CHAPTER XVII.

STARTING AND REVERSING ARRANGEMENTS.

ALL marine engines must be arranged so as to be capable of being worked in opposite directions, in order that the ship may be driven either ahead or astern. It is necessary, therefore, that suitable reversing gear should be fitted to enable the slide-valves to be placed in the proper positions to produce revolution of the crank-shaft in either direction.

Loose eccentric.-In the earlier paddle-wheel steamers this was accomplished by means of a single eccentric, fitted loosely on the shaft,

b

FIG. 165.

and driven by stops fixed in suitable positions on it, to give either ahead or astern motion as required. A sketch of this arrangement is given in Fig. 165.

This loose eccentric is balanced by means of the disc D, to prevent it falling away from its position when the slide-valve is moved by hand. The eccentric rod is attached to the slide-valve rod by means of the gab G at its end, which fits over a corresponding pin on the end of the slide-rod. In starting or reversing these engines the gab G is disconnected from the slide-rod, and the slide-valves worked by hand to start the crank-shaft revolving in the proper direction, or reverse its motion as the case may be, and cause the stop for the proper motion to come in contact with the eccentric, and drive it in the required direction. The gab end then drops over the pin and continues the

motion.

In Fig. 166, let o R represent the position of the crank on the dead point. Then, from what has been previously explained about the motion of the slide-valve, if steam be taken at the outside edges of the valve, and the eccentric radius be in the position o P, the crank

will revolve in the direction A B C, whilst if the eccentric arm be in the position shown by the dotted line o Q, the motion of the crank will be in the opposite direction a Q C. The stops on the crank-shaft must, therefore, be so arranged as to bring the eccentric pulley to the

B

position o P, or to the position oq, according as the engine is to be driven ahead or astern.

The manner in which this is accomplished is shown in Fig. 165. On the crank-shaft, c, is fixed a stop cd, extending a sufficient distance round its circumference, and the ends of D, the balancing disc on the eccentric, are arranged to form corresponding stops, a and b. When, therefore, the top of the shaft revolves from right to left, the edge c comes in contact with a, and the eccentric and shaft revolve together so long as the motion continues in this direction; but as soon as the engine is reversed, the stops become detached, and after about a quarter revolution the edge d comes in contact with b, and the engine works in the reverse direction.

FIG. 166.

Link motion. The reversing of modern marine engines is usually effected by means of the 'link motion,' which was invented by Stephenson. It is simple in construction, and not only can the engines be reversed by it, but it provides for a considerable range of expansive working of the steam, without the interposition of any other gear.

The general construction and arrangement of this gear are shown in Fig. 167. On the crank-shaft c there are keyed two eccentrics, one

SUDE
ROD

B

D

FIG. 167.

in the position to give ahead motion and the other in the position for astern motion. The eccentric rods are of equal length, and their ends are attached by working joints to the opposite ends of a curved link L,

with a slot in the centre, which slides over a brass block B, to which the end of the slide-valve rod is attached. This gear forms a ready means of throwing either of the eccentrics into gear as desired, and at the same time throwing the other out of gear.

It will be seen that when the link is moved from its middle position, so as to bring the pin A in a line with the slide-valve rod, the motion of the valve will be governed by the eccentric D, while the other simply swings the link without affecting the motion of the sliderod. If, on the other hand, the link be moved to the other side, so as to bring the end of the eccentric rod в in a line with the slide-rod, the eccentric E will govern the motion of the slide-valve, the other eccentric having no effect, and the engine will work in the reverse direction. When the link is in the central position the motion of the valve is considerably reduced, and the distribution of the steam is such that no revolution of the engine could ensue.

The centre of the link is commonly called its 'dead point.' If the link be placed in such a position that the sliding block B is between the centre and the ahead end of the link, the ahead eccentric exercises the greatest influence over the motion of the valve, so that the engine continues to work in the ahead direction; but the astern eccentric has now some effect in modifying the motion of the valve, the result being that the travel of the slide-valve will be less than when the link is at its extreme position, and all the operations of the valve will be earlier than when in full gear, as if the valve were now being worked by an eccentric with greater angle of advance and smaller throw. steam is therefore cut off earlier and worked expansively.

The

The operation of working expansively by means of the link motion

is technically called 'linking up' in horizontal

engines, or linking in' for vertical engines, or, generally, 'shortening the link.'

Varieties of links. There are three varieties of links used: (a) the slotted link, (b) the solid-bar link, and (c) the double-bar link.

The slotted link consists of a curved bar with a slot cut in it (Fig. 168), in which slot the link block is fitted. This link block is attached to the slide-rod by a pin about which an oscillating motion of the block occurs. Two projections are formed on the link on one side, with eyes to which the ends of the two eccentric rods are attached. This is the original form of Stephenson's link, and is still commonly used in small engines. The centre of the eccentric-rod end cannot in this form be made to coincide with the centre line of the link block, and the motion is not so regular or the means of adjustment so good as in plan (c), which is now the general plan used for large vertical engines.

FIG. 168.

The solid-bar link (Figs. 169 and 170) is always fitted by Messrs. Humphrys, Tennant & Co., and consists of a simple curved rectangular

bar, with eyes formed at each end for the attachment of the eccentric rods. The solid bar passes through the block, which consists of two segmental pieces of gunmetal, cylindrical on the outside, and which

have an oscillating motion in its bearing when at work. This variety also has the advantage of being easily adjusted in all parts. The end of the eccentric rod cannot be brought in line with the slide-rod, and even in full gear the motion is one with a 'shortened link.' The sketches show in detail the construction and means of adjustment of all parts.

The most general plan is the double-bar link (Figs. 171 and 172) consisting of a pair of curved steel bars joined together at the ends,

О

FIG. 171.

and kept a certain distance apart by distance pieces. Projecting pins are formed on the link bars, two on each side, for the attachment of the eccentric rods. The ends of the eccentric rods are forked, and contain each two adjustable bearings which embrace the pins on each side of the link. The link block consists of a steel or iron pin sliding between the bars, with top and bottom projections each side, which embrace the link bars. The link bars slide through these projections, and adjustable gunmetal liners are fitted as working surfaces between

the link block projections and the link bars. All parts are capable of ready adjustment, and when the link is in full gear the centre of the

eccentric rod end coincides with that of the link block. Detailed sketches of the link block and its lubrication arrangements are given in Figs. 173 and 174.

These different arrangements of link motion, however, only differ in the details of construction, the principles involved being the same in all.

For detailed construction of the eccentric sheave, strap, and rod see end of preceding chapter.

Curvature of link. If the link were used simply for reversing the engines, its amount of curvature would not be of importance, or it may even be straight; but as it is required to be used for working expansively, its shape must be such that when the block is in any intermediate position the centre of the travel of the valve will be always constant, otherwise the distribution of steam between the two ends of the cylinder would be interfered with. To effect this quite accurately the