A CNC router with an energy chain (cable carrier) started snagging after six months. The chain was sized for a 50 mm bend radius. The cables inside — a servo cable, two sensor cables, and an air hose — were snagging on the chain’s inner radius at every cycle. The customer thought the chain was defective. The chain was fine. The radius was too small for the cables.
An energy chain’s bend radius is the radius of the inner arc when the chain folds. Every cable inside the chain has a minimum bend radius — published by the cable manufacturer. If the chain radius is smaller than the cable’s minimum bend radius, the cable flexes beyond its spec. Over thousands of cycles, the copper conductors work-harden and crack. The machine fails electrically, not mechanically. The customer’s servo cable had a minimum bend radius of 75 mm. The chain was 50 mm. The cable was being bent 33% tighter than its spec. It failed.
The radius rule
The chain radius must be larger than the largest minimum bend radius of any cable inside. Not equal — larger. The rule used in practice:
Chain radius ≥ 1.1 × cable minimum bend radius
For the servo cable with 75 mm minimum bend radius, the chain needs at least 82 mm radius. A standard 100 mm radius chain was the right choice. The 50 mm chain was a mismatch from day one.
The second rule: the cables need room to move inside the chain. If the chain is packed tight (fill ratio above 50-60%), the cables rub against each other and the chain walls. The rubbing wears the jacket, then the shield, then the conductors. The fill rule: the cross-sectional area of all cables should be less than 50% of the chain’s interior cross-section. For the 50 mm chain with 4 cables, the fill was about 70%. Too much. The cables had no room to float, so they pinched at the bend.
What changed
The chain was replaced with a 100 mm radius unit. The servo cable stopped snagging. The fill ratio dropped to 45%. The machine ran for 2 years without a cable failure. The chain cost $60 more. The servo cable replacement cost $300 and 4 hours of downtime. The radius rule paid for itself in one month.
There’s also a subtlety most people miss: the cables must be laid in the chain with the correct clearance between them, and the chain must be pre-stretched (installed so the upper run is straight) before the machine runs. A chain installed with slack will bunch up at the bend and pinch the cables. The manufacturer’s installation instructions show this. Following them is not optional — the “self-supporting” chain claim only holds when the cables are laid per the fill and clearance rules.
Bend radius by cable type
| Cable type | Typical min bend radius | Recommended chain radius |
|---|---|---|
| Servo power cable (high flex) | 15x cable OD | Chain radius ≥ 1.1x that |
| Encoder / sensor cable | 10x cable OD | Chain radius ≥ 1.1x that |
| Air hose (polyurethane) | 8x OD (varies) | Check hose spec |
| Hybrid cable (power + signal) | 18x cable OD | Often the controlling case |
For a 10 mm servo cable with 15x rule, the min bend radius is 150 mm. The chain needs 165 mm minimum. Most people look at a 100 mm chain and think “that’s plenty.” It isn’t, for a 10 mm high-flex servo cable.
Length and stroke calculation
Chain length for a given stroke: L = (stroke / 2) + (π × radius) + safety margin. For a 1000 mm stroke with 100 mm radius: L = 500 + 314 + 100 = 914 mm of chain. The chain length determines the number of links. A common mistake is ordering the chain too short and stretching it to fit — the chain must not be pre-tensioned. Order by the formula.
Also check the chain’s own weight capacity. A 100 mm radius chain carrying 4 heavy cables over 1000 mm may exceed the chain’s max unsupported length. For runs over 2 meters, use a glide chain (runs on a track) instead of a free-standing arch. The arch buckles under its own weight and the cables pay the price.
Energy chains fail when the radius is smaller than the cables demand. Measure the cable min bend radius, add 10%, and check the fill ratio stays under 50%. The snagging chain wasn’t worn out — it was the wrong radius for the cables inside. Cables die quietly inside a tight chain.
