SMC Cylinders and Auto Switches, New Zealand

SMC / Cylinders and auto switches

Auto switches are not supplied with the cylinder, they have to match your PLC input type, and they only work if the cylinder has a magnet in the piston. Three separate things to get right, and all three are ordered separately from the cylinder itself.

Cylinders are the highest volume SMC enquiry we get, and most of the wrong orders come from the same handful of characters.

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Reading a cylinder part number

Take CDQ2B32-50DZ.

Part	Means
C	Cylinder
D	Built-in magnet for auto switches
Q2	The series. Compact cylinder
B	Mounting style
32	Bore, in millimetres
50	Stroke, in millimetres
Trailing characters	Action, cushioning, rod option, port thread and other options

Bore and stroke sit either side of the dash and read directly in millimetres. That answers most sizing questions immediately.

Everything else in the string is an option that can make two cylinders non-interchangeable, which is why we ask for the whole number rather than the series and size.

The D is the one that matters most

	CDQ2	CQ2
Magnet in the piston	Yes	No
Auto switches work	Yes	No
Cylinder strokes normally	Yes	Yes
Visibly different	No
Can be added later	No. The magnet is inside

The same convention runs through the range. CM2 and CDM2, CJ2 and CDJ2, MB and MDB. If the machine uses position switches, you need the D version.

Look at the cylinder before ordering. If there are small switches clamped to the body or sitting in a groove, the replacement needs a magnet.

Choosing the auto switch

The second half of the job, and the part most often ordered wrong.

Two wire against three wire

	Two wire	Three wire
Wiring	In series with the load, like a switch	Supply, common and a signal output
Polarity of output	Not applicable	NPN or PNP. Must match the input card
Leakage current	Small standing current even when off	Negligible

NPN against PNP

This has to match the PLC input card, and it is not a preference. An NPN switch into a PNP input does nothing useful.

Common suffix	Type
D-M9N	Three wire, NPN
D-M9P	Three wire, PNP
D-M9B	Two wire

So the last letter decides whether the switch works with your control system. If you do not know which your inputs are, tell us the PLC and we will work it out, or read it off an existing working switch on the same machine.

Reed against solid state

	Reed, D-A9 family	Solid state, D-M9 family
Mechanism	Mechanical contact	Electronic
Life	Finite switching cycles	Much longer on fast machines
Typical failure	Intermittent first, then stuck	Less common
Sensing band	Wider, less precise	Narrower, more repeatable

An end-of-stroke signal that works when you test it slowly and drops out in production is very often a worn reed switch. It looks like a control fault because everything checks out on the bench.

On a high cycle machine, fit solid state. They mount the same way and outlast reeds substantially.

Cable length and whether you need a flying lead or a plug are also part of the order code, so tell us which.

Speed controllers, and the judder nobody diagnoses

Cylinder speed is set by restricting air flow, and which side you restrict changes the quality of the motion.

	Meter-out	Meter-in
Restricts	Air leaving the cylinder	Air entering it
Effect	Back pressure holds the piston steady	Piston lurches, then waits for air
Motion	Smooth	Jerky, especially on light or vertical loads
Normal choice	Yes, for almost all cylinders	Rare, specific cases

So a cylinder that judders, lurches or overshoots is often a speed control problem rather than a worn cylinder. The two controller types look nearly identical and get swapped in error during a repair.

If a machine developed a lurch after somebody replaced a fitting, look here first. It costs nothing to check and the part is inexpensive.

The lubrication mistake

The one that quietly destroys cylinders across a whole plant.

Modern SMC cylinders are pre-lubricated and designed to run on clean, dry, filtered air with no added oil.

Situation	Result
Clean dry air, no oiler	Correct. Long life
Somebody adds an oiler to help	Oil washes out the factory grease
Oiler later runs dry or is removed	No lubrication at all. Rapid wear

Starting lubrication commits you to maintaining it. The failure comes later, when the lubricator empties and nobody notices for a month.

So on a plant with a rash of early cylinder failures: check whether an oiler was added at some point, and whether it is still working. That is a free check and it explains a lot of repeat orders.

Cushioning

Type	Notes
Air cushion, adjustable	A screw at each end, set on site
Rubber bumper	Fixed. Common on compact and mini cylinders
None	Short stroke, light load or externally stopped

A cylinder that slams at the end of stroke is usually an adjustment rather than a fault, on the types that have adjusting screws. They get opened fully during commissioning and never revisited.

