34 2 FARM MOTORS 



estimate. It is generally conceded that from one-third to 

 one-half square feet of grate surface is equivalent to one 

 horse power. 



STRENGTH OF BOILERS 



468. Materials used. The materials used in the con- 

 struction of boilers are mild steel, wrought iron, cast iron, 

 copper, and brass. 



In order that a boiler have proper strength for the 

 severe work required of it, sample pieces of all the ma- 

 terials used in its construction are selected and given 

 a test, and those which fail to have the proper require- 

 ments are discarded. They are tested in tension, com- 

 pression, and shear. (See Chap. Ill, Part I.) 



Steel. All present-day boilers are made up of mild-steel 

 plates. This steel is a tough, ductile, ingot metal, with 

 about one-quarter of I per cent of carbon. It should have 

 a tensile strength of about 55,000-60,006 pounds. Some- 

 times a better grade of steel plate is used for the fire box 

 and tube sheets of the boiler than for the shell. This is 

 because flanging for riveting and the variations of tem- 

 perature due to the fire require a better grade of steel. 



Blue heat. All forms of mild steel are very brittle when 

 at a temperature corresponding to a blue heat. Plates 

 that will bend double when cold or at a red heat will 

 crack if bent at a blue heat. 



Wrought-iron parts. All welded rods and stays should 

 be of wrought iron. About 35 per cent of the strength of 

 the bar is lost because of the weld. Boiler plates made 

 of wrought iron are considered more satisfactory than of 

 steel, but are used only in exceptional cases because of 

 the greater cost. Wrought-iron plates should have a 

 tensile strength of 45,000, and bolts should have 48,000. 



Rivets. Boiler rivets are either of wrought iron or mild 

 steel. The rods from which rivets are made should have a 



