If the balance wheel’s weight is not correctly distributed, then you will end up with a heavy spot, or an unpoised balance wheel.
When a watch is in either the dial up or down position, the balance wheel will be horizontal and therefore the effect of any uneven weight will be virtually negated. However if the watch was placed on its side; in the 12, 3, 6 and 9 high positions, then as the balance wheel will be oscillating the heavy spot will have a noticeable effect on the timekeeping on the watch, as it will add and subtract momentum from the oscillating balance wheel
This is called a positional error. As the watch has a fault in a certain position.
Positional error is not always caused by a poising problem. In a modern watch balance wheels are built, assembled and adjusted by machines and so they almost always leave the factory without issue. A more common cause of a positional error would be a fault with the balance spring or with the balance wheel jewels or pivots for example.
However if the balance wheel otherwise looks fine, then it is fairly straightforward to check for a poising problem and correct it if necessary. This is called static poising.
The balance wheel with the hairspring removed. In normal conditions the weight of the wheel should be perfectly balanced; giving rise to its name. The rotational symmetry of the wheel is only interrupted by the impulse jewel, however the additional weight of this is balanced by the material that is removed for the notch on the safety roller This is a poising tool. The most important part of it are the ruby jaws. You can adjust the heights of each of the legs of the tool independently , as well as open and close the jaws to allow for different heights of balance wheel Ruby does not wear, but chips when damaged. That means that you can be confident that an unchipped ruby jaw is perfectly flat. Some poising tools use steel jaws which although cheaper, will need reflattening from time to time The first thing we need to do is ensure that the jaws are parallel to one another. Here you can see that the jaw on the right is very marginally higher than the one of the length The jaws can be adjusted independently to one another, and so here we have lowered the right jaw to ensure that it is perfectly parallel to the left one. You need to make sure that the jaws are parallel along their entire length Contrary to perhaps your first impression, the spirit level should only be used on top of the jewels, rather than the position on the right where it is stored. This is because the only part of the tool that matters that is it flat are the jaws. You can adjust the legs as necessary to ensure that the jaws are perfectly flat along their entire length Before placing your balance wheel on the jewels, you should give the jaws a quick clean. In this instance we are using some pith wood, which will ensure that there is no dust or grease along the jaw’s edges Holding the balance wheel with some tweezers, it is important to not try and grab or pinch it. You can then safely lower the wheel so that the two pivots are resting on the jaws. You can adjust the width of the jaws to fit the distance the pivots are apart Here the balance wheel is in the correct position on the poising tool. The only part of the wheel touching the tool are the pivots and it is set up straight. It is critically important to be aware of your environment when testing the wheel. A draft of air from an open window will move the wheel, as will a person walking past your desk, as will your own breathing. You may therefore find it useful to test the wheel in a secluded area To move the wheel we just need a single hair. You can steal one of your cat’s whiskers, or alternatively you can bend a hair from one of your brushes as shown With the lightest of touches, you should stroke the top of the balance wheel with the hair. The wheel will rotate. What we are looking for is for the wheel to move at a steady pace, and come to a gradual stop. If it goes fast then slow, or ever swings back in the opposite direction, even a little, then that shows that the wheel’s weight is not evenly distributed. In such cases when the wheel stops the heavy part will usually rest at the bottom. Test the wheel a few times to ensure that the wheel always stops at the same place When you have identified the heavy spot you can adjust the wheel by removing material. There are specialised tools for the purpose, but in this example we will use a tungsten drill bit. With the balance wheel resting on a staking block we place the end of the drill bit at the heavy spot. Double and triple check that you are going to remove material from the underside of the wheel i.e. the side with the impulse jewel. With a slight twist of the drill bit we will remove some material The golden rule is that a balance wheel that is poised should never look like it is! Less is more, do not get carried away, as you cannot add any material back once removed. If you do make a mistake you will have to counteract your first attempt with additional spots to ensure that the wheel is balanced. What you are looking for is a spot that does not touch the edge of the width of the wheel. A wheel that is poised with one spot is superior to one that has 4 spots Recheck the wheel each time you remove material. Remember if the wheel rolls back on itself at all, or ever speeds up as it turns, then the wheel still has a heavy spot. Once you have corrected the wheel it will gradually and evenly come to a stop.
There are some additional ways that balance wheels can be poised, or the effects of an unpoised wheel can be averaged out.
Rather than removing material, some balance wheel have timing screws set around the rum of the wheel. By moving any of the screws highlighted at (1) you can have the same effect as adding or removing weight from the wheel. By moving the screws set at (2) you can adjust the speed of the oscillation rate Some of the highest end of watches have their balance wheel set in a tourbillon carriage. Simply put; the whole escapement rotates independent of the rest of the train. If a balance wheel has a heavy spot in this instance, then any negative effect will be averaged out The tourbillon is seen as one of the pinnacles of watchmaking, although the positive effect on timekeeping is generally very limited. This example is a double axis tourbillon, when the whole escapement spins both vertically and horizontally