Slamming destroys the cylinder and the mechanism it drives, so it is worth correcting before it becomes a repeat order. The cushioning type is also part of the part number, so an uncushioned cylinder fitted where a cushioned one was will hammer.

Mountings and rod ends are separate

Foot brackets, flange plates, clevises, pivot brackets, rod eyes and floating joints are all separate part numbers. A cylinder arrives as a cylinder.

On a like-for-like replacement the existing mountings normally transfer. On a new installation they all have to be ordered, and that is the most common omission on a first order.

A floating joint is worth knowing about. It absorbs small misalignment between the rod and the load, and fitting one is often the fix for a cylinder whose rod keeps scoring. Replacing the cylinder without addressing alignment just resets the clock.

Sizing a cylinder from scratch

Input	Why
Force required	Bore and working pressure decide this
Pressure at the machine	Measured under load, not at the compressor
Stroke	Plus overtravel
Direction	Pull force is lower than push, because the rod takes area off the piston
Mass and speed	Decides cushioning and whether a shock absorber is needed
Side load	Points at a guided cylinder or a slide table

The pull stroke is the one people forget. A cylinder sized on push force can be short on the return.

Before ordering a replacement

Symptom	Check first
New cylinder fitted, switches dead	The D. No magnet in the piston
New switch fitted, no signal at the PLC	NPN against PNP
Signal drops out in production only	Worn reed switch
Cylinder judders or lurches	Meter-in against meter-out
Slow and weak	Pressure at the machine under load, and the filter element
Slamming at end of stroke	Cushioning screws
Rod keeps scoring	Alignment and side load. Consider a floating joint
Cylinders failing early plant-wide	An oiler that was added and then ran dry

What to send us

The complete part number, every character.

A photo of the cylinder on the machine. It shows the mountings, the switches and the piping.

Whether switches are fitted, and what type your PLC inputs are.

Whether you need mountings, or the existing ones transfer.

The symptom.

For a new application, the force, stroke, pressure and whether there is side load.

What we can supply

Cylinders. CQ2 and CDQ2, CM2 and CDM2, CJ2 and CDJ2, MB and MDB, C95, MGP guide cylinders, MXS slide tables, MY rodless, rotary actuators and grippers.

Auto switches. D-M9 solid state and D-A9 reed, in NPN, PNP and two wire, with cable and connector options and mounting bands.

Speed controllers. Meter-out and meter-in, elbow and inline, in the port sizes.

Mountings. Brackets, flanges, clevises, rod eyes and floating joints.

Equivalents. Where an ISO 15552 cylinder from another brand is faster or cheaper we will offer it and tell you what differs.

We buy SMC

We buy surplus and unused SMC, particularly cylinders, manifolds and electric actuators from decommissioned lines.

Unused stock in original packaging is worth the most, and auto switches and mountings are worth keeping with the cylinders they came off. See Sell To Us.

Common questions

Can you get a part that is not listed on your site?
Usually. The site shows what we hold, not what we can source.

What does the D in CDQ2 mean?
A magnet in the piston, so auto switches can detect position. CQ2 has no magnet.

Do switches come with the cylinder?
No. They are ordered separately and the type has to match your PLC inputs.

How do I know if I need NPN or PNP?
It depends on the PLC input card. Read it off a working switch on the same machine, or tell us the PLC.

My cylinder judders.
Check the speed controllers. Meter-out is normally correct and meter-in causes lurching.

Should I oil the air?
Usually not. These cylinders are pre-lubricated, and added oil washes the factory grease out.

What is the lead time?
Ex stock items ship the next working day within New Zealand. We give you a firm lead time with the quote.

Do you quote in NZD?
Yes, in NZD including GST for New Zealand customers.

Send us your part numbers

All of it. And tell us whether switches are fitted and what your PLC inputs are, because those two answers decide most of the order. Quotes in NZD including GST.

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Industech is an independent supplier of new, surplus and used industrial automation equipment. We are not an authorised distributor of, approved by, sanctioned by, or affiliated with SMC. SMC and the product series named here are trademarks of SMC Corporation. All trademarks, brand names and part numbers are the property of their respective owners and are used here for identification purposes only.