Any part that moves is subject to wear. Common friction damage will be evident at pivot points, jewel holes, and setting and winding gears. When a watch is serviced all points of movement are lubricated. After years of use the lubricant dries up and jewels and pivots can experience extreme wear. Dust can also mix with the lubricant that has migrated to other parts of the watch causing malfunction. As pivots wear out, the wheels (the term used to define gears) can start “leaning” and cause damage to gear teeth. The more complicated the watch (calendar, automatic, chronograph functions) the more parts that are subject to wear. Impact damage can result in bent/broken pivots and cracked jewels.
Each time the watch is wound, or the stem is pulled out to set the watch, small levers are exposed to quite a bit of friction. This can cause some severe non-repairable wear on these parts.