LOCOMOTIVE ENGINE. 
457 
Fig. 37. 
of the two tires are given in figs. 36 and Fig - 36 - 
37, half of the real size ; they are both 
made slightly conical, being tapered 
from If inch to 1J inch thick ; and the 
flancli 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 truty circular, and to make the two in each pair 
exactly alike. 
The flancli 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