Hollow Robot Joint Module: Why Cable Routing Matters in Robotic Joints？

When looking at a robot joint specification sheet, we usually focus first on torque, speed, dimensions, and weight. There is another parameter that may look like a structural detail, but can have a significant impact on how well the joint performs once it is integrated into a robot.

That parameter is the hollow cable-routing structure.

Space inside a robot joint is already limited. The motor, reducer, drive, encoder, and bearings all need to occupy part of the available space. So why leave a passage through the center?

The answer becomes clearer when the joint is considered as part of the complete robot rather than as an isolated actuator.

Why Do Robot Joints Need a Hollow Design?

A robot is not only a mechanical system. It is also a moving system for power, signals, and other utilities.

When a robotic arm performs a pick-and-place operation, the joints are only part of the system. An electric gripper at the end of the arm needs power and control signals. A camera needs power and communication connections. A pneumatic gripper or vacuum system may require air tubing. All of these connections have to extend from the robot body to the end effector through the moving arm.

When the robotic arm is stationary, arranging these cables and tubes is relatively straightforward. Once the joints begin rotating, however, the situation changes.

A cable bundle that was previously loose may become tensioned. Excess cable may bulge outward. Cables routed around a joint may also undergo repeated bending and twisting.

If a cable becomes damaged, stretched, or develops an intermittent connection, the problem is no longer simply an issue of appearance. It can affect the reliability of the entire robotic system.

For this reason, robot joint cable routing needs to be considered together with the mechanical structure and the joint's range of motion. If the cable path is designed only after the joints and links have already been finalized, additional brackets, protective sleeves, or routing structures may be required, and the original mechanical layout may have to be modified.

A hollow robot joint provides a dedicated passage through the joint for cables, tubing, and other internal connections.

When this passage is aligned with the internal space of the robot links and the end-effector interface, cables and tubes can be routed through the robot structure rather than being exposed outside the arm. This can reduce external cable loops and contribute to a more compact overall design.

Hollow vs. Non-Hollow Joints: How Does Cable Routing Differ?

The differences between external and hollow cable routing can be considered across several engineering dimensions.

The weakness of external cable routing is not simply whether a cable will eventually fail, but when it will fail.

Cable bending life is a statistical specification. A cable rated for one million bending cycles under defined test conditions may have a substantially different service life under actual combined stresses involving bending, twisting, tension, temperature, and installation conditions.

Cable failure can also be an intermittent fault. A cable may appear intact externally while individual conductors inside have already been damaged. The result may be intermittent communication loss or unexpected power interruption at the end effector, making troubleshooting more difficult and increasing maintenance costs.

However, hollow cable routing introduces its own engineering challenges.

The internal passage is limited in space. Cables must pass through it while still being properly secured, protected against abrasion, and provided with sufficient assembly clearance.

The passage may also be located close to the motor and transmission components, which can be among the warmer areas of the joint. Cables routed through the hollow section therefore need to be selected according to the actual temperature conditions inside the passage, rather than simply the ambient operating temperature.

Cable routing is also part of the assembly process. When the available hollow bore is tight, even a small reduction in clearance can increase the time required to install a cable during production.

There is another point that should not be overlooked.

The two sides of a robot joint rotate relative to each other. If the cable bundle is fixed to different moving components on each side of the joint, the cable may still experience twisting as the joint rotates. Local bending or friction can also occur at the entrance and exit of the hollow passage.

Therefore, a hollow joint does not eliminate the need to consider cable flexibility, torsional resistance, fixation, protection, and motion allowance.

Being able to physically pass a cable through the hollow bore is only the first step. The cable should also be checked throughout the complete operating range of the joint to ensure that it does not experience excessive bending, twisting, tension, or abrasion.

How Large Should the Hollow Bore Be?

Once internal cable routing has been selected, the next question is straightforward:

How large does the hollow bore need to be?

The most obvious approach is to look at the number of cables and pneumatic tubes that need to pass through the joint. In actual mechanical design, however, simply adding up their cross-sectional areas is not sufficient.

The first consideration is what needs to pass through the bore during assembly.

The cable itself may be relatively small, while the connector at its end may be considerably larger. If a pre-assembled cable harness is used, the connector dimensions may determine whether the harness can pass through the hollow bore.

If the cable is routed first and connected afterward, the design must instead consider the available internal working space and whether future maintenance can be performed efficiently.

The second consideration is how much of the bore is actually usable.

Protective sleeves, bushings, liners, and other protection structures may occupy part of the available diameter. Multiple cables also require clearance between individual lines rather than forming a perfectly packed cross-section.

At the outlet of the passage, additional space may be required for the cables to change direction without forcing them into a bend radius that is too small.

For these reasons, selecting a hollow bore that is simply "large enough to fit everything" is not an appropriate engineering criterion.

The routing design should also provide sufficient allowance for assembly, motion, and maintenance.

Is a Larger Hollow Bore Always Better?

Not necessarily.

When the overall joint diameter and length are constrained, increasing the hollow bore reduces the space available around it for the motor, reducer, bearings, and structural support components.

To maintain the required output capability and structural rigidity, the internal architecture may need to be redesigned, or the overall dimensions of the joint may need to increase.

The appropriate hollow bore therefore needs to be determined according to the actual application.

Important factors include:

• What devices will be connected to the end effector

• The specifications and quantity of cables and pneumatic tubes

• How connectors will be installed

• The required joint rotation range

• The required cable bend radius

• Assembly requirements

• Future maintenance or cable replacement requirements

Once these requirements are clear, the required bore size can be evaluated more accurately.

For a robot joint, the hollow bore is a dimensional parameter.

For the complete robot, however, it is a passage that must participate in the mechanical, electrical, assembly, motion, and maintenance design.

Hollow Design Should Be Considered at the Beginning of Joint Design

The value of a hollow robot joint is not simply that it provides a passage through the center of the actuator.

Its real value lies in integrating the joint with the cable-routing requirements of the complete robot.

When the internal routing path is considered from the beginning of the mechanical design process, the robot can be designed so that the joint has sufficient motion capability while cables and pneumatic lines remain properly protected and controlled.

A well-designed hollow joint actuator therefore needs to balance hollow bore size, structural rigidity, motor and transmission packaging, thermal conditions, cable requirements, assembly, and maintenance.

The goal is not simply to make the bore as large as possible.

The goal is to create a routing path that works together with the robot's mechanical architecture, so that the joint can move as required while cables and tubes have enough space to operate reliably throughout the robot's service life.

Conclusion: Designing a Hollow Robot Joint for the Complete Robot System

A hollow robot joint should not be evaluated only by the diameter of its central opening.

The hollow bore is part of the robot's mechanical and electrical architecture. Its dimensions affect cable routing, connector installation, cable protection, thermal conditions, structural packaging, and maintenance.

For robot manufacturers and system integrators, the right question is therefore not simply whether a joint has a hollow bore, but whether the hollow joint actuator provides a routing path that matches the complete robot design.

When cable routing is considered from the beginning, the robot joint can be designed around the actual requirements of the end effector, motion range, cable configuration, structural rigidity, and service life.

A well-integrated hollow design allows the joint to provide the required motion while giving cables and tubes a controlled path through the robot structure. That makes the hollow bore more than a structural feature—it becomes an important part of the overall robot joint architecture.