_a._ The lost motion at the valves and the setting of them must be
determined by a solid hub on the rod, finished in the pump shop to
standard dimensions, so that no adjustment is possible after the pump
is once set up.
This hub may be forged on the rod and then lathe-finished to standard
dimensions, or it may be made by turning down a rod of the size of
the hub. It is believed that Tobin bronze can be safely forged after
a little experience, if care is taken to maintain the proper heat.
It is recognized that the practice of making adjustable valve tappets
located outside of the steam chest is a good thing in a large pump
in constant service and operated by a skilled engineer, but for
the infrequently used ordinary fire-pump, the utmost simplicity is
desirable, and it is best not to tempt the ordinary man to readjust
the valve gear.
The common form of lost motion adjustment consisting of nut and
check nut at each end of the slide valve is not acceptable, as these
nuts are liable to become loose and may be incorrectly reset by
incompetent persons. A long rectangular nut in the center of the
valve is also not acceptable, as it can be moved out of adjustment.
A solid hub made as a part of the rod is required, as it absolutely
avoids the possibility of the hub becoming loose, an accident
possible with a separate hub attached to the rod.
The amount of lost motion should generally be such that admission
takes place at about 5/8 of the stroke of the piston, _i. e._, for
12-inch stroke R. H. valve will be about to open when L. H. piston
has moved 7-1/2 inches to 8 inches from the beginning of stroke. When
piston is at end of stroke the ports should be full open.