High vacuum shaft seals, flanged joints and the gassing and the permeability of rubber-like materials

Abstract

Various designs of high vacuum shaft seals are given and their performance measured by the pressure rate of rise in a closed system. Experiments show that there is little to choose between them from their vacuum performance, although the washer in which a tension spring is embedded is at times to be preferred. A large portion of the leak is condensable indicating something more complicated than the direct passage of air through the seal. Experiments give measurements of the gassing from rubber-like materials including neoprene and butyl rubber when totally enclosed in a vacuum system, and also measurements of their permeability to gases. All the synthetic materials tested gassed considerably more than did rubber. In the permeability experiments it was found that the time required for gases to pass through diaphragms varies from several minutes to an hour, depending on the gas. The permeability, unlike the gassing effect, is considerably less with the synthetic rubbers tested than with rubber. Data are given of the testing of flanged gasket joints and their effect on a vacuum system during the first twenty hours of pumping when the joint gassing is liable to be confused with a leak. Data are also given on the flange clamping pressure necessary for a tight joint.