[Illustration:  FIG. 18 ROVING FRAME By Permission of Messrs. Douglas Fraser & Sons, Ltd.]

A roving machine at work is shown in Fig. 19, and it will be seen that the twisted sliver or rove on emerging from the drawing rollers passes obliquely to the top of the spindle, through a guide eye, then between the channel-shaped bend at the upper part of the flyer, round the flyer arm, through an eye at the extreme end of either of the flyer arms, and finally on to the bobbin.  Each bobbin has its own sliver can (occasionally two), and the sliver passes from this can between the sides of the sliver guide, between the retaining rollers, then amongst the gill pins of the fallers and between the drawing (also the delivery) rollers.  Here the sliver terminates because the rotary action of the flyer imparts a little twist and causes the material to assume a somewhat circular sectional form.  From this point, the path followed to the bobbin is that described above.

As in all the preceding machines, the delivery speed of the sliver is constant and is represented by the surface speed of the periphery of the delivery rollers, this speed approximates to about 20 yards per minute.  The spindles and their flyers are also driven at a constant speed, because in all cases we have—­

  spindle speed = delivery x twist.

There is thus a constant length of yarn to be wound on the rove bobbin per minute, and the speed of the bobbin, which is driven independently of the spindle and flyer, is constant for any one series of rove coils on the bobbin.  The speed of the bobbin differs, however, for each complete layer of rove, simply because the effective diameter of the material on the bobbin changes with the beginning of each new layer.

The eyes of the flyers always rotate in the same horizontal plane, and hence the rove always passes to the bobbins at the same height from any fixed point.  The bobbins, however, are raised gradually by the builder during the formation of each layer from the top of the bobbin to the bottom, and lowered gradually by the builder during the formation of each layer from bottom to top.  In other words, the travel of the builder is represented by the distance between the inner faces of the flanges of the rove bobbin.

[Illustration:  FIG. 19 ROVING FRAME FAIRBAIRN’S ROVING FRAME IN WORK]

Since every complete layer of rove is wound on the bobbin in virtue of the joint action of the spindle and flyer, the rotating bobbin, and the builder, each complete traverse of the latter increases the combined diameter of the rove and bobbin shaft by two diameters of the rove.  It is therefore necessary to impart an intermittent and variable speed to the bobbin.  The mechanism by means of which this desirable and necessary speed is given to the bobbin constitutes one of the most elegant groups of mechanical parts which obtains in textile machinery.  Some idea of the intricacy of the mechanism, as well as its value and importance to the industry, may be gathered from the fact that a considerable number of textile and mechanical experts struggled with the problem for years; indeed 50 years elapsed before an efficient and suitable group of mechanical parts was evolved for performing the function.