NASA Langley Research Center developed a square structural joint design that enables robotic assembly and has been designed for high strength-to-weight, multifunctionality, and reduced manufacturing cost. The joint not only provides increased axial stiffness compared to round tubes of the same width but torsional and bending strength is increased as well. Electrical conductivity and provisions for routing of wiring or tubing through the joints have been incorporated in the design, all enabling greater capability and a stronger structural design.
The square joint has several novel features to improve reliability, performance, and robustness. Most simply, the square tubes are stronger than round for a specified maximum cross-section dimension. Structural benefits include nearly complete perimeter contact geometry for improved structural efficiency, improved cantilever beam response via linear bending response about the y and z axes, and linear torsional response about the x axis. Additionally, there is better linear axial response along the x axis due to simple geometry and large contact surfaces, higher torsional/torque capability (about the x axis), higher bending capability about all axes, and higher axial capability. It also offers a bonding strap and treated contact surfaces that provide electrical conductivity through the joint.
Switching to square cross-section joints provides packaging efficiency, along with numerous improvements for robotic assembly applications such as providing rotational registration, robotically compatible tool designs, both mechanical and visual indicators to verify locking operation, and preload and capture spring forces with a unique stop plate in the drive train that can be designed to default to the assembled condition without a preload, yet spring back if forced toward unlocked. After assembly, preload can be adjusted for security. Designed for robust assembly, the robotic tools are built to actuate the joint.