Modern Machine-Shop Practice Part 257

A s.h.i.+p's side air pump discharge valve is an ordinary dead weight mitre valve that opens to let the water pa.s.s out into the sea, but seats itself and closes if the water attempts to pa.s.s inwards. It differs from a common stop valve in being weighted, and therefore self-acting. It requires to be lifted before starting the engine, as such valves are liable to stick in their seats.

The course of the main injection water of a jet condenser is as follows: From the rose plate or strainer, through the injection valve and pipe to the condenser, where it mingles with the exhaust steam and from which it is pumped with the products of condensation into the hot well. From the hot well it pa.s.ses mainly overboard through the Kingston valve, but that part of it used for the boiler feed pa.s.ses through the suction pipe and valve into the pump, and thence through the delivery valve, pipe and check valve into the boiler.

The course of the main circulating water of a surface condenser is through the Kingston valve (on the s.h.i.+p's side or bottom), and the circulator inlet pipe, either direct to the condenser, from which it is _drawn_ by the circulating pump, or else it pa.s.ses through it, and is _forced_ through the condenser. It circulates through the condenser twice or thrice according to the construction, and is forced overboard by the action of the circulating pump, pa.s.sing through a valve on the s.h.i.+p's side or bottom.

The advantages of surface condensation are, first, that the feed water is obtained at a higher temperature than if injection water was fed to the boiler. Second, the feed water is purer, and therefore less water requires to be blown out of the boiler in order to keep it clean. Third, the boiler does not scale so much, hence its heating surface is maintained more efficient; and fourth, the boiler suffers less from expansion and expansion strains when hot feed water is used.

Surface condensers foul from the grease with which the cylinders are lubricated and from the salt in the injection water. The condenser is cleaned by the admission of soda with the exhaust steam and by was.h.i.+ng out.

A condensing engine has the following c.o.c.ks and valves on the skin of the s.h.i.+p in the engine room: The main Kingston valve for the injection, or circulating water, the main delivery valve from the condenser, the bilge delivery valves, and the water service c.o.c.ks for keeping the main bearings of the engine cool with streams of cold water.

A donkey engine is a small engine used to feed the boiler, and has the following connections: A steam pipe from the boiler to drive the donkey engine; and exhaust pipe into the condenser; a suction pipe from the hot well or from the sea, as the case may be; and a delivery pipe to the boiler; a suction pipe from the bilge, so that the donkey pump can a.s.sist in pumping the bilge out; a suction pipe to the condenser, to circulate the water when the main engines are stopped, and thus maintain the vacuum; and a suction pipe from the water ballast tanks, to pump them out when necessary.

The pipes that lead from, or go to, the sea are: Boiler blow off pipe, sea injection or circulator pipe, condenser discharge pipe, and, in some cases, donkey feed suction pipe.

The parts of an engine that are generally made of wrought iron are those in which strength with a minimum of weight and size is desired; for example, the piston rod, cross head, connecting rod, crank shaft, crank, eccentric rods, link motion, valve spindle pump rods, and all studs, bolts, and nuts.

The parts generally made of cast iron are those where strength and rigidity are required, and which are difficult to forge, while weight or size is of lesser importance, such as the bed plate, cylinders, pistons, condensers, and pumps.

The parts sometimes made of steel are those subject to great wear, and for which strength with a minimum of size is necessary, as piston springs, piston rods, connecting rods, cranks, crank pins, and valve rods.

The parts generally made of bra.s.s are those subject to abrasion or corrosion, as the connecting rod bra.s.ses, the bearings for the crank shaft, the pump plungers or pistons, and their rods, linings for the pump barrels or bores, the bores of the glands, the condenser tubes, and all c.o.c.ks and valves.

White metal or babbitt metal is sometimes used in place of, or in connection with, bra.s.ses, serving as an anti-abrasion surface. It is easily renewed, as it is cast into its place, but will melt and run out at a temperature of about 600 Fahrenheit.

Muntz metal is used where iron or steel would suffer greatly from corrosion when in contact with salt water. It can be forged.

