Steam and the Marine Steam-engine by John Yeo

Author:John Yeo
Language: eng
Format: epub
Publisher: Macmillan
Published: 1894-03-25T05:00:00+00:00

\

Fi(4. 73.

As the block is placed nearer the centre of the link the effects referred to become more marked, until, in the central position, the valve travel is largely reduced, and the operations are so much in advance of the piston's requirements that no stroke can be completed.

Centring the links has thus the effect of stopping the engines; while by " linking-up," that is, by making the block approach the centre, we are able to advance the point of cut-off, and consequently to increase the expansion given to the steam to a moderate

^ It is assumed for simplicity that OE is the ahead eccentric. This would be the case in a return-connecting-rod engine, the direction of rotation being that shown by the arrow. In a direct-acting horizontal engine OE' would be the ahead eccentric ; and the link would be up for ahead if connected with the eccentrics as shown in the tigure. If, however, the eccentric OE were connected with the lower end of the link and OE' with the tipper end, so that the eccentric-rods were crossed in the position of the gear shown, the link would then be down for ahead, as shown, in direct-acting engines. If so connected the link would be said to have **crossed rods"; while if connected as in the figure, i.e. with the rods uncrossed when the eccentric radii pmnt towards the link, it has ** open rods." Open rods are preferable for working ** linked-up," the desired alteration in the cut-off being effected with less reduction of travel.

SLIDE-VALVE GEAR

103

extent; and this is now the ordinary plan of working at reduced powers. The principal objection to linking-up beyond a moderate degree consists in the reduction of travel, and the consequent contraction of the port openings both for steam and exhaust.

Forms of Link. —Three forms of Stephenson's link may be found at work in the service: the open or slotted link, the solid-lav link, and the double-lar link. The last is the form now in general use.

In the slotted link. Fig. 72, the block is fitted into the curved slot passing down the centre of the link, and is free to turn about the pin, jp, which connects it with the slide-valve rod. In the solid-bar form the link is a simple curved bar passing through the block, which is in the form of two sectors bearing upon its opposite edges, and cylindrical on their outer surface. This block is carried in a cylindrical recess in the slide-rod head, so that it can turn about its axis and give the necessary freedom to the link.

In the double-bar variety. Fig. 74, two curved steel bars, BB, are secured together, as shown, to form the link; distance-pieces, D, being fitted between the bars. The block consists of a steel

ELEVATION

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.