rings to be adjusted at any time from the outside, whether the engines are working or not, which cannot be done if the ring is fitted on the slide-valve.

Details of various relief rings.-The details of these relief rings vary considerably, but the principle is the same in all.

Fig. 155 shows the plan adopted by Messrs. Humphrys, Tennant & Co., in which steam is prevented from passing the back of the relief ring by means of a copper spring ring, which keeps the back of the relief ring steamtight. Details of this ring are shown on an enlarged scale in Fig. 156. Stops are fitted at each end of the ring to prevent the

friction of the valve on the relief ring straining the copper ring. These stops are shown at A.

Other plans of relief ring are also shown, in which different methods of preventing leakage past the back of the relief ring are used. In Fig. 157 this consists of small Ramsbottom rings. In Fig. 158 a turn of soft packing is used as shown. Fig. 159 shows a ring working on the valve casing known as Church's relief ring.

A series of spiral springs is generally fitted in recesses, pressing the relief ring against the back of the slide-valve. In two or more of these recesses the springs are omitted and stop pins fitted, a sketch of one being given in Fig. 160. These are so fixed that their points are about

inch clear of the relief ring, and they prevent the slide-valve from leaving the cylinder face by more than this distance. A washer should be fitted, so that these cannot be inadvertently screwed up beyond their

J

correct positions. In Fig. 159, showing one form of Churcn's ring, a flat spring is used for pressing it against the valve-casing cover.

Unfortunately, however, relief rings for the back of flat slide-valves do not behave in an entirely satisfactory manner. They are troublesome to make efficient when new, and are difficult of accurate adjustment, and it is often found that they do not remain efficient for long periods, especially with the higher pressures.

slide-valves. To overcome this defect, the slide-valves of the high and usually the intermediate cylinders of engines using high-pressure steam are fitted with cylindrical or piston slidevalves, instead of flat slide-valves, so that the steam does not cause any pressure between the rubbing faces, and no relief arrangements are necessary. The valve is formed by two pistons connected together, which work in cylin drical chambers that contain the steam ports, and are generally kept steamtight by spring rings in the usual manner. The face of each of the pistons corresponds to the bars of the single ported flat slide-valve, and is of the same length, i.e. it is just long enough to cover the steam ports and allow the necessary lap.

These valves
single-ported, as double-
ported piston-valves

FIG 161.

would be very complicated, and are unnecessary, owing to the considerable length of port secured by the cylindrical form.

The face of the circular slide-valve may be imagined to be formed from a flat valve face by curving the latter into the circular form. The steam is either admitted to the spaces on the outside of the two pistons with the exhaust space between them, or vice versa. In the form shown in Fig. 161 the steam is admitted to the space between the two pistons around the tube that connects them together, while the exhaust takes place at the outer ends of the pistons, and the exhaust steam from the opposite ends of the valve-chest is in communication through the tube that connects the two pistons together. In Fig. 162, which

shows the lower half of a pistonvalve at the centre of its stroke, a steam pipe connecting the two ends is carried outside the cylinder. The steam is admitted to the cylinder from the outsides of the valves, the exhaust taking place on the inside. The steam ports are formed in separate cylindrical faces secured to the cylinder ports, as shown, and the openings are stayed with bars, that run diagonally across and serve also as guides for the piston-valves, and prevent the packing-rings springing out into the ports. Balance pistons, &c.-In vertical engines the weight of the slide-valve rod, and link-gear will all be taken by the eccentric and its strap, unless means are fitted to prevent this. With large slide-valves in vertical engines balance pistons are therefore fitted to take the weight of valves and gear off the links and eccentrics. Small cylinders with pistons are fitted on top of the slide casing immediately over the slide-valve, as shown in Figs. 153 and 161. The lower side of the balance piston is in connection with the slide casing, the steam pressure in which acts on the balance piston, the area of which is so arranged that the total steam pressure is sufficient to balance the weight of the gear. The top of the balance cylinder is connected by a pipe with the exhaust steam from the cylinder.

FIG. 162.

With piston slide-valves the balancing can be effected by making the diameter of one end of the valve a little greater than that of the other, so that the steam pressure acting on the excess area balances the weight of valves and gear.

Momentum cylinder. With very fast-running engines the momentum of the moving slide-valves brings considerable forces to bear on the eccentrics and link motion, so that it is important to reduce the weight of the valves to the lowest point consistent with strength. In fastrunning engines of torpedo boats and destroyers the valves are often of gunmetal, or similar composition, to effect this object; but even when all is done that can be in this direction, the momentum forces at speeds of, say, 400 revolutions per minute are very considerable. A simple momentum cylinder is often fitted above the slide-valve to neutralise this. This consists of a small piston and cylinder, with arrangements

for compressing a certain volume of steam or air at the end of each stroke of the valve, to absorb the forces due to the velocity of the valves and bring them gradually to rest without strain.

Joy's assistant cylinder.---Sometimes arrangements are supplied with vertical slide-valves, not only to support the weight of the valve, but to also relieve the eccentrics and link-gear of most of the work required to move the valve to and fro. One such arrangement is Joy's assistant cylinder, Fig. 163.

This consists of a small cylinder and steam-piston attached to the valve-spindle. The cylinder has a central inlet for steam, A, and two exhaust ports, B, one for

each end, leading to a common exhaust pipe, and the piston is so constructed that by its motion the operations of steam admission, cutoff, release, and compression are performed on each side of the piston. The apparatus is, therefore, a small engine which exercises a force on the valve to move it up or down, and cushions steam at each end to absorb the momentum forces. These assistant cylinders give diagrams similar to that of an ordinary engine; they exert from 15 to 25 I.H.P. each for the sizes fitted in marine engines, and the amount of power developed can be adjusted by means of a valve on the steam-pipe. If the main valve be linked in, the assistant cylinder is also automatically similarly affected.

EXHAUST

-----
HALF TRAVEL

OF VALVE

BEHIND B

LENAUST P

BANIND

VALVE SPINDLE

FIG. 163.

STEAM

Eccentrics and rods for large marine engines.-Eccentric sheaves for large marine engines are of castiron, and are generally made in two parts, owing to the couplings, &c., preventing their being put on the shaft in one piece. The parts are firmly secured together by bolts. The rod is of wrought-iron or steel, and generally has a T end, by means of which it is secured to the eccentric strap by studs and nuts.

The two halves of the eccentric strap were for many years made of gunmetal. In some examples the eccentric strap, as well as the rod, is made of wrought-iron or steel, one-half of the strap being forged solid with the rod, with gunmetal liners fitted to the strap to form the rubbing surfaces.

In the most recent practice the strap is of forged or cast-steel, lined with white metal, which forms the rubbing surface working on the castiron sheave. This combination of metals has been found to give excellent results, and it is now specified by the Admiralty.

Fig. 164 shows the details of a modern eccentric sheave, strap, and

rod, the sheave being lined with white metal. In some examples, in order to reduce the diameter of the eccentric sheave, the smaller of the two parts is made of wrought-steel, which enables the least thickness

++

FIG. 164

of the sheave around the shaft at A B to be diminished. Set screws, C, are fitted for purposes of adjustment before the key is fitted.