The difference in the composition of cast iron and steel has never been determined; the difference lies in the percentage of carbon they contain and the structure of the metal. Cast iron will not weld.

Cast iron is brittle, of granular structure, and always breaks short, having a very low elastic limit.

Wrought iron is tough and fibrous, will weld but will not harden, and is stronger than cast iron.

Steel is stronger than wrought iron, and will weld and harden and temper. The breaking strain of wrought iron varies from about 42,000 to 60,000 lbs. per square inch of section.

Steel is tempered by first being heated red hot and suddenly cooled (usually by plunging it into cold water), which hardens it. The surface is then brightened, and on being reheated the tempering colors appear, beginning at a pale yellow, and deepening into red, brown, purple, and blue, the latter gradually fading away as the metal is re-heated to a red heat. The higher the temperature to which the hardened steel is reheated the softer or lower it is tempered.

These colors merely indicate the temperature to which the piece is reheated, since they will appear on steel not hardened and upon iron.

Case hardening is a process that converts the surface of wrought iron into steel, which is accomplished by placing them in a box filled with bone dust, animal charcoal, or leather hoofs, etc. The box is sealed with clay, heated red hot for about 12 hours, and the pieces are quenched in water.

The parts usually case hardened are the link motion, and other light working parts that are of wrought iron.

The forgeable metals used in engine work are wrought iron, steel, copper, and Muntz metal. The brittle or short metals are cast iron and bra.s.s.

Welding is the joining of two pieces solidly together. Wrought iron, steel, and Muntz metal can be welded.

All the metals used in the construction of marine engines expand by heat, and this is allowed for in adjusting the lengths of the eccentric rods, or of the valve spindles when setting the valve lead. In the case of two marine boilers being connected together, the steam pipe is fitted with an expansion joint, one pipe end having an enlarged bore to receive the other. The joint is made by packing, which is squeezed up by a gland, whose bore fits on the outside of the pipe which moves through the gland bore, from the expansion and contraction.

The piston of a marine engine steam cylinder is a disc of cast iron, into which the piston rod is secured. Its body is cored out to lighten it. Around its circ.u.mference is a recess to receive the packing ring or rings, each of which is split across so that it may be expanded (to fit the bore of the cylinder) by means of the packing or of the springs. The split is closed in the centre by a tongue piece let into the ring, and fastened to one end of the ring.

To hold the piston rings or ring in place, a junk ring is employed, being an annular ring bolted to the piston. The piston rings are set out to fit the cylinder bore by suitable springs. The round plugs seen on the piston face merely fill the holes used to support the core in the mould and to extract it from the finished casting.

Cylinder drain c.o.c.ks sometimes have a check valve upon them, so that while the water may pa.s.s out of the cylinder the air cannot pa.s.s in and destroy or impair the vacuum.

Cylinder escape or relief valves are provided at the top and at the bottom of the cylinders, and consist of a spring loaded valve with an adjusting screw to regulate the pressure at which they shall act. They are most needed when the boiler primes heavily, and the water might knock out the cylinder heads or covers. They should be enclosed in a case with a pipe to lead the water away, thus preventing it from flying out and scalding the engineer.

A link motion is a valve gear by which the engine may be reversed (caused to run in either direction), or which may be used to vary the point of cut off. The advantage of the link motion is its simplicity and durability.

A link motion for a marine engine is usually of the Stephenson type, and consists of two eccentrics or eccentric sheaves fixed upon the crank shaft, and so set as to give more lead at the bottom than the top ports, because the wear of the journals, bra.s.ses, and pins gradually increases the lead at the upper, and correspondingly diminishes that at the lower port. In addition to this, however, more lead is required at the bottom port, to counterbalance the weight of the piston at the end of its descending stroke. The eccentric hoops or straps drive the rods which connect to the ends of the link.

The link may be a curved, solid, or a slotted bar, and in either case has fitted to it a block or die which connects to the valve spindle.

