to be elevated and lowered. These two last are undoubtedly the most powerful of the examples given. The systems at present adopted for guiding the slide valve rod are of three kinds. First, the dove-tailed guide, similar to that used by tool makers for the arm of a shaping machine. Second, a block of gun metal sliding on two fixed turned rods as guides over and under the valve rod. Third, the valve rod secured to a square bar, working in a bracket, and cap to correspond. This last may be said to be the most simple, but perhaps not so rigid as the first example. The double guides are complicated, but at the same time produce the rigid resistance to the strains imposed on the valve rod by the vibration of the link. Some makers of marine engines prefer to allow the link to rest or hang on the block pin inserted in the lever of the slide rod weigh-shaft. Such a practice dispenses with guides. Excessive vibration of the link on or in its block greatly deteriorates the action of the valve, it being understood that whilst the link has an ascending or descending motion, as well as sliding, the strain on the valve rod is increased, and at the same time the stroke is effected. The excess of the vibratory motion is painfully perceptible in the ordinary slotted link; the eccentric rods being connected beyond the block pin, a direct-action cannot ensue. The distance between the centres of the eccentric rods and block regulates the amount of indirect action. Links of this kind are often hung from a rod connected in the centre to the link, either to the clip or at the back. This is far better than at the lower end, as the connexion of the suspension rod regulates the ascending and descending motion of the link whilst at work. The link resting on the block when for going ahead, obviates to a certain extent some of the evils alluded to. The gain by the introduction of the solid link, with the eccentric rods connected at its extremities, is strength with less material, but the vibrating motion is not decreased. In order to obtain a more direct, and, if possible, a perfect action, the eccentric rods have been secured to the link, so that the centre of connexion may be on that of the block, and by this the vibratory motion is effectually got rid of. There have been two distinct modes for accomplishing this, which I have had the opportunity of observing. The first example is-two solid links, one on each side of the block, the eccentric rods being connected to pins on the outer face of each link, the inner face and sides being sustained in a groove in the block, which oscillates on its axis, in the eye of the valve rod, the links being one on each side. The second example is like the first in principle, but one solid link only is used, of a dove-tailed form in section, at the inner face, to prevent the link from slipping out of the groove in the block; the eccentric pins are fixed in the extremities of the link, and the rods are attached as in the last example, but with a single eye. The writer has designed a solid link and connexion, which, although not superior in principle of action to the two last examples, is more simple in construction, and has less working portions; therefore it may be held to be worthy of introduction. A solid bar of iron is slotted at each end, to receive the single eye of each eccentric rod, so that the entire surface of the link remains unbroken; it is secured in a block with an adjusting portion and key at the back, the front being open sufficiently to admit of the ascent and descent of the eccentric rods; adjustment in front can be obtained by loose portions and setscrews, but this last is not imperative, as the wear of the link and block is very slight when the acting eccentric rod is on the centre of the block; the block has provisions on each side for suspension, the valve rod having portions formed to receive the block; the back part of the rod works in a dove-tailed guide of the ordinary kind. It now becomes necessary to treat of the suspension or lifting rods for solid links; for this a few words will suffice. As the ascent and descent of the link whilst in motion are governed by the length and position of the rod, it is almost needless to state that the suspension rod should be connected in the centre of the connexion of the eccentric rod. The link, when for going ahead, should be down. It may now be argued that the vibration of the link, when for going astern, must be excessive. Granted; but as the forward motion of the ship is of the most importance, it is not unfavorable to economy to adopt the connexion alluded to. In some cases the solid link is guided at the top or bottom, but this is only required when an overhanging or outside connexion of the eccentric rods is resorted to. The next portion for consideration is the expansion valve and gear; the use of this valve is to allow the steam to be cut off at the early or given part of the stroke of the piston, and the expansion or elasticity of the steam completes the power required. Now, it is certain that the use of high pressure steam for large cylinders and short strokes, produces excessive shocks at the commencement of the strokes, and thereby entails an increase of strength in the materials used, so that the proportions are larger than when for ordinary purposes. It is clear also