Wheels for carriages are used to diminish friction, by causing the
“tire” or smooth outer edge to roll upon the surface instead of
being rubbed; all the friction in wheels is in the centre or axle,
which being turned smooth, and greased or oiled, works very easily.
Carriage wheels are made to revolve upon a fixed axle, and each wheel
revolves independently of the other, but in railway-carriages and
engines, the wheels are united in pairs, and the axle revolves with
them, the weight being borne outside of the wheel on a small part of
the axle which projects. The various parts of a wheel are the box or
“nave” which is the centre part, the “spokes” or those bars which
connect it with the centre edge or felloes, and the “tire,” an iron
band binding the whole together. Wheels for gun-carriages are made
at Woolwich Arsenal by machinery. The following is a description of
them, taken from the “Times” newspaper:--
“Here a few unskilled labourers superintending the machines produce
forty complete gun-carriage wheels a day, though all their component
parts are made of the hardest woods--viz., elm for the naves, oak
for the spokes, and ash for the felloes. The novelty here was the
new mode in which a wheel is fitted together. Instead of by hand,
as formerly, the pieces are all laid together on the ground, and
of course in a circle, around the outside of which are six small
hydraulic rams, with the head of the piston of each curved so as to
form a segment of a circle touching the outside portion of the wheel.
One small steam-engine pumps the water into all these with an equal
pressure, which, as it increases, forces the felloes into the spokes
and the spokes into the nave of the wheel, with such force as to
compress the whole, by a strain of 250 tons, into the solidity of
one piece.”
Paddle-wheels are made to revolve with their lower part in water,
and are furnished with a series of short boards fixed to the tire of
the wheel, which is generally double, that they may be better held
on; these boards or paddles take a great hold on the water and cause
the resistance which is necessary to move the vessel. The wheel of a
watermill is constructed in the same way.