Engineering reminiscences contributed to "Power" and "American machinist"

by Charles T. Porter

“In the Allen engine the action of high speed causes all the practical difficulties which lie in the way of the successful employment of high grades of expansion combined with high pressure of steam completely to disappear. The crank receives as little pressure on the centers as we please; none at all if we like; the force is applied to it as it advances, in a manner more gradual than the advocates of graduated openings and late admission ever dreamed of, and a fair approximation is made to a uniform rotative force through the stroke. So that, in a properly constructed engine, the higher the speed the smoother and more uniform and more silent the running will be.”

After a page or more devoted to a demonstration of this action, Mr. Colburn sums up the advantage of high speed in the following illustration:

“Let us suppose that, in an engine making 75 revolutions per minute, the reciprocating parts are of such a weight that the force required at the commencement of the stroke to put them in motion is equal to a pressure of 20 pounds on the square inch of piston. This will not modify the diagram of pressure sufficiently to produce much practical effect. But let the number of revolutions be increased to 150 per minute, the centrifugal force of these parts as the crank passes the centers is now equal to 80 pounds on the square inch of piston, and any pressure of steam below this amount acts only as a relieving force, taking the strain of these parts partly off from the crank. It makes no matter how suddenly it is admitted to the cylinder, not an ounce can reach the crank; but as the latter advances, and the acceleration of the reciprocating parts becomes less, the excess of force not required to produce this becomes, in the most gradual manner, effective on the crank.

“It will be observed how completely the designer has this action of the reciprocating parts under control. He can proportion their speed and weight to the pressure of steam in such a manner as to relieve the crank from the blow on the center to whatever extent he may wish. The notion that the reciprocating parts of high-speed engines should be very light is therefore entirely wrong. They should be as heavy as they can be made, and the heavier the better.

“The advantages of more rapid rotation are largely felt in the transmission of power. Engineers understand very well that, theoretically, the prime mover should overrun the resistance. Motion should be not multiplied but reduced in transmission. This can seldom be attained in practice, but high speed gives the great advantage of an approximation to this theoretical excellence. On the other hand, slow-speed engines work against every disadvantage. Coupled engines and enormous fly-wheels have to be employed to give a tolerably uniform motion; often great irregularities are endured, or the abominable expedient is resorted to of placing the fly-wheel on the second-motion shaft. Then comes the task of getting up the speed, with the ponderous gearing and the enormous strains. Slow motion also prevents the use of the belt, immeasurably the preferable means of communicating power from a prime mover.

“But how about the wear and tear? The question comes from friends and foes alike. The only difference is in the expression of countenance, sympathetic or triumphant. The thought of high speed brings before every eye visions of hot and torn bearings, cylinders and pistons cut up, thumps and breakdowns, and engines shaking themselves to pieces. It is really difficult to understand how so much ignorance and prejudice on this subject can exist in this day of general intelligence. The fact is, high speed is the great searcher and revealer of everything that is bad in design and construction. The injurious effect of all unbalanced action, of all overhanging strains, of all weakness of parts, of all untruth in form or construction, of all insufficiency of surface, increases as the square of the speed. Put an engine to speed and its faults bristle all over. The shaking drum cries, ‘Balance me, balance me!’ the writhing shaft and quivering frame cry, ‘See how weak we are!’ the blazing bearing screams, ‘Make me round!’ and the maker says, ‘Ah, sir, you see high speed will never do!’

“Now, nothing is more certain than that we can make engines, and that with all ease, in which there shall be no unbalanced action, no overhanging strains, no weakness of parts, no untruth of form or construction, no insufficiency of surface; in which, in short, there shall be no defect to increase as the square of the speed, and then we may employ whatever speed we like. ‘But that,’ interposes a friend, ‘requires perfection, which you know is unattainable.’ No, we reply, nothing unattainable, nothing even difficult, is required, but only freedom from palpable defects, which, if we only confess their existence, and are disposed to get rid of, may be easily avoided. It is necessary to throw all conceit about our own work to the dogs, to lay down the axiom that whatever goes wrong, it is not high speed, but ourselves who are to blame, and to go to high speed as to our schoolmaster.

“Among the many objections to high speed, we are often told that the beam-engine will not bear it, and the beam-engine, sir, was designed by Watt. In reverence for that great name, we yield to no one. The beam-engine, in its adaptation to the conditions under which it was designed to work—namely, a piston speed of 220 feet per minute and a pressure of one or two atmospheres—was as nearly perfect as any work of human skill ever was or will be; but we wonder why the outraged ghost does not haunt the men who cling to the material form they have inherited, when the conditions which it was designed to meet have been all outgrown, who have used up his factor of safety, and now stand among their trembling and breaking structures, deprecating everything which these will not endure.

“A journal and its bearings ought not only never to become warm, but never even to wear, and, if properly made, never will do so with ordinary care to any appreciable extent, no matter how great speed is employed. It is well known that there exists a very wide difference in bearings in this respect, some outlasting dozens of others. Now, there need be no mystery about this: the conditions of perfect action are so few and simple that it seems almost idle to state them. The first is rigidity of a shaft or spindle between its bearings; but everybody knows that if this is flexible, just in the degree in which it springs, the journals must be cast in their bearings, though in actual practice this perfect rigidity is not once in a thousand times even approximated to. The point of excellence in the celebrated Sellers bearing for shafting is that it turns universally to accommodate itself to this flexure of the shaft, and the result is a durability almost perfect.

“The second requirement, when we have a shaft capable of maintaining perfect rigidity under all the strains it may be subjected to, is abundant extent of bearing surface both in length and circumference, a requirement, it will be seen, entirely consistent with the first. It is a mistake to use journals of small diameter with the idea that their enlargement will occasion loss of power on account of the increased surface velocity, as, in fact, the coefficient of friction will diminish in a greater ratio than that in which the velocity is increased. In the Allen engine it is intended to make all shafts and journals too large.