Power Chain for Machine Tool: Conductor Sizing and Chain Load

What a Power Chain Means in Machine Tool Practice

A power chain for machine tool duty is the section of the cable carrier that carries the conductors doing real work: servo motor cables, spindle cables, brake lines and auxiliary supplies. Signal conductors — encoder, temperature sensor, proximity switch — travel in the same chain but should not share the same compartment. The reason is not tidiness. It is heat, electromagnetic noise and mechanical load, three problems that behave differently for a 35 mm² motor cable than for a 0.2 mm² encoder pair.

Why a Power Chain Is Sized by Conductors, Not by Cables

Most enquiries describe the chain by the number of cables. That is the wrong unit. Two spindle cables of 16 mm² carry far more copper than six 1.5 mm² cables, and it is the copper that generates heat and resists bending. Size the chain on total conductor cross-section, cable outer diameter and the stiffness of the largest conductor.

A chain that is comfortably sized on cable count can still be overloaded on copper. When in doubt, quote the sum of conductor cross-sections in square millimetres per compartment — it is the number that decides whether the bundle overheats in a thirty-metre run.

Ampacity Derating Inside the Chain

A motor cable rated 40 A in free air does not carry 40 A inside a closed carrier. The chain is a duct: heat leaves only through the side plates and the gaps between links, and the fill ratio controls how much air moves. Three factors reduce the usable current.

• Fill ratio. Above roughly 40 % of the inner cross-section occupied by cables, the bundle starts to insulate itself. Keep power compartments at 30 – 40 % when the duty cycle is continuous.
• Grouping. Four loaded conductors side by side in one compartment derate each other. The derating is worse when the cables touch along their full length, which they do in a carrier.
• Ambient. A chain running inside a machine enclosure sees 45 – 55 °C, not 25 °C. Ask for the cable's derating table at the real ambient temperature, not the catalogue headline current.

The practical consequence: if a spindle cable is specified at the edge of its ampacity in free air, it will run hot in the chain, and a hot PVC or PUR jacket hardens, cracks in the bend, and exposes the shield. Oversizing the conductor by one step is usually cheaper than replacing a spindle cable inside a machine that is already at the customer's site.

Separating Power from Signal in the Same Chain

Putting power and encoder conductors in one carrier is normal and acceptable, provided the separation is physical, not nominal.

• Use a vertical divider of at least 2 mm between the power and signal compartments, and fix it — a divider that slides leaves a gap exactly where the bundle bends.
• Run shielded encoder cable with the shield bonded at the drive end, and never rely on the chain itself as a screen.
• Keep a minimum of 20 mm between the power compartment and the encoder compartment on drives above 400 V.
• Route the protective earth conductor in the power compartment and bond the chain's fixed end to the machine frame.

Most "false encoder alarm" complaints on machine tools are not encoder failures. They are a 3 mm gap between two compartments and an unshielded run of forty centimetres.

Matching the Chain to Spindle and Servo Duty

Servo and spindle axes load the chain in different ways and are rarely served well by the same series.

Servo axes

High acceleration, short stroke, many reversals. The dominant load is fatigue at the bend, so the radius and the pin quality matter more than the width. Keep the fill ratio under 40 % so the motor cable can dissipate heat during rapid cycling.

Spindle axes on gantry and travelling-column machines

Long travel, heavy bundle, and a spindle cable that is stiff by nature. Here the inner width, the load per metre and the support method decide the outcome. A spindle cable bent below its minimum radius once will not fail immediately; it will fail in the third month, at the point where the shield was compressed.

Tool changer and auxiliary axes

Very high cycle counts at low load. Use the smallest series that satisfies the radius, and check cycle rating rather than static strength.

Installation Rules for Large Conductors

• Clamp the motor cable at the moving end with a proper strain relief, not a cable tie through a link. A 35 mm² cable pulling on one link will ovalise the pin bore.
• Leave 10 – 20 % slack inside the chain so the conductor is never in tension at the end of the stroke.
• Never stack a stiff spindle cable on top of small signal cables; put the heavy conductor at the bottom of the compartment.
• Respect the cable's own minimum bend radius, which for a 35 mm² motor cable is often 6 – 8 times its diameter — larger than the chain radius quoted for the same size.

Fault Signs Specific to Power Chains

Power conductors fail differently from signal conductors. Watch for a jacket that has hardened and no longer flexes, a bend that has visibly flattened, copper that has broken inside an intact jacket after repeated bending, and a drive that trips on overcurrent only after a long run at high ambient. Any of these means the chain is either undersized on fill ratio or the conductor was bent below its radius from day one.

What to Send for a Power Chain Quotation

• Motor and spindle cable list with cross-sections, outer diameters and shield type
• Continuous current, duty cycle and the ambient temperature inside the enclosure
• Travel stroke, speed, acceleration and mounting orientation
• Encoder and sensor lines that must share the chain, with their own diameters
• The bend radius you can accommodate, and the support method

SONDA manufactures plastic and hybrid power chains for machine tool axes, with fixed dividers, shielded compartments and series that accept inner height and width independently. Send the conductor cross-sections rather than the cable count and you will get a chain sized for heat, not just for fit.