A Catechism of the Steam Engine Part 32

632. _Q._--How are the trunnion plummer blocks made?

_A._--The trunnion plummer blocks are formed in the same manner as the crank shaft plummer blocks; the nuts are kept from turning back by means of a pinching screw pa.s.sing through a stationary washer. It is not expedient to cast the trunnion plummer blocks upon the lower frame, as is sometimes done; for the cylinders, being pressed from the steam trunnions by the steam, and drawn in the direction of the condenser by the vacuum, have a continual tendency to approach one another; and as they wear slightly toward midships, there would be no power of readjustment unless the plummer blocks were movable. The f.l.a.n.g.es of the trunnions should always fit tight against the plummer block sides, but there should be a little play sideways at the necks of the trunnions, so that the cylinder may be enabled to expand when heated, without throwing an undue strain upon the trunnion supports.

633. _Q._--What kind of paddle wheel is supplied with these oscillating engines?

_A._--The wheels are of the feathering kind, 9 feet 8 inches in diameter, measuring to the edges of the floats; and there are 10 floats upon each wheel, measuring 4 feet 6 inches long each, and 18-1/2 inches broad. There are two sets of arms to the wheel, which converge to a cast iron centre, formed like a short pipe with large f.l.a.n.g.es, to which the arms are affixed.

The diameter of the shaft, where the centre is put on, is 4-1/2 inches, the external diameter of the pipe is 8 inches, and the diameter of the f.l.a.n.g.es is 20 inches, and their thickness 1-1/4 inch. The f.l.a.n.g.es are 12 inches asunder at the outer edge, and they partake of the converging direction of the arms. The arms are 2-1/4 inches broad and half an inch thick; the heads are made conical, and each is secured into a recess upon the side of the f.l.a.n.g.e by means of three bolts. The ring which connects together the arms, runs round at a distance of 3 feet 6 inches from the centre, and the projecting ends of the arms are bent backward the length of the lever which moves the floats, and are made very wide and strong at the point where they cross the ring, to which they are attached by four rivets. The feathering action of the floats is accomplished by means of a pin fixed to the interior of the paddle box, set 3 inches in advance of the centre of the shaft, and in the same horizontal line. This pin is encircled by a cast iron collar, to which rods are attached 1-3/8 inch diameter in the centre, proceeding to the levers, 7 inches long, fixed on the back of the floats in the line of the outer arms. One of these rods, however, is formed of nearly the same dimensions as one of the arms of the wheel, and is called the driving arm, as it causes the cast iron collar to turn round with the revolution of the wheel, and this collar, by means of its attachments to the floats, accomplishes the feathering action. The eccentricity in this wheel is not sufficient to keep the floats in the vertical position, but in the position between the vertical and the radial. The diameter of the pins upon which the floats turn is 1-3/8 inch, and between the pins and paddle ring two stud rods are set between each of the projecting ends of the arms, so as to prevent the two sets of arms from being forced nearer or further apart; and thus prevent the ends of the arms from hindering the action of the floats, by being accidentally jammed upon the sides of the joints.

Stays, crossing one another, proceed from the inner f.l.a.n.g.e of the centre to the outer ring of the wheel, and from the outer f.l.a.n.g.e of the centre to the inner ring of the wheel, with the view of obtaining greater stiffness. The floats are formed of plate iron, and the whole of the joints and joint pins are steeled, or formed of steel. For sea-going vessels the most approved practice is to make the joint pins of bra.s.s, and also to bush the eyes of the joints with bra.s.s; and the surface should be large to diminish wear.

634. _Q._--Can you give the dimensions of any other oscillating engines?

_A._--In Messrs. Penn's 50 horse power oscillating engine, the diameter of the cylinder is 3 feet 4 inches, and the length of the stroke 3 feet. The thickness of the metal of the cylinder is 1 inch, and the thickness of the cylinder bottom is 1-3/4 inch, crossed with feathers, to give it additional stiffness. The diameter of the trunnion bearings is 1 foot 2 inches, and the breadth of the trunnion bearings 5-1/2 inches. Messrs. Penn, in their larger engines, generally make the area of the steam trunnion less than that of the eduction trunnion, in the proportion of 32 to 37; and the diameter of the eduction trunnion is regulated by the internal diameter of the eduction pipe, which is about 1/5th of the diameter of the cylinder.

But a somewhat larger proportion than this appears to be expedient: Messrs.

