into the room?
Are there requirements for material, grease, air tubing, wiring, ESD?

If you do not ask from the start, you very easily have to redo the design later.

2. Why can a pneumatic cylinder cause contamination?

A pneumatic cylinder has two main sources of contamination.

2.1. Dust generated inside the cylinder

Inside the cylinder there are the piston, seals, packing, rod packing and sliding surfaces.

When the piston rod runs back and forth, there is friction between the seals and the metal surfaces. However small, this process can still create very fine wear particles.

These particles can travel out with the exhaust air.

For an ordinary machine, this is not too serious. But in a clean room, even very tiny particles can affect the product.

Pay special attention to mechanisms that:

Run many cycles
Run at high speed
Have short strokes repeated continuously
Have an offset load causing side load on the rod
Have misalignment between the rod end and the guide mechanism
Lack a floating joint
Use an ordinary cylinder instead of a clean-room type

If the rod is pushed off-centre, the packing wears faster. The cylinder then not only loses life, but also increases the risk of particle generation.

2.2. Unclean supply air exhausted into the room

The second source is the compressed air supplied to the cylinder.

Factory compressed air is not always clean. It can contain:

Dust from the ambient air
Water vapour
Oil from the compressor
Oil mist
Rust from the piping
Debris or dirt in the air system

If the supply air is not clean, when the cylinder operates, this air is exhausted through the exhaust port of the valve or cylinder.

For a clean room, this is a very dangerous point. You cannot simply think "air is air" so it can be exhausted anywhere.

3. Managing supply-air quality: it has to start from the source

To use a pneumatic cylinder in a clean room, you need to start from the supply-air quality.

In high-requirement environments, Clean Dry Air (CDA) is often used.

CDA is not just dry compressed air. It needs to be controlled for:

Solid particles
Water vapour
Oil / oil mist

A common reference standard is ISO 8573-1, used to classify compressed-air quality by particles, water and oil.

For a clean room, depending on the product requirement, you may need to aim for a very clean class, for example Class 0 or Class 1 for some items. Check against the customer's real requirement and the factory system.

A clean-air system usually consists of:

Oil-free compressor: reduces the risk of oil contamination right at the source.
Air dryer: removes water vapour, avoiding condensation.
Pre-filter: filters coarse dust and water.
Mist separator / micro mist separator: removes oil mist and fine particles.
Final filter: fine filtration before supplying the clean area.
Piping management: avoid old, rusty, dusty or unsuitable-material pipes.

The important point is that the machine designer must clearly state the supply-air requirement in the specification.

Do not just write:

Air supply: 0.5 MPa

Write more clearly, for example:

Clean dry air, oil-free, filtration level per the clean-room requirement; dew point and quality class per ISO 8573-1 to be confirmed with the customer.

The content differs by company and project. But the thinking is: the supply air is part of the design conditions, not a side matter.

4. Choose a low-particle / clean-room cylinder

Even if the supply air is clean, an ordinary cylinder can still generate particles from internal friction.

So in a clean room, prioritise types such as:

Clean-room cylinder
Low-particle-generation cylinder
A cylinder with a vacuum-suction structure at the rod
A cylinder using low-dust-generating grease
A cylinder with seals / packing suitable for a clean room

Large pneumatics makers such as SMC, CKD, Koganei all have product lines for clean rooms or low-particle applications.

Some common features of clean-room cylinders:

Seal material with better wear resistance.
Grease that evaporates little and generates little dust.
A vacuum-suction option to collect particles at the rod region.
Grading by cleanliness level or application.
Documentation on how to handle exhaust air.

When choosing, do not just take a standard cylinder, add an external filter and think it is done.

Check the catalogue:

Is the cylinder a clean-room series?
Can it be used in the required ISO Class?
What grease is used?
Is there a vacuum port / relief port?
Does the exhaust need special treatment?
Are there limits on speed, load, temperature?
Is air purge or vacuum suction needed?

5. Grease and seals are also contamination sources

In a clean room, grease does not only lubricate.

Grease is also related to:

Particles
Outgas
Volatility
Dust adhesion
Compatibility with the food, medical, electronics industries
Compatibility with the seal material

For example, some clean-cylinder lines use fluorine grease. Some others may use grease meeting the NSF H1 standard for food or medical environments.

Do not change the grease on your own without checking the maker's instructions.

There are cases where a maintenance person sees the cylinder dry and adds ordinary grease. For an ordinary machine this may be fine, but in a clean room it can increase particles or contaminate the product.

So the maintenance document should clearly state:

The allowed grease type
The inspection cycle
The cleaning method
Whether additional greasing is allowed
When the cylinder must be replaced
When the filter must be replaced or the vacuum line checked

6. Treating exhaust air: a point very easily missed

With a pneumatic cylinder, air goes in, so air comes out.

