Why Do Many Customers Choose HONPINE HPJM Harmonic Joint Modules for Just 2 mm More Hollow Bore Diameter?

Why do some customers choose the HONPINE HPJM harmonic joint module for just 2 mm more hollow bore diameter? In compact robotic joints, a small increase in bore size can make a significant difference to internal cable routing, wiring flexibility, and overall system integration. Because bore area increases with the square of its diameter, an extra 2 mm can provide much more usable space than expected. This article explores the engineering impact of hollow bore diameter, compares selected HPJM models with comparable products, and explains why bore size matters in humanoid robots, collaborative robots, and industrial automation.

1. An Extra 2 mm Means a 78% Increase in Area, Not Just 25%

Hollow bore diameter is not a linear measure of usable space. The cross-sectional area of a circular bore is calculated using the formula A = πD²/4. As a result, even a 2 mm increase in diameter can deliver a substantial gain in available space, especially in smaller bores.

For example, increasing the bore diameter from Ø6 mm to Ø8 mm increases the cross-sectional area by approximately 78%. The same 2 mm increase delivers a 44% gain when going from Ø10 mm to Ø12 mm, a 27% gain from Ø16 mm to Ø18 mm, and a 14% gain from Ø30 mm to Ø32 mm.

2. Why Does Hollow Bore Diameter Determine Whether a Robot Joint Can Support Internal Cable Routing?

In robotic joints, hollow bore diameter directly affects whether power cables, communication cables, and brake or sensor wiring can be routed internally. This is particularly important for humanoid robots, collaborative robots, and compact robotic arms, where external cables can interfere with movement, increase the risk of snagging, and complicate system integration.

A typical fully integrated cable-routing arrangement may need to accommodate three groups of cables:

• Power cables: Two conductors, with an equivalent cable diameter of approximately 3.5 mm.

• EtherCAT communication cable: A shielded cable with an outer diameter of approximately 4.5 mm.

• Brake and sensor wiring: With an approximate diameter of 3 mm.

Protective sleeves and bushings can further reduce the available space. Assuming a combined allowance of approximately 2 mm for these components, the effective internal diameter can be estimated as:

Effective bore diameter ≈ nominal bore diameter − 2 mm

This is a simplified engineering estimate. Actual cable capacity depends on cable construction, connector dimensions, routing geometry, insulation, shielding, and the required clearance.

Ø6 mm: Limited Space for Internal Wiring

With a nominal bore diameter of Ø6 mm, the estimated effective diameter is only about Ø4 mm. This may accommodate a single small cable, but routing power, communication, and brake or sensor wiring through the same joint becomes extremely difficult.

For designs that require all three cable groups to pass through the joint internally, a Ø6 mm bore may make the proposed routing arrangement impractical without a different cable configuration or additional external routing.

Ø8 mm: A Small Increase That Can Enable Internal Cable Routing

Increasing the bore diameter to Ø8 mm provides an estimated effective diameter of approximately Ø6 mm. Depending on cable dimensions and the routing arrangement, this additional space may allow multiple cable groups to pass through the joint.

This can make a more integrated internal cable-routing design feasible, although the actual fit must be verified against the cable bundle, connector sizes, protective components, and assembly tolerances. Cable packing ratio alone does not guarantee that a bundle will fit or remain reliable during operation.

For robotic joints with repeated bending, cable flexibility and minimum bend radius are equally important. A commonly used preliminary guideline for dynamic cable routing is a minimum bend radius of approximately four times the cable's outer diameter, although the cable manufacturer's specifications should take precedence.

For a cable with an outer diameter of 3.5 mm, this guideline gives a minimum bend radius of approximately 14 mm.

Consequently, a Ø8 mm bore may be suitable for certain wrist joints or other applications with limited reciprocating rotation, potentially around ±90° to ±120°, depending on the complete cable-routing design. However, this does not guarantee that rotation range: cable length, internal routing geometry, joint construction, and operating cycles must also be considered.

Continuous 360° rotation generally requires a more carefully designed cable-management solution, such as a larger hollow bore, a suitable rotary electrical interface, or a slip ring where appropriate.

This is also one reason why larger HPJM models offer hollow bores of Ø25 mm or Ø32 mm: greater internal space can provide more flexibility for cable routing and system integration in demanding robotic and industrial automation applications.

3. How Does the HONPINE HPJM Series Compare with Other Harmonic Joint Modules in Hollow Bore Diameter?

Hollow bore diameter is only one factor in selecting a harmonic joint module. Overall dimensions, torque capacity, module length, weight, encoder configuration, and integration requirements also affect the final decision.

However, when internal cable routing is a major design requirement, even a modest difference in bore diameter can influence the entire joint architecture.

The following comparison presents selected HONPINE HPJM models alongside publicly listed specifications for comparable products from other manufacturers. The figures are based on the available product information and should be verified against the latest manufacturer datasheets before making a direct engineering comparison.

4. Why Does a Larger Hollow Bore Matter for Robotic and Automation Applications?

The HPJM harmonic joint module combines a compact integrated design with a relatively large hollow bore in selected model sizes. This can help engineers accommodate internal wiring while reducing the need for external cable routing and additional structural modifications.

More Flexible Cable Integration

Additional space can make it easier to accommodate power, communication, brake, and sensor wiring, subject to the cable dimensions and routing requirements.

Cleaner Robotic Joint Design

Reduced dependence on external cable routing can help minimize interference with moving structures and support a more compact robotic design.

Simplified System Integration

A larger bore can provide greater flexibility when designing compact joints for humanoid robots, collaborative robots, and robotic arms.

More Room for Application-Specific Wiring

A larger hollow bore can help accommodate different cable bundles and integration requirements, subject to dimensional verification.

High Torque Density in a Compact Package

The HPJM series combines harmonic transmission technology with integrated motor and feedback components to support space-constrained motion-control applications.

A larger hollow bore does not automatically make one joint module superior in every application. Engineers should evaluate the bore diameter together with rated torque, peak torque, torsional stiffness, positioning accuracy, thermal performance, weight, and cable-bending requirements.

5. Explore HONPINE HPJM Harmonic Joint Modules

HONPINE HPJM harmonic joint modules are designed for applications that require compact dimensions, integrated functionality, and high torque density. Their hollow-bore configurations offer additional flexibility for internal cable routing, particularly in robotic joints where installation space is limited.

If you are evaluating harmonic joint modules with a large hollow bore for humanoid robots, collaborative robots, robotic arms, or industrial automation equipment, contact HONPINE to request the latest HPJM product catalog and technical datasheets.

Our team can help you compare model dimensions, hollow bore diameter, torque specifications, and integration options to identify a suitable solution for your application.