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Humidity & The Piano Action

I remember, as a boy, my grandmother putting an ax head into a bucket of water. When I asked her why she did that, she told me that the ax head was loose and needed to be tightened. Soaking it in water caused the wood of the ax handle to swell against the iron head thus tightening it.

This same principle is at work in a piano to a much lesser & time-sensitive degree. Most piano parts are wooden & are sensitive to moisture. Too much humidity causes wood to swell, too little, to contract.

First of all, the soundboard of the piano is belt with a slight 'belly' or crown as we call it in the industry.
(picture a guitar or violin with bridge attached)

As moisture increases, the soundboard swells and lifts up. This usually happens in summer when there is more heat, causing water to condense in the cool evenings. When moisture is less, during the winter, for example, the soundboard contracts. When soundboards expand, the bridges rise, and strings tighten or go sharp.

When the soundboard contracts,(dry conditions) the soundboard contracts, strings lose tension & become flat. This is one of the reasons why pianos go flat or sharp over time.

Secondly, there are a number of bearings in pianos.
A bearing is basically a small hole in a wood part lined with felt in which a 'center pin' is inserted.
The pin revolves in this felt-lined bearing.

When wood swells, it closes around the center pin, & the action parts slow down or stick.
The results of all this is that hammers become sluggish, jacks don't return to their position, repetition is hindered, and dampers can remain lifted causing an 'echoing' sound.

Piano keys also have bearings except that they are lined on both sides with felt. The rail pin glides within these bearings. When moisture is excessive, keys pinch against the rail pin causing the key to stick down. Conversely, a dry condition can cause keys to clatter & become loose.

How does the piano technician solve some of these problems?
Most sticky actions can be treated with lubricants that remove moisture & lubricate the bearing. If the humidity is excessive, center pins are removed, flanges are reamed and burnished and a new center pin is installed. In extreme cases, bearings are drilled out
and replaced.

Preventive measures include installing a “Dampp Chaser” humidity control system to regulate the humidity inside the piano. These are purely prophylactic & don't 'cure' sticky actions although they will keep most pianos in better tune since the humidity remains at more of a constant. If you are moving your instrument to a more humid environment, a humidity control system is a wise investment.