Machine Design #12: How to Use Pneumatic Cylinders in Machine Design – The Points Easily Overlooked

In automated machines, the pneumatic cylinder is one of the most widely used mechanisms.

Pushing a workpiece out. Pulling a mechanical unit back. Stopping a product. Light lifting and lowering. Temporary clamping. Opening and closing a cover. Lightly pressing a part into position.

Motions like these look simple at a glance. Pick the cylinder bore, pick the stroke, attach a solenoid valve, tune the speed controller, add a stroke sensor, and the machine can run.

But when you get into real design, the pneumatic cylinder has quite a few points to watch.

There are faults that do not show up at first. The trial run is fine. But after a few months, when the factory air pressure is unstable, the cushion starts to weaken, the rod takes side load, the air tube is longer than planned, or the operator has tuned the speed controller differently — that is when the fault appears.

This article records the basic points when using a pneumatic cylinder in machine design.

Not to make the problem complicated. But to avoid the very common mistakes when designing pneumatic mechanisms.

If you are torn between air and electric, you can also read this article: Replacing a pneumatic cylinder with an electric cylinder – what to check before you choose.

1. Do not choose thrust based on a too-nice pressure

When choosing a pneumatic cylinder, many people look at the factory air pressure and then calculate the thrust.

The basic formula is:

Force = pressure × piston area

If the factory runs at around 0.5 to 0.7 MPa, the catalog will show a fairly good thrust.

But machine design should not look only at the nice-condition case.

In a real factory, air pressure can drop for many reasons:

Many lines running at once
A leak somewhere
Workers using extra air blow
A long air-supply line
A clogged filter or valve
A compressor that is no longer as stable as it was

So when designing an important mechanism, do not choose the force tightly to 0.5 or 0.6 MPa.

A safer approach is to check whether the mechanism still has enough force at around 0.3 MPa.

You do not always have to design at 0.3 MPa, but you should at least check. If the mechanism already cannot move at a slightly lower pressure, the chance of a field failure later is high.

This point is very practical.

A machine newly assembled in the test shop may run well. But once it goes into the customer's factory — different air system, different piping, different air usage — the result can be different.

2. The extend force and the retract force are not the same

A point easily forgotten is that the extend-side force and the retract-side force of a cylinder are not the same.

The reason is that the rod side loses part of its area because the rod takes up space.

Put simply:

The extend side usually has a larger effective area
The retract side usually has a smaller effective area

So at the same pressure, the extend force and the retract force can differ.

If the mechanism only pushes lightly, it is fine. But if the cylinder carries load in both directions, or the retract direction also needs large force, you have to check the force on both sides.

Do not just look at one force figure in the catalog and assume both directions are the same.

In addition, at the same cylinder bore and the same pressure, the real force between brands can also differ slightly due to construction, friction, efficiency and how the catalog is written. When designing a real production machine, it is best to use the catalog data of exactly the brand you are choosing.

3. When used for pressing or contact pushing, do not use the whole stroke

If you use a pneumatic cylinder for contact pushing, light pressing, or pushing a part into position, do not use the entire stroke of the cylinder.

For example, you need to push about 100 mm.

If you pick a 100-stroke cylinder and use exactly the full 100 mm, then at the end of the stroke the internal mechanical part of the cylinder can become a stop. At that point the cylinder no longer has spare travel to keep producing force the way you want.

The design should leave spare stroke.

For example:

The real push needed is 100 mm
Do not pick a 100 stroke and use it all
Pick a 125 stroke or a slightly larger stroke
Use about 100 mm, leaving the rest as spare for the pressing mechanism / tolerance / adjustment

An easy rule of thumb is that when using a cylinder for contact pushing, leave a spare of a few millimeters, usually about 5 mm or more depending on the mechanism.

The number 5 mm is not for every case, but the thinking is right: do not let the cylinder go dead-solid at the end of the stroke if you still need working force.

4. Too small a stroke and even the sensor can cause trouble

Pneumatic cylinders usually use body-mounted sensors to confirm the out / in position.

