SMOKE-JACK. This ingenious machine is of German origin, and Messinger, in his Collection of Mechanical Performances, says it is very ancient, being represented in a painting at Nuremberg, which is known to be older than the year 1350.
Its construction is abundantly simple. An upright iron spindle GA (fig. 5.), placed in the narrow part of the kitchen chimney, turns round on two points H and I. The upper one H passes through an iron bar, which is built in across the chimney; and the lower pivot I is of tempered steel, and is conical or pointed, resting in a conical bell metal socket fixed on another cross bar. On the upper end of the spindle is a circular fly G, consisting of 4, 6, 8, or more thin iron plates, set obliquely on the spindle like the sails of a windmill, as we shall describe more particularly by and by. Near the lower end of the spindle is a pinion A, which works in the teeth of a contrate or face wheel B, turning on a horizontal axis BC. One pivot of this axis turns in a cock fixed on the cross bar, which supports the lower end of the upright spindle HI, and the other pivot turns in a cock fixed on the side wall of the chimney; so that this axle is parallel to the front of the chimney. On the remote end of this horizontal axle there is a small pulley C, having a deep angular groove. Over this pulley there passes a chain CDE, in the lower bight of which hangs the large pulley E of the spit. This end of the spit turns loosely between the branches of the fork of the rack or rax F, but without resting on it. This is on the top of a moveable stand, which can be shifted nearer to or farther from the fire. The other end turns in one of the notches of another rack. The number of teeth in the pinion A and wheel B, and the diameters of the pulleys C and E, are so proportioned that the fly G makes from 12 to 20 turns for one turn of the spit.
The manner of operation of this useful machine is easily understood. The air which contributes to the burning of the fuel, and passes through the midst of it, is greatly heated, and expanding prodigiously in bulk, becomes lighter than the neighbouring air, and is therefore pushed by it up the chimney. In like manner, all the air which comes near the fire is heated, expanded, becomes lighter, and is driven up the chimney. This is called the draught or suction, but would with greater propriety be termed the drift of the chimney. As the chimney gradually contracts in its dimensions, and as the same quantity of heated air passes through every section of it, it is plain that the rapidity of its ascent must be greatest in the narrowest place. There the fly G should be placed, because it will there be exposed to the strongest current. The air, striking the fly vanes obliquely pushes them aside, and thus turns them round with a considerable force. If the joint of meat is exactly balanced on the spit, it is plain that the only resistance to the motion of the fly is what arises from the friction of the pivots of the upright spindle, the friction of the pinion and wheel, the friction of the pivots of the horizontal axis, the friction of the small end of the spit, and the friction of the chain in the top pulleys. The whole of this is but a mere trifle. But there is frequently a considerable inequality in the weight of the meat on different sides of the spit: there must therefore be a sufficient overplus of force in the impulse of the ascending air on the vanes of the fly, to overcome this want of equilibrium occasioned by the unskillfulness or negligence of the cook. There is, however, commonly enough of power when the machine is properly constructed. The utility of this machine will, we hope, procure us the indulgence of some of our readers, while we point out the circumstances on which its performance depends, and the maxims which should be followed in its construction.
The upward current of air is the moving power, and should be increased as much as possible, and applied in the most advantageous manner. Every thing will increase the current which improves the draught of the chimney and secures it from smoking. A smoky chimney must always have a weak current. For this particular, therefore, we refer to what has been delivered in the article PNEUMATICS, No 359; and the article SMOKE.
