To understand the workings of a player piano, it is necessary to first understand the key component parts and how they are arranged within the instrument. A basic appreciation of the standard upright piano action is also desirable. To see cross-sectional views of a typical player, click on the thumbnail images below.

Cross-sectional view of an entire player pianoDiagram of piano action

The Roll Playing System
The player mechanism, typically referred to as the 'pneumatic stack', is located in the upper portion of the instrument and connected to the piano action by series of push-rods. For each note of the piano, there is a small pneumatic, a valve assembly, a soft flexible leather pouch and a bleed. The tracker bar has a row of equally spaced holes, each of which is connected to a channel, via a small pipe. The chamber is kept under vacuum by the foot operated bellows. The pneumatic is connected to the piano action by a simple linkage. The diagram below shows the whole assembly, firstly in resting and then playing positions.

When a roll perforation passes over a tracker bar hole, air is admitted, causing the pouch to move upwards under suction inside the chamber. The valve rises accordingly, uncovering the top of the chamber and resting against its upper seat.  This in turn connects the pneumatic to the vacuum supply, forcing it to collapse under atmospheric pressure and to operate the piano action.

The bleed, which connects the channel to the chamber, allows the pouch to assume its correct resting position after a note has finished playing, by equalising the pressure above and below. The diameter of the bleed is necessarily much smaller than that of the corresponding tracker bar hole.

The take-up spool, which is driven by an air powered motor, transports the music roll over the tracker bar at a speed that may be varied by means of a control lever. The tempo of the music is unaffected by variations in the force at which the pedals are operated, due to the presence of a governing device.

Large diameter rubber hoses provide a connection between the main bellows and pneumatic stack, in addition to other auxiliary devices. Rubber tubing of small diameter is also used throughout, particularly in the case of the tracker bar, each port of which has an individual connection to its corresponding valve assembly.

The Exhauster Bellows
Player piano bellows diagramThe purpose of the exhauster bellows is to provide suction, which is the motive force for virtually all player systems. Located in the lower section of the piano, it usually consists of  a large bellows assembly connected to two foot treadles. The bellows are of wooden construction, covered with heavy duty rubberised cotton cloth.

As the exhausters are alternately opened and closed by the action of the feet upon the treadles, air within the system is exhausted into the atmosphere. This naturally has the effect of lowering the pressure within the player mechanism. The purpose of the external leather flap valves is to allow the exhausters to discharge the air extracted during the forward stroke of the treadles. Upon the return stroke, the internal valve prevents any feeback of air to the bellows system. The reservoir provides a necessary measure of equalisation for the vacuum level within the player mechanism.

The more sophisticated and fully automatic 'reproducing' pianos, described later, utilise an electrically operated suction pump instead.

Auxiliary Devices
Most players also incorporate a number of auxiliary devices, the most common of which is a system that keeps the music roll in perfect alignment with the tiny holes of the tracker bar.

Others include a means of automatically operating the soft and sustain pedals of the piano from perforations within the roll. Another popular feature, known in Aeolian terminology as the 'Themodist', allows selected notes to be accented, a concept known as 'theming' and also provides some control over the dynamics of the music in general.