With a long enough stroke, there is no major problem.

But if the motion is too small — say only a few millimeters — there are cases where the sensor is still in the ON zone, or the states before and after moving cannot be clearly separated.

This often happens with:

Short-stroke cylinders
Air-chuck clamping mechanisms
Mechanisms that only nudge slightly to position
Mechanisms whose assembly tolerance makes the sensor hard to adjust
Sensor positions that are too close together

In that case, the PLC may not read the state clearly. The machine thinks it has moved, or thinks it has not moved, while the real mechanism moved only very little.

So when designing, do not only ask "can the cylinder travel that stroke," but also "can the sensor confirm that motion stably."

If the stroke is too small, sometimes it is better to use an external sensor, a separate stopper, or a different way to confirm the position.

5. Cushioning: do not think a cushion is the end of it

A pneumatic cylinder running fast or carrying load will have impact at the end of the stroke.

There are usually a few ways to handle this:

Use an external mechanical stopper
Use a mechanical stopper combined with a shock absorber
Use the cylinder's optional stopper
Use an internal cylinder cushion such as a rubber cushion or air cushion

But one point must be understood clearly.

If the mechanism does not use the full cylinder stroke, or the stop point is at an external stopper, then the internal cylinder cushion may not actually take effect.

Many real mechanisms stop at a separate stopper. In that case the external shock absorber is the part that mainly takes the impact.

And a shock absorber also has a life.

It is not something you fit and then forget.

When choosing a shock absorber, you need to look at:

The load mass
The impact speed
The absorbed energy
The impact frequency
The ambient temperature
The mounting orientation
Oil, dust, water, metal chips
The expected life
The replaceability during maintenance

There are mechanisms that run very smoothly at the trial, but after some time the shock absorber weakens, the machine starts to hit hard, causing position drift, loose bolts or cracked plastic parts.

This is a rather annoying fault because the design still looks correct at first.

6. Side load is a very common cause of cylinder damage

A pneumatic cylinder does not like taking side load.

Its main job is to push / pull along the axis of the rod. If the rod takes much lateral force, over time it tends to develop problems.

Some faults you may see:

The rod bends
The seal wears quickly
The cylinder leaks air
The motion becomes heavy
The mechanism jams
The rod gets scratched
In the worst case, buckling can occur

Especially with a long-stroke cylinder, the side-load risk is even greater.

If the rod end pulls a mechanical unit with a poor guide, or the workpiece applies an off-center force, the rod is very easily pushed sideways.

When there is side load, do not make the cylinder bear it alone.

You should use:

A linear guide
A guide shaft
A guided cylinder
A slide table
A separate guiding mechanism
A floating joint if small misalignment must be absorbed

A simple principle:

The cylinder produces force. The guide takes the side load.

Do not make the cylinder rod also do the job of the main guide, especially with a long stroke or an off-center load.

7. Be careful with single-acting cylinders and two-position-stroke cylinders

Depending on the application, spring-return single-acting cylinders still have their place. But in a continuously running production machine, without a clear reason, I usually do not favor this type.

The reason is that the return force depends on the spring. Over a long time, the return state may no longer be as stable as at the start. In addition, when there is dust, rising friction, or a heavier mechanism, the return motion tends to develop faults.

The same goes for two-position-stroke cylinders.

It sounds convenient, but when brought into machine design it sometimes makes the mechanism harder to understand, harder to buy, longer in lead time and harder to maintain.

If you really need two intermediate positions, in many cases using two separate cylinders or switching to an electric actuator is easier to control.

It is not that a two-position cylinder cannot be used. But do not choose it just because "it looks compact."

You have to consider the procurement lead time, the spare parts, and whether the maintenance staff can understand that mechanism a few years later.

8. A jammed slide mechanism can make the cylinder move very dangerously

A rather dangerous real-world fault is a slide mechanism that jams slightly.

When the cylinder is moving and the sliding part gets stuck, the pressure in the cylinder chamber can rise. When the jam point suddenly releases, the mechanism can shoot off very fast and hit hard.

Put simply, it "holds back the force" and then bursts out.

