Page:A Treatise on the Steam Engine (1847).djvu/187

Varieties of the Steam Engine. catch-pins at both ends of the beam. Winches are placed above the cylinder, and pumps to raise the piston, and pump buckets easily. There is a prejudice among the Cornish engineers to metallic packing in the pistons, and the use of hemp is attended with much trouble, from the continual screwing down and renewal it requires, especially if the pressure of the steam be considerable ; but this prejudice is now wearing away, and metallic packing is gaining a gradual introduction. Notwithstanding all the ingenuity, however, expended upon the Cornish engine, and the very efficient performance to which it has attained, it appears to us to stand on the verge of early supercession, and we question whether it can maintain its ground for many years. The centrifugal pump is now under trial as a pump for the drainage of mines, and should the results which have been realized be confirmed by more extended experience, there cannot be a doubt that the whole of our present system of raising water must undergo a revolution. Small engines working at a high speed will then perform the functions of the cumbrous and lethargic pumping engine ; smaller pipes will suffice, as the flow of water through them will be continuous, and pump valves, with which there is much trouble at present, will cease to be necessary. The present Cornish engines, though rudely made, are very expensive, as there is an immense quantity of iron in them ; besides which, the engine-house and foundations require to be so strong and massive, that they add largely to the cost.

THE ROTATIVE ENGINE.

Of the rotative land-engine, the sixteen-horse power engine of Messrs. Maudslay & Field, represented in plate 8, may be taken as a good example. It is our conviction, however, that this species of engine must go altogether out of use, as several kinds of direct-action engine are far more simple and compact, and appear to be in every way entitled to a preference. One of them is the oscillating engine ; another is the species of engine attached to Robinson's cane-mill, represented in plate 9, and there are several other varieties. We shall not, therefore, here dwell upon any description of the ordinary rotative or mill engine, as we look upon it now as a mere piece of antiquity, and although as a necessary part of our plan, we give in pages 206-214, a full account of the details of the ordinary beam rotative engine in its most approved form, we have not a word to say in commendation of that species of engine. The jacket in Messrs. Maudslay's engine, plate 8, it will be observed, is cast in the same piece with the cylinder. The valve casing is provided with packing ports on the back, upon the lowermost of which the blow-through valve seat is cast. The condenser is situated in the cold water cistern, beneath the cylinder. Mr. Fairbairn's 60-horse power engine, plates 10 and 11, has the peculiarity of a toothed fly-wheel : in the rest of the arrangements there is little unusual. In some cases the double-cylinder engine is used for driving mill-work, and some very excellent specimens of this variety of engine have been produced by Messrs. Renuie, Messrs. Hall of Dartford, and Mr. Hick of Bolton. In engines for driving cotton-mills, the double cylinder plan may perhaps be an expedient one, for then the equability of motion is of the highest importance ; but in ordinary cases, where expaansion is desired, it appears to be the preferable way to use either two smaller engines, set at right angles on the shaft, or a single cylinder, with a heavier and swifter fly-wheel. There cannot be a doubt, moreover, that engines working at twice the usual speed, after the manner we have already recommended, must come into use for driving cotton mills ; and if the speed be greater, the expansion may be carried further, without impairing the regularity of motion. The details of this species of engine we need not here enter upon, as the neccssary information respecting them will be found in pages 206 to 215.

THE MARINE ENGINE

The marine engine has now become the most important variety of the steam engine, not merely on account of the great extension of steam navigation, but because it is fast superseding the ordinary steam engine, even for land purposes. We shall therefore enter into the consideration of the structure and operation of this engine with considerable fulness of detail, and much of what we say will also be found to illustrate the merits of the other varieties of engine. Of the side lever marine engine, the engines of the City of London steamer, by Mr. Robert Napier, plate 12, may be taken as an illustration. We may take this engine as a text to suggest the few remarks upon side lever engines we here desire to record, postponing to a more advanced stage of our progress, (pages 215 to 230), any extended enquiry into the details of that description of engine.

The framing shown in plate 12. is of malleable iron. This is a wise innovation, for it was a difficult thing to prevent cast iron framing from being broken by the working of the ship, though we believe Mr. Robert Napier's framing was more exempt from such accidents than that of any other maker. This malleable iron framing is a judicious one ; yet we think it might have been an improvement if the diagonal stays had run on to the cylinder, the parallel and valve motion shafts being supported on a column tied to the diagonal stay by a bar running from the top of the column to the junction of the lower diagonal stay, with the crank shaft pillar adjoining the air-pump. Tbe piston rod is secured into the cross head with a nut on the top, as well as a cutter through, w hich is a good practice. There is too little taper in the part of the rod which fits into the cross head, so that it will jam, and cannot be got out without great difficulty in the event of such disconnexion being wanted. There is also too little taper in the pistonrod where it passes through the piston, though there is a counter-sink to take the strain, which will prevent the piston rod from splitting the piston. If the cone on the piston rod end be made slight, and nothing be added to take the strain, the rod will be drawn up through the hole, and the piston will be split: this accident has happened to several of Mr. R. Napier's pistons. The piston packing consists of a double tier of rings. We think a single ring turned, of an eccentric form, and fitted with a tongue-piece, to be a preferable packing.

