The fundamental problem in watchmaking is friction. There are loads of ways to reduce the friction in a watch, such as jewelled bearings, correct oiling, new materials such as silicon etc. but one of the most straightforward is the shape of the pivots themselves.
In wristwatches in particular, there are two main shapes of pivot: conical and straight.
Straight pivots are used for the workhorse parts of a movement, such as the gear train and the barrel arbour. At ‘larger’ sizes, a straight pivot is perfectly functional and enables the end-shake of meatier parts to be adjusted accurately. But on the finest parts of the watch, which require pivots of an incredibly small size, a straight pivot would be too brittle – it could snap at the shoulder – and so conical pivots are used instead.
Conical pivots are used on Balance Staffs because they are stronger and can be manufactured smaller than straight pivots, which reduces friction. Also, the end shake of a conical pivot is decided by the tip of the pivot, rather than the shoulder on a straight pivot. This also reduces friction. Additionally, some watches use conical pivots on the escape wheel.
Theoretically, you could use conical pivots all over a watch, but this would require every jewelled bearing to have a cap-stone. Not only would it be a servicing nightmare, it is also totally unnecessary.
