LOCOMOTIVE ENGINE. 



457 



FIG. 37. 



of the two tires are given in figs. 36 and FlG - 3G - 



37) half of the real size ; they are both 



made slightly conical, being tapered 



from If inch to 1-| inch thick ; and the 



flanch projects one inch and a quarter, 



and is three quarters of an inch thick 



at the edge and an inch thick at the base. 



The rims and tires are both turned, and 



the tires are heated when put on, and 



contract on cooling so as to hold firmly on 



the wheel; great care is required in 



fitting them, that they may not be 



loose upon the wheels nor shrunk too 



tight, so as to injure their texture. 



They are held in their places by three bolts with countersunk heads in the tires 



and nuts screwed on against the inner side of the rims. The tires are turned when 



fixed on the wheels to make them truly circular, and to make the two in each pair 



exactly alike. 



The flanch wheels, like the wheels of all railway carriages, require to be made a 

 little conical, in order to prevent the flanches being continually in contact with the 

 rails and rubbing against them, which would cause a great deal of friction ; as a 

 wheel, when running towards one side and bringing the flanch in contact with the 

 rail, will bear upon a larger circumference than the other wheel, and will tend to run 

 towards the opposite side and make the wheels central again ; the flanches are 

 thus hardly required on a straight line, and only necessary upon sharp curves to 

 keep the wheels from running off the line. The rails are laid inclined a little, so as 

 to fit the conical wheels, and for this reason the driving wheels have to be made 

 also conical, although they have no flanch. The driving wheels are made without 

 flanches that they may always have firm hold on the rails, as a flanch on the inner 

 one, when the engine is turning round a curve, would be forced against the inner 

 rail, and would interfere with the bearing of the wheel and cause friction ; and 

 flanches upon the front and hind wheels are sufficient to keep the engine upon the 

 rails. For the improvement of making the middle driving wheels without flanches, 

 Mr. Stephenson has a patent. 



The wheels of the first engines were made entirely of cast iron, but it was found 

 difficult to make them sound in consequence of the unequal contraction in cooling, 

 and they were too brittle to bear the shocks produced in running fast ; the cast iron 

 was also found to be too soft and to wear in a groove on the edge with running on the 



3 M 



