Balance weight transfer device for a vibrator
FIELD OF THE INVENTION 
     This invention relates to balance weight transfer device for a vibrator, and more particularly a weight for a vibrator of which the vibration speed is freely changed within a given range. 
     BACKGROUND OF THE INVENTION 
     In the field of construction of buildings and the like, a vibrator or vibrators are used for providing certain vibrations into wet concrete, cement and the like. In this case, the vibration speed (v.p.m.) of the vibrator is selected according to the kinds of the cement or slab structures. For example, vibration speeds of 1,500 v.p.m., and 9,000 v.p.m., are provided for rough particle structures and finely divided particle made structures, respectively. Provision of such vibration causes air gaps in the interior of the concrete, cement and the like to be eliminated and causes an increase of strength of the concrete. A fine aesthetic finish is achieved. 
     Conventional, known vibrators for such use employ a weight of a given weight on a rotary shaft of a motor and the motor is arranged to be rotated in a given direction, such vibrators have a given vibration speed, centrifugal force and vibration bandwidth. When the vibration speed is intended to be changed because the particle sizes of the structure materials are different, different vibrators conventionally have been needed, and such a requirement has proven quite uneconomic. 
     SUMMARY OF THE INVENTION 
     To overcome said conventional defects, a primary object of this invention is to provide a balance weight for a vibrator to permit a selection of variable vibration speed according to the object materials to be vibrated. This invention provides such a weight transfer device for vibrators comprising a motor with varible speed rotation having an output shaft, a balance weight of generally sectorial shape of which a major portion is securely mounted on the output shaft, a projection having a hollow chamber therein extending beyond said output shaft and on a radial line of said weight, a further weight, e.g. in the form of a rigid iron steel ball slidably and rotatably received in the chamber at a position close to said output shaft, a slidable plunger of which one end is slidably mated in said hollow chamber and of which another end is slidably received through an opening of the radially outer tip end of said projection outwardly from said projection, and a spring giving a transfer characteristic performance to said slidable plunger toward said ball. As a modification, the slidable plunger is provided at its outermost end with nuts which are threadedly mounted so as to have adjustable positions. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     FIG. 1 is a perspective view of a embodying principles of this invention; 
     FIG. 2 is a front view of the weight transfer device; 
     FIG. 3 is a cross sectional view of the FIG. 2 along line 3--3; 
     FIG. 4 is a operational state frontal view; 
     FIG. 5 is a perspective view of the slidable plunger; 
     FIG. 6 is a perspective view of the weight transfer device of a modification; 
     FIG. 7 is a cross sectional view of FIG. 6 along line 7--7; 
     FIG. 8 is a operational state view of the modification; and 
     FIG. 9 is a perspective view of the slidable plunger and the nuts. 
    
    
     The vibrator 1 is shown including an electric motor 2, of which a side face has a terminal box 3 through which an electric power supply cord 4 is connected. A cover 2a is also is provided for the rotary power output shaft end of the motor. 
     Provided on the output shaft 5 of the motor 2 is the balance weight 6 which is generally sectorial and made of steel or the like. A slit 6b is formed to reach a mating circular opening 6a for the output shaft 5 near either the left or right (clockwise-most or counterclockwise-most) end of the weight 6. For said slit 6b, a resilient piece 7 is formed. The weight 6 is tightly squeezed onto the output shaft 5 by inserting a bolt 8 from the outside of the resilient piece 7 toward the weight body 6. A key 9 is inserted between the weight 6 and the output shaft 5 for preventing a slip. Two weight change bolts 10,10 are shown detachably provided on a side face of the weight 6. These bolts 10,10 are for meeting a requirment for territories where either one of 50 or 60 hertz is employed and changing the weights adjusts for the change in the revolution speed of the motor 2 caused by the change of frequency of the power source. 
     Protrudedly provided on a radial line of the weight 6 is a projection 11, of which base portion fixedly receiving said output shaft 5. The hollow chamber formed in the projection 11 receives rotatably the weight e.g. in the form of a steel ball 13 close to the output shaft 5. A cavity or chamber 12 which is provided has an opening at its radially outer end as clearly shown in FIGS. 2 and 3 and said opening is closed on one side by fixing a closing plate 14 with a screw 15. A screw hole 11a is also shown. 
     Formed at an outer end of the projection 11 is a slit 11b through which a slidable shaft or plunger 16 is slidably received. The slidable shaft 16 is at one end is inserted into the chamber 12 and a disk 16a which is normally circular is fixed on the radially inner end of the shaft 16. Mounted in compression between the disk 16a and the slit 11b side within the chamber 12 is a spring 17, which always presses the slidable shaft 16 toward the steel ball 13. 
     An invertor is provided at the power supply side, although not shown, to adjust and control the frequency of the current to be supplied to the motor and it freely changes the frequency within a given range for supplying the motor 2, as requested. 
     Now, an operation of the device as discussed above will be discussed in detail. When electrical power is not supplied to the motor 2, the slidable shaft 16 presses the steel ball 13 with an aid of stored or repelling energy of the spring 17, through the disk 16a and further presses it against the end of the chamber 12 which is nearest the output shaft 5. When electrical power is supplied to the motor 2, at the given frequency, the motor 2 is driven at a given revolution speed. In response to said revolution speed, centrifugal force is created and in response to said centrifugal force, the ball 13 moves radially away from the output shaft in the chamber 12. As a result, the slidable shaft 16 is outwardly pushed out, compressing the spring 17. This moving or transfering amount is in response to the centrifugal force amount. This transferring amount of the slidable shaft is not only caused by said transfer of the ball 13, but also by a centrifugal force acting on the weight of the slidable shaft 16 itself. As a result, the inertia moment of the weight 6 itself changes and then the vibration speed of the vibrator is determined. Thus, if the frequency of the current to the motor 2 is changed, the frequency of the vibrator is correspondingly changed. 
     Now, referring to FIGS. 6-9 a modification will be hereinafter discussed as to its different features. The like members are designated by like numerals, raised by 100, and the functions are almost similar unless specifically otherwise specified herein. An outer end of a slidable shaft 116 passes out through a slit 111b, and threads 116b are provided around the emergent portion of the shaft. Threadedly mounted on said shaft by the threads 116b are two nuts 118, 119. Threaded adjustment of the nuts 118,119 causes the spring 117 to be more compressed or more released, so that the pressing energy of the slidable shaft 116 against a steel ball 113 is adjusted. In addition to the first embodiment, due to the existence of the nuts, 118,119 the weight changes to a great extent, and hence, the intertia moment changes more greatly, and thus the vibration adjustability is greatly enhanced.