Device for regulating the tractive force in lower linkages of tractor lifting gear

The invention provides a device for regulating the tractive force in the lower linkage system of a tractor lifting gear which serves, for example, to hold a plough. The lower linkages on which the tractive resistance, acting on the plough, is exerted transmit the tractive force to a bending bar, the bending of which is recorded as a measurement of the tractive force. In accordance with the invention the bending bar is rigidly coupled to a bearing frame which is rigidly mounted on the tractor and which absorbs the force acting on the lower linkages while the bending bar is subjected to bending strain. This means that the lower linkages do not exert a force directly on the bending bar, and the bending bar can therefore be supported free from play.

The invention relates to a device for regulating the tractive force in the 
lower linkages of a tractor lifting gear. 
Conventional devices of the above-mentioned type have an actual or 
instantaneous value recorder for the force in the lower linkages in the 
form of a bending bar which is mounted on the rear of the tractor in such 
a way that, when loaded, for example by the tractive force during 
ploughing, the bending bar is deflected in a quite specific manner. The 
bending bar is mounted symmetrically on the rear of the tractor and it 
forms, with its ends projecting above the bearings, the link pins for the 
lower linkages. The deflection of the bending bar is dependent on the 
tractive force, and it is determined by the variable distance measured 
between a fixed point on the tractor and the maximum 
deflection/displacement of the bending bar. The measured distance is the 
controlled variable for the feedback transducer. 
This device has the disadvantage that, during this measurement of force, 
the bearing clearance in the event of load changes (from the tractive to 
the compressive load range, and vice versa) is included as a measurable 
variable for the tractive force or compressive force to be measured. 
The object underlying the invention is therefore to provide a device for 
regulating the tractive force in the lower linkage of a tractor lifting 
gear, in which the bearing clearance of the bending bar does not have any 
influence on the function of the controller. 
The replacement, in accordance with the invention, of the bending bar by a 
bracing, which is rigidly connected to a tractor-mounted bearing frame 
consisting of longitudinal sections or a linkage frame and which can be 
deformed while the forces of the lower linkages are initiated, ensures 
that the measurement transducer measuring the deformation of the bending 
bar no longer exhibits a `jump` in the recording of measured values when 
the lower linkage force changes from the compressive to the tractive force 
range, or vice versa. 
In this case the measurement transducer can as before be mounted on the 
towing vehicle. 
However, it is particularly advantageous if the measurement transducer, 
uncoupled from the tractor, is seated directly on the bending bar, because 
in this way relative movements between the tractor and bearing frame or 
lower linkage cease to have any effect on the measurement transducer. 
A preferred embodiment of the present invention results in a particularly 
simple and judicious arrangement of the device. In this connection the 
longitudinal trailing links or swinging arms absorb a major part of the 
tractive and compressive forces of the lower linkages; only a smaller 
proportion of the force of the lower linkages causes deformation of the 
bending bar. As a result, the latter can have smaller dimensions in 
relation to prior art without the reliability or durability of the device 
being reduced. Since the bending bar must be machined from a special 
material and must have a special quality, this device also provides an 
economic or commercial advantage. 
A further particularly advantageous result may be obtained in accordance 
with a preferred embodiment of the invention. Any transverse movements of 
the bending bar which may occur upon deflection of the bearing frame at 
right angles to the direction of the lower linkages-for example, when 
loads are transported-can no longer transmit transverse shearing forces to 
the measurement transducer. As a result, the measurement transducer is 
protected from the introduction of undefined shearing stresses, and the 
reading or indicating accuracy of the device is increased.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
FIG. 1 shows a hydraulic control circuit for regulating the tractive and 
compressive forces Z and D, respectively, in the lower linkages U of a 
tractor lifting gear SH. Each lower linkage system U is articulated to the 
tractor 1 by one end via a bearing frame T and connected to a 
hydraulically actuated working cylinder 5 via a lifting mechanism 
consisting of a lifting linkage 2, a bell-crank lever 3 and a slide rod 4. 
The other end of the lower linkage U is connected to a working equipment, 
for example a plough, the force (tractive force) of which is proportional 
to the ploughing depth, i.e. to the position of the lower linkage U. In 
order to keep a constant ploughing depth, the working cylinder 5 is 
controlled by a directional control valve 10 integrated in a control 
circuit RK which also operates hydraulically. However, it is also 
conceivable to have a control circuit which operates electrically and 
which, for example, displaces the valve by way of servomotors or controls 
the working piston directly by electrical means so that the control system 
is no longer dependent on the hydraulic system of the tractor. 
In the illustrated case the working cylinder 5 is supplied with hydraulic 
fluid from a storage reservoir 7 via a working line 11. For this purpose a 
working pump 8 takes in hydraulic fluid through a suction line 9 and pumps 
it via the directional control valve 10 (in the illustrated case, a 
3/3-way valve) to the working cylinder 5 where the fluid performs work 
while displacing a lifting piston 6. 
The three-way directional control valve 10 is controlled by a control 
pressure circuit via control devices 12 and 13 arranged on both sides of 
the valve. The control pressure circuit has a control pump 14 which 
supplies the control circuit with hydraulic fluid via a supply network 15. 
