for controlling bearing preload value application_number US 73622200 A description referring to fig4 applying an electric field to a piezoelectric material 3 usually will cause the piezoelectric material 3 to produce a small displacement their relationship may be calculated by the following equation where d is piezoelectric constant u03b5 is strain value e is electric field applied to the piezoelectric material 3 based on above equation a greater value of d means that strain has a higher sensitivity in the electric field in the general applications of piezoelectric material 3 most considerations are focusing on extension deformation application along the electric field direction and neglect the contraction of the piezoelectric material 3 in the direction normal to the electric field referring to fig4 in the coordinates composed of xyz axes when an electric field is applied to a piezoelectric material 3 at the z axis direction the piezoelectric material 3 will produce an extension in the electric field direction i e z axis direction and will have contraction in the x and y axes directions shown by phantom lines this invention uses the contraction deformation characteristics of the piezoelectric material 3 to control bearing preload referring to fig5 this invention includes two main portions the first portion includes a preload.