that, when steam is admitted at an excessive pressure against the piston suddenly, it (the piston) receives an impetus equivalent to the power imposed, and in no case whatever could an engine of proportions for low pressure resist the strains imposed by the use of high pressure steam. The ordinary pressure adopted by marine engineers is from 20 to 30 lbs. per square inch, more often the former than the latter. I am not aware, however, of any cause why 60 to 80 lbs. should not be adopted, with a great increase of economy and power. Of course, the present proportions of engines and boilers would have to be increased, if the same materials were used, but steel boilers, shafts, and rods might be introduced with considerable advantage, embracing great strength with less weight. Having alluded to the ordinary pressures at present used, it will be well now to advert to the expansion valves. These valves are of three kinds-throttle, slide, and tubular. The motions imparted to the throttle valve are oscillating and revolving. The latter is now most generally adopted, but with this disadvantage, that the action is equal both for supply and cutting off. The slide valves are of the ordinary and gridiron type, the latter may be said to be the better on account of the stroke being so short in comparison to that of the former. Tubular valves are tubes inserted in each other, with ports to correspond, a sliding or rotary motion accomplishing the desired effect. The motions imparted to these several valves are generally uniform, either by mitre gearing or eccentrics, consequently the action of the valves is not perfect. The proper motion for an expansion valve is to open gradually and close suddenly; to obtain this the old but correctly working cam must be resorted to; this useful arrangement is too often discarded to make place for newer but less correct productions. It may, of course, be urged that the cam is not applicable for high velocities, but undoubtedly its use might be attained by introducing stiff gear and perfect equilibrium double beat valves; by dividing this valve centrally a more correct action can be attained, in relation to that of the steam, on the valve whilst closed and open. The merits and demerits of the expansion valves here alluded to are almost equal. The ordinary throttle valve has less friction than any yet introduced, but it possesses the great evil of throttling the steam when closing; also when this valve is worked by levers, or has a vibratory motion, should the stroke be lessened, the full area cannot be attained. The last evil is dispensed with in the remaining example, as the ports or openings are much larger than required when the valve is at full stroke, and not too small when the least motion is given. The friction of the gridiron valve is perhaps in excess of the other examples, as in the case of the tubular valves, the action of the steam is neutralized. The means adopted for altering the grades of expansion valves whilst in motion are various. A spiral motion is the one universally adopted, and there is not the least doubt it is correct. I will now call attention to the following description of an expansion valve and gear which I have designed for high velocities: A cylindrical casing has within it projections at given positions; two of these projections act as spaces between the ports of ordinary tubular valves. The valve now explained is tubular, but the area centrally is half of that of the ends, which are parallel for given lengths, due to the stroke of the valve. These parallel lengths also regulate the neutrality of the valve whilst in action. At the present time the means adopted to impart the motion is a disk of metal with a circular slot; within this slot is a brass nut into which is screwed a pin. The connecting rod of the valve is attached to the pin in the ordinary manner. The means for altering the grades of expansion is by loosening the pin by its handle, and allowing the nut to slide in the slot to the required position. It is almost needless to add that the steam enters at the side of the casing, and escapes around and through the valve, keeping it in equilibrium. The valves next in requisition are those for the ends of the cylinder, commonly known as relief valves. The usual kinds adopted are disks, with springs or weights to resist the given pressure of the steam. The action of these valves is, of course, due to the excess of pressure within the cylinder over that of the resistance caused by the springs or weights. It has been proved that in the case of excessive priming of the boilers, the cylinders are suddenly flooded; in order to release the water, cocks are sometimes used, but in many instances the springs or weights are lifted by levers. Now, in the case of cocks, if not provided with valves beyond them they must be worked by hand at each return stroke of the engine, or the vacuum will be destroyed. The spring valves will close naturally, or by the spring on its release from the hand lever. High pressure steam has been lately introduced, with great advantage, in the place of springs, but with an entirely differently arranged valve and casing. I have arranged a relief valve, so that the spring is not tampered with by levers or hand power, and an instantaneous opening can be effected without cocks, &c. The