Rennie make the area of their eduction pipes, in oscillating engines, 1/22d of the area of the cylinder. In the oscillating engines of the Oberon, by Messrs. Rennie, the cylinder is 61 inches diameter, and 1-1/2 inch thick above and below the belt, but in the wake of the belt it is 1-1/4 inch thick, which is also the thickness of metal of the belt itself. The internal depth of the belt is 2 feet 6 inches, and its internal breadth is 4 inches. The piston rod is 6-3/4 inches in diameter, and the total depth of the cylinder stuffing box is 2 feet 4 inches, of which 18 inches consists of a bra.s.s bush--this depth of bearing being employed to prevent the stuffing box or cylinder from wearing oval.

635. _Q._--Can you give any other examples?

_A._--The diameter of cylinder of the oscillating engines of the steamers Pottinger, Ripon, and Indus, by Miller & Ravenhill, is 76 inches, and the length of the stroke 7 feet. The thickness of the metal of the cylinder is 1-11/16 inch; diameter of the piston rod 8-3/4 inches; total depth of cylinder stuffing box 3 feet; depth of bush in stuffing box 4 inches; the rest of the depth, with the exception of the s.p.a.ce for packing, being occupied with a very deep gland, bushed with bra.s.s. The internal diameter of the steam pipe is 13 inches; diameter of steam trunnion journal 25 inches; diameter of eduction trunnion journal 25 inches; thickness of metal of trunnions 2-1/4 inches; length of trunnion bearings 11 inches; projection of cylinder jacket, 8 inches; depth of packing s.p.a.ce in trunnions, 10 inches; width of packing s.p.a.ce in trunnions, or s.p.a.ce round the pipes, 1-1/2 inch; diameter of crank pin 10-1/4 inches; length of bearing of crank pin 15-1/2, inches. There are six boilers on the tubular plan in each of these vessels; the length of each boiler is 10 feet 6 inches, and the breadth 8 feet; and each boiler contains 62 tubes 3 inches in diameter, and 6 feet 6 inches long, and two furnaces 6 feet 4-1/2 inches long, and 3 feet 1-1/2 inch broad.

636. _Q._--Is it the invariable practice to make the piston rod cap of bra.s.s in the way you have described?

_A._--In all oscillating engines of any considerable size, the cover of the connecting bra.s.s, which attaches the crank pin to the connecting rod, is formed of malleable iron; and the socket also, which is cuttered to the end of the piston rod, is of malleable iron, and is formed with a T head, through which bolts pa.s.s up through the bra.s.s, to keep the cover of the bra.s.s in its place.

637. _Q._--Is the piston of an oscillating engine made deeper than in common engines?

_A._--It is expedient, in oscillating engines, to form the piston with a projecting rim round the edge above and below, and a corresponding recess in the cylinder cover and cylinder bottom, whereby the breadth of bearing of the solid part of the metal will be increased, and in many engines this is now done.

638. _Q._--Would any difficulty be experienced in keeping the trunnions tight in a high pressure oscillating engine?

_A._--It is very doubtful whether the steam trunnions of a high pressure oscillating engine will continue long tight if the packing consists of hemp; and it appears preferable to introduce a bra.s.s ring, to embrace the pipe, cut spirally, with an overlap piece to cover the cut, and packed behind with hemp.

639. _Q._--How is the packing of the trunnions usually effected?

_A._--The packing of the trunnions, after being plaited as hard as possible, and cut to the length to form one turn round the pipe, is dipped into boiling tallow, and is then compressed in a mould, consisting of two concentric cylinders, with a gland forced down into the annular s.p.a.ce by three to six screws in the case of large diameters, and one central screw in the case of small diameters. Unless the trunnion packings be well compressed, they will be likely to leak air, and it is, therefore, necessary to pay particular attention to this condition. It is also very important that the trunnions be accurately fitted into their bra.s.ses by sc.r.a.ping, so that there may not be the smallest amount of play left upon them; for if any upward motion is permitted, it will be impossible to prevent the trunnion packings from leaking.

DIRECT ACTING SCREW ENGINE.

640. _Q._--Will you describe the configuration and construction of a direct acting screw engine?

_A._--I will take as an example of this species of engine, the engine constructed by Messrs. John Bourne & Co., for the screw steamer Alma, a vessel of 500 tons burden. This engine is a single steeple engine laid on its side, and in its general features it resembles the engines of the Amphion already described, only that there is one cylinder instead of two.

