The action - the mechanism linking the keys to the hammers - is the most complicated major component of a piano, with the largest number of parts and the greatest chance for many different things to go wrong.
The action is made of several major groups of parts: action rails and screws; flanges; the wippen assembly; the hammer assembly; and the damper assembly. The action rack must be firmly fastened to the keyboard frame, or the keyboard and action will not function properly.
Action parts need to be repinned when loose, sluggish, or noisy. Loose and dislocated centre pins usually indicate that the action has been exposed to harsh seasonal climate changes.
Other causes include corroded capstans, swollen key bushings, rusty or broken vertical hammer springs, or lack of lubricating graphite on dowel capstans, jack tips, hammer butts and knuckles. Installing a humidity-control system is the best long-term solution.
Felt bushings in action parts should be replaced when over-reamed, worn, missing, hardened by inadvertent spills and glue, or when treated with a lubricant that interacts with the centre pin. Replacing the bushings may be the only solution in some newer Asian pianos, in which centre pin friction tends to increase with use.
Verdigris is a buildup of a greasy green substance on the ends of centre pins and bushings, and is found in older American pianos. Verdigris can increase the friction in shanks and wippens to the point of the action seizing up. A short term solution is applying Protek CLP; a long term solution is to replace the affected parts.