In a clean room, the important question is:

Where does the exhaust air go?

If it is exhausted straight into the clean-room environment, all the particles and oil mist in the air line or inside the cylinder can enter the clean space.

There are two common treatment approaches.

7. Approach 1: Fit an exhaust filter at the exhaust point

The simplest approach is to fit a clean exhaust filter to the exhaust port.

Advantages:

Easy to apply.
No need to run a long exhaust line.
Simpler design.
Suits small mechanisms or not-too-extreme requirements.

For example, some filters such as the CKD FAC line use a fine filtration structure that can handle very small particles. According to the maker's documentation, some can filter to a very small level such as 0.01 µm at high efficiency. In real use you still have to check the exact model, flow conditions and class requirement.

Disadvantages:

Filters have a life and must be replaced periodically.
If the filter clogs, the cylinder speed can change.
You need to calculate the exhaust flow.
The filter mounting position must be easy to maintain.
If the filter is right above the product area, you still need to consider the air direction.

An exhaust filter is a simple approach but should not be fitted just for show. You must consider flow, pressure, exhaust position and replacement cycle.

8. Approach 2: Vacuum suction to capture particles at the source

For applications with a stricter cleanliness requirement, you can use a cylinder with a vacuum-suction / relief port.

This structure usually has a suction port near the rod-packing region. This port is connected to a vacuum pump or ejector to continuously draw off the air and particles generated near the sliding position.

The biggest advantage is capturing particles right at the generation source, before they disperse into the clean room.

This approach suits:

High-requirement clean rooms
Semiconductors, precision electronics
Positions near a sensitive product
Mechanisms running many cycles
Mechanisms placed above the workpiece
Cases where you do not want to exhaust air on the spot

Some lines such as SMC clean series 11-/13-/22-, or the CKD P53 series have solutions related to vacuum suction / clean-room applications. When choosing you must read the latest catalogue carefully, because each series has a different purpose and level.

Disadvantages:

Needs a vacuum system.
Adds more piping.
Needs the suction flow checked.
If the vacuum is weak or the line is blocked, the effect drops.
More complex design and higher cost.

If the product is very sensitive to particles, vacuum suction is usually more worth considering than a simple filter.

9. A quick comparison of exhaust-treatment approaches

Approach	Advantage	Note
Direct exhaust	The simplest	Usually unsuitable for a strict clean room
Exhaust filter	Easy to fit, less piping complexity	Must check flow, filter clogging, replacement cycle
Ducting exhaust out of the clean room	Reduces exhaust in the clean area	Costs piping, affects speed if the run is long
Vacuum suction at the rod/seal	Captures particles at the source, high effect	Needs vacuum, more cost and complex design

No approach is right for every case.

If the cleanliness requirement is moderate, an exhaust filter may be enough. If the requirement is high or near a sensitive product, consider vacuum suction or ducting the exhaust outside.

10. Comparing SMC, CKD, Koganei for clean-room applications

The table below is only a general view. When choosing a real model, check the latest catalogue and the specific project conditions.

Maker	Common approach	Strength	Suitable application
SMC	Many clean series, many selection levels, vacuum-suction options	Easy to select to requirement, rich documentation, wide line-up	Semiconductors, electronics, clean automation in general
CKD	Clean cylinder, P53 series, FAC clean exhaust filter	Strong in exhaust-treatment and source-capture solutions	Machines needing clear exhaust treatment, high-requirement clean rooms
Koganei	Clean system components, CS series, food/medical-suitable grease in some lines	System-oriented, suits a clean pneumatic system	Food, medical, pharma, cosmetics, small-to-medium clean rooms

Do not choose a maker just out of habit.

Choose by:

The required class
Whether vacuum suction is needed
Whether an exhaust filter at the exhaust point is needed
Whether the valve, speed controller, fittings, tubing can be matched
Whether there is industry-suitable grease
Whether the parts are easy to buy and replace
Whether there is clear technical documentation
Whether there is support in the machine-installation market

11. Not only the cylinder — the solenoid valve must also be clean

Many people only change the cylinder to a clean-room type but still use an ordinary solenoid valve.

This is an oversight.

The solenoid valve also has an exhaust port. When the valve switches, the exhaust air from the cylinder usually escapes through the valve. If the valve is placed in the clean room and exhausts directly, it can also become a particle-dispersion source.

For a clean-room machine, consider:

Is the valve placed in the clean area?
Should the valve be moved outside the clean room?
Is the valve exhaust collected?
Is an exhaust filter needed for the valve?
Is a manifold used to concentrate the exhaust lines?
Are the coil, wiring and valve-body material suitable for the environment?