The cylinder is of 42 inches diameter and 42 inches stroke, and the vessel has been propelled by this single engine at the rate of fourteen miles an hour.

641. _Q._--Is not a single engine liable to stick upon the centre so that it cannot be started or reversed with facility?

_A._--A single engine is no doubt more liable to stick upon the centre than two engines, the cranks of which are set at right angles with one another; but numerous paddle vessels are plying successfully that are propelled by a single engine, and the screw offers still greater facility than paddles for such a mode of construction. In the screw engine referred to, as the cylinder is laid upon its side, there is no unbalanced weight to be lifted up every stroke, and the crank, whereby the screw shaft is turned round, consists of two discs with a heavy side intended to balance the momentum of the piston and its connections; but these counter-weights by their gravitation also prevent the connecting rod and crank from continuing in the same line when the engine is stopped, and in fact they place the crank in the most advantageous position for starting again when it has to be set on.

642. _Q._--Will you explain the general arrangement of the parts of this engine?

_A._--The cylinder lies on its side near one side of the vessel, and from the end of the cylinder two piston rods extend to a cross head sliding athwartships, in guides, near the other side of the vessel. To this cross head the connecting rod is attached, and one end of it partakes of the motion of the cross head or piston, while the other end is free to follow the revolution of the crank on the screw shaft.

643. _Q._--What is the advantage of two discs entering into the composition of the crank instead of one?

_A._--A double crank, such as two discs form with the crank pin, is a much steadier combination than would result if only one disc were employed with an over-hung pin. Then the friction on the neck of the shaft is made one half less by being divided between the two bearings, and the short prolongation of the shaft beyond the journal is convenient for the attachment of the eccentrics to work the valves.

644. _Q._--Will you enumerate some of the princ.i.p.al dimensions of this engine?

_A._--The bottom frame, on which also the condenser is cast, forms the base of the engine: on one end of it the cylinder is set; on the other end are the guides for the cross head, and in the middle are the bearings for the crank shaft. The part where the cylinder stands is two feet high above the engine platform, and the elevation to the centre of the guides or the centre of the shaft is 10 inches higher than this. The metal both of the side frames and bottom f.l.a.n.g.e is 1-1/4 inch thick. The cylinder has f.l.a.n.g.es cast on its sides, upon which it rests on the bottom frame, and it is sunk between the sides of the frame so as to bring the centre of the cylinder in the same plane as the centre of the screw shaft. The opening left at the guides for the reception of the guide blocks is 6 inches deep, and the breadth of the bearing surface is 11 inches. The cover of the guides is 8 inches deep at the middle, and about half the depth at the ends, and holes are cored through the central web for two oil cups on each guide. The bra.s.s for each of the crank shaft bearings is cut into four pieces so that it may be tightened in the up and down direction by the bolts, which secure the plummer block cap, and tightened in the athwartship direction, which is the direction of the strain, by s.c.r.e.w.i.n.g up a wedge-formed plate against the side of the bra.s.s, a parallel plate being applied to the other side of the bra.s.s, which may be withdrawn to get out the wedge piece when the shaft requires to be lifted out of its place. The air pump is bolted to one side of the bottom frame, and a pa.s.sage is cast on it conducting from the condenser to the air pump. In this pa.s.sage the inlet and outlet valves at each end of the air pump are situated, and appropriate doors are formed above them to make them easily accessible. The outlet pa.s.sage leading from the air pump communicates with the waste water pipe, through which the water expelled by the air pump is discharged overboard.

645. _Q._--Is the cylinder of the usual strength and configuration?

_A._--The cylinder is formed of cast iron in the usual way, and is 1-1/8 inch thick in the barrel. The ends are of the same thickness, but are each stiffened with six strong feathers. The piston is cast open. The bottom of it is 5/8ths of an inch thick, and it is stiffened by six feathers 3/4 of an inch thick; but the feather connecting the piston rod eyes is 1-1/4 inch thick, and the metal round the eyes is 2 inches thick. The piston is closed by a disc or cover 5/8ths of an inch thick, secured by 15 bolts, and this cover answers also the purpose of a junk ring. The piston packing consists of a single cast iron ring 3-1/2 inches broad, and 1/2 inch thick, packed behind with hemp. This ring is formed with a tongue piece, with an indented plate behind the cut; and the cut is oblique to prevent a ridge forming in the cylinder. The total thickness of the piston is 5-1/2 inches. The piston rods are formed with conical ends for fitting into the piston, but are coned the reverse way as in locomotives, and are secured in the piston by nuts on the ends of the rods, these nuts being provided with ratchets to prevent them from uns.c.r.e.w.i.n.g accidentally.

