disc which comprises a substantially square metal frame with a toothed wheel on either side of the frame and a pivoted spring activated pawl 27 that engages the teeth on both of the toothed wheels simultaneously this rotation control means 26 is shown in fig2 or 4 the embodiment of the apparatus shown in fig4 includes a latch 29 that can be used to disengage the pawl 27 from the ratchet 26 and thereby allow free rotation of the shaft 24 in one embodiment the rotation control means 26 functions to prevent the unrestricted rotation of the shaft 24 in one direction only and can be used therefore to prevent the blade from falling when it is being lifted towards the moldboard in an alternative embodiment the rotation control means 26 functions to prevent the unrestricted rotation of shaft 24 in a selected one of both directions of rotation and can therefore be used to control both the lifting and the lowering of blade 16 although shown and disclosed herein as a ratchet any device or combination of devices that would control the rotation of shaft is intended to be included herein an example of the assembly of embodiment 10 a that the inventors have used is herein provided.