BOILERS AND GENERATION OF STEAM 307 



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 



(Copyrighted by Millers Falls Co.) 



used to any great extent m FlG> 16L _ Butt Joint> 



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 out- 

 side, 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 



(Copyrighted by Millers Falls Co.) faree TOWS 



FIG. 162. Double-Riveted 



Butt Joint. 



, . , . , , . . 



a triple-riveted butt joint. 



348. Thickness of Boiler Plate. The Boiler Inspection 

 Department of Massachusetts recommends the following 



