the intermediate wheel 15 to allow the yarn y to be threaded round the yard feed wheel 14 the yarn feed wheel is then returned to its operating position to nip the yarn between the yarn feed wheel 14 and intermediate wheel 15 thus the yarn is driven upon the rotation of these two wheels in the position shown in fig1 the left hand end of the arm 9 is in its uppermost position in which the intermediate wheel 15 engages with the first drive roller 5 such that the yarn y is driven at a speed determined by the first drive roller 5 when the pneumatic actuator 12 is actuated the left hand end of the arm 9 is moved downwardly bringing the intermediate wheel 15 into engagement with the lower drive roller 6 hence driving the yarn y at a speed determined by the second drive roller 6 upon leaving the yarn feed roller assembly the yarn y is fed through a pair of gear type puller rolls 16 which brush against the yarn rather than driving it and serve to maintain the tension in the yarn while isolating the yarn feed assembly from variations in the yarn tension caused by a reciprocation of the needles 3 the yarn then extends through a pair of guide plates 17 18 and then to the needles 3 the way in which the machine is controlled in order to produce the various pile heights will now be described with reference to fig2 the tufting machine is provided with pattern data which contains information on the required height of each stitch of the pattern a control means receives this data and controls the timing of actuation of the pneumatic actuator 12 accordingly in the.