PURPOSE: To accurately regulate the quantity of pre-load in a spindle unit for a machine tool or the like by insertedly providing an axially movable bearing case for supporting a spindle inside an outer sleeve and an axially expansible piezoelectric element between the outer sleeve and the bearing case, and changing voltage applied to the piezoelectric element according to the change of the number of revolutions of the main spindle.
CONSTITUTION: In establishing heavy pre-load in a low speed rotational domain, high voltage is applied to a piezoelectric element 11 to elongate it to the maximum, and oil is thereby drawn from a left hydraulic chamber 23, and supplied to a right hydraulic chamber 24 for its pressurization. A middle ring 10 is thereby moved left to make a regulating member 16 abut on the end face 1a of an outer sleeve 1 for fixing the member 16, and a functioning member 9 abut on stepped parts 20,22 for giving pre-load P1 to bearings 4-7. A voltage applying device 13 computes the quantity of pre-load on informations given by a revolution detector 14 and a temperature detector 15 as the number of revolutions of the spindle 2 increases to obtain the movement quantity of a bearing case 8, and the dimensions of the piezoelectric element 11 are therefore determined for the control of the applied voltage.
HIBI KENJI
FUJII KENJI
