214
OF THE PARTS OF
[sect. VII.
Pistons may be rendered tight by an elastic packing of vegetable or animal
matter; but the latter kind of packing cannot be used for steam, on account of
the heat destroying it.
Pistons may also be made wholly of metal, constructed so as to admit of a
certain degree of elasticity.
After considering some particulars common to all pistons, we will treat of
pistons, as below, dividing them into two classes.
on wood,
on metal.
f Common leathered
Packed <( Atmospheric engine piston.
Common.
Woolf’s.
Hemp
Pistons <(
f Cartwright’s 1797.
J Barton’s 1816.
| Jessop’s 1823.
Perkins’ 1823.
463. When a piston rod is to be pushed as well as drawn, unless it be of a
certain thickness in proportion to the diameter, it is liable to stick if there be the
slightest inequality in the friction, or in the centring of the rod. If it were a thin
plate, nothing but its connexion to the rod would prevent it turning with the
slightest inequality of its friction, on being pushed; and as we make it thicker,
the thickness interferes more and more with any tendency to turn. The propor
tions which will secure us from the risk of this evil are not difficult to ascertain.
Let the pressure on the piston A B move the rod C D.
Then, in order that the piston may move steadily, its friction
at the circumference multiplied by half the diameter of
the piston, should be equivalent to the pressure producing
that friction, multiplied by half the thickness of the piston ;
consequently, the thickness should be to the diameter, as the
friction is to the pressure of the rubbing surfaces.
Fig. 18.
c
¿s
The friction of brass on iron is at an average one-eighth
of the pressure; hence the thickness of metallic pistons
should not be less than one-eighth of the diameter.
The friction of hemp packing on iron is about one-sixth of the pressure, hence
the thickness of the packing should be one-sixth of the diameter. Practice is