STEAM BOILERS 343 



tensile strength of 55,000 pounds for steel and 48,000 for 

 iron. When cold they should bend around a rod of their 

 own diameter, and when warm bend double without a 

 fracture. The shearing strength is about two-thirds of 

 the tensile strength. 



Cast iron is used in boilers for those parts where there 

 are no sudden changes of temperature and where there is 

 no great tensile strength required. Couplings, elbows, 

 etc., are better of cast iron, for when they become set and 

 can be removed in no other way they can be broken. 



469. Stay bolts and stay rods. In some parts of the 

 boilers the flues act as stays. In horizontal tubular 

 boilers the flues hold the ends of the shell together. In 

 the fire box and in vertical boilers they act in the same 

 way between the flue sheets. Wherever there are flat 

 surfaces and no other means of supporting them, special 

 stay bolts or braces must be put in. In nearly all boilers 

 above the flues stay rods are used to support the ends. 

 Around the fire box stay bolts are put in. These bolts 

 are threaded full length, then screwed through the outer 

 shell and through the water leg and into the fire-box 

 lining, then they are riveted on both ends. Their size and 

 distance apart depends upon the pressure to be carried. 



Example. If the stay bolts are 4 inches apart and the maximum 

 pressure to be carried is 120 pounds they should be large enough 

 to hold 



4 X 4 X 120 = 1,920 



pounds. If we use a factor of safety of 10 that is, make it ten times 

 as strong as necessary to avoid accidents it will have to be large 

 enough at the base of the thread to hold 



1,920 X 10 = 19,200 

 pounds. If a wrought-iron bolt is used it would have to have 



19,200 -r- 48,000 = 0.40 



square inches area at the base of threads. A %-inch bolt has about 

 this area. 



