Piano tuning pins are basically screws. They are the most sensitive part of the piano, as they carry out the most important job
Their job is to keep the string at a certian pitch, they are sensitive because they are screwed into wood. Humidity changes can make the wood that holds the pin contract, causing the pin to go loose, they can also expand. This exansion and contraction causes the pin to loosen.
The best was to keep your pins healthy is to have the piano tuned regularly at between 6 months to a year intervals. So that they do not need over-moving to compensate for dropped pitch in the strings. If they are left for a very long time the tuner willl need to overcompensate for the drop in pitch, causing the pin to go even further and push harder against the wood.
Normal to and throw causes the pin to loosen over a period of several years, even with 6-month or yearly tuning. However, regular pitch-raises will have a more severe effect.
For a piano tuner in Oxfordshire England contact us at (01865) 240 634, for other areas of the UK search the PTA, for us try the PTG.