A crimping technique has been developed for fastening polytetrafluoroethylene (PTFE) tubes of 0.032-in. (0.81-mm) outside diameter and 0.0105-in. (0.267-mm) inside diameter onto stainless-steel hypodermic tubes of 0.0105-in. (0.267-mm) outside diameter. The technique was developed to provide lightweight connections that would not slip or leak when subjected to rapid, repeated movements. The technique is needed because connections made with such conventional means as wrapped and/or twisted wires, flared tubes, ringed hose barbs, and combinations of these eventually slip and/or leak when handled or moved repeatedly.

Figure 1. The Crimping Tool is a modified pair of crimping pliers containing a hexagonal crimping die. The hexagon was chosen over other shapes because it minimizes the amount of copper extruded from the sleeve during the crimping process.

The development of the technique included tests that revealed that the best crimp reduction for PTFE is about 18 percent. The dimensions of the crimping sleeves and the crimping tool were chosen accordingly. The crimping sleeves were made from copper rods of 0.047-in. (1.2-mm) diameter, cut to 1/8-in. (3-mm) length, and drilled to an inside diameter of 0.033 in. (0.84 mm). The crimping tool was made by machining a pair of crimping pliers down to a thickness of about 0.100 in. (2.54 mm) and cutting a 0.038-mm (0.97-mm) hexagonal crimping die into its tip (see Figure 1).

Optionally, one can prepare the crimp area on the stainless-steel tube with a light grit blast prior to joining to improve the subsequent grip on the tube. To make a connection, one first slips the copper crimping sleeve over the PTFE tube, then inserts the stainless-steel tube into the PTFE tube a sufficient distance to enable proper positioning of the crimping sleeve as described next. The crimping sleeve is positioned so that PTFE and stainless-steel tube segments 0.05 to 0.06 in. (1.3 to 1.5 mm) long protrude from opposite ends of the crimping sleeve. The tool is then applied to effect the crimp. Figure 2 shows a completed connection.

Figure 2. A Crimped Connection made by the present technique grips more strongly than does a connection made by the traditional twisted-wire technique. One reason for the superiority of the present technique is that the grip is spread over a larger area.

Limited experience thus far seems to indicate that a connection made by this technique does not leak measurably when exposed to vacuum or to pressure up to 115 psi (0.79 MPa). The technique can readily be modified, by appropriate changes in dimensions of the sleeves and tool, for PTFE and stainless-steel tubes of different diameters. Potential uses for this technique could include joining fuel lines for gas-powered models, joining pneumatic and gas lines in general, and perhaps joining tubes in medical equipment.

This work was done by Myron M. Joseph (retired) and George J. Saad of John H. Glenn Research Center.

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

John H. Glenn Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7-3
21000 Brookpark Road
Ohio 44135.

Refer to LEW-16689

NASA Tech Briefs Magazine

This article first appeared in the May, 1999 issue of NASA Tech Briefs Magazine.

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