The joint should be always inside the line of bolts, and if any joint material extends beyond, it would only help to support the flange in case it should spring. This, of course, indicates faulty design, for flanges ought to bear the strains of jointing without perceptible spring. Male and female flanges are best for high pressures.
A very popular joint is made with a planed or turned surface and a sheet of paper of the quality used to wrap bales of paper. This is the last survival of the millboard. Rubbed over the flange with a dirty hand and cut out with a penknife on a board, this is one of the cheapest jointings known. This paper has no lumps or grit in it, and if smeared with mineral cylinder oil it may be separated several times before it is spoiled. It is largely used on the faced joints of small engines and steam pumps. The mineral oil increases the life of the paper when exposed to high steam. Sheet asbestos is better.
Hydraulic joints for high pressure require greater rigidity than those of steam, but they do not have to bear high temperatures. The jointing material may be more or less plastic, such as leather, rubber or gutta percha. It is generally inclosed in a groove in the flange, and compressed by a projection fitting the groove, so that expansion of the jointing is arrested and the space is completely filled. There is no better principle for joints than this where packing is used between flanges. At a pressure of three tons to the inch, every square sixteenth of an inch must resist a power equal to twenty-six pounds; the joint must therefore be non-porous.
Fig. 646.
Fig. 647.
Fig. 648.
