346 FARM MOTORS 



Hence the ratio of the strength of the joint to the strength of the 

 plate is 



0.588 X ioo = 58.8 per cent = the efficiency. 



Now, if the original shell on page 344 is referred to, it will be 

 seen that instead of having a boiler with a factor of safety of 10 it 

 will have only 58.8 per cent of this factor, or approximately 6, which 

 is about the usual factor. 



473. Double-riveted lap joint (Fig. 239). 



1. Tearing between two rivets. The resistance to tearing is 



(p d) tSt = (2*/ 2 i ) X 7/i6 X 55,ooo = 36,095. 



2. Shearing two rivets. Instead of shearing one rivet as in the 

 single-riveted lap joint, two are sheared. Hence 



X 3.14.6 </ X S, = 'X3..4-6XIXI x ^^ 



4 4 



is equal to 70,686. 



3. Crushing two rivets. Here again two rivets are considered 

 instead of one, hence 



2^5*0=78,750. 



The efficiency of this joint would then be 



ioo X 36,095 -r- 60,155 =60 per cent. 



The same dimensions have been used in this joint as in the pre- 

 vious one for simplicity and comparison. By using a smaller rivet 

 this joint can be made much more efficient. 



474. Test of boilers for strength. There are two dis- 

 tinct methods of testing boilers for strength. The one 

 which is generally conceded to be best is the hydraulic 

 test; and the other, which is about as safe and sure, and 

 in some cases more so, is the hammer test. 



Hydraulic test. This test consists in filling the boiler full 

 of cold water and then putting pressure upon it to the 

 desired point. This pressure is generally about one and 

 one-half times the working pressure. Since some boilers 

 are designed with a factor of safety of only four or five, 

 if twice the working pressure be put on it there will be 

 danger of rupture to the boiler. With new boilers this 



