The Piano Action Train: From Key To String

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Joseph Meehan is an active member of the Piano Technicians Guild and a full-time piano craftsman. He resides in Maine, and is Secretary/Treasurer of that state's chapter of the Guild.

[This article originally appeared in the April 1977 issue of Contemporary Keyboard magazine.]

DAY IN and day out, in hundreds of cities and towns from New York to Timbuktu, countless thousands of people sit down at the piano and use their fingers on the keys. Some of them breeze through Rachmaninoff while others have trouble with Chopsticks, but all of them, if they're playing on a modern piano, initiate pretty much the same series of mechanical movements within the instrument itself. What happens when you push a key down is called the action train, because it's a chain of quick movements of finely crafted parts, each dependent on the one before it, which gives rise to the wonderful (or dreadful) sounds that come from that mammoth monster of metal, wood, and felt, the piano.

Before we can discuss the action train, we have to understand how the piano as a whole is constructed. Since its invention by Bartolomeo Cristofori in the early 1700s, the instrument has naturally evolved, but it hasn't changed all that radically. A modern piano, like its ancestors, consists of three main sections: the case, the harp, and the action.

The case is the outer housing. It may take different forms—the grand from baby to concert, and the vertical from spinet to large upright. Most musicians are familiar with the exterior parts of the case. They know that the music rack holds the music that is being played, while the top of the case is for piling everything else up on, including a lifetime collection of music, a picture of Aunt Jessica, a candelabra, and whatever else lands there. The inner parts of the case help hold the keys and the action.

The other part of the piano, which we won't discuss in detail, but which we would have very little music without, is the harp. The harp is made up of an iron plate, which takes the combined pull of the 230-odd strings, and a soundboard. The vibration of the strings is transmitted to the soundboard by way of a bridge much like a violin or guitar bridge. At one end, the strings are attached to tuning pins (which are firmly embedded in the pin block and extend through appropriately placed holes in the plate), and at the other end they are held by hitch pins embedded in the plate.

Now that you have a fuzzy picture of how a piano is put together, we can discuss the action, that bit of ingenuity that takes up where our fingers leave off. Since the action on a grand piano is slightly more complex than that on a vertical, we will discuss the latter first. As we proceed, please refer often to Figure 1.

Modern vertical pianos are equipped with one of two types of action—direct blow (Fig. 1) or drop (Fig. 2). In addition, older uprights sometimes used an action identical to the direct blow except for the addition of a vertical stick, called a sticker, which connected the inside end of the key with the wippen. (Incidentally, it must be noted that you may find things when you look inside your piano that aren't mentioned here. Bear in mind that some manufacturers use slightly different designs—but it's safe to say that they're all variations on what we're discussing.) The drop is like the sticker in that they both use an extra part to transfer the motion of the key to the wippen, whereas in the direct blow the wippen rests directly on the capstan screw. The purpose of drop action is to allow the piano to be lower in overall height: This is the type of action used on spinet models.

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The finger pushes down on the front of the key. At rest, the key sits on the balance rail and the back rail, both of which run horizontally along the key bed. But being a lever, when the key is pressed it rocks on the balance rail, rising off the back rail and pressing up on the wippen. The wippen itself then rocks, moving upwards in back (the end nearer the player) and downwards in front.

Everything now depends on the wippen, which causes two things to happen almost simultaneously. The jack moves upward and pushes against the hammer butt, causing the hammer to travel toward the string. The speed of travel is determined by the finger on the key. In addition, the damper spoon moves forward, pressing against the damper lever and thus rocking the damper away from the string.

As the jack continues upward, the jack knuckle contacts the regulating button. The wippen is still moving upward with the motion imparted to it by the key, but its motion now carries the jack backwards and away from the hammer butt, so that the hammer continues forward to the string with whatever momentum has already been imparted to it. You can prove to yourself that your finger doesn't actually cause the hammer to go all the way to the string by depressing a key very slowly. You will see the hammer let off and drop back slightly before it reaches the string.

Since on an upright, gravity and recoil are not necessarily sufficient to bring the hammer back, the hammer butt spring and the bridle strap are provided for the purpose. But it is not desirable that the hammer should travel all the way back to the hammer rail every time it is returning from the string, so the back stop, which is falling backward with the hammer butt, contacts the back check, which has been rocked forward by the wippen.

As the key is released, the wippen returns toward its rest position, incidentally lowering the back check and the damper spoon, but more importantly lowering the jack. At this point, the jack spring pushes upward on the jack knuckle, rotating the jack forward so that its upper end slides underneath the hammer butt. Even before the hammer has returned to its rest position on the hammer rail, the jack is in position to impel it forward again, thus allowing for a quick repetition of the note. The means of allowing repetition on a grand piano action is even more sophisticated, as we shall see.

A close scrutiny of the action diagram will show that the moving parts are attached to flanges, which are usually made of wood, though sometimes brass or even plastic is used. The attachment is effected by means of a brass pin known as the center pin. The center pin is free to rotate in the felt bushing built into the flange. Some modern pianos are now using Teflon bushings instead of the traditional felt, but regardless of the type of material used, it is of the utmost importance that the center pin be neither too tight nor too loose in the bushing, so that it will rotate smoothly.

