Hollow-shaft joints and cable routing through robot joints

By Tolerance Labs engineering team · Updated 18 October 2026

Every joint in a robot must pass power and data to the joints beyond it. Badly routed cables fail by fatigue, pick up noise or snag. Plan the routing with the joint, not after it.

What has to pass through

In a robot built from joint modules with integrated drivers, each joint needs only a DC bus and a communication bus. A typical bundle has two power conductors (bus positive and return), a CAN twisted pair with its ground, and sometimes a brake supply, an emergency-stop loop or a cable for a tool. Daisy-chaining the buses from joint to joint keeps the bundle the same size all along the arm.

If the drivers sit in the base instead, every joint carries three motor phase wires and an encoder cable for each motor beyond it. The bundle grows at each joint, and the phase wires carry fast switching edges next to the signal lines. That is one of the main reasons integrated joint modules are popular. Wiring a CAN bus for robot joints and power supplies and batteries cover the bus side.

Hollow-shaft modules and bore sizing

A hollow-shaft joint leaves a clear bore along its axis. Cables on the axis move least as the joint turns: they twist but do not swing. Harmonic reducers with hollow shafts, inrunner frameless motors with a large inner diameter, and off-axis ring encoders make it possible; encoders for robot joints covers the encoder side. The bore costs diameter: larger bearings and a larger housing for the same torque.

Size the bore from the bundle, then check the connectors. Worked example for two power wires of 2.2 mm outer diameter, a shielded CAN cable of 4.5 mm and a four-core sensor cable of 4.0 mm:

• Bundle cross-section: π/4 × (2 × 2.2² + 4.5² + 4.0²) = 0.785 × (9.68 + 20.25 + 16.0) = 0.785 × 45.93 ≈ 36 mm².
• Cables that twist in a bore need room to move. Keeping the fill below about 35% of the bore area is a common rule of thumb.
• Bore area: 36 ÷ 0.35 ≈ 103 mm². Diameter: √(4 × 103 ÷ π) = √131 ≈ 11.5 mm, so 12 mm or more.

Then check whether the cables must pass through with connectors attached. A small circular connector can be 10 to 15 mm across, larger than the bore you calculated. If so, fit the connectors after threading the cables, or pick a larger bore. Connector size decides the bore more often than cable area does.

Cable twist for limited rotation

Most arm and leg joints turn less than one revolution, so cables can twist instead of sliding. A torsion-rated cable is specified in degrees of twist per meter of free length, and figures around ±180° to ±360° per meter are typical for torsion-rated robot cables. Clamp the cable at both ends of the free length so the twist spreads evenly instead of collecting at one clamp.

Example: a wrist joint turns ±90°, and the cable is rated for ±360°/m. It needs at least 90 ÷ 360 = 0.25 m of free length between clamps. If only 0.10 m fits, the cable sees 90 ÷ 0.10 = 900°/m, 2.5 times its rating, and the conductors will fatigue early.

For ranges beyond what a straight twist allows, use a clock-spring layout: a flat cable coiled in an annular drum that winds and unwinds as the joint turns. Or lay the cable in a loop around the axis, so rotation changes the loop’s radius instead of twisting the cable.

Service loops for external routing

Where a cable runs outside the joint, it needs a service loop: slack that takes up the joint’s motion as bending. The loop must cover the arc the cable travels. A cable crossing a joint 40 mm from the axis, with a range of ±135°, travels 0.040 m × (270 × π ÷ 180) = 0.040 × 4.71 ≈ 0.19 m. Give the loop that length plus margin, and check that at both ends of the range it stays above the minimum bend radius and clear of pinch points. External loops are simple to build and service, but they snag and are exposed to damage.

Continuous rotation needs slip rings

A joint that turns without limit, such as a wheel, a turret or a continuous wrist roll, cannot use twist or loops. It needs a slip ring, which passes current through sliding contacts. Capsule slip rings are compact and mount on the axis. Through-bore slip rings leave the center open.

• Rate each ring for its current with margin, and give the power return its own ring or rings.
• Contact resistance varies as the ring turns. Put signals on rings rated for data, keep each pair on adjacent rings, and check the data rate the maker supports. CAN at 1 Mbit/s usually passes a good slip ring; Ethernet needs rings rated for it.
• Give the cable shield its own ring, or the shield is broken at the slip ring.
• Slip rings wear. Check the rated life in revolutions against the duty, and plan access for replacement.

Bend radius and flex-life cable

Cables have two bend limits. The static minimum bend radius applies to a cable bent once at installation. The dynamic minimum bend radius applies to a cable that flexes in service, and it is larger. For flex-rated cables, figures of about 7 to 15 times the outer diameter for continuous flexing are typical; the cable’s datasheet has the real number.

• Use fine-stranded conductors. Solid or coarse-stranded wire work-hardens and breaks.
• Use cables rated for flexing or torsion, with PUR or TPE jackets. General-purpose PVC cable stiffens in the cold and cracks under repeated flexing.
• Check the cable’s temperature rating. Cables in a motor bore run warm.
• Do not mix stiff and flexible cables in one bundle. The stiff one takes the motion and the flexible ones kink.

Shielding, strain relief and connectors

Motor phase wires commonly switch the full bus voltage in tens of nanoseconds, and those edges couple into anything nearby. Keep phase wires inside the joint where possible. Where they must pass through, use a shielded motor cable, bond the shield 360° at both ends, and keep it apart from encoder and CAN cables. Where cables must cross, cross at right angles.

Run CAN as a twisted pair of about 120 Ω characteristic impedance, terminated at both ends. Run encoder signals as differential pairs where the encoder allows. The DC bus carries ripple from every driver on it, so twist the bus pair as well. For strain relief and connectors:

• Clamp the cable on both sides of every moving section, so motion never reaches a crimp, solder joint or connector.
• Use latching or screw-locking connectors. Friction-fit connectors back out under vibration.
• Key connectors so power and CAN cannot be swapped.
• Check mating-cycle ratings if joints are swapped often during development.
• Keep connectors out of the moving section and outside the bend.

Checklist

• List every conductor through each joint, with its current and signal type.
• Size the bore from bundle area, then check connector size.
• For limited rotation, compute twist per meter and compare it with the cable rating.
• For external routing, size service loops from arc length and bend radius.
• Use slip rings for continuous rotation, with separate rings for power, signals and shield.
• Use fine-stranded, flex- or torsion-rated cable within its dynamic bend radius.
• Keep phase wires away from CAN and encoder lines, with shields bonded at both ends.
• Strain-relieve both sides of every moving section.

Browse power and control cables and harmonic joint modules, and check which offer a hollow shaft.