block 38 the said bands are guided by the sections 60 to facilitate this guiding action and avoid large frictional forces the inner surface of the sections is covered by an antifriction material 62 generally a plastics material such as nylon teflon or the like furthermore it is contemplated that said covering 62 should be provided with two grooves 64 each of which is adapted to receive one end of the cross members 36 the central block 38 is suspended by way of support means formed for each suspended pulley by a set of at least three support pulleys 65 and 66 situated generally regularly around the corresponding suspended pulley 40 and duly attached directly or indirectly to the frame 68 the lower pulleys 65 are attached to brackets 69 of the frame between each support pulley 66 and the corresponding suspended pulley 40 there is at least one transmission belt 80 running in the peripheral grooves 46 of the pulley 40 the thus suspended central block 38 is adapted to rotate around the axis thereof and preferably the corresponding motor 82 is arranged coaxially with a pulley 66 of each set of pulleys whereby these pulleys 66 have the dual support and drive function each of the rods 54 of the compound disc 50 as said above support one end of a spring 56 the other end of which is attached to the frame 68 in corresponding anchor points 81 during the orbital movement of the of the central block 38 the aperture 48 also obviously orbits and therefore the said compound discs 50 nevertheless the effect of the springs 56 is that during the orbiting movement thereof all the positions of the compound discs 50.