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Upright Piano Action

Felt items are shown in green. The red and pale green sections of the hammer and damper are also felt. Felt is a matted fabric of wool, made by working the fibres together under pressure. Because they have a slightly soft surface while being sufficiently rigid felt parts can transfer (or absorb) mechanical energy without producing annoying scraping or impact noises.

The brown and yellow parts are wood. Where wood grain is shown the part is sectioned (effectively sawn in two) for purposes of illustration. The sectioned parts extend the length of the keyboard.

Except for the key lead, all grey, black or blue items are steel.

The bridle strap is made of fabric.

Although key facing is referred to as ivory, that material has been replaced by plastic owing to animal welfare concerns regarding the source of ivory - elephant tusks.

Notes:

The animation in the illustration is in slow motion. The actual time from when a finger begins to depress a key to the point where the hammer strikes the string can be less than tenth of a second.

The damper lift (2-D) extends over all keys - driven by the sustain pedal

Operation:

The  main function of the piano action is to cause the hammer (1-A) to deliver a single and brief blow to the string the strength of which is dependent on the rate of depression of the key (6-B).

As can be seen in the diagram the piano action is fairly complicated. This is mainly because that at the time of contact with the string the hammer (1-A) must be in free motion and mechanically detached from the rest of the piano action once it has been set in motion. This assures that the time of contact with the string is as short as possible and occurs only once. 

A way of understanding this is to picture a person bouncing a tennis ball off a brick wall and then catching it. The ball is in free motion from the time it leaves the thrower's hand and it is caught. The ball is in contact with the wall only for a brief instant.

Once the impact has taken place, the motion of the hammer (1-A), after the hammer has been pulled back by the bridle strap (5-E), is arrested by the backcheck (5-C).  Mechanical energy is transmitted to the hammer via the sticker (4-A) and the jack (4-A). Immediately after the hammer has delivered its blow to the string, the jack is forced to swing back by the letoff button (3-C).