[Illustration: Fig. 49.]
In addition to palm and gusset stays, there are in use riveted or
screwed stays, as shown in Fig. 49.
This would not answer in furnaces, owing to the burning off of the
heads, hence driven stays are used there.
[Illustration: Fig. 50.]
These screwed stays, shown in Fig. 50, are used (in marine and similar
boilers) between the combustion chamber back and boiler back and also
between the sides of the combustion chambers.
The general plan is to have a large nut and washer inside and outside
the boiler with the outside washer considerably larger than the inside,
so as to hold more efficiently the back and front ends together.
In marine boilers it is customary to place the stays 15 to 18 inches
apart for ease of access to the parts of the boiler, and to make them
of 2-1/4 to 2-1/2 inch iron of the best quality.
INSPECTOR’S RULES RELATING TO BRACES IN STEAM BOILERS, ALSO TO BE
OBSERVED BY ENGINEERS.
Where flat surfaces exist, the inspector must satisfy himself that the
spacing and distance apart of the bracing, and all other parts of the
boiler, are so arranged that all will be of not less strength than the
shell, and he must also after applying the hydrostatic test, thoroughly
examine every part of the boiler.
No braces or stays employed in the construction of marine boilers shall
be allowed a greater strain than six thousand pounds per square inch
of section, and no screw stay bolt shall be allowed to be used in the
construction of marine boilers in which salt water is used to generate
steam, unless said stay bolt is protected by a socket. But such screw
stay bolts, without sockets, may be used in staying the fire boxes
and furnaces of such boiler, and not elsewhere, when fresh water is
used for generating steam in said boiler. Water used from a surface
condenser shall be deemed fresh water. And no brace or stay bolt used
in a marine boiler will be allowed to be placed more than eight and
one-half inches from centre to centre, except that flat surfaces,
other than those on fire boxes, furnaces and back connections, may
be reinforced by a washer or =T= iron of such size and thickness as
would not leave such flat surface unsupported at a greater distance,
in any case, than eight and one-half inches, and such flat surface
shall not be of less strength than the shell of the boiler, and able to
resist the same strain and pressure to the square inch, and no braces
supporting such flat reinforced surfaces, will be allowed more than 16
inches apart.
In allowing the strain on a screw stay bolt, the diameter of the same
shall be determined by the diameter at the bottom of the thread. Many
State laws and City ordinances allow a strain of seven thousand five
hundred pounds per square inch of section on good bracing without
welds. The following table gives the safe load of round iron braces or
stays.