_a._ These must be of hard close iron with metal so distributed as to
ensure sound castings and freedom of shrink cracks. The following are
the minimum thicknesses acceptable:
14″ Diam. 7/8″ thick
16″ „ 15/16″ „
18″ „ 1″ „
20″ „ 1-1/8″ „
_b._ The inside face of the steam cylinder heads and the two faces of
the piston must be smooth surfaces, fair and true so that if the piston
should hit the heads it will strike uniformly all around, thus reducing
to a minimum the chances of cramping the piston rod or injuring the
pump.
_c._ All flanged joints for steam must be fair and true and must be
steam tight under 80 lbs. pressure if only a packing of oiled paper
1/100 inch thick covered with graphite were used. Jenkins, “Rainbow”
or equivalent packing of not exceeding 1/32 inch original thickness is
acceptable. Oiled paper is not acceptable as a final packing, as it
burns out.
For size of steam and exhaust pipes, standard flanges and bolting, see
Art. 39.
The specifications originally required machine facing for all these
surfaces. The art of machine molding from metal patterns with draw
plates, etc., has, however, attained such excellence in certain
shops, that in regular practice “foundry faced” cylinder heads and
piston faces can be made true and fair, and steam joints can be made,
tight under 80 lbs. pressure with a packing of oiled paper only 1/100
inch thick.
Under proper assurance that this precision can be obtained in regular
practice at the shop in question, foundry finish may be accepted on
cylinder heads and piston faces, steam chests and steam-chest covers.
In the case of _built-up pistons_, of separable form, it must be
conclusively shown that the boring and finishing are carried on by
such methods as will ensure the faces of pistons being exactly square
to the piston rod and exactly parallel to the cylinder head.
In the case of _solid pistons_ the two faces must be machine faced,
as proper parallelism cannot well be obtained by foundry methods.
Ordinary foundry finish secured by the old methods and wooden
patterns is not acceptable and acceptance of any foundry-finish can
only be secured after a special investigation of shop practices.
_d._ Heads at both ends of cylinder must be beveled off very slightly
over a ring about one inch wide, or equivalent means provided to give
steam a quick push at piston, should it stand at contact stroke.