Limit switches (position switches) are the simplest components on a machine — until one fails or trips at the wrong moment. A switch with the wrong actuator wears out in weeks, a housing rated IP40 dissolves in a washdown area, and a contact block without positive opening fails a safety audit. This guide walks through the four decisions that actually matter when specifying a limit switch, using the Schmersal range as the reference series.
1. What a Limit Switch Has to Do
A limit switch detects the presence or position of a moving machine part and converts that mechanical event into a electrical signal for the control system. Typical jobs: end-of-travel detection on conveyors and lifts, home-position sensing on rotary tables, over-travel protection, and counting parts on a line. It is a pure position signal — it tells the PLC where something is, not whether a guard is safe to open. That distinction matters later.
The governing standard for this component class is EN IEC 60947-5-1 (low-voltage switchgear — control circuit devices). Any reputable position switch you buy today — for example the Schmersal M 687-11-1 — carries this certification, and you should treat it as a non-negotiable line on the datasheet.
2. Step 1 — Choose the Actuator Type
The actuator is the part the machine physically strikes. It determines mounting accuracy, wear life, and how forgiving the switch is to overspeed or misalignment. This is the single most common specification mistake.
| Actuator | Best for | Watch out for |
|---|---|---|
| Plunger (push-rod) | End stops, precise repeat positions | Needs accurate approach alignment; side loads kill it |
| Roller plunger | Cam-driven detection, higher approach speeds | Roller adds height; check cam profile |
| Roller lever | Directional approach, uneven targets, wide tolerances | Lever arm is the wear part — order spares |
| Spring rod (flexible) | Rough environments, imprecise machine motion | Less precise switching point |
In the Schmersal modular range the actuator family is coded in the model number — for instance the plunger-type PS116-Z11-LR200-S200 and its roller-plunger variant PS116-Z11-LR200-R200 share one switch body but behave completely differently against a cam. Where approach direction varies or the target is a sliding rather than a striking part, the roller-lever PS116-T11-LR200-S200 is usually the safer specification. The full matrix of body and actuator combinations is listed in the Modular Limit Switches category.
3. Step 2 — Contact Configuration and Positive Opening
Three contact decisions matter:
Snap-action vs slow-action. Snap-action contacts change state at a defined trip point regardless of actuator speed — essential when the target moves slowly, because slow-action (translating) contacts can chatter or burn under creeping approach speeds.
NO / NC arrangement. Most machine-position duties use one normally-open contact for the position signal and one normally-closed for the counter-signal, so the PLC can cross-check plausibility. Specify both rather than relying on a single contact.
Positive opening. If the switch feeds a safety-related function (over-travel that protects people, not just the machine), the NC contact must be positively linked to the actuator — the contact physically cannot stay closed if the mechanism welds. This is marked on the datasheet and on the switch itself. Plain limit switches without positive opening are fine for pure position feedback, but they are not safety components.
4. Step 3 — Match the Housing to the Environment
The two failure modes are ingress and temperature. Check the datasheet rather than the series name: within one manufacturer's catalogue you will find everything from compact thermosetting-resin housings rated IP40 and -30 to +120 °C — such as the M 687-11-1 — through to sealed versions for dust and splash zones.
- Dry control cabinets, indoor machinery: IP40 to IP54 is sufficient and keeps cost down.
- Machine-level mounting with dust or coolant mist: IP65 minimum.
- Washdown, outdoor, or high-vibration areas: IP66/67 with metal or reinforced housings — the heavy-duty M 330-11-1-P4 class of switch is built exactly for this.
Chemical compatibility is the forgotten criterion: thermosetting resin housings are excellent against oils and most coolants but check the list before specifying near solvents.
5. Step 4 — Standard Modular vs Heavy-Duty
This is the main fork in the Schmersal range, and the decision is driven by duty cycle and mechanical stress, not by price alone.
| Criteria | Modular series (PS116 class) | Heavy-duty series (M 330 / M 687 / M 697) |
|---|---|---|
| Mechanical life | High, at moderate switching frequencies | Highest — built for continuous cam actuation |
| Housing | Compact thermosetting resin | Reinforced, with metal and sealed variants |
| Best application | OEM machinery, end stops, position feedback | Heavy industry, steel, conveyors, outdoor plant |
| Footprint & cost | Smaller, lower | Larger, higher — but lower lifetime cost in harsh duty |
The decision rule: if the switch is actuated by a machine-driven cam every cycle, or it lives outside a cabinet, go heavy-duty. The M 697-11-1 family exists precisely for high-frequency, high-stress positions where a compact switch would be a maintenance item. Both families are listed side by side in the Heavy Duty Limit Switches category.
6. When a Limit Switch Is Not Enough
A limit switch signals position. It does not make a guard safe. If the detected position protects a person — an interlocked gate, an access hatch, a guard that must stay locked until a spindle has coasted down — you need a dedicated safety switch with positive-opening contacts and a rated safety level, not a general-purpose position switch. The selection logic for that class of component is covered in our safety door switch procurement guide, and the interface side — how the switch signal reaches the contactor — is explained in how to select a safety relay module.
If you are replacing switches on an existing machine and the original brand is obsolete, our Schmersal cross-reference against EUCHNER, SICK, Steute, Schneider, Omron and Pilz maps the common equivalents model by model.
Quick Selection Checklist
Before sending an enquiry, write down these six lines — they define 90% of a limit switch specification:
- Approach direction and speed of the target (→ actuator type)
- Position signal only, or safety-related function (→ positive opening, or a safety switch)
- Environment: dust, coolant, washdown, outdoor (→ IP rating and housing material)
- Ambient temperature range at the mounting point
- Switching frequency and expected cycles per day (→ modular vs heavy-duty)
- Contact arrangement the PLC expects (NO/NC, snap or slow action)
With those six lines you can match any Schmersal PS116 or M-series part number directly. For pricing, stock and lead times on any limit switch in this guide, request a quote through our quote form — we reply within 24 hours with a firm offer.
Frequently Asked Questions
What is the difference between snap-action and slow-action contacts?
Snap-action contacts switch rapidly regardless of actuator speed, ideal for slow-moving actuators and preventing contact arcing. Slow-action contacts switch at a speed proportional to the actuator and are required for positive-opening safety contacts, because the contact is mechanically forced open.
Which actuator type should I choose for a limit switch?
A plunger suits precise end-stop detection; a roller plunger or roller lever handles cam-driven and higher-speed approaches; a spring rod tolerates rough, imprecise machine motion. Match the actuator to how the machine part approaches and strikes the switch.
What IP rating do I need for my environment?
IP67 is the standard for dusty or wet industrial environments. For washdown areas use IP69K; for dry indoor cabinets IP40-IP54 can suffice. Choose the rating that protects the switch against the specific dust, moisture and spray in the application.