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Now let's turn our attention to the grand piano action, as shown in Fig. 3. At first glance, we can see some old friends. The key, capstan, wippen, jack, and hammer are all present and doing their accustomed duties, although the hammer must now travel upward to strike the string. This is an advantage when it comes to returning the hammer to a position where another stroke can be made, as we now have gravity on our side, and in consequence don't need the bridle strap and the hammer butt spring. Another difference is that the damper lever is now activated not from a spoon on the wippen but from the key itself. A vertical wire leads from the damper lever up to the damper, and when the key is released, gravity pulls the damper back onto the string, a motion which must be accomplished in the upright with a spring.

The most critical difference in the grand action is, however, the presence of the repetition lever, which was invented in 1821 by Sébastian Érard. What this does basically is to hold the hammer slightly higher as it falls back, allowing the jack to slide back into striking position under it (in this case under the hammer roller, also called the hammer knuckle) before the key has returned to its rest position. Needless to say, this makes possible a very rapid repetition of a note.

Let's look at the process in detail. The back of the key rises and the capstan pushes up on the wippen, as before. The wippen transmits this motion to the jack, which passes through the repetition lever to contact the hammer knuckle. The hammer now moves toward the string, and as on the upright, the jack is tripped by the regulating button and pulled away, so that the hammer strikes the string under its own momentum.

The repetition lever has also been moving upward under the impetus provided by the wippen, until it is halted by the drop screw. When the hammer rebounds downward from the string, the jack is no longer beneath the knuckle, so it lands on the repetition lever instead. The downward force is greater than the tension in the repetition lever spring, so the lever pivots on its flange and the downward motion continues until the hammer lands on the back check, which is still above rest position because the key is still depressed.

As the key is released, the hammer is released by the back check, and now the repetition lever and its spring push upward on the hammer knuckle. Simultaneously, the wippen and the jack are being lowered, and as the jack falls away from the regulating button, its spring pulls it back in under the hammer knuckle, which has been raised to accommodate it by the repetition lever. The key still has not returned to its rest position, but it can now be depressed again and the hammer will respond.

Confused? Don't be: Just reread the last three paragraphs a few times while referring to the diagrams, and some of the haze should lift. But considering that there are more than eighty working parts, some of them moving faster than the eye can follow, let alone the time it takes to read about them, connected with each key, you shouldn't expect to develop a perfect understanding of the action train all at once. Even diagrams can't tell the whole story. Suffice it to say that it is the repetition lever that gives the grand piano action that certain subtle feel.

Now we come to the pedals. On virtually all pianos, the pedal on the right is the sustain (or "loud") pedal. When depressed, this pedal activates (by a series of rods and levers) the damper lift rod, which lifts all the dampers away from the strings simultaneously.

The left pedal, called the soft pedal, has different methods of operation on the grand and the upright, but in both, the intent is the same. On the upright, the soft pedal moves the hammer rail and the hammers closer to the strings. In this position, the jack cannot impart as much momentum to the hammer, so it strikes the string more softly. In the grand, the soft pedal is also known as the una corda pedal, because when it is depressed the entire action, including the keys, is shifted to one side. As a result, the hammer strikes only two of the strings of a unison instead of all three, or one (in the bass register) instead of both. The latter method of inducing softness offers two advantages over the former, first that the touch of the action, the response of the key to the pressure of a finger, is unchanged, and the second that the third string in the unison, although not struck, is not damped either, and thus vibrates sympathetically with the two strings that are struck, imparting a pleasing mellowness to the tone.

Some pianos have only two pedals, and the function of the middle pedal when it is present may differ widly from one piano to another. On most grands, the middle pedal is the sostenuto pedal. It allows the pianist to select any combination of notes (by playing the appropriate keys) and sustain their sound without affecting the staccato sound of the remainder of the keyboard.

This mechanism works as follows: When a key is played, or even de-pressed silently, its damper rod moves upward, lifting the damper off the string. On the damper rod is a small tab or lip called the sostenuto lip. When the sostenuto pedal is depressed, the sostenuto rod moves forward, and if at that moment a given key is depressed, its sostenuto lip will catch on the sostenuto rod, preventing the damper from falling back onto the string. But once the sostenuto rod is forward, no further damper rods can be affected by it, because their sostenuto lips will remain below the sostenuto rod.

The middle pedal on an upright may do several things—or it may serve a merely decorative function, and have no musical effect whatsoever! Some middle pedals act as a sustain pedal for the bass register without affecting the treble. Others lower a felt mute between hammers and strings to make quiet practicing possible.

In addition to being tuned, your piano should be regulated as often as necessary to insure that all the moving parts work together precisely in the way that the manufacturer intended. It isn't always as obvious that a piano is out of regulation as it is that it is out of tune, but the long-term effects on your musicianship if you continue to play on it without having its innards seen to could be just as severe. By all means, however, call a qualified piano tuner/technician rather than resorting to a screwdriver and a pair of pliers. The Piano Technicians Guild ( has members in all fifty states and abroad who have passed rigorous exams and are qualified to work on your piano. With their help (and a piano), all you need to make beautiful music is talent, determination, and lots of practice.

Here is a video showing the action of an upright piano in slow motion, with each section named.

This video shows the various parts of a grand-piano action in slow motion.

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