Boiler Explosions. 



157 



often compressed and cracked, and pieces of the plate may be 

 forced out. Fig. 1, is intended to illustrate the overlapping holes, 

 and figs, 2, 3 and 4 the effects of the use of the " drift pin." An- 

 other difficulty here presents itself, arising chiefly from careless- 

 ness and poor workmanship. Often the sheets do not come in 

 contact, and especially at the heads or ends of the boilers, on which 

 the flanges are turned, is this the case, and also on internal fire-box 

 work. When the rivets are driven, the iron acts as a spring, and 

 vibrates back and forth from the blows of the hammer. The riv- 

 ets too, will " upset" in between the plates if much apart. 



Eivets driven in this way can never be made tight, neither will 

 Fig. 5. the caulking chisel 



remedy the defect 

 for when the caulk, 

 ing is done, the iron 

 is driven back be- 

 tween the plates 

 forming a thin nar- 

 row ridge under 

 which the pressure 

 will soon force the 

 water or steam. Fig. 5, is a fair illustration of the case. 



To this defect are due, many of the mysterious leaks in new 

 boilers, when but a short time in use. Often rivets are improp- 

 erly supported or " backed " when being riveted, which causes 

 leaks ; or riveted when too cold, causing crystalization to such an 

 extent that often a slight jar will cause the heads to drop off. 

 The outer corner of the outside cylinder must be chamfered to an 

 angle of about fifteen degrees, thus leaving a sharp edge where 

 the cylinders join, for caulking. In many large shops this is 

 done by machinery before the plates are rolled, in others before 

 the cylinders are placed together. In many, it is done after the 

 riveting, and thus the lower sheet is more or less cut by the corner 

 of the chisel, the greatest care cannot prevent it. With many 

 boiler-makers, this is of minor consideration, but the fact that 

 many exploded boilers have given way at this point should draw 

 attention to it. The following account of an exoeriment made at 



