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If the .05 is a fair sample of the value of the coefficient of friction for practice, it appears that compound engines have a decided advantage over duplex, as applied to propellers, in the matter of the crank alone; the percentage gained being about four and a half in the case of the actual crank length adopted in the "Iris," and with a twelve to twenty-fold expansion. The thirteen per cent. loss in the extreme case of the duplex is nearly that due to all causes in the turbine water-wheel, and is surely too much to be absorbed by one member of the steam engine, under any circumstances.

It appears that the large crank-bearings necessary in the modern style of marine engine, where several cylinders are put to work on one quick-running crank-shaft, should be compensated for by every means possible.

Probably too little attention has been given to the arrangement of the cylinders in these marine engines. The arrangement of the cranks has been made a subject of much study by marine engineers, with a view to steadiness of motion and of strains. This should not be overlooked as regards efficiency, for reduction of strains reduces bearings, and consequently increases the efficiency; but arrangement of cranks and arrangement of cylinders should be considered together, both leading to important advantages in the efficiency; the part belonging to crank-arrangement being reduction of bearing, while that due to cylinder-arrangement being increased equivalent crank length.

For instance, if the two cylinders of a duplex be situated at right angles, say one with axis vertical, and the other horizontal, and working on the same crank-pin, instead of both being vertical, each with a crank, and at the necessary distance asunder along the shaft; the loss at the crank would be less in the ratio of 1/2 to 2 for the case of non-expansion, but not quite as much in expansion. This result is for a given diameter of crank-bearings, and is seen to be true from the fact that the indicated power is the same each way, but the pressure or thrust against bearings is the diagonal of the square of which the sides, added together, make the thrust when the cylinders are side by side.

Again, if the one high-pressure cylinder could be situated diametrically opposite the two low-pressure ones of a compound engine, with cranks at 180°, and the former have its crank between the two other cranks; also if the work be divided equally between the high and low-pressure, the thrusts against the main crank bearings would be nearly all avoided, and the equivalent single crank would be one of twice the length of the actual cranks. This arrangement, as compared with that where the cylinders are side by side, with main bearings between, will reduce the work lost at crank to one-half, and the

efficiency would be changed from 1-.09 to 1-045; true for nonexpansion, and nearly so for expansion.

Duplex engines, with cylinders directly opposite, and cranks at 180°, would realize nearly the same advantage, but not quite, because the cranks do not come to balance in a plane of rotation.

Again, in a gang of several cranks, like the eight in the "Iris," those remote from the propeller, can be smaller in bearing.

Second-Method by Temperature.

One cylinder of the "Iris " appears, by data given in King's Report, to develop about four hundred horse-power, and the aggregate surface of its crank and box appears to be about twenty-two square feet. Taking the temperature excess at 80° Fah. Eq. 23 gives the

Effy 1-074,

a result lying among those given in Table IV.

EXAMPLE OF ANGLE OF REPOSE.

From quantities given in Tables III. and IV., for the "Iris," we find

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Eq. 21, which gives 0, for the point at which the useful work is just neutralized by the prejudicial, makes 0,=.1=5° 44'.

DISCUSSION.

Mr. Porter-I would like, Mr. President, to gain some information about this angle of repose. That was the basis, if I understand the paper correctly, of the computations. Is that a permanent angle?

Prof. Robinson-It depends upon the co-efficient of friction. This angle, I believe, in the case of the "Iris," is 2° 52', and the angle at which it is desirable 'to admit steam is about twice that. Mr. Porter-The angle depends, I suppose, also on the relation of the steam-pressure to the resistance, does it not?

Prof. Robinson-It is independent of the pressure.

Mr. Porter-With large pressure and very light resistance any engine, whatever the size of the crank-pin or the bearings, will start from a position very close to the center line, very much within one degree. On the contrary, if the resistance is great and the steam pressure is small the angle of repose will be very large. It might be 5° or it might be 10°, or, as suggested by the paper, it might be 90°.

Prof. Robinson-You can see by examining the diagrams, which I gave for illustrating the action of the force of the stroke, that this point would be independent of the pressure; that it would depend upon the value of the crank-radius and crank-bearings, it being in

creased as the value of the crank-radius decreases; and increases as the radii of the crank-pin and crank-bearing are increased.

A Gentleman-Mr. President: May a person, not a member of the Society, be permitted to ask a question?

The President-Yes, if no objection is made.

The Gentleman-As I understand the paper, the friction caused by the pressure on the crank-bearing, has a moment to prevent the rotation of the crank. The pressure of the steam is less than this moment of friction, consequently under any load whatever the engine would never start with any pressure of steam within the angle of repose.

Prof. Robinson-Yes, sir.

