402

MODERN STEAM PRACTICE.

is to compress the packing ring, by the insertion of a wedge betweenthe tongue piece at the fast end and the bridle at the loose end, sothat when the wedge is driven in between the two the packing ringis drawn together, and can be readily placed in the piston when inthe cylinder; the cotter is then drawn out, and the packing ringexpands to its original size, and fills the cylinder somewhat tightly.The projections left on the junk rings and the body of the pistonmust have recessed parts in the cover and end of the cylinder, withsufficient clearance at the end and round the projections; and it isadvisable to leave recessed parts at both ends of the cylinder,making the part bored out somewhat shorter than the actual strokeof the piston, so that the rubbing surface travels over it at each end,and prevents a groove forming in the cylinder that would eventu-ally prove destructive to the engine, in the event of the connectingrods requiring lining up in the brasses, by causing the piston tostrike hard against the projections. Sometimes the piston fordirect-acting single piston-rod engines is dished out or formed of acurved shape, with the view of getting more room for the crosshead,the gland for the rod being recessed into the end of the cylinder.This is the only advantage to be derived from this plan, and it isbut rarely adopted. The fittings of such pistons are identical withthose for the plain-ended arrangement. When annular cylindersare adopted for high and low pressure combined engines, the smallpiston for the high-pressure cylinder is similar to an ordinary one,but the piston for the annular cylinder must have two packingrings, the outside ring bearing on the cylinder surface, as in ordinaryarrangements, but the internal packing ring bears on the insidediameter of the ring, the ring being pressed up against the barrelof the high-pressure cylinder with strong Steel springs. Thesepistons are generally connected to the crosshead by a central pistonrod for the high-pressure cylinder, and two side ones for the low-pressure cylinder, with one crosshead common to both. Theremust be block pieces fitted between each packing ring and thebody of the pistons, to keep them all fair with one another. Thepacking rings of all pistons are generally made thicker at the bottomwhere these blocks are fitted, and thinner at the top where thering is cut; this is necessary, as in all horizontal arrangements theseverest strains and wear are undoubtedly at the bottom of thepiston.

As bearing on this subject the following extracts are from a