GB1319621A - Power lift mechanism for an agricultural tractor - Google Patents

Abstract

1319621 Power lift linkages DAVID BROWN TRACTORS Ltd 6 May 1971 [7 Feb 1970] 5978/70 Heading A1P [Also in Division G3] In a tractor power lift linkage 11, 12 control of draught load, weight transfer or position is achieved by controlling the flow of hydraulic oil to and from the lifting ram 14 by a spool valve (21, not shown) operated via a rack and pinion by a split field motor (29) in response to signals from A.C. or D.C. transducers fitted to the linkage or the ram 14. The valve (21), which may alternatively be rotary type or operated by a rotary actuator or two solenoids, has self-centering springs and when in the central position seals the oil in ram 14 to "hold" the linkage. The transducers form arms in resistance bridges (one for each form of control) which are adjustable to set their output signals to zero at the required points of control. The output from one of the bridges is fed by a selector switch (60) to an amplifier (63) and trigger circuit (66, 74, 75) which activates either of the field coils of the motor (29) when the signal is above a preset level. Indicator lights may be connected in parallel with the field coils. The outputs of the bridges may be combined and the transducers may be of capacitive, inductive or piezo-electric types. A manual overide switch (64) enables the control signal to be replaced by a positive or negative signal to "raise" or "lower" the linkage or the trigger circuit to be opencircuited to allow the valve (21) to centre itself and "hold" the linkage. The weight transfer transducer 55 is a potentiometer operated by a pressure sensitive element 53 connected to the oil chamber of ram 14. Alternatively a slave cylinder may be connected to the oil chamber and act to displace the centre of an end-mounted beam carrying strain gauges. Position Control is achieved by cam 45 on rock shaft 18 which deflects cantilever 46 carrying strain gauges 47 or by a potentiometer operated by rock shaft 18. Draught load is sensed by strain gauges 39, 40 on arm 18 engaging the midpoint of transverse beam 37 mounted to the tractor by leaf springs, not shown, and to which lower links 12 are connected and also by strain gauges 35, 36 fixed to the mounting of upper link 11. Alternatively all four gauges may be mounted on the upper link mounting or on arm 18.