While SAE double inverted flares have been in use for decades, leaking joints continue to be a problem for OEMs in production settings consuming time and energy to detect and correct them before releasing vehicles from the assembly plant. It should be noted that this issue is limited to first-time vehicle assembly; once a flared brake tube joint is sealed at the assembly plant it remains sealed during normal customer usage. From their inception through the late 1980s most brake tubes have been 3/16″ nominal diameter. With the advent of higher flow requirements of Traction Control and Yaw/Stability control systems, larger tubes of 1/4″ and 5/16″ size have also been introduced. While it was known that the first-time sealing capability of the 3/16″ joint was not 100%, leakers were generally containable in the production environment and the joint was regarded as robust. However, with the incorporation of larger diameter tubes, chronic leak issues on these joints have appeared more frequently, and have been more difficult to troubleshoot and correct than with the 3/16″ tubes. In some cases, production repairs of large diameter leaking joints have been sustained at undesirably high levels for months at a time even though all parts are consistently reported to be manufactured within the required specifications.
In-depth investigation of the specifications defining the joint component relationships of the brake tube, nut and port have shown several areas where it is questionable whether current dimensions and tolerances are adequately set up to maximize likelihood for joint sealing during assembly. For example, it can be shown that as tube diameter increases, the relationship between the parts changes in ways that appear to contradict the encouragement of robust joint sealing.
This paper explores observed features in leaking and non-leaking parts, current specifications for brake tube joints, and apparent relationships between the two. Additionally, elements of the relationship between the tube joint components and how this relationship changes as tube diameter increases is investigated. Finally, the impact of revisions to industry-standard specifications that change manufacturing, gauging and function will be addressed.