Abstract: Tractor hydraulic power lift mechanisms may have automatic draft control signals fed back to them from lower link draft force sensing systems. One such system has two bowed rearwardly-extending leaf springs anchored to the tractor frame at their front ends, two laterally-spaced arms pivoted at their upper ends to the tractor frame and at their lower ends to the rear ends of the respective springs, a cross-bar universally supported by the lower ends of the respective arms and having two attachment points laterally offset from said arms to which the front ends of respective lower hitch links are universally connectible, and linkage for deriving a control signal from the fore and aft displacement of the mid-point of the cross-bar. However, the cross-bar is stressed in bending to the detriment of the control signal accuracy. To avoid this, a summating bar is universally connected between said arms and the control signal derived from a point on the unstressed summating bar.

Abstract: A very heavy vehicle may tend to run away on a steep gradient and as a result the engine is liable to be overspeeded with consequential risk of damage especially to its valve mechanism. The invention aims to prevent overspeeding by providing a servo mechanism operable automatically above a predetermined engine speed to apply an exhaust brake. The servo mechanism preferably comprises a centrifugal clutch normally held disengaged by springs and disposed on a shaft driven by the engine. Angular movement of the driven member of the clutch due to its engagement by engine speed in excess of the predetermined value is limited to, say, 60.degree. by a stop, and said member is mechanically linked to the operating means of the exhaust brake.