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MANAGERS AND PROJECTIONISTS 525
FOUNDATION — The manufacturer recommends a foundation such as is shown in figure 155, The floor line should be at the bottom of the concrete, so that the bottom of the base of the machine itself will be two feet above the top of the floor, which will bring the set at a height convenient for examination and handy to work on.
Such a foundation will assist in absorbing the vibration and hum incident to all motor generator sets, thus preventing it from reaching the structure of the building and being carried to the theatre auditorium. The manufacturer recommends that the concrete base be made hollow, with 2-inch-thick layers of cork on top of the concrete, and two 2-inch-thick layers of planking, layed in opposite directions on top of the cork, the two layers of boards to be securely nailed or bolted together or fastened together with Y% by 3 inch lag screws, their heads countersunk into the bottom of the planking.
The set is then mounted on the boards and may be bolted thereto, if thought necessary, by means of previously placed anchor bolts, or by heavy lag screws.
NOTE — An equal thickness of heavy felt may be substituted for cork, if desired, but cork is much the better of the two, because felt packs down after a time, and loses the greater part of its resiliency, whereas cork retains its resiliency almost indefinitely.
Ballast Rheostat Figure 156
TOP PERFECTLY LEVEL— It is very important that the top of the base to receive a motor generator set be perfectly level. If it is not, then the oiling system may not function well; also, except in certain types of machines, it is best that the armature of horizontal sets be perfectly level, since when it is thus it will "float" — have slight end motion, which tends to prevent grooving the commutator and bearings.