The valve is of the long D variety, of which we do not approve much for large engines, and the method of packing it is not convenient. There are no packing ports formed in the back of the valve casing, but the packing has to be put in from the inside, and the valve has to be drawn every time that it is packed. It is a defect we conceive for the eduction passage to enter the condenser at so low a level. The faucett joint in the valve casing is a good arrangement, and ought to be applied to all engines above a moderate size. The air-pump bucket is uuprovided with a junkring for screwing down the packing, the want of which is, in large engines, a weighty fault. The bucket valve is of the common pot-lid description, which strikes hard. There are two delivery valves, one in the mouth of the pump, and the other in the hot well. We do not see the use of this multiplication, especially when there are engines working well without any delivery-valve at all. The same objection that we brought against the want of sufficient cone in the ends of the piston rod applies also to the airpump rod. The connecting-rod is needlessly heavy: the connecting-rod at the smallest part need not be so thick as the piston rod, and here it is much thicker. The starting gear in the City of London, consists merely of a long lever, by which it is difficult to obtain sufficient power for moving so large a valve : a great number of men are required to start the engine, and the travel of the starting lever is so great, when the engine is thrown into gear, as to be a source of danger to persons in the vicinity. This is not a common fault of Mr. R. Napier's engines; in the engines of the Precursor, the starting gear is the most elegant and convenient that has come under our observation; and the engines of the City of London, notwithstanding the slight imperfections we have mentioned, are sound and good engines in most respects, and will, we have no doubt, cost but little for repair. In the Precursor and British Queen steamers, the engines of both of which vessels are by Mr. R. Napier, the starting gear consists of a wheel, like the steering wheel of a ship, which, by means of a pinion working into a sector on the valve-shaft, gives motion to the valve, and the power is thus so multiplied that one man can start the engines. To prevent inconvenience from being experienced by the engineer from the rapid whirling round of the starting-wheel, when the engines are thrown into gear, the disconnecting apparatus it is so contrived that the act of throwing the eccentric rod into gear, throws the starting wheel out of gear, and vice versa. In the Precursor the eccentric rod is thrown out of gear by means of a lever, with a pulley on the end of it, and which, being forced up against the under side of the eccentric rod, lifts the rod out of a notch. The centre of this lever is attached to an eccentric stud in the framing, which is so contrived, that when the lever is in the position which allows the eccentric rod to fall into gear, the eccentric stud draws back the valve-shaft, so that the teeth of the pinion are no longer in gear with the teeth of the sector; but when the lever is forced up into the position which throws the eccentric rod out of gear, the valve shaft is pushed forward until the connexion of the pinion and sector is again established. It is very desirable that this or some equivalent contrivance should be adopted in all engines where the starting handle has a large travel, as the rapid movements of the starting handle, when the engine is thrown into gear, will otherwise be a source of much danger to the engine attendants.

The engines of the steamers Clyde, Tweed, Tay, and Teviot, plate 13, by Messrs. Caird and Co., are clear of many of the defects which we have just mentioned, but they have others of their own. Of these, the most remarkable is the obstruction offered by the valve to the entrance of the steam into the valve casing, when the valve is at the lower part of its stroke, so that during the upward stroke of the piston the steam is effectually throttled. This, of course, was an oversight of construction, but it is one that must seriously impair the efficacy of the engines. The diagonal stay appears to be too weak where it joins the cylinder. It is a bad plan to have the main centre going through the condenser, as it is difficult to keep the joint tight, unless a pipe be cast in for putting the main centre through; and if that be done, the inequality of contraction in the metal makes the sides of the condenser very liable to be cracked, by heating with steam in blowing through, and cooling with cold water when the injection is admitted. Many of Messrs. Caird's condensers have been cracked in this way. It appears to us preferable to make the condenser in the form of a large and tall pipe, the main centre being supported by pillow blocks of a suitable construction; and this is the plan followed by the London engineers in their side lever marine engines.

Paddle-wheels. If a body moves through a quiescent fluid with a given velocity, or if a fluid moving with a given velocity impinge against a body at rest, the resistance will in either case be as the square of the velocity, while the power requisite to overcome that resistance will be as the cube