between the legs at the bottom at the bottom end of the legs within the housing 51 the drive system 50 comprises a main drive roller 60 and a plurality of idler rollers 61 62 63 and 64 it is most convenient to provide four such idler rollers as shown so as to provide effective driving for the belt while wrapping the belt around the majority of the peripheral surface of the main drive roller 60 the idler rollers and the main drive roller are carried on suitable bearings attached to the side walls 52 and 53 the main drive roller 60 has a drive shaft 65 which projects through the side wall 52 in one embodiment the shaft carries a conventional pto coupling 66 which is thus fixed to the side wall 52 and substantially at a fixed position relative to the axle 13 the pto coupling is therefore substantially stationary even as the tube 10 is raised and lowered between the extreme positions the stationary position is obtained since the distance between the shaft 65 and the axle 13 is relatively small the leg assembly 22 is relatively long and the amount of movement of the bracket 23 is relatively small in an alternative arrangement shown in dashed line.