Narrow surfaces make better and safer joints than wide ones; they are more quickly repaired with file or scraper, and they are less liable to catch dirt at the moment of making a joint. The limit of narrowness is that required to resist strains that might crush the metal and spoil the face of the joint.
Unless the joint is made metal to metal, fitting without any orifice, the jointing material is always softer than the pipes or other things to be joined. In this way the jointing need not have dead-true surfaces, but, yielding under pressure, it adapts itself to the space it has to fill. It must be dense enough and hard enough to resist all the working strains and influences that are likely to act upon it. The jointing of a steam pipe must resist the temperature of the steam, the water it carries with it, the changes of temperature when the pipe cools during intervals of work, and the strains due to the weight of the pipe, and also the internal pressure of the steam. If it expands differently from the metal in the pipe, it must be sufficiently elastic to compensate for this expansion, otherwise it will leak each time the pipes cool down.
Note.—In proportion as steam pressure gets higher joints are made thinner and flanges smoother. In the past rough turning succeeded chipping, rough filing followed with an application of the surface plate, and finally the scraper was used to produce a dead-true surface, which is now only cleaned and wet with heavy mineral oil to withstand any pressure whatsoever.
Fig. 643.
Fig. 644.
Fig. 645.