What happens to clocks as time passes by? The main things are:
Airborn fluff and dust get into the works then act as a grinding medium. Few clock cases are anything like dust proof and many have to be opened to wind the clock.
Oil thickens with age increasing friction in the clock mechanism. Springs do not unwind evenly so no longer supply steady power.
Old oil, thickened and contaminated
Oil can creep away from bearings onto the teeth of the wheelwork and attract dust to them.
Oily gears on a turret clock
Dust particles get into the oil and are carried around bearings grinding away the metal.
Bearings can become worn elliptical so that gears no longer mesh correctly, their teeth begin to wear away and there is significant increase in friction.
A worn hole in a striking mechanism
Mainsprings can tear their fixing holes. If they break they can do significant damage to other parts of the clock.
Teeth stripped from a wheel when the mainspring broke
Steel parts can begin to rust especially if the clock is in a location which can be damp.
Rusty balance spring on a carriage clock
Hands can work loose then catch on each other either stopping the clock or causing damage
Old repair to a longcase (grandfather) clock’s hour hand which had collided with a date indicator
More about some these clocks and others can be seen on the examples of work done page..