126 PKACTICAL INSTRUCTIONS REGARDING THE USE OF 



in the direction of the thread on the shaft, guiding it, and 

 with them the block E and the rollers RR, between which 

 the cable C runs. 



The space between the rollers RR, projected on a plane 

 perpendicular to the cable, is exactly the diameter of the 

 cable, but the space is greater in any other direction, so as 

 to allow the free passage of any irregularity or splice in the 

 cable preventing the rollers from being torn away. 



The winding guide works as follows : Let us assume the 

 machine starting when the rod K is in a ; this arm comes 

 in contact with the thread of this shaft, and its speed is 

 calculated so that the guide M travels at each turn of the 

 drum a distance equal to the diameter of the cable. When 

 the rod K reaches , it is arrested by a block b on the 

 threaded shaft V. The stop forces it to pivot round and 

 engage the other thread. It then travels in the reverse 

 direction, carrying the guide M ; the rod then meets the 

 stop a, pivots and returns, and so on until the winding of the 

 cable is completed. The rod K is made of bronze, and wears 

 out more rapidly than the steel screw, but is easily replaced. 



The cable being the principal cause of the expense in 

 steam ploughing it is necessary to take every precaution to 

 diminish its wear, and, for this purpose, to support the cable 

 on pulleys, so that it is only submitted to rolling friction. 

 When the cable unwinds it is necessary to pass it over a 

 pulley placed near the drum, as it has a tendency to come 

 out of the groove on account of irregularities in the unwind- 

 ing. A good method of keeping the cable in position consists 

 in placing a small roller r in front of the pulley P, entering 

 slightly into the groove (Fig. 82). 



It is not sufficient to protect the cable against sliding fric- 

 tion on the frame of the drum ; it must also be protected 

 against friction on the ground. This is prevented by cable 

 runners. Most of the old systems used for cable supports were 

 not practical ; they could only be kept in certain positions, 

 and were very unsteady when the cable was being moved from 

 furrow to furrow. The cable runner shown in Fig. 81 has 

 proved satisfactory in practice. It is a small truck on three 

 wheels, on which is a frame, carrying four rollers on top ; twa 

 have a vertical axis, the two other rollers revolving horizon- 

 tally ; and they are arranged in such a way that the juxta- 

 position of the groove of the four rollers forms a complete 

 circle. The axle of the top roller can pivot round the spindle 