With respect to the manner of applying this force, it is evident that the best construction of a windmill sails will be nearly the best construction for the fly. According to the usual theory of the impulse of fluids, the greatest effective impulse (that is, in the direction of the fly's motion) will be produced if the plane of the vane be inclined to the axis in an angle of 54 degrees 46 minutes. But, since we have pronounced this theory to be so very defective, we had better take a determination founded on the experiments on the impulse of fluids made by the academy of Paris. These authorise us to say, that 49 or 50 degrees will be the best angle to give the vane: but this must be understood only of that part of it which is close adjoining to the axis. The vane itself must be twisted, or weathered as the millwrights term it, and must be much more oblique at its outer extremity. The exact position cannot be determined with any precision; because this depends on the proportion
proportion of the velocity of the vane to that of the current of heated air. This is subject to no rule, being changed according to the load of the jack. We imagine that an obliquity of 65 degrees for the outer ends of the vanes will be a good position for the generality of cases. Messinger describes an ingenious contrivance for changing this angle at pleasure, in order to vary the velocity of the motion. Each vane is made to turn round a midrib, which stands out like a radius from the spindle, and the vane is moved by a stiff wire attached to one of the corners adjoining to the axle. These wires are attached to a ring which slides on the spindle like the spreader of an umbrella; and it is stopped on any part of the spindle by a pin thrust through a hole in the spindle and ring. We mention this briefly, it being easily understood by any mechanic, and but of little consequence, because the machine is not susceptible of much precision.
It is easy to see that an increase of the surface of the vanes will increase the power: therefore they should occupy the whole space of the circle, and not consist of four narrow arms like the sails of a windmill. It is better to make many narrow vanes than a few broad ones; as will appear plain to one well acquainted with the mode of impulse of fluids acting obliquely. We recommend eight or twelve at least; and each vane should be so broad, that when the whole is held perpendicular between the eye and the light, no light shall come through the fly, the vanes overlapping each other a very small matter. We also recommend making them of stiff plate. Their weight contributes to the steady motion, and enables the fly, which has acquired a considerable velocity during a favourable position of things, to retain a momentum sufficient to pull round the spit while the heavy side of the meat is rising from its lowest position. In such a situation a light fly soon loses its momentum, and the jack staggers under its load.
It is plain, from what has been said, that the fly should occupy the whole of that section of the vent where it is placed. The vent must therefore be brought to a round form in that place, that none of the current may pass uselessly by it.
It is an important question where the fly should be placed. If in a wide part of the vent, it will have a great surface, and act by a long lever; but the current in that place is slow, and its impulse weak. This is a fit subject of calculation. Suppose that we have it in our choice to place it either as it is drawn in the figure, or farther up at , where its diameter must be one half of what it is at . Since the same quantity of heated air passes through both sections, and the section has only one-fourth of the area of the section , it is plain that the air must be moving four times faster, and that its impulse is 16 times greater. But the surface on which it is acting is the fourth part of that of the fly ; the actual impulse therefore is only four times greater, supposing both flies to be moving with the same relative velocity in respect of the current; that is, the rim of each moving with the same portion of the velocity of the current. This will be the case when the small fly turns eight times as often in a minute as the large fly: for the air is moving four times as quick at , and the diameter of is one-half of that of . Therefore, when the small fly is turning eight times as quick as the great
one, there is a quadruple impulse acting at half the distance from the axis. The momentum or energy therefore of the current is double. Therefore, supposing the pinion, wheel, and pulleys of both jacks to be the same, the jack with the small fly, placed in the narrow part of the vent, will be 16 times more powerful.
By this example, more easily understood than a general process, it appears that it is of particular importance to place the fly in an elevated part of the vent, where the area may be much contracted. In order still farther to increase the power of the machine, it would be very proper to lengthen the spindle still more, and to put another fly on it at a considerable distance above the first, and a third above this, &c.
As the velocity of the current changes by every change of the fire, the motion of this jack must be very unsteady. To render it as adjustable as may be to the particular purpose of the cook, the pulley has several grooves of different diameters, and the spit turns more or less slowly, by the same motion of the fly, according as it hangs in the chain by a larger or smaller pulley or groove.
Such is the construction of the smoke-jack in its most simple form. Some are more artificial and complicated, having, in place of the pulleys and connecting chain, a spindle coming down from the horizontal axis . On the upper end of this spindle is a horizontal contrate wheel, driven by a pinion in place of the pulley . On the lower end is a pinion, driving a contrate wheel in place of the pulley . This construction is represented in fig. 6. Others are constructed more simply, in the manner represented in fig. 7. But our first construction has great advantage in point of simplicity, and allows a more easy adjustment of the spit, which may be brought nearer to the fire or removed farther from it without any trouble; whereas, in the others, with a train of wheels and pinions, this cannot be done without several changes of pins and screws. The only imperfection of the pulley is, that by long use the grooves become slippery, and an ill-balanced joint is apt to hold back the spit, while the chain slides in the grooves. This may be completely prevented by making the grooves flat instead of angular (which greatly diminishes the friction), and furnishing them with short studs or pins which take into every third or fourth link of the chain. If the chain be made of the simplest form, with flat links, and each link be made of an exact length (making them all on a mould), the motion will be as easy as with any wheelwork, and without the least chance of slipping.
