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 21⁄4 to 21⁄2 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.