The Limit Switch That Broke (Roller Arm)
We used a limit switch (article 114) on a moving slide. The slide hit the switch’s roller (a plastic roller). On the floor, the roller broke (the slide hit it hard, the plastic roller cracked). The problem: the roller was plastic (cheap), and the slide hit it fast (and hard). We either used a metal roller (stronger) or added a cam (the slide engages the roller slowly, not a hard hit). We used a metal roller (steel). The roller held. The mistake was using a plastic roller (on a fast, hard hit).
Limit switch roller lever — pick the right actuator. This article covers the selection.
The Limit Switch Actuators
- Pin plunger (top): The switch is hit (a direct push). For a vertical motion (the slide pushes down). Simple. No lever.
- Roller lever (side): A lever (with a roller) extends. The slide hits the roller (the lever pivots, the switch trips). For a horizontal motion (the slide brushes past).
- Roller arm (adjustable): The arm is adjustable (set the length, the angle). For a flexible setup.
Step 1: The Roller Material (Plastic vs Metal)
A plastic roller is cheap (but it breaks on a hard hit). A metal (steel) roller is strong (but it’s louder, and it wears the cam). For a fast, hard hit (a slide moving at 1 m/s), use a metal roller. For a light touch (slow), a plastic roller is fine.
The roller rule: For a fast/hard hit, use a metal roller. The roller that broke was plastic (on a fast slide). Use steel. For a light touch, plastic is fine. Add a cam (the slide engages the roller slowly, not a hard hit).
Step 2: The Cam (Engage Slowly)
Don’t let the slide hit the roller directly (a hard hit). Add a cam (a shaped plate) on the slide. The cam engages the roller (slowly, over a distance). The roller is not hit (it’s pushed gradually). The switch trips (smooth). The roller lasts.
Step 3: The Lever Length (Over-Travel)
The lever (with the roller) has over-travel (the slide can push past the switch’s trip point). The lever absorbs the over-travel (it pivots, no damage). Don’t mount the switch (so the slide pushes directly on the plunger, no over-travel). Use a lever (it absorbs the over-travel).
Step 4: The Bounce (Article 211)
The limit switch (a mechanical contact) bounces (when it trips). The PLC (article 181) sees multiple pulses. Debounce (article 181, a 20 ms delay). Or use a proximity (article 108, no bounce). For a clean signal, a proximity (no bounce) is better. A limit switch (mechanical) needs debounce.
| Actuator | Use |
|---|---|
| Pin plunger (top) | Vertical push |
| Roller lever (plastic) | Light touch (slow) |
| Roller lever (metal) | Fast/hard hit |
| Cam (slow engage) | Protect the roller |
A Limit Switch Checklist
- Is the hit fast/hard? (Metal roller?)
- Is there a cam? (Slow engage?)
- Is the lever adjustable? (Length?)
- Is there over-travel? (Lever pivots?)
- Is the switch debounced? (PLC? Article 181?)
- Does the roller break? (Test?)
- Is the switch rated? (Cycles?)
- Is the mounting solid? (No flex?)
- Is the IP rating right? (Article 77?)
- Is the actuation consistent? (Repeat?)
The Bottom Line
Limit switch roller lever uses a metal roller (for a fast hit). The roller that broke was plastic. Use steel. Add a cam (slow engage). Debounce (article 181). The switch that lasted wasn’t the cheapest roller — it was metal (with a cam).