It is always of importance to avoid this slipping of the chain by balancing the loaded spit. For this purpose it will be extremely convenient to have what is called a balance-skewer. Let a part of the spit, immediately adjoining to the pulley, be made round, and let an arm be made to turn on it stiffly, so that it may be made fast in any position by a screw. Let a leaden ball be made to slide along this arm, with a screw to fasten it at any distance from the spit. When the meat is spitted, lay it on the racks, and the heaviest side will immediately place itself undermost. Now turn round the balance-skewer, so that it may point straight upwards, and make it fast in that position by the screw. Put the leaden ball on it, and slide it inwards or outwards
Smoke-Jack. wards till it exactly balances the heavy side, which will appear by the spit's remaining in any position in which it is put.
The greatest difficulty is to keep the machine in repair. The essential part of it, the first mover, the fly, and the pinion and wheel, by which its motion is transmitted to the rest of the machine, are situated in a place of difficult access, and where they are exposed to violent heat and to the smoke and soot. The whole weight of the fly, resting on the lower pivot I, must exert a great pressure there, and occasion great friction, even when this pinion is reduced to the smallest size that is compatible with the necessary strength. The pivot must be of hardened steel, tapered like an obtuse cone, and must turn in a conical socket, also of hardened steel or of bell-metal; and this seat of pressure and friction must be continually supplied with oil, which it consumes very quickly. It is not sufficient that it be from time to time smeared with an oiled feather; there must be an iron cup formed round the socket, and kept filled with oil. It is surprising how quickly it disappears; it soon becomes clammy by evaporation, and by the soot which gathers about it. The continued rubbing of the pivot and socket wears them both very fast; and this is increased by hard powders, such as sandy dust, that are hurried up by the rapid current every time that the cook stirs the fire. These, getting between the rubbing parts, cause them to grind and wear each other prodigiously. It is a great improvement to invert these rubbing parts. Let the lower end of the spindle be of a considerable thickness, and have a conical hollow nicely drilled in its extremity. Let a blunt-pointed conical pin rise up in the middle of the oil cup, on which the conical hollow of the spindle may rest. Here will be the same steady support, and the same friction as in the other way; but no grinding dust can now lodge between the pivot and its socket: and if this upright pin be screwed up through the bottom of the cup, it may be screwed farther up in proportion as it wears; and thus the upper pivot G will never desert its hole, a thing which soon happens in the common way. We can say from experience, that a jack constructed in this way will not require the fifth part of the repairs of one done in the other way.
It is of importance that the whole be so put together as to be easily taken down, in order to sweep the vent, or to be repaired, &c. For this purpose, let the cross bar which carries the lower end of the upright spindle be placed a little on one side of the perpendicular line from the upper pivot hole. Let the cock which carries the oil cup and the pivot of the horizontal axis BC be screwed to one side of this cross bar, so that the centre of the cup may be exactly under the upper pivot hole. By this construction we have only to unscrew this cock, and then both axles come out of their places at once, and may be replaced without any trouble. We have sketched in fig. 8. the manner in which this may be done, where M represents a section of the lower cross bar. BCDE is the cock, fixed to the bar by the pins which go through both, with finger nuts a and b on the opposite side. F is the hard steel pin with the conical top i, on which the lower end I of the upright spindle AG rests, in the manner recommended as the best and most durable. The pivot of the horizontal axis turns in a hole at E the top of the cock.
After all, we must acknowledge that the smoke-jack is inferior to the common jack that is moved by a weight. It is more expensive at first, and requires more frequent repairs; its motion is not so much under command; it occasions soot to be thrown about the fire, to the great annoyance of the cook; and it is a great encumbrance when we would clean the vent.