While the lower pressures are in general more efficient, the loss due to higher compres sion is not serious. If the compressed air be used without expansion, it will be seen that there is a material loss in efficiency; but, on the other hand, if it be used expansively with out reheating, trouble may be experienced, due to the drop in temperature below the freezing point. When moisture is present, this will cause the formation of ice, which may clog the passages if proper precautions are not taken to i prevent it. The low temperature will not in itself cause trouble; if, therefore, the moisture which the compressed air holds in suspension be allowed to settle in a receiving tank, placed near the motor or other air apparatus and frequently drained, less trouble will be experi enced from this cause. The greater adaptability of compressed air to various purposes causes its use to increase along with that of the elec tric motor, for it has a different field of use fulness, independent of power transmission ; at the same time, when the requirements are prop erly observed in its production and use, its economy as a motive power in special cases compares favorably with other systems. But compressed air possesses so many advantages that, however inefficient it may be as a motive power, its application to shop processes will be continually extended as its usefulness becomes better known. See PNEUMATIC Toms.
Hydraulic Mention has been made of the use of hydraulic motors as a factor in the subdivision of power but these are being used to such a.limited extent for this purpose that they will not be considered here. There is, however, a growing field of usefulness for hy draulic power in manufacturing operations which is peculiar to this agent alone, namely, its use in forging and similar work. Where hy draulic power exists for this purpose it is also generally used for a variety of work which could be accomplished just as well, and often more economically, by steam or compressed air; but in forging operations where heavy pressures are 'required hydraulic power is in finitely better than either. The compressibility of air is an objection in many lines of work, and it is now well recognized that the effect of a hammer blow is oftentimes merely local. As H. F. J. Porter has so ably shown, the pressure applied in forging a body of iron or steel should be sufficient in amount and of such a character as to penetrate to the centre and cause flowing throughout the mass; as this flowing of the metal requires a certain amount of time, the pressure should be maintained for a correspond ing period. Hydraulic pressure, instead of a hammer, should, therefore, be used to work the metal into shape. Under its influence the forg ing is slowly acted upon and the pressure is distributed evenly throughout the mass, where as under the high velocity of impact of the hammer the metal does not have time to flow, and thus internal strains are set up in the mass, which may cause serious results, especially with certain steels which have not the property of welding. Moreover the quality of the steel is
very much improved by the processes of hy draulic forging. There exists at present a marked tendency to substitute this method in a wide variety of work in which presses are employed varying in capacity from 20 tons to 14,000 tons.
It is an interesting fact that the magnificent hammer, designed and built by the Bethlehem Steel Company, of a nominal capacity of 125 tons and an actual of 145 tons falling weight has been superseded by hydraulic presses for forging purposes. This hammer is no more in existence. The largest hammer at the Bethle hem plant to-day is a 10-ton hammer. Among the numerous hydraulic presses at Bethlehem under which forgings are made they have one of 14,000-ton capacity, operated by the world's largest land engine of 16,000 horse power.
The different hydraulic systems are as fol lows: (1) A pump or continuous intensifier using pressure corresponding to the resistance of the press. This pressure runs up to 7,000 pounds per square inch; (2) single stroke steam or air intensifiers where the hydraulic cylinder is filled with water, under a pressure of 2 to 50 pounds per square inch and then the pressure increased by means of the intensifier from 3,000 to 9,000 pounds per square inch.
The latest development at Bethlehem has been in high speed processes where the water is pumped, electrically, to an accumulator and from there used in the presses. This is the most economical. In presses of this nature, the cylin der is filled from a tank through a valve of very large dimensions (to date up to 16-inch in diameter) withstanding a pressure of 10,000 pounds per square inch. A water intensifier is then used; this may be one, two or three-pres sure intensifier. The nominal pressure used in the lines for operating auxiliaryparts of the press, together with the intensifier, is 2,500 pounds per square inch line pressure. A line pressure lower than this makes the machinery bulky and cumbersome and does not take ad vantage of modern steel construction, nor does it give high speed.
Any one of these presses will run up to 100 or more strokes per minute when needed. A press at Bethlehem has been tested up to 60 strokes of three feet length per minute. Most die forging presses are made of this last type. The speed of these presses may be understood when it is stated that as many as 10 projectiles are turned out per minute.