LECTURE XXIII.

CONTENTS.-Double-ported Slide Valve-Piston Valve-Expansion Valves -Gridiron Expansion Slide Valve-Expansion Valve on Back of Slide Valve-Starting and Reversing Gears-Single Loose Eccentric Gear— Stephenson's Link Motion-Link Motion of the s.s. St. Rognvald, and Specification for Valves and Link Motion.

In the last Lecture we drew attention to the great power absorbed in simply driving the ordinary D slide valve. To impress this fact upon the student, suppose the case of a locomotive D slide valve, 16" broad by 10" long, with a travel of 6", working under a steam pressure of 160 lbs. per square inch. What power will be absorbed in driving each of the locomotive slide valves, when the crank shaft is making 200 revolutions per minute, and assuming the co-efficient of friction between the face of the valve and steam-port bars to be o°1 ?

This calculation neglects the power absorbed in moving the mere dead weight of the valve, as well as that due to the friction of the valve spindle in its stuffing box and guides, since these quantities may be supposed to be constant for any kind of valve used so far as our purpose is concerned-which is simply to impress upon the student that a very large proportion of the power developed by an engine is absorbed in merely working one of its most essential parts, viz., the slide valve. This fact has led engineers to devise many forms of slide valves, with the object of overcoming this great waste of power. One of these devices is known as the "Double-ported Valve,"

Double-ported Slide Valve. This form of slide valve aims at lessening the power required to drive the valve by reducing the travel.

This is effected by having two steam openings to each steam port of the cylinder, instead of only one, as in the case of the common D slide valve. The outer opening admits steam direct from the valve casing, and exhausts through the outermost opening in the valve by a passage formed round the back of it into the exhaust port, while the inner opening to the steam-port gets steam from the valve casing through a transverse opening in the valve, and exhausts directly into the exhaust port. A valve of this kind may, therefore, afford the same opening to the steam as an ordinary locomotive slide valve with half the travel, and, consequently, with half the expenditure of power to drive it, as far as regards

COMMON DOUBLE-PORTED VALVE AT ITS MID-STROKE.

the effect of the direct pressure to steam on the back of it, but this is sometimes still further reduced by causing the back of the valve to bear evenly upon a steam-tight opening or pressure relieving frame, in direct communication with the condenser. In very large marine engines, treble-ported and even quadruple-ported slide valves have been resorted to with the same object, viz., a reduction in the power required to drive them, by lessening the travel. Neither of these plans is thoroughly satisfactory, for they only partially diminish the power required to drive the valve.

The above illustration shows a longitudinal section, with the valve at its mid-stroke, whilst that on the next page shows the valve when opening the steam port by the full amount, as well as a plan of the face of the valve and an end view (half outside view, half section).

Specification for the s.s. "St. Rognvald's" Slide Valves.-To be placed on the forward side of their respective cylinders. The low-pressure cylinder valve to be double ported, the high-pressure cylinder valve to be single ported, and both to be made of hard close-grained cast iron. These valves to be carefully fitted to valve face by scraping, and rendered perfectly steam-tight,

PB

Piston Valve.-As we remarked before, no thoroughly satisfactory plan has yet been devised of relieving the ordinary D slide valve from the pressure of steam upon its back surface, and, consequently, in the case of large engines, using very high-pressure steam, recourse is had to the piston

valve.

The accompanying figure of a piston valve is from Seaton's Manual of Marine Engineering. It will be seen from the figure that there are two steam ports and one exhaust port, as with the ordinary slide valve; but, owing to the circular construction of this valve, the steam-port area is nearly three times that of an ordinary flat valve of the same transverse dimensions (since the circumference of a circle is πd = 3.1416 times the diameter). Further, the pressures on the sides of the valve are perfectly balanced, since they act equally all round it, and, consequently, the valve, if properly made and fitted, simply floats in a bath of steam. The valve consists of two pistons in one casting, connected together by a pipe, which is fixed to the valve spindle. There is a balance piston and cylinder, fitted immediately above them, to relieve the eccentrics from the dead weight of the valve, link motion, and eccentric rods. The piston valves are rendered steam-tight by having stiff cast-iron liners and bronze spring rings, which extend beyond the turned surface of the pistons sufficiently far to permit the steam having a free passage to the ports. These spring rings are prevented from forcing themselves into the steam ports by a series of narrow diagonal bearing bars being cast in and around the port liners, thus →W. These bars are truly bored out along with the liners. Steam exhausts from the cylinder round the outside of the pipe between the pistons of the valve, and enters the cylinder from each end, so that the lap of the valve is on the outside edges of the valve in the same way and to the same extent as with an ordinary slide valve. To avoid, as far as may be, the naturally large clearance

THE PISTON SLIDE VALVE.

space with this form of valve, the pistons are kept as far apart as possible, so that they may be almost directly opposite to the positions where the steam enters the cylinders. All the latest

engines for Transatlantic steamers, as well as many others, have been fitted with piston valves. The chief objection or disadvantage usually urged against these valves is their first cost, which naturally prevents their being commonly fitted to the low-pressure cylinder of compound, or of multiple expansion engines.

Expansion Valves. We have already mentioned that the ordinary slide valve is not well suited for high grades of expansion, because sufficient opening to steam cannot be obtained with it for an earlier cut off than, say, half stroke, without increasing the travel and the lead, or using unduly long ports, or else subjecting the steam to considerable wire-drawing. This defect is, to a certain extent, overcome by the double-ported valve, and also by the piston valve, from the fact that they each give a larger area of opening to steam with less travel than the ordinary D slide valve.

There are, however, many special devices, called “expansion valves," which have been invented to remedy or assist the ordinary slide valve in overcoming this defect. One of the earliest arrangements was to fix a cam upon the crank shaft, with a bearing-roller on the cam, connected to an ordinary throttle valve, fixed in the steam pipe close to the valve casing. This cam was so set with respect to the ordinary eccentric working the ordinary slide valve, that it elevated the bearing roller, and consequently closed the throttle valve just before the steam had forced the piston through the desired length of stroke at which cut-off should take place. The obvious defect in this arrangement lay in the fact that the throttle valve could not be conveniently fixed sufficiently near to the slide valve, and thus the valve casing was left full of steam after the throttle valve had closed the steam pipe, which steam was not cut off by the ordinary slide valve until half stroke, or later. In order to so far remedy this defect, what are called " gridiron expansion valves were at first fitted to seatings fixed near to the steam-pipe opening in the main valve casing, but latterly a modification of this plan has been fitted directly to the back of the main ordinary slide valve.