. The Locomotive . Fig. 2. The elementary lap joint is illustrated in Fig. 3. If tensionis applied as indicated in Fig. 4, the plates, in an attempt to alignthemselves with the load, will bend along a line running underthe outer edge of the rivet heads. VJVJ. Fig. 3. Fig. 4. The combined effect of all these conditions, together with theconstant bending of these joints by changes of pressure when inuse, is to impose excessive stresses in the surface of the boilerplate along the line just mentioned. This has produced, in manyboilers, a crack which starts always from the inside or coveredsurface
. The Locomotive . Fig. 2. The elementary lap joint is illustrated in Fig. 3. If tensionis applied as indicated in Fig. 4, the plates, in an attempt to alignthemselves with the load, will bend along a line running underthe outer edge of the rivet heads. VJVJ. Fig. 3. Fig. 4. The combined effect of all these conditions, together with theconstant bending of these joints by changes of pressure when inuse, is to impose excessive stresses in the surface of the boilerplate along the line just mentioned. This has produced, in manyboilers, a crack which starts always from the inside or coveredsurface of the inside or the outside plate of the joint as indi-cated at A and B in Fig. 4. This crack may eventually workits way through the plate until it shows itself by leakage. Butin many cases it may develop for some distance along the jointand yet remain absolutely invisible. Eventually the weaknessmay develop to the point of complete failure and a disastrousexplosion. 144 THE LOCOMOTIVE. [January, Inspection is generally accepted as being safe for the determina-tion of the fitness of a boiler for use. The lap seam crack, how-ever, is invisible to all methods of inspection, except cuttingout the rivets and separating the plates or the method describedi
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