Abstract: A vibration generator has a driven excitation shaft on which are fixed two axially-spaced first eccentric weights. A second eccentric weight positioned between the two first eccentric weights also rotates with the excitation shaft but is angularly adjustable thereon about the longitudinal axis of the shaft in order to adjust the vibration amplitude. Such adjustment employs a pawl mounted on one of said first eccentric weights for two-and-fro movement tangentially of a gear ring mounted on said second eccentric weight and with the toothed periphery of which the pawl engages. The second eccentric weight is lockable in adjusted position by a friction disc mounted on said one first eccentric weight and into frictional engagement with which the second eccentric weight is urged by a set of annular disc springs disposed in compression between the second eccentric weight and the other of said first eccentric weights.