The figure presents a partial cross section of two poly(methyl methacrylate) (PMMA) tubes with machined ends butted and sealed together in a special coupling fixture. This coupling scheme, in conjunction with the careful selection of PMMA tubes to match inner diameters, ensures the precise alignment of the inner tube surfaces. The scheme was devised to satisfy a requirement in a liquid-flow experiment to ensure a smooth, continuous inner tube surface to prevent both flow disturbances and trapping of bubbles. If the inner diameters were not matched and/or the inner tube surfaces not aligned precisely, the junction between the tubes would feature small, sharp corners that could give rise to waves and could trap bubbles.

Two Tubes Are Sealed at a butt joint and held in alignment by a special coupling fixture.

The end surfaces of both tubes are machined flat and perpendicular to the inner surfaces. Cylindrical alignment surfaces referred to the inner surfaces are machined on the adjacent exterior end portions of the tubes. Facing halves of a seal groove are machined on the outer surfaces of the tubes at the butt joint. A retaining ring is placed in a groove on each tube at a short distance from the end. A male threaded collar is placed around one tube and a female threaded collar around the other tube, each collar covering and abutting the retaining ring on its respective tube.

The tubes are butted together along with an alignment ring and with an O-ring placed in the seal groove. The alignment ring is machined for a snug fit with the alignment surfaces on the tubes, thereby ensuring the precise alignment of the inner tube surfaces with each other. The two collars are threaded together until the force on the retaining rings pushes the ends of the tubes together. At this point, the O-ring is squeezed tightly between the tubes and the alignment ring, forming a tight seal.

In an alternative coupling scheme (not shown in the figure), the threaded collars are replaced by a combination of unthreaded collars and a two-piece ring clamp that engages the collars. The clamp features tapered surfaces that exert a longitudinal force to push the tubes together when the two halves of the clamp are bolted together.

This work was done by Robert Mate of the University of Houston for Lewis Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Ohio 44135

Refer to LEW-16255.