Abstract: An unbalance vibrator having a shaft 2, with a flyweight 3 rigidly connected thereto, which shaft 2 is mounted in a bearing housing 1, comprises on both sides of this connecting flyweight movable flyweights 5 with flyweight bodies 6 nearly semicircular in cross section and guiding rings 7 nearly semicircular in cross section. The shaft 2 comprises a radial bore on diametrically opposite sides, into which a dog 8 is screwed, and a blind bore 11, into which a locking member 12 is retracted by action of a spring 17. The radial bore 3 and the blind bore 11 are angularly spaced by 90.degree. with respect to the center plane of the flyweight 3 rigidly connected. When the shaft 2 is rotating, the locking member 12 extends from the blind bore 11, such that the movable flyweights 5 are held between the dogs 8 and the locking members 12. The locking members 12 are subjected to hysteresis and are retracted at a rotary speed, which is lower than the loading rotary speed.