A common design is to place the valve manifold in a less-affected area, then route the air to the cylinder. However, long piping can affect the response speed, so you have to balance cleanliness and performance.

12. Speed controllers, fittings and tubing must not be chosen carelessly either

In a clean room, even small pneumatic accessories need careful attention.

Speed controller

A speed controller can be near the cylinder. If it has an exhaust or an air-leak point, ensure it does not disperse contamination into the product area.

For a clean application, use a clean-room-suitable type or design so the exhaust air is treated.

Fitting

Ordinary fittings may be enough for an ordinary machine, but in a clean room pay attention to:

Material
Air leakage
Dust generation from installation/removal
Resistance to cleaning chemicals
Compatibility with clean-room tubing

Tube

The air tubing is also an important part.

Materials such as PFA, PTFE or dedicated clean-room tubing are often considered in a clean environment. Some applications also need anti-static tubing.

If the tube rubs against the machine frame while running, the tube itself can generate dust. So fix the tube properly, avoiding sharp edges or moving parts.

13. A floating joint reduces wear and dust generation

The cylinder rod should not be pushed off-centre.

If the rod end is connected directly to a mechanism but the two sides are not concentric, the rod takes side load. The rod packing is then pushed off-centre, friction rises, wear rises and particles rise too.

For a clean room, this is a very notable fault.

A floating joint absorbs the small misalignment between the rod and the guided mechanism. It does not turn a wrong design into a right one, but it helps reduce the off-centre load on the rod.

Use a floating joint when:

The cylinder pushes a slide table with its own guide
There is a chance of misalignment during assembly
The mechanism has accumulated tolerance
You want to reduce side load on the rod
You want to increase seal life

However, a floating joint is also a mechanical part. If it has clearance, friction or an unsuitable surface, it too needs consideration in a clean application.

14. Sensor switches and wiring also need checking

The auto switch / sensor switch on a cylinder is often seen as a small accessory.

But in a clean room, the wiring, sensor housing, cable jacket and routing can all have an effect.

Check:

Is the cable suitable for a clean room?
Does the cable jacket easily generate dust when rubbed?
Is the cable pulled back and forth continuously?
Is an anti-static cable needed?
Is resistance to cleaning chemicals needed?
Is the sensor near the airflow passing over the product?

If the cable moves continuously, use a suitable cable carrier or route it so it does not rub against other surfaces.

15. Comparing a pneumatic cylinder and an electric actuator in a clean room

When designing linear motion in a clean room, a very common question is:

Use a pneumatic cylinder or an electric actuator?

There is no general answer for every case.

Pneumatic cylinder

Advantages:

Simple structure
Low initial cost
Compact
Fairly large push force for its size
Easy to replace
Suits simple two-point motion

Disadvantages:

Has exhaust air
Has a risk of particle generation from the seal
Hard to accurately control intermediate positions
Hard to control speed and force as smoothly as a servo
Compressed air is not very energy-efficient
The air-supply quality must be managed

Electric actuator

Advantages:

No exhaust air
Good control of position, speed, acceleration
Easy to create soft motion with little impact
Suits multiple stop positions
Can reduce shock when contacting the product
Usually better energy efficiency than pneumatics

Disadvantages:

Higher initial cost
More complex structure
Needs a driver, cable, parameters
The internal ball screw / linear guide can still generate dust
Needs a clean-room grade if the cleanliness requirement is high

A quick comparison:

Criterion	Pneumatic cylinder	Electric actuator
Exhaust air	Yes, needs treatment	No exhaust air
Particles	From seal, packing, exhaust air	From ball screw, guide, grease
Position control	Mainly two-point	Good, many positions
Speed control	Limited	Good
Push force / size	Good	Depends on type, usually larger for the same force
Initial cost	Lower	Higher
Maintenance	Easy to replace, but needs clean air	Needs checking driver, mechanism, grease
Suits	Clamp, stopper, simple lift/lower	Position alignment, controlled pressing, soft motion

If it is just simple clamping, stopping or pushing and the clean treatment is done well, a pneumatic cylinder is still very reasonable.

If you need multiple stop positions, reduced impact, force or smooth speed control, an electric actuator is more worth considering.

16. Lay out by downflow: place the dust source below the product

A clean room usually has a clean airflow from top to bottom, called downflow or vertical laminar flow.

The important design thinking is:

Limit placing dust sources above the product.

If the cylinder, motor, cable carrier, guide, valve or sliding mechanism is above the workpiece, generated particles can fall or be carried by the airflow down onto the product.

So if possible:

Place dust-generating mechanisms lower than the workpiece.
Do not place the exhaust right above the product.
Do not let tube/cable rub over the product area.
Duct the exhaust away from the product area.
Use a cover or local exhaust if needed.
Design so the airflow does not drag particles across the product.

