Piston & Valve Rod
Multiple Element Floating Rod Packing
Fully floating five-element metallic piston rod packings replaced the single element Paxton-Mitchell type which did not guarantee steam tightness over long periods without attention. Some time previously Porta had sent me a drawing made in 1975 of a piston rod packing to his design for the Indian Railways WG class 2-8-2. ….. The principle was to put as many sealing elements in series as possible, the sealing function being shared by all elements, it being the same principle that dictated the maximum number of piston and valve rings. The lower load taken by each element reduced its wear rate correspondingly and enabled steam tightness to be maintained over much longer periods than with simple packings.
The proposed WG class packing was used as the basis for the new design [was] produced both the piston rod and valve spindle packing drawings. A split packing housing derived from the British ‘Britimp’ packing was used. This elegant arrangement minimized the number of steam-tight faces required and made assembly and removal of the packings extremely simple. The split housing halves were located by dowels and tightened together by Allen screws, and could be fitted in place of the existing packings without any alterations to the back cylinder heads. Each packing element consisted of two rings made up of four segments located together by a peg and groove arrangement which held the joints in each pair of segments at 90° to the joints in the adjacent pair, thereby eliminating any leakage path past the joints. The segments were made from cast P66 alloy (a lead – copper – sulphur alloy), the same material as used for the Paxton-Mitchell packings. …. Plate 35 (below) shows the packing rings and one split housing half in position on a piston rod during assembly.