is threaded into the lower portion 46 also the screw 68 is used to adjust the biasing force of the spring 52 referring to fig3 and 6 there is shown a pawl and ratchet mechanism generally indicated by the reference numeral 70 for providing incremental rotational movement to clamping rollers 18 and 22 respectively pawl and ratchet mechanism 70 comprises a first ratchet wheel 72 integrally formed with clamping roller 18 and a second ratchet wheel 74 integrally formed with clamping roller 22 referring particularly to fig3 and 5 clamping roller 18 sets on top of a first arm 76 and the ratchet wheel 72 extends downwardly into an annular cavity 75 in the first arm 76 clamping roller 18 and arm 76 are mounted to the supporting frame 16 by means of a screw or axle 78 screw 78 is screwed into the supporting frame 16 however roller 18 and arm 76 are free to rotate relative to the screw or axle 78 and relative to each other the vertical axis 20 extends through the center of the screw or axle 78 the annular periphery of the first ratchet wheel 72 is provided with a plurality of teeth 86 which are engaged by a pawl 88 pawl 88 is mounted in a bore 90 in arm 76 for axial movement toward and away from the teeth 86 in a direction transversely of the vertical.