a hollow support body having a first end provided with a securing plate and a second end provided with a cylindrical cavity, said cavity having an inner bottom which has a first contact surface, said first contact surface having a circumference provided with a plurality of protruded teeth, said bottom further having a circular recess provided with a limiting block and surrounded by said teeth, said circular recess having a center through hole;

a hollow rotating section having a bottom which has a second contact surface having a circumference provided with a plurality of protruded teeth engageable with said protruded teeth of said first contact face, said bottom of said hollow rotating section further having a circular protrusion which is provided with a center threaded hole, said circular protrusion having a circumference provided two protruded lugs, said circular protrusion being rotatably engaged with said circular recess of said hollow support body and range of said hollow rotating section being restricted when a respective one of said outwardly protruded blocks is in contact with said limiting block of said hollow support body, said hollow rotating section being connected with a circular disc provided with a hexagonal recess, said circular disc having a third contact face provided with a plurality of teeth;

a first bolt extending through said center hole of said hollow support body to engage with said threaded hole of said rotating section;

a hollow securing section having a top provided with a securing plate, said securing section having a lateral side provided with a fourth contact face which has a plurality of protruded teeth rotatably engageable with said teeth of said third contact face of said hollow rotating section; and

a second bolt extending through said hollow securing section to engage with a nut.