The Cable That Broke in the Drag Chain
We routed a servo cable (motor + encoder) through a drag chain (cable carrier, article 43). After 3 months, the cable failed — the encoder signal cut out intermittently. We pulled the cable out. The conductors (inside) were broken (fatigue cracks). The problem: the drag chain’s bend radius was too small for the cable. The cable flexed at a tight radius on every cycle. After thousands of cycles, the copper conductors fatigued and broke. We checked the drag chain’s bend radius vs the cable’s minimum bend radius. The chain’s radius was 50 mm; the cable needed 75 mm (minimum). We upsized the drag chain (radius 75 mm). The cable stopped failing. The mistake was choosing a drag chain without checking its bend radius against the cable’s.
Drag chain bend radius matches the chain to the cable’s minimum bend radius. This article covers the selection.
The Cable’s Minimum Bend Radius
Every cable has a minimum bend radius (from the manufacturer’s spec). Below that radius, the cable flexes too tight — the conductors fatigue. For a flexible (chain-rated) cable, the minimum bend radius is about 10× the cable’s OD. For a 10 mm OD cable, the minimum bend radius is 100 mm. For a non-flexible (static) cable, it’s 15× OD (or more) — don’t put a static cable in a drag chain.
The Drag Chain’s Bend Radius
The drag chain (cable carrier) has a bend radius (the radius it turns at). It’s a fixed value (e.g., 50 mm, 75 mm, 100 mm). Pick a chain whose bend radius is at least the cable’s minimum (or larger). If the chain’s radius is smaller than the cable’s minimum, the cable fails (fatigue).
The bend radius rule: Pick a drag chain whose bend radius is ≥ the cable’s minimum bend radius. The cable that broke had a 50 mm chain on a cable needing 75 mm. Use a chain with radius ≥ 75 mm (10× cable OD for flexible cable). For a non-flexible (static) cable, don’t use it in a chain at all (it fatigues fast).
Step 1: Use Chain-Rated (Continuous-Flex) Cable
A static cable (for fixed wiring) isn’t designed to flex. Put it in a drag chain, and it fatigues in months. Use a chain-rated (continuous-flex) cable — the conductors are stranded finely (for flexing), and the jacket is abrasion-resistant. It’s more expensive, but it lasts (millions of cycles).
A servo motor cable (power) and an encoder cable are available in chain-rated versions. Buy them that way (don’t use a standard cable in a chain).
Step 2: Fill Ratio (Article 43)
The drag chain has a fill ratio (how full it is). If it’s over 60% full (cables packed tight), the cables rub (abrasion) and can’t move freely. Keep the fill under 60%. Leave room for the cables to flex. For a multi-cable chain, use separators (divide the chain into lanes) so the cables don’t tangle.
Step 3: Fixed End (Article 43)
The drag chain has a fixed end (on the machine) and a moving end (on the axis). The cables attach at both ends. The fixed end should be in the middle of the chain’s travel (so the chain bends equally on both sides). If the fixed end is at one end, the chain flexes more on one side (uneven wear). Mount the fixed end at the center (article 43).
Step 4: No Pull (Drag, Not Pull)
The cables should “lay” in the chain (they’re carried, not pulled). The chain carries the cables. Don’t pull the cables tight (they add tension). Leave a little slack (the cables flex in the chain, they don’t stretch). If the cables are pulled tight, they wear (and break).
| Cable Type | Min Bend Radius | Use in Chain? |
|---|---|---|
| Chain-rated (continuous-flex) | ~10× OD | Yes (millions of cycles) |
| Standard (static) cable | ~15× OD | No (fatigues fast) |
| Pneumatic hose (PU) | ~10× OD | Yes (but don’t overbend) |
| Thick cable (16 mm OD) | ~160 mm | Need a big-radius chain |
A Drag Chain Checklist
- What is the cable’s minimum bend radius? (10× OD?)
- Is the drag chain’s radius ≥ the cable’s minimum?
- Is the cable chain-rated (continuous-flex)?
- Is the fill under 60%? (Article 43?)
- Are separators used? (For multiple cables?)
- Is the fixed end centered? (Article 43?)
- Are the cables carried (not pulled)?
- Is there slack? (Not tight?)
- Does the chain run smoothly? (No kinks?)
- Has the cable failed? (Check the bend radius?)
The Bottom Line
Drag chain bend radius matches the chain to the cable. The cable that broke had a 50 mm chain on a 75 mm-minimum cable. Pick a chain with radius ≥ the cable’s minimum (10× OD for flexible cable). Use chain-rated (continuous-flex) cables. Keep the fill under 60%. The cable that lasted wasn’t the cheapest one — it had the right bend radius.
