to the hooks and feed devices not shown the said needle rods 7 and 8 as can be more clearly seen from the enlarged drawing of fig2 are connected to the shaft 19 by means of two known kinematic mechanisms 22 and 23 respectively which transform the rotary motion of the shaft 19 into the vertical alternating movement of the needle rods 7 and 8 which slide in the bushings 24 25 26 and 27 respectively according to the new features of the present invention the top section 2 in the zone lying between the needle rods 7 and 8 is divided into two parts 28 and 29 provided with adjusting means which are located both at the top and at the bottom of the top section 2 and consist of the guide slots 30 provided in the part 28 and of the locking screws 31 mounted on the part 29 the shaft 19 is constructed in two parts and is formed with coaxial mating end parts 32 and 33 to provide connecting means consisting respectively of a grooved area 34 on the outside and of a grooved area 35 on the inside by this means the two parts are permanently locked during rotation of the shaft 19 but are also able to slide axially relative to each other with the distance between the bottom sections 5 and 6 needs to be varied the screws 31 are loosened and the parts 28 and 29 are slid relative to each other until the abovementioned.