includes upper and lower bodies 60 and 70 that are joined to each other the upper body 60 has a plurality of grooves 61 formed concentric with the axis spaced radially from each other on the lower surface the innermost space between the grooves 61c and 61d is the smallest narrowest of all and the outermost space between the grooves 61a and 61b is the largest widest the lower body 70 has on the upper surface a plurality of upwardly projecting ribs 71 that are axially formed to correspond to the grooves 61a 61b 61c and 61d seals 50 are respectively interposed between the grooves 61 and the ribs 71 for forming a seal a plurality of screw holes 65 and 75 are respectively formed along the sides of the outermost groove 61a and the outermost rib 71a the ribs 71a 71b 71c and 71d of the lower body 70 are fitted into the corresponding grooves 61a 61b and 61c of the upper body 60 together with the seals 50 and screws are tightened to easily and firmly couple the upper and lower bodies 60 and 70 together as shown in fig9 b according to the third preferred embodiment of the present invention employ a plurality of balls 74 and liquid are provided in the respective tracks 72a 72b and 72c formed to be different in size by the ribs 71a 71b 71c and 71d the balls are designed to be of different sizes corresponding to each track in which the balls are contained the smallest ball size 74c is located in the innermost track 72c and the biggest ball size 74a is in the outermost.