This is a very important layout part. It is not enough to just use clean-room components.

17. Machine materials: stainless, surface-treated aluminium and rust prevention

In a clean room, the machine materials also need careful selection.

Rust is a very obvious particle source. So parts exposed in the clean room usually prioritise:

SUS304
SUS316
Suitable anodised aluminium
Low-dust engineering plastic
Easy-to-clean materials
Materials that do not peel paint or coating

Do not use ordinary steel exposed to the clean environment without suitable surface treatment.

For frames, covers, brackets, plates, pay attention to:

Is it easy to clean?
Are there gaps that hold dust?
Are there dead corners that accumulate dust?
Does the surface peel?
Is it corroded by cleaning chemicals?
Does it generate static that attracts dust?

18. Static electricity is also an issue in a clean room

In semiconductors, precision electronics or sensitive products, ESD / static electricity is a big issue.

Static can:

Attract particles onto the product surface
Damage electronic components
Make dust stick to covers, tubes, cables
Cause unwanted discharge

So consider:

Grounding the machine frame
Using conductive or anti-static materials when needed
Using anti-static tubing if suitable
Managing cables and moving parts
Avoiding large plastic surfaces that easily charge near the product

A clean room is not only about dust. For many industries, clean but without ESD control is still not enough.

19. Design so it can be maintained

A clean-room machine has to maintain cleanliness over a long time. So maintenance is part of the design.

For a pneumatic cylinder, think ahead:

Is the exhaust filter easy to replace?
Is the vacuum line easy to check?
Can you see the state of the tube?
Can the cylinder be removed without removing too many machine assemblies?
Is there space to clean?
Are there dead corners that accumulate dust?
Is periodic leak checking needed?
Is there documentation stating the grease type and replacement parts?

A design that is clean when newly installed but hard to clean, hard to change the filter, hard to check the tube, can still become a contamination source after a while.

20. Checklist for using a pneumatic cylinder in a clean room

Before finalising the design, you can quickly check the following.

20.1. Clean-room requirement

What is the required ISO Class?
Is the requirement at-rest or in operation?
Where is the product in the airflow?
Is there an ESD requirement?
Are there requirements for material, grease, outgas?

20.2. Supply air

Is CDA used?
Is the air quality per ISO 8573-1?
Is oil-free required?
Is there a dryer?
Is there multi-stage filtration?
Is the supply-air requirement written in the specification?

20.3. Cylinder

Is a clean-room cylinder used?
Is a low-particle type needed?
Is a vacuum-suction / relief port needed?
Is the grease suitable?
Are the seals suitable?
Are the speed, load, side load within limits?

20.4. Exhaust air

Where does the exhaust go?
Is it exhausted directly into the clean room?
Is an exhaust filter used?
Does the filter have enough flow?
Is it necessary to collect the exhaust outside?
Is vacuum suction at the source needed?

20.5. Surrounding equipment

Is the solenoid valve a clean-room type?
Is the valve exhaust treated?
Is the speed controller suitable?
Are the fittings and tubing clean-room suitable?
Do the cable and sensor generate dust or static?
Is a floating joint used to reduce side load?

20.6. Overall layout

Is the dust source above the product?
Is the downflow used to push dust away from the product?
Does the exhaust blow across the work area?
Do tube/cable rub when moving?
Are the materials rust-resistant and easy to clean?
Is there anti-static if needed?

20.7. Maintenance

Is the filter easy to replace?
Is the vacuum line easy to check?
Is the filter replacement cycle specified?
Is the grease type specified?
Is there cleaning space?
Can the cylinder be replaced without greatly affecting cleanliness?

21. Conclusion

Using a pneumatic cylinder in a clean room is not forbidden.

But you cannot use it the way you would in an ordinary machine.

For a clean-room machine, look at the cylinder as part of the whole system:

The supply air must be clean.
The cylinder must generate few particles.
The exhaust air must be treated.
The valve, tubing, fittings and speed controller must also be suitable.
The mechanism must avoid side load so the seal does not wear fast.
The layout must account for downflow.
The materials must be rust-resistant and easy to clean.
If needed, ESD must be accounted for.
Maintenance must be thought of from the design stage.

In short:

A clean room does not only require clean components. It requires the design, the layout and the maintenance to all be clean.

A pneumatic cylinder is still a good choice for many simple mechanisms needing large push force at reasonable cost. But for motions needing accurate position, smooth speed, reduced impact or a very high cleanliness requirement, an electric actuator should also be considered from the start.

Good design is choosing the right mechanism for the right environment, not forcing one solution onto every case.

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