_a._ The “inside plunger and bushing” is preferred for all situations
where the water is free from grit or mud.
_b._ Water-plungers must be of solid brass or bronze, and the bushing
in which they slide must also be of brass or bronze. The composition of
the plunger and its bushing should be of very hard, though dissimilar
alloys, to ensure good wearing qualities.
For material and size of piston rods and lock for nuts, see Art. 20.
With poor alignment or bad workmanship or lack of skill in mixing the
alloys, brass plungers are liable to score and give trouble; but with
proper selection of alloys and true cylinders accurately aligned,
they can be made to run all right wherever iron ones can. It is quite
a fine point to get these wearing surfaces just right; and _this
is wherein the experience, skill and shop practice of one maker is
likely to be much superior to that of another working under the same
specification_.
_c._ The length of machined cylindrical bearing within the partition
must be not less then 2 inches. The plunger bushing must have a faced
seat transverse to its axis against partition, forming a water-tight
ground joint not less than one-half inch wide.
Any rubber gasket or other compressible packing for making this joint
water-tight is not acceptable.
_d._ The construction of bushing and hole in partition must be
such that a cylindrical shell for use with a packed piston can be
interchangeably inserted in its place and secured by the same bolts.
This can readily be arranged and enables a packed piston to be
inserted in place of a plunger subsequent to the installation of the
pump with a minimum of expense, should this become desirable from
change of conditions at any future time.
_e._ Small transverse grooves cut within the sliding surface of the
plunger bushing, with a view to lessen the leakage, are not acceptable.
Although a slight advantage in this respect for clean water, they are
a disadvantage on the whole, as dirt catches in them in the ordinary
situation and cuts the plungers.