The Sensor That Cut Out After a Month
We routed a photoelectric sensor cable (article 157) along a moving axis (the sensor moved with the slide). The cable was laid loosely (on the frame). After a month, the cable failed (the signal cut out intermittently). We pulled the cable. The conductors (inside) were cut (fatigue from the repeated flex). The problem: the cable wasn’t in a drag chain (article 43, 148). It flexed every cycle (bending at the same point). After thousands of cycles, it fatigued. We routed the cable in a drag chain (with the right bend radius, article 148). The cable flexed in the chain (evenly). It lasted. The mistake was laying the cable loose (no drag chain) on a moving axis.
Sensor cable routing protects the cable on a moving axis. This article covers the design.
Cable Routing Rules
A cable on a moving axis (the sensor moves with the slide) flexes every cycle. It needs:
- A drag chain (article 43, 148): The cable is carried in the chain (it flexes evenly, at the chain’s bend radius).
- The right bend radius (article 148): The chain’s radius is ≥ the cable’s minimum (10× OD).
- No fixed flex point: The cable doesn’t bend at the same point (which fatigues it).
Step 1: Use a Drag Chain (Article 43)
For a moving axis, route the sensor cable in a drag chain (article 43). The chain carries the cable (it flexes at the chain’s bend, not at a fixed point). The cable lasts. For a stationary sensor (the sensor doesn’t move), the cable can be tied to the frame (no drag chain).
The cable routing rule: On a moving axis, route the sensor cable in a drag chain (article 43, 148). The cable that cut out was laid loose (flexed at a fixed point). The drag chain flexes evenly. For a stationary sensor, tie the cable to the frame (no chain).
Step 2: Separate from Power Cables
A sensor cable (signal) runs near a power cable (servo motor, 24V). The power cable’s electrical noise interferes with the sensor signal. Route the sensor cable separately (a separate drag chain lane, or 100 mm away). Don’t bundle the sensor cable with the power cable. For a shielded sensor cable (article 182), ground the shield (article 169).
Step 3: Strain Relief
At the sensor (and at the connector), the cable needs strain relief (a cable gland, a tie). The cable doesn’t pull on the sensor’s connector (which can break). A loose cable pulls on the connector (it fails). Use a cable gland (or a tie) at both ends.
Step 4: Cable Length (Don’t Over-Leave)
Don’t leave extra cable (a loop) hanging. The loop sways (and catches). Trim the cable to the right length (with a little slack for the drag chain’s fixed end, article 43). For a fixed sensor, tie the cable (a neat loop, not a loose hang).
| Situation | Routing |
|---|---|
| Moving axis (sensor moves) | Drag chain (article 43, 148) |
| Stationary sensor | Tie to the frame |
| Near power cables | Separate lane (100 mm away) |
| At the connector | Strain relief (gland or tie) |
| Extra length | Trim (no hanging loop) |
A Cable Routing Checklist
- Is the sensor moving? (Drag chain?)
- Is the bend radius right? (Article 148?)
- Is the cable separated from power? (Noise?)
- Is there strain relief? (At the connector?)
- Is the cable length right? (No hanging loop?)
- Is the cable shielded? (Article 182?)
- Is the shield grounded? (Article 169?)
- Does the cable flex at a fixed point? (No?)
- Is the drag chain fill under 60%? (Article 148?)
- Does the signal cut out? (Check?)
The Bottom Line
Sensor cable routing uses a drag chain on a moving axis. The cable that cut out was laid loose. Route it in a drag chain (with the right bend radius). Separate from power cables. Add strain relief. The sensor that never cut out wasn’t the best cable — it was routed in a chain.
