The bursting pressure of a solid shell or pipe can be determined from the following rule: "Multiply together the thickness of the metal and its strength in lbs., and divide by the shell radius in inches."

Thus, suppose a welded cylindrical boiler shell is 7 ft. diameter and § in. thick steel plate. Assuming that the strength of the metal is 28 tons per square inch, we have -

Bursting pressure = 3/8 X 28 X 2240 / 42 = 560 lb.

If the shell is riveted the above would have to be multiplied by the percentage strength of the joint to obtain its correct bursting pressure.

The strengths of steel or copper steam pipes can be found in the same way.

The above calculations will also serve to get out the required thickness of metal for a shell of given diameter to stand a given pressure.

As a somewhat curious fact it is worth noting that a spherical vessel of same thickness and material will stand just twice the pressure of a cylindrical vessel of the same diameter.

Length of Angle, Tee=Bars, etc., for Rings.

The method of finding the lengths of flat-bars, etc., explained in Chapter XXXII (Plater'S Work, Tanks, Shells, Etc. Allowance For Metal Thickness)., can also be applied to bars of irregular section. The important point is to find the position of the neutral axis. This will always pass through the centre of gravity of the section. The centres of gravity (see Chapter XXVIII (Solid Pans, Jugs, Expansion Bulbs, Etc. Solid Round Pan).) can be found either geometrically, or better still for practical purposes, by the method of suspension. A section of the bar should be cut out of cardboard or sheet metal and suspended from a point (such as S in Fig. 335) and a vertical line drawn down. It should then be hung from another point (such as H) and another vertical line drawn. Where these lines intersect will give the centre of gravity of the section. In Fig. 335 this is marked by the letter G, and when the bar is being bent. as shown in Fig. 336, either to form a ring with an outside or inside flange the neutral line will pass through the point N. To get the required length of the bar in the straight, the ring will be set out and the neutral circle drawn, and its length measured or calculated. For an inside flange part of the neutral circle is shown as N N N. Fig. 335. Fig. 336.

For the above to be true it should be remembered that uniformity of heating and bending is demanded.

In punching angles it might be here mentioned that it is usual for the centre line of the holes to run down the middle of the inside of angle-iron.

To Calculate the Increased Length of a Bar when it is made Red=hot.

Problems on the expansion and contraction of metal bars are important, hence we give one example below.

Suppose a bar of iron is 10 ft. long and its temperature (that of the atmosphere), say, 30° C. It is placed in a furnace and got red-hot. How much will it lengthen?

A red heat is generally reckoned to be about 1,000° C. So that the increase in temperature would be 970°.

Now turning to the table of multipliers for linear expansion on page 451 we get that for iron, and our calculation works out as follows: -

Increase in length = 10 x 12 x 970 x .000013 = 1.5 in.

So that the increase in length comes to about 1 1/2 in. Calculations like above come in useful in making allowance when rings, bands, etc., have to be shrunk on.