You press a key, and a note sounds.
In a nutshell, here’s how an upright works: Pressing the key down lifts the back of the key up. This raises the whippen, forcing the jack to tilt the hammer mechanism toward the string. Halfway toward the string, a spoon on the back of the whippen lifts the damper off the string, and just before it hits, the force on the hammer is removed when the jack is tripped out from underneath the butt. When you let go of the key, everything in the action returns to its original position because of springs. After the string has been struck by the hammer, it vibrates across its entire length in a sine-wave. This vibration is transferred to the soundboard through the bridge, and the soundboard, like a speaker, amplifies it so that you can hear the sound loud and clear.
A grand works in a different way. This is because instead of using a spring to return the action to a reset position, it uses gravity. There is also an extra part to the whippen called a “repetition lever” which resets the jack so quickly that you can play the note again, with having to return the key to the fully “up” position. This allows for a much faster repeat, and is one of the main benefits of a grand over an upright. Because it uses gravity instead of springs, the touch is more consistent. Also, because of the repetition lever, each key has a noticeable little click that you can feel near the bottom of the key stroke. You can feel it, but you can’t hear it. Good pianists can use this to increase the accuracy of their touch.
That was the nutshell, here’s the meat: Every moving part in a piano is adjustable to a certain extent – some have adjustment mechanisms built in, others are made to tolerances that can only be changed using very specialized tools. There is a roughly universal value to some of the movements of the action parts, however they can differ from piano to piano, make to make. How high a key sits, for example, is based on having a small gap between the top of the keytop tail, and the bottom of the fallboard. However, wood warps and wears, so the height is adjustable by using small paper washers of different thicknesses underneath the fulcrum of the key.
How far it depresses is also (relatively) standard, but the felt washer underneath the key not only can vary slightly in thickness, but it compresses. Therefore, paper washers underneath that felt Front Rail Punching regulate the “Dip” of the key to the desired amount.
The key body is weighted with small lead weights to not only ensure that the key has a certain amount of resistance, but to also ensure that it will return quickly to the rest position quickly. These weights are installed in the factory, and although they can be removed or repositioned, it is a complicated process which should only be performed by a technician specifically trained to weight keys.
At the back of the key, an adjustable capstan “connects” the key to the action. If it is too low, there is space between the jack and the butt, which creates “Lost-Motion”. If the capstan is too high, the jack won’t reset underneath the butt.
There are adjustments all through the action like this, and although some tolerances can be reasonably large (such as spring tension), most have to be finely adjusted or the action will not play the way it was intended.
The overall geometry of the action is set during the design and building stage, and changing any of these measurements can literally make the piano unplayable. For example, the hammers must hit the string at a very precise point in its length, and a precise point on the hammer itself. If these are altered even a fraction of an inch, the tone of the piano will be negatively affected.
Even a seemingly small thing, like altering the size of the whippen heel by a few millimetres, can change the weight of the key from normal, to unbearably heavy.
Like the action, the tolerances and adjustments of the back and belly are many, varied, and critical. In order for a single string to have the correct tone, it must be exactly the right length, tension, and thickness. All these measurements must be in balance not only for that one string, but across all 240+ strings.
If you have to have a shorter string for the same note in a smaller piano, you have to increase the thickness, and this can negatively affect the tone – likewise if you increase or decrease the tension as well. If there is too much tension in one part of the back, that area could go out of tune faster than the rest of the strings. As you can see, getting all of these things (called “Scaling”) right, is complicated and critical. Each string is under a lot of tension – ranging from 120 – 160 lbs per inch. Across the whole scale, that adds up to 14 to 16 tons of pressure. That’s enough force to tear a two-car garage off its foundation, and because of this, not only does the plate and the back have to be very strong, it has to be designed to hold that tension, and more. Each string is held in place, and up to tension, by the tuning pin on one end, and by the anchor pin on the other end. A few (very terrible) manufacturers experimented with having anchor pins being cast into the plate. They soon found out that they could break off during stringing, shooting the pin like a bullet toward the stringer.
The tuning pin has to be tight enough so that it will hold the tension, but loose enough to be able to turn, enabling the string to be tuned. The pin has to be on a slight angle and set at a certain depth so that as the string is wrapped around itself, it makes a neat, tight wrap, and doesn’t wrap on top of itself.
The point at which the string interacts with the bridge is so critical, that not only does it have to be at an exact point on each string, it has to be at an exact height. An error in these measurements can not only ruin the pianos tone, but its ability to sustain the tone as well.
– Jamie Musselwhite