_a._ Piston rods for their entire length must be of solid Tobin Bronze,
and the distinguishing brand of the manufacturers of this metal must be
visible on at least one end of each rod.
_b._ The sizes must be not less than in table below.
=========+==========+===========+============+============
=Size of=| | | |
=Pump.= |=500 gal.=|=750 gal.= |=1,000 gal.=|=1,500 gal.=
---------+----------+-----------+------------+------------
Diameter | | | |
of rod | 2 inch. |2-1/4 inch.|2-3/8 inch. | 2-1/2 inch.
---------+----------+-----------+------------+------------
_c._ The size and form of connection of rod to piston plunger and
cross-head must be such that the stress in pounds per square inch at
bottom of screw thread, or at such other point of reduced area as
receives the highest tensile strain, shall not exceed 8,000 lbs. per
square inch, when the steam pressure acting on the piston is 80 lbs.
per square inch.
_d._ Piston rod nuts, in both steam and water ends, must be tightly
fitted, and preferably of a finer thread than the United States
Standard. This is to avoid as much as possible the unnecessary
weakening of the rod at the bottom of the thread, and to reduce the
tendency of the nut to work loose.
In practice 8 threads per inch has been found to give good
satisfaction.
_e._ In addition to a tightly fitting nut, some reliable device must be
provided, in both steam and water ends, for absolutely preventing these
nuts from working off.
[Illustration: FIG. 6.]
Fig. 6 shows one form of such a locking device and illustrates the
kind of security desired.
This device combines the advantage of a taper key and a split pin,
and the elongated key-slot gives sufficient leeway to always insure
that the key can be driven up tight against the nut and thus prevent
it from even starting to work off. Other methods will be approved in
writing, if found satisfactory.