TEST PIT. A test pit may be used for many purposes, but in its generally accepted meaning the term refers to a heavily lined pit sunk in the floor of an industrial factory building, and employed in making strength and balancing tests on the revolving parts of such machines as large electric generators, motors, and steam and hydraulic turbines.
The necessity for such pits arises from the fact that machines are usually tested at speeds considerably in excess of those at which they will normally run, and consequently there is the risk of disruption, with the likelihood of great damage to property and loss of life if proper protection is not provided. While it is not intended that such tests shall be carried to the point of destruction, a certain factor of safety must be present in the machines, and this can only he demonstrated by actual over-speed tests.
Test pits are therefore sunk beneath the shop floor level, and are heavily lined with reinforced concrete and steel. All up-to date manufacturers of large revolving machines are equipped to make such tests with safety to their employees and equipment. There are cases on record of very serious accidents resulting from the disruption of rapidly revolving machines improperly protected, and the modern tendency is toward more heavily armoured pits located in isolated sections of the manufacturing plants.
The largest test pit in the world is that designed and built by the General Electric Company in 1927, at their principal plant which is situated at Schenectady, N.Y. This pit is located in a special building with other testing equipment, a quarter of a mile from the nearest factory building. It is Soft. deep and 45ft. in in side diameter, and is capable of enclosing the rotating member of the largest electrical machine likely to be built for many years. The walls are of double concrete construction with a layer of soft sand in between, and with a total thickness of 14ft. The floor is a concrete slab faced with steel, and is supported by wooden piles driven 6oft. into the ground. The entire construction is reinforced
with one-inch steel bars cast in the concrete, giving even greater rigidity than is necessary in the case of large dams and bridges.
The top of the pit is closed by a removable lid made of steel plates and girders, and is held in place by eight steel wedges each a yard long. For testing purposes the rotating machine is lowered into the pit by a travelling crane, and is supported in a huge roller bearing at the bottom, capable of carrying a load of 500 tons. The shaft of the machine is held in the vertical position by a small upper bearing, and is connected by a coupling to an electric motor installed upon the top of the lid. Motors of as high as 3,00o h.p. are used to revolve the largest rotors at test speeds.
The pit may be used for any tests where bomb-proof protection is required. Its principal use, however, is in performing "double speed" tests on large hydro-electric generator rotors. The object of such tests is to establish the fact that rotors will not fly apart even under the maximum "runaway" conditions which might exist during an emergency in service. Huge centrifugal forces and enormous rim velocities are developed in these tests, the rim speed of the largest machine (1928) being of the order of 2o,000ft. per min. (about 227m. per hour).
During the test runs no one is allowed to remain in the pit building. The observers retire to a control house loo yards away and there manipulate the speed of the electric motor driving the machine under test, by means of a switchboard connected with the pit through underground cables. Indicating meters record the speed and degree of vibration, and telephonic communication with a microphone in the pit transmits to the observers the sounds made by the machine as it revolves. By these means it is expected that sufficient warning of a breakdown will be given, so that the test can be stopped before the machine or pit is damaged. (D. 0. W.)