Question

A steam boiler is to have a 300-mm-diameter hand hole placed on one end. If the maximum internal pressure is to be 1,3 MPa, how many 15-mm bolts will be required to keep the cover plate in position? The root diameter of a 15-mm bolt is 12 mm. The allowable tensile stress in the bolts is 100 MPa and the initial stress in the bolts when they are tightened is 60 MPa.

Answer 1

Tightening stress + Additional stress on bolt = Allowable stress for bolt

60+ $\frac{F}{Ab} = 100$ ( here Ab = root area of bolt)

60 + $\frac{F}{(\pi)/{4}\times 12^2}$ = 100

solving we get

F = 4523.89342 N

Now

F^' = P (A) [ pressure p active on area A]

F^' = 1.3$\times \frac{\pi}{4}$ $\times 300^2$

F^' = 91891.58512 N

n = $\frac{F'}{F}$

n= $\frac{91891.58512}{4523.89342}$

n= 20.31250

so in whole number n=21 bolts are required