344 FARM MOTORS 



470. Strength of boiler shell. To determine the ten- 

 sion upon one side of a boiler shell, let 



p pressure in pounds per square inch, 

 t = thickness in inches, 

 r radius, 



s = stress in pounds per square inch ; 

 then 



Example. A boiler has a diameter of 3 feet, a thickness of 

 7/16 inch and the steam pressure is 125 pounds. How many pounds 

 per square inch pull is there on each side? 



pounds. This is about one-tenth the tension which boiler plate 

 will stand, hence we have a factor of safety of 10, which is greater 

 than need be. 



471. Riveted joints. If a boiler shell could be made 

 of one continuous piece, the above tension would be the 

 safe working load, but since the steel has to be riveted 

 and a riveted joint is not as strong as the original plate, 

 we must consider the ratio of this strength of the whole 

 plate. This ratio is commonly called the efficiency of a 

 riveted joint. 



There are three general ways that a riveted joint may 

 give way : 



1. By tearing the plate between the rivets. 



2. By shearing the rivets. 



3. By crushing the rivets or plate at the point of contact. 



Since only single-riveted and double-riveted lap joints 

 are used in small boilers, these styles will be considered 

 only. 



472. Single-riveted lap joint. In the joint shown by Fig. 

 238. let t be the thickness of plate, d the diameter of rivet, p the 

 distance between rivets, commonly called pitch, the tensile strength 