The Gentleman-Because the friction, caused on the crank-pin and the crank-bearing, would have a greater moment to prevent the rotation of the crank than the tangential component of the pressure transmitted through the connecting rod to turn the crank-pin. That I understood to be the character and quality of this angle of repose. Prof. Robinson-Yes, sir.

Mr. Sterling-I think one of the questions that enter into this discussion is the inertia of moving parts of the-engine, and Mr. Porter has brought before us very clearly the important position that this inertia plays in the movements of the steam-engine. It is true that the engine will not start within this angle of repose, but that is a condition that we do not design our engines for, and in the running of an engine the inertia of the moving parts would largely destroy the effect of this friction, because the power of the steam would be used in changing the direction of the inertia before it could reach the crank-pin.

Prof. Thurston-I would like to ask Prof. Robinson if he obtained any figures for the efficiency where the values of the co-efficients were very much lower than five per cent. I ask that because all the co-efficients that I have obtained-which were nothing like so high as five per cent.-have been from journals in about the condition of well-finished journals in new machinery; and the co-efficient of one-fourth of one per cent. which I have obtained with very high pressures, was with journals in the condition in which we have good journals after they have been running for months. The surface was a steel surface, the polish was perfect, and with that polish and the condition of surface that we actually have in the crankpins of steamers running for some time, we got a co-efficient of one-quarter of one per cent.-one-twentieth of the figure given here.

Prof. Robinson-As to computation, I have examined the figures obtained for those values of the co-efficient of friction in the last column. Those figures are based on the value .05, of the co-efficient of

friction. If we introduce a co-efficient of friction one-half of that which I assumed for the purpose of getting at the numerical quantities, the values of these quantities will be reduced to one-half; so that for the co-efficient .005, the value of these losses would be reduced to one-tenth. That is the first figure there would be 1-.008. The total loss would be less than one per cent. on that supposition. Prof. Thurston-I have no question in my own mind that the figure one-half of one per cent. is very common; I have not any question about that. These low co-efficients were obtained with every condition favoring extreme light friction.

Mr. Gordon-I would like to ask Prof. Robinson if all his deductions were not based on the idea that the brass around the crank pin was loose, and that there was no obliquity in the connecting rod. If the rod were in a line with the crank pin, parallel with a horizontal line, would not all your calculations fail? Would not you have nothing to base your calculations on?

Prof. Robinson-Taking the diagrams, which present the figure of the action of the work for a stroke, it will readily appear that if we were drawing a diagram of that kind we could easily modify it from the case of a parallel connecting rod, to the case of a connecting rod with an obliquity. I think, from my study of the case, the value of the efficiency would not be changed. It would be, perhaps, increased slightly, but not, I think, more than a fraction of one per cent. of the value that I find. It might be, possibly, two or three per cent. That first figure, 1-.08, might be modified two or three per cent. I don't think it would be more than that. Attention should, perhaps, be called to the fact that the actions taken into account are those continuing throughout the stroke, and on account of this we obtain a counterbalancing of the effects. On account of this, I think the result for efficiency would be but slightly modified. The effect upon the efficiency would be much less, I am confident, than it would be in the case of the steadiness of motion of the flywheels as depending on the difference of action.

TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL EMGINEERS. |

MECHANICAL CORRECTNESS.

BY CHARLES A. HAGUE.

Read at the Annual Meeting, 1880.

Mechanical correctness is the adaptation of the proper means to the accomplishment of the end in view: hence the success of mechanical engineers must necessarily depend upon the extent to which they satisfy the conditions governing the production of the effect sought.

Throughout the long list of industries calling for the services of engineers, the character of the different operations constantly change to suit the varying demands.

Correctness of manipulation in machine construction does not always mean extreme closeness of fit. The automatic cut-off gear of steam-engines, for example, requires to be sufficiently sensitive, to enable it to adjust itself to fluctuate in speed to the slightest attempt of the balance-wheel. This action should resemble, as near as possible, the operation of the human will controlling the muscles through the medium of the nerves.

A tolerably clear conception of the action necessary to regulate speed, under varying demands for energy, may be obtained by attempting to walk at a perfectly uniform rate of speed against a very strong but unsteady wind; the muscular exertions under such conditions would constantly vary in intensity, to meet the fluctuations in pressure of the gusts of wind against the body.

The principal obstacles to close regulation in steam-engines are friction and lost-motion of the regulating-gear; friction, of course, being the greater, because upon the effort of the friction depends the extent to which lost-motion is realized.

In some of the best types of automatic engines the controlling mechanism is fitted up with a looseness of joints that would be entirely out of place in the slide-rest of a lathe, or in the equally important parts of other machine-tools. But this looseness of fit in the engine-gear is the guarantee of precise automatic action; each

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