A Catechism of the Steam Engine

by C. E. John Bourne

A.--The tube plates are generally made from five eighths to three fourths of an inch thick, but seven eighths of an inch thick appears to be preferable, as when the plate is thick the holes will not be so liable to change their figure during the process of feruling the tubes: the distance between the tubes should never be made less than three fourths of an inch, and the holes should be slightly tapered so as to enable the tubes to hold the tube plates together. The tubes are secured in the tube plates by means of taper ferules driven into the ends of the tubes. The ferules are for the most part made of steel at the fire box end, and of wrought iron at the smoke box end, though ferules of malleable cast iron have in some cases been used with advantage: malleable cast iron ferules are almost as easily expanded when hammered cold upon a mandrel, as the common wrought iron ones are at a working heat. Spring steel, rolled with a feather edge, to facilitate its conversion into ferules, is supplied by some of the steel- makers of Sheffield, and it appears expedient to make use of steel thus prepared when steel ferules are employed. In cases where ferules are not employed, it may be advisable to set out the tube behind the tube plate by means of an expanding mandrel. There are various forms of this instrument. One form is that known as Prosser's expanding mandrel, in which there are six or eight segments, which are forced out by means of a hexagonal or octagonal wedge, which is forced forward by a screw. When the wedge is withdrawn, the segments collapse sufficiently to enable them to enter the tube, and there is an annular protuberance on the exterior circle of the segments, which protuberance, when the mandrel is put into the tube, just comes behind the inner edge of the tube plate. When the wedge is tightened up by the screw, the protuberance on the exterior of the segments composing the mandrel causes a corresponding bulge to take place in the tube, at the back of the tube plate, and the tube is thereby brought into more intimate contact with the tube plate than would otherwise be the case. There is a steel ring indented into the segments of Prosser's mandrel, to contract the segments when the central wedge is withdrawn. A more convenient form of the instrument, however, is obtained by placing the segments in a circular box, with one end projecting; and supporting each segment in the box by a tenon, which fits into a mortise in the cylindrical box. To expand the segments, a round tapered piece of steel, like a drift, is forced into a central hole, round which the segments are arranged. A piece of steel tube, also slit up to enable a central drift to expand it, answers very well; but the thickness of that part of the tube in which there requires to be spring enough to let the mandrel expand, requires to be sufficiently reduced to prevent the pieces from cracking when the central drift is driven in by a hammer. The drift is better when made with a globular head, so that it may be struck back by the hammer, as well as be driven in. An expanding mandrel, with a central drift, is more rapid in its operation than when the expansion is produced by means of a screw.

421. Q.--Will you explain the means that are adopted to regulate the admission of steam to the cylinders?

A.--In locomotives, the admission of the steam from the boiler to the cylinders is regulated by a valve called the regulator, which is generally placed immediately above the internal fire box, and is connected with two copper pipes;--one conducting steam from the highest point of the dome down to it, and the other conducting the steam that has passed through it along the boiler to the upper part of the smoke box. Regulators may be divided into two sorts, viz., those with, sliding valves and steam ports, and those with conical valves and seats, of which the latter kind are the best. The former kind have for the most part consisted of a circular valve and face, with radial apertures, the valve resembling the outstretched wings of a butterfly, and being made to revolve on its central pivot by connecting links between its outer edges, or by its central spindle. In some of Stephenson's engines the regulator consists of a slide valve covering a port on the top of the valve chests. A rod passes from this valve through the smoke box below the boiler, and by means of a lever parallel to the starting lever, is brought up to the engineer's reach. Cocks were at first used as regulators, but were given up, as they were found liable to stick fast. A gridiron slide valve has been used by Stephenson, which consists of a perforated square moving upon a face with an equal number of holes. This plan of a valve gives, with a small movement, a large area of opening. In Bury's engines a sort of conical plug is used, which is withdrawn by turning the handle in front of the fire box: a spiral grove of a very large pitch is made in the valve spindle, in which fits a pin fixed to the boiler, and by turning the spindle an end motion is given to it, which either shuts or opens the steam passage according to the direction in which it is turned. The best regulator would probably be a valve of the equilibrium description, such as is used in the Cornish engine: there would be no friction in such a regulator, and it could be opened or shut with a small amount of force. Such valves, indeed, are now sometimes employed for regulators in locomotives.

CHAPTER VIII.

CONSTRUCTIVE DETAILS OF ENGINES.

PUMPING ENGINES.

422. Q.--Will you explain the course of procedure in the erection of a pumping engine, such as Boulton and Watt introduced into Cornwall?

A.--The best instructions on this subject are those of Mr. Watt himself, which are as follows:--Having fixed on the proper situation of the pump in the pit, from its centre measure out the distance to the centre of the cylinder, from which set off all the other dimensions of the house, including the thickness of the walls, and dig out the whole of the included ground to the depth of the bottom of the cellar, so that the bottom of the cylinder may stand on a level with the natural ground of the place, or lower, if convenient, for the less the height of the house above the ground, the firmer it will be. The foundations of the walls must be laid at least two feet lower than the bottom of the cellar, unless the foundation be firm rock; and care must be taken to leave a small drain into the pit quite through the lowest part of the foundation of the lever wall, to let off any water that may be spilt in the engine house, or may naturally come into the cellar. If the foundation at that depth does not prove good, you must either go down to a better if in your reach, or make it good by a platform of wood or piles, or both.

423. Q.--These directions refer to the foundations?

A..--Yes; but I will now proceed to the other parts. Within the house, low walls must be built to carry the cylinder beams, so as to leave sufficient room to come at the holding down bolts, and the ends of these beams must also be lodged in the wall The lever wall must be built in the firmest manner, and run solid, course by course, with thin lime mortar, care being taken that the lime has not been long slaked. If the house be built of stone, let the stones be large and long, and let many headers be laid through the wall: it should also be a rule, that every stone be laid on the broadest bed it has, and never set on its edge. A course or two above the lintel of the door that leads to the condenser, build into the wall two parallel flat thin bars of iron equally distant from each other, and from the outside and inside of the wall, and reaching the whole breadth of the lever wall. About a foot higher in the wall, lay at every four feet of the breadth of the front, other bars of the same kind at right angles to the former course, and reaching quite through the thickness of the wall; and at each front corner lay a long bar in the middle of the side walls, and reaching quite through the front wall; if these bars are 10 feet or 12 feet long it will be sufficient. When the house is built up nearly to the bottom of the opening under the great beam another double course of bars is to be built in, as has been directed. At the level of the upper cylinder beams, holes must be left in the walls for their ends, with room to move them laterally, so that the cylinder may be got in; and smaller holes must be left quite through the walls for the introduction of iron bars, which being firmly fastened to the cylinder beams at one end, and screwed at the other or outer end, will serve, by their going through both the front and back walls, to bind the house more firmly together. The spring beams or iron bars fastened to them must reach quite through the back wall, and be keyed or screwed up tight; and they must be firmly fastened to the lever wall on each side, either by iron bars, firm pieces of wood, or long strong stones, reaching far back into the wall. They must also be bedded solidly, and the residue of the opening must be built up in the firmest manner.