646. _Q._--What species of slide valve is employed?

_A._--The ordinary three ported valve, and it is set on the top of the cylinder. The cylinder ports are 4-1/2 inches broad by 24 inches long; and to relieve the valve from the great friction due to the pressure on so large a surface, a balance piston is placed over the back of the valve, to which it is connected by a strong link; and the upward pressure on this piston being nearly the same as the downward pressure on the valve, it follows that the friction is extinguished, and the valve can be moved with great case with one hand. The balance piston is 21 inches in diameter. In the original construction of this balance piston two faults were committed.

The pa.s.sage communicating between the condenser and the top of the balance piston was too small, and the pins at the ends of the link connecting the valve and balance piston were formed with an inadequate amount of bearing surface. It followed from this misproportion that the balance piston, being adjusted to take off nearly the whole of the pressure, lifted the valve off the face at the beginning of each stroke. For the escape of the steam into the eduction pa.s.sage momentarily impaired the vacuum subsisting there, and owing to the smallness of the pa.s.sage leading to the s.p.a.ce above the balance piston, the vacuum subsisting in that s.p.a.ce could not be impaired with equal rapidity. The balance piston, therefore, rose by the upward pressure upon it momentarily predominating over the downward pressure on the valve; but this fault was corrected by enlarging the communicating pa.s.sage between the top of the balance piston and the eduction pipe. The smallness of the pins at the ends of the link connecting the valve and balance piston, caused the surfaces to cut into one another, and to wear very rapidly, and the pins and eyes in this situation should be large in diameter, and as long as they can be got, as they are not so easily lubricated as the other bearings about the engine, and are moreover kept at a high temperature by the steam. The balance piston is packed in the same way as the main piston of the engine. Its cylinder, which is only a few inches in length, is set on the top of the valve casing, and a trunk projects upwards from its centre to enable the connecting link to rise up in it to attain the necessary length.

[Ill.u.s.tration: Fig 52. CONNECTING ROD. Messrs. Bourne & Co.]

647. _Q._--What is the diameter of the piston rods and connecting rod?

_A._--The piston rods, which are two in number, are 3 inches diameter, and 12 feet 10 inches long over all. They were, however, found to be rather small, and have since been made half an inch thicker. The connecting rod consists of two rods, which are prolongations of the bolts that connect the sides of the bra.s.s bushes which encircle the crank pin and cross head. The connecting rod is shown in perspective in fig. 52. The rods composing it are each 2-3/4 inches in diameter.

648. _Q._--Will you describe the configuration of the cross head.

_A._--The cross head, exhibited in fig. 53, is a round piece of iron like a short shaft, with two unequal arms keyed upon it, the longer of which _b_ works the air pump, and the shorter _c_ works the feed pump. The piston rods enter these arms at _a A._ The cross head is 8 inches diameter where it is embraced by the connecting rod at _e_, and 7 inches diameter where the air pump and feed pump arms are fixed on. The ends of the cross head _d d_, for a length of 12 inches, are reduced to 3 inches diameter where they fit into round holes in the centre of the guide blocks. Those blocks are of cast iron 6 inches deep, 11 inches wide, and 14 inches long, and they are formed with f.l.a.n.g.es 1 inch thick on the inner sides of the blocks. The projection of the air pump lever from the centre of the cross head is 1 foot 9 inches, and it is bent 5-3/4 inches to one side to enable it to engage the air pump rod. The eye of this arm is 6 inches broad and about 2 inches thick. At the part where one of the piston rods pa.s.ses through it, the arm is 8 inches deep and 6 inches wide; but the width thereafter narrows to 3 inches, and finally to 2 inches; and the depth of the web of the arm reduces from 8 inches at the piston rod, to 4 inches at the eye, which receives the end of the air pump rod. The feed pump arm is only 3 inches thick, and has 9 inches of projection from the centre of the cross head; but the eye attached to it on the opposite side of the cross head for the reception of the other piston rod is of the same length as that part of the air pump arm which one of the piston rods pa.s.ses through. The piston rods have strong nuts on each side of each of these arms to attach them to the arms, and also to enable the length of the piston rods to be suitably adjusted, to leave equal clearance between the piston and each end of the cylinder at the termination of the stroke.