This is bad for both people and the machine.

The cause may come from:

A misaligned guide
A dirty sliding surface
Metal chips present
A mechanical unit tightened too hard
Off-center load
No floating joint
Assembly tolerance pushing the rod sideways
Lack of lubrication
A workpiece caught in the travel area

With a pneumatic mechanism, do not only look at whether the cylinder force is enough. You have to look at whether the sliding path is smooth.

When trial-running by hand, feel the mechanism before supplying air. If it is already heavy to pull by hand, supplying air will not naturally turn it into a good mechanism.

9. A lifting cylinder needs checking in both the up and down directions

When using a pneumatic cylinder for a lifting mechanism, many people only check whether the lifting force is enough.

But the downward direction matters too.

With a vertical load, the downward direction can be pulled along by gravity. If you only adjust with a speed controller, it is sometimes very hard to make the downward motion fast, smooth and stable all at once.

Some points to look at:

How heavy is the load?
Will the load fall on air loss?
Does the downward direction free-fall?
Can the speed controller be tuned stably?
Is a counterbalance needed?
Is a lock cylinder needed?
Is a load-holding valve needed?
Is a mechanical stopper needed?
Is there a risk of pinching a hand or dropping the product?

In particular, if you want to supply low pressure on the downward direction for smoother motion, the adjustment will not be simple. Air is not always easy to control smoothly in vertical motion.

For a vertical axis with a load-drop risk, also see: Drop-prevention design for a vertical pneumatic cylinder.

10. A mechanism with an operator must put safety first

If the cylinder runs in an area where a person works, you cannot just think "press the button and it runs."

Especially with semi-automatic machines, hand-operated jigs, or simple pressing mechanisms, the operator may put a hand into the danger zone.

If you use only a one-hand single push button to activate the cylinder, the pinching risk is very large.

Depending on the level of danger, consider:

A two-hand push button
A safety cover
An area sensor
A light curtain
An interlock
A guard cover
A pressure-release valve when the door opens
A safe speed during setup
A separate manual mode for maintenance

This is a design area that many beginners easily take lightly.

A machine that runs is not enough. The machine must run under real user conditions and still be safe.

If the operator has to place the product by hand and then press a button for the cylinder to press down, then at the very least you must review the pinching risk, the danger zone, and how to prevent the machine from running while a hand is still in that zone.

11. Hanging a heavy load with a pneumatic cylinder means you must think about air loss

When using a pneumatic cylinder to lift a heavy assembly, you must think of the worst case: air loss, pressure drop, power loss, a valve fault, or a tube coming off.

If the load is hanging and the pressure drops, the holding force may no longer be enough. At that point the mechanical unit or the workpiece can fall.

This is not just a machine fault. This is a safety problem.

Some common ways to handle it:

Use a lock cylinder
Use a mechanical stopper mechanism
Use a load-holding valve
Use a suitable perfect-block or anti-drop valve
Use an auxiliary tension spring to reduce the drop risk
Use a counterweight if the mechanism suits it
Design a safe home position
Do not let the load hang freely above the operator

The key point is to design on the assumption: what happens if the air is lost?

If the answer is "the assembly will fall," then that mechanism is not yet acceptable.

12. Do not use two pneumatic cylinders for synchronization if you need precise motion

There are mechanisms that at a glance look very easy to design with two pneumatic cylinders running in parallel.

For example, lifting a long plate with two cylinders on each side.

But in reality, two pneumatic cylinders cannot guarantee precise synchronization.

The reason is that each cylinder has its own friction, the load on the two sides can differ, the air-tube lengths differ, the valves differ, the speed controllers are tuned differently, and the pressure and flow also vary over time.

Even if you tune them fairly similar at first, over time they can still drift apart.

If the mechanism only needs relative pushing and does not require precise parallelism, it may still be usable. But if you need to lift a flat surface, press evenly, keep it parallel, or avoid guide jamming, you have to be careful.

