Most simply stated, an exhauster assembly (or pumper bellows assembly) consists of two exhauster (or 'pumper') bellows and at least one reservoir bellow whose function it is to create and store vacuum. The unit is typically called an 'Exhauster Assembly" because it exhausts, or gets rid of, air from inside the working components of the player mechanism. (Those components consist of a stack -or windchest-, an air motor and it's associated governor, and a tracking device. Most player systems also have an automatic sustain device and a bass and treble soft device.) By exhausting the air from these devices -which are basically sealed units- a partial vacuum is created inside the devices. And it is that partial vacuum which is the force that allows a player piano to function.
The operation of the exhauster assembly is pretty straightforward. Both of the exhauster bellows are designed as you see below.
The pump includes the two pumping bellows connected to the pumping pedals, and one or more reservoirs which serve to smooth the suction output between pumping strokes. The pumping bellows and reservoirs are usually all fastened to a common suction chamber called the trunk. Each pumping bellows has one or more external springs which hold it closed and hold the pumping pedal up. Each reservoir has one or more internal springs which hold it open.
The pumping bellows have internal and external flap valves which control the flow of air as the bellows are pumped. Each flap valve allows air to pass through only in one direction. When a pumping pedal pushes the bellows open, suction inside the bellows pulls the external flap valve shut, causing air to be pulled out of the trunk. When the pedal is released, the spring pushes the bellows closed again, creating air pressure inside the bellows. This higher pressure pushes the internal flap valve shut, trapping the lower pressure inside the trunk, and exhausting the contents of the pumping bellows through the external flap valve. Thus, each time the bellows is pushed open, it pulls air out of the player action through the internal flap valve, and each time it is released, it dumps this air into the atmosphere through the external flap valve.
To keep the suction output of the pump at a constant level, one or more reservoirs are included. When each pumping pedal is pushed down, the suction level inside the trunk increases, and the reservoirs pull shut, absorbing some of the increase. Between pumping strokes, the suction level decreases, and the internal reservoir springs push the reservoirs partially open, compensating for the decrease. The result is that as the pumping pedals are pumped alternately, the reservoirs pulsate in and out, and the suction output of the pump remains fairly constant.
The apparatus in the lower part of the piano supplies the necessary suction to operate the player mechanism. Each foot pedal has its own pump bellows, to which it is connected by linkage. When a pedal is pressed, one of the pumps opens, drawing air from the pneumatic stack into it. When the pedal is released, the air with which the pump has filled itself is expelled into the atmosphere. This is accomplished by two simple valves, usually in the form of flaps of leather, which act as check valves to control the direction of flow of the air. Figure 6 is an illustration of the principle of operation of the pumps. 'A' represents the fixed board of the pump, and 'B' represents the movable hoard, attached through linkage to the foot pedal. Channel 'C' leads through the reservoir to the player action. The leather flap valves D are attached to the boards at their ends, but are free to move away from the boards at their centers. When the pedal is pressed and the pump opens, valve 'D' on the movable board is drawn tightly against its seat, while valve 'D' on the fixed board is pulled away from its seat, drawing air from channel 'C', When the pedal is released, the pump has filled itself with air from channel 'C', and it begins to close. Valve 'D' on the fixed board is then pressed against its seat, while valve 'D' on the movable board moves away from its seat, expelling the air in the pump into the atmosphere.
The reservoir acts as a cushioner and equalizer for the supply of air. The two foot pump bellows supply vacuum only while they are being pushed open, and this would lead to very erratic performance of the piano if there were no means to smooth out the vacuum supply and assure a reasonably constant suction, The reservoir is a large spring-loaded bellows which "stores" the generated suction inside it. When the piano is playing, the reservoir is partially collapsed. The movable board of the reservoir is in a continually "floating" state, moving in and out quickly as variations in suction occur. If a heavy chord is struck, the reservoir springs snap the movable board out instantaneously to maintain the working vacuum and to keep the piano playing normally. Pianos with small reservoirs usually give the player pianist better opportunity to accent certain notes by quick, hard strokes of the pedals, However, pianos with large reservoirs usually maintain their playing level more uniformly and are better able to handle sudden demands for vacuum. Some player pianos are equipped with two reservoir bellows at the ends of the bellows unit, rather than a single reservoir extending across the unit-but the function is the same in both cases.
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