The Reed Switch That Dropped Off the Cylinder
We used a pneumatic cylinder with a reed switch (magnetic sensor) to detect the end of stroke. The switch clamped onto the cylinder’s groove. On the bench, it worked. On the floor, after a week, the switch fell off. The cable pulled on it. The clamp was a simple plastic band that loosened from vibration. The cylinder was on a fast cycle (1 Hz), vibrating. The plastic clamp stretched. We switched to a metal (stainless) clamp with a screw. No more dropping off. The mistake was using a plastic clamp on a vibrating, fast-cycling cylinder.
Pneumatic cylinder reed switch mounting is about the switch position and the clamp. The switch must detect the piston’s magnet at the end of stroke. This article covers the design.
What a Reed Switch Does
A reed switch (magnetic sensor) clamps onto the pneumatic cylinder’s tie-rod groove (or a T-slot). It detects the piston’s internal magnet when the piston reaches the switch. The switch closes (a contact output), telling the PLC the cylinder is extended (or retracted).
No mechanical contact — the switch senses through the cylinder wall. Good for dirty environments (no moving parts to jam).
Step 1: Mounting Position
The switch mounts near the end of stroke. The piston’s magnet passes over the switch. The switch triggers.
- Fully extended: The switch is at the cap end (the end the rod comes out to). The piston triggers it when fully out.
- Fully retracted: The switch is at the rear end (the cap opposite the rod). The piston triggers it when fully in.
Adjust the switch position: slide it along the groove until it triggers exactly at the end of stroke. A small LED on the switch shows when it’s triggered (green = on). Tune it on the machine.
Step 2: Clamp Type
| Clamp Type | Holding | Best For |
|---|---|---|
| Plastic band (slide-on) | Light (loosens with vibration) | Slow cylinders, light vibration |
| Metal screw clamp | Strong | Fast cycles, vibration, standard |
| T-slot (built-in) | Strong (cylinder has a slot) | Modern cylinders, clean look |
| Bracket (external) | Adjustable | Cylinders without a groove |
The switch that dropped off had a plastic band on a vibrating cylinder. Use a metal screw clamp (or a T-slot mount) for fast cycles.
The reed switch rule: Use a metal screw clamp (or T-slot), not a plastic band. The switch that fell off was on a vibrating, fast-cycling cylinder. A plastic clamp loosens. A metal clamp (with a screw) holds. Mount the switch near the end of stroke, tune it with the LED, and lock it.
Step 3: Switch Output Type
The reed switch has an output:
- Reed (mechanical contact): A physical contact (NO, normally open). Simple, but the contact wears (mechanical life, about 100 million operations). Good for low cycles.
- Hall effect (electronic): A solid-state sensor (no moving parts). PNP or NPN output. Longer life (for fast cycles). Standard for modern machines.
For a fast-cycling cylinder (over 1 Hz), use a Hall-effect switch (no wear). For a slow cylinder (occasional), a reed switch is fine (cheaper).
Step 4: Cable Strain Relief
The switch’s cable shouldn’t pull on the switch. Route the cable with a loop (slack) and anchor it (a cable clip near the cylinder). If the cable pulls (from the moving cylinder or the drag chain), the switch might shift or pull off.
For a moving cylinder (the whole cylinder moves), the cable moves with it (article 105). Use a flexible cable and a drag chain. Don’t let the cable pull on the switch.
Step 5: Switch Spacing (Multiple Switches)
If the cylinder has multiple positions (extended, retracted, and a mid-position), use multiple switches spaced along the groove. The switches must be far enough apart (about 20 mm) so the piston’s magnet doesn’t trigger two at once.
For a cylinder with a mid-stroke stop (via external stops), the switch at the mid-position detects the piston there.
A Reed Switch Checklist
- Is the cylinder tie-rod grooved (or T-slot)?
- Where is the switch mounted? (End of stroke?)
- Is the clamp metal (not plastic band)?
- Is the switch position tuned with the LED? (Triggers exactly at end?)
- Is the clamp screw tightened?
- Is the output type right? (Reed for slow, Hall for fast?)
- Is the cable strain-relieved? (No pull on the switch?)
- For multiple switches: spaced apart? (No double-trigger?)
- Is the IP rating correct? (Dust, oil?)
- Is the LED visible? (For troubleshooting?)
The Bottom Line
Pneumatic cylinder reed switch mounting is a tight metal clamp at the end of stroke. The switch that dropped off had a plastic band on a vibrating cylinder. Use a metal screw clamp (or T-slot), tune the position with the LED, and lock it. Use Hall-effect (electronic) switches for fast cycles. Anchor the cable so it doesn’t pull. The switch that stays put wasn’t the cheapest one — it had a metal clamp.