Better options may be:

One cylinder pulling through a mechanical linkage
Using a synchronizing shaft
Using a rack and pinion
Using a stiff enough guide
Using an electric actuator when position control is needed
Using a cam or linkage mechanism depending on the purpose

I have split this particular issue into its own article: Synchronizing two axes with pneumatic cylinders – why you should be careful.

13. The air piping and the air-preparation unit also matter a lot

Whether a pneumatic mechanism runs stably does not depend only on the cylinder.

The air line matters too.

When designing, watch for:

Is there a suitable air filter?
Is there a regulator for each unit that needs adjustment?
Is a lubricator needed?
Is the main line large enough?
Is the supply tube to the cylinder too long?
Does it go from a small tube to a large tube and back to a small tube?
Is the valve placed too far from the cylinder?
Is there a quick exhaust when high speed is needed?
Is it easy to check the pressure at the machine unit?

A simple principle is that the air line should flow in a sensible direction:

Large main line → smaller branch → valve / air unit → cylinder

If you design the air line in a patched-together way, it may still run at first. But when the machine runs many mechanisms at once, the speed can become unstable.

For a machine with many units, divide the air into clear groups. Each important group should have an air-combination unit, or at least a suitable filter / regulator, to make maintenance easier.

14. Maintenance: design it so the next person can still understand it

Pneumatic design is not just choosing devices and drawing a circuit.

You also have to think about the maintenance staff later.

A few small but very useful points:

Gather the pressure gauges in an easy-to-see location
Label the air tubes clearly
Color-code the tubes if needed
Note the set pressures
Use the same brand of cylinder / valve where possible
Leave space to replace sensors
Leave space to adjust the speed controller
Leave space to replace the shock absorber
Do not put fittings in positions that are too hard to remove
Do not let the air tube fold or stretch tight as the unit moves

Many machines fail not because the design principle is wrong, but because maintenance is too hard.

A speed controller placed deep inside the machine — where adjusting it means removing the cover, the wiring, the jig — will very easily be skipped or wrongly tuned later.

Good design is one where others can also maintain it, not only the person who originally designed it.

15. When to keep air, and when to look at other options?

The pneumatic cylinder is still very good if the mechanism:

Runs only 2 points
Does not need to stop at many positions
Does not need precise force control
Does not need to log data
The load is not too heavy
The speed is not too sensitive
There is no high positioning requirement
Maintenance needs to be simple
Cost needs to be low

But reconsider the option if the mechanism:

Needs multiple positions
Needs to change position by model
Needs pressing-force control
Needs to log OK / NG data
Needs to synchronize multiple axes
Has a dangerous vertical load
Has large side load
Has much end-of-stroke impact
Needs a very stable takt time
Frequently has to be tuned in the field

There is no fixed answer for every machine.

Air is not bad. Electric is not always better either.

What matters is what task that mechanism is doing, how demanding the requirement is, and where the risk lies after it runs in the real world.

Conclusion

The pneumatic cylinder is a very easy mechanism to use, but it should not be designed in an overly simple way.

When using a pneumatic cylinder, you need to check at least these points:

The real air pressure in the factory
The extend force and the retract force
The safety factor when pressure drops
The spare stroke when used for pressing / contact pushing
Whether the sensor can confirm a small stroke
End-of-stroke impact and shock-absorber life
Side load acting on the rod
The guide for the slide mechanism
The risk of the mechanism jamming and then bursting out
Vertical motion and the load-drop risk
Safety when an operator is near the travel area
Synchronization capability if using multiple cylinders
The air piping, regulator, filter and maintenance positions

If these points are looked at carefully from the start, the pneumatic cylinder is still a very practical choice in automated machine design.

Cheap, easy to buy, easy to replace, easy to understand.

But if you overlook side load, spare stroke, drop prevention, cushioning, real pressure and operator safety, the pneumatic mechanism very easily becomes a source of faults after the machine goes into operation.

Machine design is not just making the mechanism run during the trial.

Good design is one where, a few years later, when the machine runs in a real factory, with a real operator, real air conditions and real maintenance, that mechanism is still easy to understand, easy to tune and safe enough.

View all MINATA technical articles