[Ill.u.s.tration: Fig. 53.

CROSS HEAD AND PUMP ARMS. Messrs. Bourne & Co.]

649. _Q._--Will you recapitulate the main particulars of the air pump?

_A._--The air pump is made of bra.s.s 12-1/2 inches diameter and 42 inches stroke, and the metal of the barrel is 9/16ths of an inch thick. The air pump bucket is a solid piston of bra.s.s, 6-1/2 inches deep at the edge, and 7 inches deep at the eye; and in the edge three grooves are turned to hold water which answers the purpose of packing. The inlet and outlet valves of the air pump consist of bra.s.s plates 1/2 inch with strong feathers across them, and in each plate there are six grated perforations covered by india rubber discs 7 inches in diameter. These six perforations afford collectively an area for the pa.s.sage of the water equal to the area of the pump. The air pump rod is of bra.s.s, 2-1/2 inches diameter.

650. _Q._--What are the constructive peculiarities of the discs and crank pin?

_A._--The discs, which are 64 inches diameter, are formed of cast iron, and are 2-1/2 inches thick in the body, and 5 inches broad at the rim. The crank shaft is 8-1/2 inches diameter, and the central boss of the disc which receives the shaft measures 10 inches through the eye, and the metal of the eye is 3 inches thick. In the part of the disc opposite to the crank pin, the web is thickened to 10 inches for nearly the whole semicircle, with the view of making that side of the disc heavier than the other side; and when the engine is stopped, the gravitation of this heavy side raises the crank pin to the highest point it can attain, whereby it is placed in mid stroke, and cannot rest with the piston rods and connecting rod in a horizontal line. The crank pin is 8-1/2 inches diameter, and the length of the bearing or rubbing part of it is 16 inches. It is secured at the ends to the discs by f.l.a.n.g.es 18 inches diameter, and 2 inches thick. These f.l.a.n.g.es are indented into thickened parts of the discs, and are each attached to its corresponding disc by six bolts 2 inches diameter, countersunk in the back of the disc, and tapped into the malleable iron f.l.a.n.g.e. Besides this attachment, each end of the pin, reduced to 4-1/2 inches diameter, pa.s.ses through a hole in its corresponding disc, and the ends of the pin are then riveted over. The crank pin is perforated through the centre by a small hole about 3/4 of an inch in diameter, and three perforations proceed from this central hole to the surface of the pin. Each crank shaft bearing is similarly perforated, and pipes are cast in the discs connecting these perforations together. The result of this arrangement is, that a large part of the oil or water fed into the bearings of the shaft is driven by the centrifugal action of the discs to the surface of the crank pin, and in this way the crank pin may be oiled or cooled with water in a very effectual manner. To intercept the water or oil which the discs thus drive out by their centrifugal action, a light paddle box or splash board of thin sheet bra.s.s is made to cover the upper part of each of the discs, and an oil cup with depending wick is supported by the tops of these paddle boxes, which wick is touched at each revolution of the crank by a bridge standing in the middle of an oil cup attached to the crank pin. The oil is wiped from the wick by the projecting bridge at each revolution, and subsides into the cup from whence it proceeds to lubricate the crank pin bearing. This is the expedient commonly employed to oil the crank pins of direct acting engines; but in the engine now described, there are over and above this expedient, the communicating pa.s.sages from the shaft bearings to the surface of the pin, by which means any amount of cooling or lubrication can be administered to the crank pin bearing, without the necessity of stopping or slowing the engine.

[Ill.u.s.tration: Fig. 54. DOUBLE DISC CRANK. Messrs. Bourne & Co.]

651. _Q._--What is the diameter of the screw shaft?

_A._--The screw shaft is 7-1/2 inches diameter, but the bearings on each side of the disc are 8-1/2 inches diameter, and 16 inches long. Between the side of the disc and the side of the contiguous bearings there is a short neck extending 4-3/4 inches in the length of the shaft, and hollowed out somewhat to permit the pa.s.sage of the piston rod; for one piston rod pa.s.ses immediately above the shaft on the one side of the discs, and the other piston rod pa.s.ses immediately below the shaft on the other side of the discs. A short piece of one piston rod is shown in fig. 54.