Page:Popular Science Monthly Volume 45.djvu/638

618 hydraulic pressure admitted to the under side of the piston. This pressure is derived simply from the water in the penstock supplied to the turbine, and when the latter is working under full gate—that is, is taking water to its full capacity—the pressure in the penstock is decidedly less, just as the pressure in a water pipe is partly relieved by the opening of a faucet. This causes the supporting force on the under side of the piston to materially decrease, and a thrust bearing—that is, a bearing adapted to withstand either pressure or pull, so as to hold the shaft against the tendency to end play—has to be resorted to in order to take up the difference. As a matter of fact, the difference between the supporting force when the flow is a minimum and that when the gate is wide open is about two tons in the seventy-six. The way this is handled is to arrange the area of the piston and the depth below^ the upper water level so that at minimum flow the supporting pressure will be about one ton more than the total weight, and at full gate about the same amount less. At the normal rate of working there is very little to be taken up by the thrust bearings.

An idea of the magnitude of the proportions of the generators may be gathered from the fact that the designers were limited in the size of base plates that they could use by the inability of the railways to transport, even by especially large and powerful cars, pieces of proportions originally designed from the factories to he falls.

It is stated that, had it not been for the tariff restrictions imposed on the importation of electrical machinery, the generators would probably have been purchased abroad. As it was, they, as well as the motors which will operate on their circuits, are the work of a great Pittsburg company. In the case of the turbines the design was by a Geneva firm, and the construction mainly done in Philadelphia. Certain of the fittings are French, and the governors Swiss.

One of the details in the power house is a traveling crane capable of handling pieces weighing up to fifty tons, which commands every portion of the floor of the building. The presence of this piece of apparatus is of the greatest importance in the case of anything going wrong with one of the generators or turbines. With its assistance any portion of either of these ponderous pieces of mechanism which may need repair can be moved with the greatest expedition, and a spare interchangeable part put in its place. Frequently in an installation of heavy machinery, although perhaps much less ponderous than these in question, a break occurs which may cause a shut-down of many hours, when, if sufficiently powerful means of moving heavy parts were at hand, the damaged piece could be replaced in a