The Limit Switch That Broke: Roller Arm Life, Not the Switch
A mechanical limit switch on a conveyor. It was actuated by a cam on the conveyor chain. The switch broke every 2 months. The customer thought the switch was cheap. The switch was fine. The roller arm was being hammered 60 times per minute. The switch’s mechanical life was about 1 million actuations. At 60 per minute, that’s 16,666 minutes = 278 hours = about 2 months of 24/7 operation. This is about switch selection for high-cycle applications, not about the switch being defective.
The switch life ratings
| Switch type | Mechanical life | Electrical life | Use for |
|---|---|---|---|
| Standard micro switch | 1 million cycles | 100,000 cycles | Low cycle (few per minute) |
| Heavy-duty limit switch | 10 million cycles | 1 million cycles | Medium cycle (up to 60/min) |
| Inductive proximity (no contact) | 100 million+ cycles | 100 million+ | High cycle (100+/min) |
| Safety switch (no contact) | 100 million+ | 100 million+ | Safety circuits |
What we did
1. Replaced with an inductive proximity sensor. The limit switch was mechanical (a roller arm hitting a cam). We replaced it with an inductive proximity sensor detecting a metal flag on the chain. No contact. No wear. The sensor’s life is rated at 100 million+ cycles. At 60 per minute, that’s 19 years. The switch broke every 2 months; the sensor will last years.
2. Added a debounce filter. The sensor was triggered by a passing flag. We set the PLC input filter to 20 ms. The sensor doesn’t chatter as the flag passes. The PLC sees a clean signal.
3. Kept the mechanical switch for low-cycle uses. For other stations (where the switch is actuated a few times per hour), the standard micro switch is fine. It’s cheap and reliable. We only upgraded the high-cycle station.
The cost comparison
The standard limit switch cost $15. It lasted 2 months. That’s $90 per year. The inductive proximity sensor costs $50. It lasts 19 years. That’s $2.60 per year. The sensor is cheaper over its life. The customer thought the switch was cheap; it was actually expensive because it kept breaking.
The limit switch rule: For high-cycle applications (60+ actuations per minute), don’t use a mechanical limit switch. Use an inductive proximity sensor (no contact). The switch’s mechanical life will be exceeded. The sensor lasts 100x longer. The switch that broke every 2 months wasn’t defective — it was the wrong type for the cycle rate.
