It is very important that a boiler should safely withstand the pressure of steam for which it has been constructed. Though the tensile strength of the boiler plate is marked on it, it is necessary to test it when the boiler is completed. When a rivet hole is punched, the plate is weakened proportionally because a quantity of metal has been removed. Therefore an additional piece of metal, known as a strap, must sometimes be placed around such rivet holes to make up this deficiency to some extent. At the present time, most fire-tube boilers made to generate steam for engines have the different portions of the shell overlapping one another, as shown in Fig. 160, and these are held with a single row of rivets. This arrangement forms what is called a lap joint. Lap joints are not used to any great extent in joining the two ends of the same sheet. In this case the ends are brought together and one strap is placed on the inside and another on the outside, as shown in Fig. 161. This method forms what is called a butt joint. These straps and the plate are joined by riveting, as shown. If a single row of rivets is used on each side of the joint through the outer plate, as shown at A A, it is called a single-riveted butt joint. If a double row is placed on each side of the joint through the outer strap, as shown in Fig. 162, it is called a double-riveted butt joint; if three rows are used, it is called a triple-riveted butt joint.

Fig. 160.   Lap Joint.

(Copyrighted by Millers Fall Cos.)

Fig. 160. - Lap Joint.