spring valve has an opening in it centrally to receive on its outer side a flat disk, termed the vacuum valve. On the inner side is a provision mitred to receive a solid disk valve, which, on being pressed inwards by a spindle and lever, allows a free exit for the steam and water. On a vacuum being caused the vacuum valve, which is guided on the spindle alluded to, closes the opening air-tight; by this it will be understood that the spring has not been in requisisition, but on closing the inner disk the spring valve becomes one of the ordinary kind. Previously to starting the enginesit is well known a vacuum should be caused in the condensers, also the cylinders and slide casings should be warmed, and the condensed water be allowed to escape through the relief valves and cocks. The valves used for the purpose alluded to are termed the "blowthrough valves." It may be here observed that, in some cases, the ordinary plug-cock is preferred for this purpose. When valves are introduced, they are generally of the ordinary disk kind, but one firm adopts a common slide valve for the purpose, with the advantage of simplicity of levers, &c., and easy manipulation. The piston rods of marine engines are subject to excessive strain; consequently, the use of guides is imperative. For the single piston rod engine, the universal system is a channel underneath the rod, the guide block being generally of gun metal, and the upper portion attached to the piston rod by bolts Its and nuts. For double piston rod engines, the guides are of two kinds: the first arrangement is as the last, and the second, as for high pressure engines, or double guides. To say which is the preferable mode of arrangement of guides will perhaps be deemed bold, but I may venture to state that I deem that for the single piston rod the best of any yet introduced. I cannot close this portion of the present paper without alluding to the admirable arrangement for tightening the gland of the piston rod stuffing boxes, introduced by the firm of Messrs. Maudslay, Sons & Field. The screws are of the ordinary kind, but, in the place of nuts, worm wheels are used, worms being fitted to correspond; and motion can be given by a box spanner while the engines are at work. This is one of the most important improvements tending to accelerate the progress of a ship during a voyage, say three or four months. Imagine the engines requiring stoppage during a gale in order to tighten VOL. L. THIRD SERIES. - No. 4. - ОстOBER, 1865, 21 the glands, and a fair estimation can be formed of the value of the improvement alluded to. Having commented, though somewhat briefly, on the cylinder appendages, attention may now be given to the main frames and crank shaft. The main frames may be said to undergo a continuous strain, and must, consequently, be of a certain strength in order to preserve the requisite rigidity. The cylinder is attached to the one end, and the condenser at the other, whilst the crank shaft has to be supported in its bearings. Not many years ago a celebrated firm used to make the condenser and main frame in one casing: since that, we have had the well known frame like the letter A laid on its side, also the hollow frame, with a raised projection for the crank shaft, and a stay from the upper portion connected to the cylinder; this last may be said to be the most simple, and, at the same time, of less material than the A frame. As before stated, the strains on the frames are continuous, yet, when sudden shocks occur, from the racing of the engines or priming of the boilers, the tenacity of the cast iron is severly tested. As this is the case, wrought iron might be used with great advantage, both as to increase of strength and decrease of weight. The crank shafts of marine engines are generally of wrought iron, in one mass, the cranks being double, and forged with the shafts. Three bearings are deemed imperative, so as to equally distribute the strains. Now, this is correct in theory and practice, and the writer will be deemed committing a grave error no doubt in mathematics, when he assumes that the forward frame and half crank can be dispensed with, in order to reduce the weight and material. He is, of course, aware that the thrust and pull of the connecting rod will be thrown on the centre crank and bearing, but, in order to counteract this, the length and diameter of the shaft at that part should be increased. He would also prefer, in this case, to extend the frame and connect the upper portion to the condenser, the cap being on the top instead of at the end, as now used. Screws might be employed to adjust the side brasses; the eccentrics could be within the cranks, or between them and the bearings. (To be continued.) Roof of the London Terminus of the Midland Railway. From the London Mechanics' Magazine, June, 1865. Up to the present time the roof of the Imperial Riding House, at Moscow, has held its place as the largest ever executed, its span being 235 feet. But we understand that it is contemplated to cover in the London terminus of the Midland Railway with a roof of wrought iron 240 feet clear span. It will be of an arch construction, springing from the level of rails, and having a versed sine of about 100 feet. When built, this roof will, therefore, rank as the largest of one span in the world. |