How is thermal expansion influence in watches material
The influence of temperature changes in one material is measured by thermal expansion. This expansion affects the volume and length, measured by the coefficient of thermal expansion. This ratio measures the heat for solid energy stored in the atoms, which affects a change linearly as expansion and contraction. The ratio is dL / dT measured longitudinally in microstrain or mm if multiplied exponentially.
Temperature changes in the materials of watches affecting the balance wheel, the hairspring and plates depending on the material. At first, the wheel must be made of an alloy with a low coefficient of thermal expansion as the Nivarox of 11 microstrain / ° C approximately. Also the balance wheel is very sensitive to temperature changes so that its constitution may be in nickel or Glucyidur. Nickel expansion range is 8-13 microstrain / ° C than is the same coefficient of the spiral and is chosen to avoid distortions.
The linear expansion that would result in a brass plate whose distance between axes is 10 mm, depend on the value of the linear coefficient of thermal expansion. The brass is 16.5 microstrain / C °, which increased 10 mm and 20 ° would give a linear expansion between the axes of 0,00033mm. This amount is low to disrupting the movement in respect of plates and bridges.
The wheel balance is made of Glucydur or Nickel. Nickel has a thermal expansion of 13 microstrain x 10-6 what varies in +-3,5 seconds per degree in 24 h with respect of it, but nickle balance are mede of Nivarox, that allows an expansion thermal enough for movement. Glucydur is a cooper-beryllium alloy of high quality, with a very low thermal expansion, so its varying is on 0,6 seconds per degree in 24 h. The hairsprings alloys compensates are made of Nivarox, an alloy of different qualities like Nivarox I, II and III.
Policarbonate or PPMMA have a thermal expansion about 70 microstrains x 10-6. This is a feature for considering in order to design parts like crystal. Anyway, policarbonate much thicker and flat can be made like natural or sapphire crystal. There is to consider for PPMA to put some kind of metal ring for compensating the elasticity or thermal expansion if this part is thinner.
The Theory of Horlogery