Presses are now built at the Bethlehem plant of 10,000 and more tons capacity for the manu facture of die forgings, such as car wheels, which may be produced at the rate of one a minute.
The high efficiency of the presses has chiefly been accomplished by means of the large valves and the high initial pressure in the lines, the cushioned accumulator, and the shifting tables (both in top and bottom platen). These have all been developed by the Bethlehem Company, which, at the present time, stands practically alone in the building of these enormous ma chines, a single one of which will weigh as much as 1,000 tons. All handling of pieces under these presses is done mechanically by hydraulic or electric power. The success which has attended the use of hydraulic power in forging is causing it to be applied to other and similar work to an increasing extent. In boiler works, railroad and locomotive shops, bridge works and shipyards it is used along with compressed air, but where heavy pressures are desired hydraulic power is greatly to be pre ferred; hence we find it operating machines for punching and shearing heavy plates and sec tional beams, riveting machines, stationary and portable, flanging and bending machines, tube upsetting machines, wheel and crank-pin presses, lifting jacks and hoists of all kinds. For heavy boiler work hydraulic riveting seems especially well adapted, since an intensity of pressure can be brought to bear upon the plates which is obtained by no other method.
In the transmission of power by compressed air, within reasonable limits, the loss in transmission if the pipes be tight need not be considered, for although there is a slight loss in pressure due to the frictional resistances of the pipes, yet there is a corre sponding increase in volume due to drop in pressure, so that the loss is practically inap preciable. It has already been stated that com pressed air as now used without reheating is not at all efficient as a source of motive power, since the combined efficiency of compressor and motor, even under favorable conditions, is not more than 50 per cent of the available =era. put into the compressor. In other cases the efficiency is as low as 20 per cent. There should be no comparison between the cost of power by compressed air and its brilliant rival, elec tricity, since each has its own field of useful ness. With a three-wire 220-volt-system, which
is very suitable for ordinary shop transmission when both light and power are to be taken off the same circuit, the combined efficiency of generators and motors working intermittently with fluctuating loads will be about 60 per cent of the power delivered to the engine. For greater distances than those which obtain in plants of this character the loss in transmis sion will be greater, and higher voltage must be employed in order to down the line loss. While it is possible to put in conductors suffi ciently large to carry the current with any as sinned loss, yet the cost of the line may become prohibitive with low voltage. Where cheap fuel is available it is found in many cases that electric power can be generated at the works more cheaply than it can be purchased from a central station; especially is this the case if the exhaust steam be used for heating purposes. In isolated plants the cost of transmission is very small as compared with the total cost of generation; whereas in the average central sta tion the cost of transmission, which includes interest and depreciation on pole line, may constitute a large percentage of the operating cost.
As already stated the last census shows that 63 per cent of the electric power used in the industries is generated by the user. In those localities where the cost of fuel is high, elec tric power can often be purchased more cheaply from a central station which possibly obtains its current from a water-power development many miles distant and transmits it electrically to a convenient distributing centre, where it is used for power and light. The recent ad vances in electrical transmission are very marked, and one constantly hears of some new achievement more wonderful than anything previously accomplished. Distances have been gradually increased until it is now not un common to transmit electrical energy economi cally and in commercial quantities up to 150 or 200 miles; in one recent case the transmission extends from the Sierras to Los Angeles, Cal., a distance of 240 miles.