the lead screw 28 independent of the carriage 16 by trapping the lugs 74 in the chamber 90 and by trapping the side wall 92 between the projections 72 and the arm 76 the nut 50 is free however to 34 float 34 specifically due to the presence of a predetermined circumferential clearance between the annular enlargement 70 and the aperture 94 the nut 50 is free to assume an infinite number of positions at right angles to the theoretical axis of the lead screw 28 the key 78 is free to slide within the slot 98 and acts as a sliding pivot for this universal floating of the nut 50 before the lead screw 28 is threaded into the nut 50 a coil spring 102 is placed either over the cylindrical body 54 where the nut carriage assembly is followed by nut screw assembly and the arm 76 is either stiff or flexible or over the screw 28 and the body 54 where nut screw assembly precedes nut carriage assembly the ends of the spring 102 are so dimensioned that one end thereof rests against the shoulder 66 while the other end of the spring rests against the inside of the left carriage side wall 82l the spring 102 is chosen so that it normally urges two of the lugs 74 against the interior of the side wall 92 referring again to fig1 it may be seen that the spring 102 thus urges the drive nut 50 in the same direction as that taken by the carriage 16 as it is moved by rotation of the lead screw 28 from left to right that is to say as the lead screw 28 rotates it pulls the nut 50 along which in turn pulls the.