On the Theory of Shrink Fits with Application to Waveguide Pressure Seals

The derivations and conclusions reported in this paper have been stimulated by the current interest in light-weight microwave antennas for missiles and space vehicles. These antennas must be capable of maintaining a pressure seal in temperature environments from -- 80°F to +400°F or higher. A typical seal is intended to maintain a pressure of one atmosphere. Such pressures are required to prevent electrical breakdown of the waveguide at nominal operating power. A common method of achieving such a seal is to provide a radome over the antenna. This scheme is usually effective but requires gaskets * " T e f l o n " is a registered t r a d e m a r k of the E. I. du Pont D e N e m o u r s and Co. T h e t y p e referred to in this p a p e r is a p o l y t e t r a f l u o r e t h y l e n e resin. 885 88() THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1962 and screws and, moreover, is an additional structure to be carried during flight. A design is proposed in which a suitable dielectric plug, or core, necessary to give the required antenna pattern, is inserted into the waveguide opening of the antenna. A pressure seal between the plug and the waveguide is accomplished by first chilling the plug to a temperature well below that of the waveguide piece. The plug is then inserted into the waveguide. As the plug warms up, it expands and exerts sufficient pressure 011 the walls of the waveguide to cause an effective seal. The purpose of this study is to determine theoretically as well as experimentally the feasibility of such a shrink fit.