The link is pivoted at its centre to a swinging arm or suspension link,[58] and by this arm may be moved endways to bring the required end of the link beneath the valve rod or spindle. From the positions in which the eccentrics are set, one end of the link operates the valve to go ahead, while the other end operates it to go astern; hence all that is necessary (so far as the link motion is concerned) to reverse the engine is to move the link endwise to the requisite amount, which, for full gear, is so that the block is at or near the end of the link.

[58] See page 383 for the construction of a link motion.

In proportion as the link block is (by moving the link endways) brought nearer to the middle of the link, the valve travel is reduced and the point of cut off is hastened, thus increasing the expansion.

When the link block is in the middle of the link, the latter is in mid gear, and the valve only opens the ports to the amount of the lead, and the link action is the same, whether the engine moves backwards or forwards.

The motion of the link is as follows:

The two ends are vibrated by the eccentrics from the central pin of the link hanger (or suspension link) as a centre of motion, while at the same time this end of the link hanger swings in an arc of which its other end is the centre of motion.

In small engines the link is sometimes used for varying the expansion as well as for reversing the direction of engine revolution.

In large engines it is used for reversing only, a separate expansion valve being used for varying the point of cut off.

In small engines the link is moved endwise for forward or backward gear by a simple arrangement of hand levers. In large engines these levers are supplemented by a worm and worm gear, and in still larger engines a steam reversing gear is used for s.h.i.+fting the links from forward to backward gear, or vice versa.

When there is no link motion, a Joy valve gear, a Marshall valve gear, or a loose eccentric may be used. A loose eccentric is one that can be moved around the shaft to reverse the engine. It may be moved around the shaft by mechanical means, or the eccentric rods may be disconnected, and the valve worked by hand, to cause the engine to run in the required direction, until a pin fast in the shaft meets a lug on the eccentric and drives it, there being two such lugs or shoulders s.p.a.ced the requisite distance apart on the eccentric. This plan is obviously only suitable for small engines.

A separate expansion valve is a valve employed to effect the cut off and vary the expansion. It does not affect either the admission or exhaust of the steam to the cylinder.

It is used because by its means an early point of cut off and high rate of expansion may be obtained with a fixed point of exhaust, a fixed amount of compression, and a fixed amount of lead, whereas with the link motion alone the exhaust occurs earlier in the stroke, and the compression and the lead increase as the link is moved from full gear towards mid gear. The expansion valve should, when the engine is to be started, be set for the latest point of cut off. The eccentric for the expansion valve is set opposite to the crank, in order that its action may be the same, whether the engine runs backward or forward.

The small cylinders on top of the steam chests are for the purpose of guiding the upper ends of the valve spindles, and are fitted with pistons having steam beneath, the s.p.a.ce above being in communication with the condenser. The steam pressure on the piston supports the weight of the valves and valve gear.

The friction of a slide valve may be relieved or reduced by excluding the steam from its back, which is done by various means, such as by a ring cast on its back and working steam tight against a plate held independently of the valve. The interior of the ring should be open to the exhaust.

The friction of a slide valve is caused by the steam pressing it to its seat, the amount of this pressure varying with the fit of the valve to its seat, and its position over the ports, or, in other words, upon how much of the valve area has steam pressing on one side only.

The travel of the eccentric rod is the distance it moves measured on a straight line. It is equal to twice the throw of the eccentric.

The throw of an eccentric is the distance between the axis of its bore and the centre or axis from which its circ.u.mference was turned in the lathe.

Double beat valves are composed of two discs or mitre valves, one above the other on the same stem, so that as the steam presses on the opposite faces of the two discs the valve is balanced. The objection to their use as safety valves is, that they are balanced and would not lift unless the area of the upper disc was made larger than that of the lower one, in which the objection would remain that the two discs do not expand equally, hence they are apt to leak. They are sometimes used instead of slide valves, but are objectionable because a separate admission and exhaust valve is required at each end of the cylinder, and because at quick speeds of revolution they fall to their seats with a shock or blow which wears out both the valve and the seat. When a high piston speed is obtained by great length of piston stroke, and not by high rotative speed, their use is less objectionable.