When control devices 12 or 13 are not being actuated, the directional 
control valve 10 is maintained in the illustrated central position by a 
set of control springs 16 and 17. On one side of the directional control 
valve 10 a pressure is applied which is determined by two fixed jets 18 
and 19. The pressure on the other side of the directional control valve 10 
is determined by the drop in pressure at the two variable throttles 20 and 
21. The throttle 20 functions as a set-point adjuster and the throttle 21 
as a feedback transducer of the control circuit. The throttle 21 is 
connected to a force transducer KA (not shown) for the lower linkage U by 
mechanical, electrical or hydraulic coupling means (indicated by reference 
numeral 22) so that this connection serves as a feedback for the control 
circuit. 
The illustrated position represents the adjusted or reset position of the 
directional control valve 10. In this case the throttle 20 determines the 
height of the lower linkage. The tractive force Z or the compressive force 
D in the lower linkage U is such that the pressure between the two 
throttles 20 and 21 is equal to the pressure between the two fixed jets 18 
and 19. 
If for some reason the force in the lower linkage deviates from the 
adjusted value, for example when the plough penetrates too deeply into the 
soil when ploughing over a rise in the ground, the throttle cross-section 
of the feedback transducer (throttle 21) changes and the pressure which 
becomes effective on the control device 13 increases. Since the pressure 
on the side of the control device 12 is constant, the directional control 
valve 10 is in this case displaced into a position in which hydraulic 
fluid is pumped into the working cylinder 5 so that the lower linkage U is 
lifted by way of the lifting mechanism. The force in the lower linkage U 
is reduced thereby and the throttle 21 is re-opened slightly so as to 
establish a new state of equilibrium in which the force in the lower 
linkage is reset, but the position of the lower linkage is changed. 
FIG. 2 shows the construction of a device for generating the controlled 
variable, which is proportional to the tractive force in the lower 
linkages, for the throttle valve of the control circuit. The device is of 
symmetrical construction and has a bearing frame T consisting of two 
longitudinal linkage sections 30 which are arranged parallel to one 
another, which are aligned in the direction of travel, and which are 
connected to the tractor 1. A bending bar 31, which is welded to the 
linkage sections 30, acts as a bracing for the latter and forms a closed 
frame. The linkage sections 30 project slightly above the bending bar 31 
and, in these projecting sections, they each receive a pin 32 which 
extends through a further bearing plate 33 rigidly connected, for example 
welded, to the bending bar 31. Each lower linkage U is mounted by way one 
of the pins 32. When the lower linkages U are under load, the closed frame 
is deformed elastically and symmetrically, the bending bar undergoing the 
greatest deformation. This displacement caused by deformation is recorded 
by a measurement transducer 34 which is mounted on the tractor 1 and 
which, by way of a key 38, transmits the deflection of the bending bar 31 
to the feedback transducer (throttle 21) via the coupling means 22. 
FIG. 3 shows a device having a construction substantially identical to that 
of the device shown in FIG. 2. However, the measurement transducer 34' is 
uncoupled from the tractor 1 and mounted on the centre of the bending bar 
31. The key 38 transmits via the coupling means 22 a signal which is 
dependent on the deflection of the bending bar. 
The measurement transducer 34' can operate, for example mechanically, by 
determining via a knife-edge bearing 35 a reference point on the bending 
bar 31 and determining, via the key 38 engaging resiliently on the bending 
bar 31, the relative deformation of the bending bar 31 and thus the force 
in the lower linkage U. The measurement transducer can, however, also 
operate electrically by determining directly the deformation of the 
bending bar 31 by inductive or capacitive means and transmitting it as an 
electric signal to the feedback transducer. 
FIG. 4 shows an embodiment of a mechanically operating measurement 
transducer which is connected to the hydraulically operating control 
circuit via a mechanically operating coupling. 
The measurement transducer is formed by a housing 40 which is mounted on 
the bending bar 31 by way of a double knife-edge bearing 41 in the form of 
fitted-on toroidal knife-edges 42. The housing 40 has a bore 43 which runs 
symmetrically relative to the bearings 41 and in which a slide valve 44 is 
slidably mounted to create a seal on its end sections. With the aid of a 
control edge 45, the slide valve throttles the hydraulic line, which is 
also integrated in the measurement transducer and leads to the storage 
reservoir 7, and thereby determines the pressure before the control device 
13 (see FIG. 1). The outwardly curved end section 48 of the slide valve 44 
is pressed against the bending bar 31 by means of a compression spring 46 
which is supported on the housing 40 of the measurement transducer by way 
of a screw 47 screwed into the housing 40. The housing of the measurement 
transducer is mounted on the bending bar 31 in such a way that the bore 43 
for the slide valve 44 points in the direction of maximum deflection of 
the bending bar. 
When a tractive force F which exceeds the adjusted force of the linkages 
acts on the lower linkages U, the bending bar 31 is deflected away from 
the tractor 1 and the slide valve 44 which follows this deformation 
throttles the stream of hydraulic control fluid. The directional control 
valve 10 is displaced thereby and the working cylinder 5 is supplied with 
hydraulic fluid. As a result of this, the lower linkage U is lifted and 
the force transmitted thereby is reduced so that a state of equilibrium is 
re-established at the directional control valve 10. 
In accordance with FIG. 2 the measurement transducers can also be connected 
rigidly to the tractor frame 1. As a result, the knife-edge bearings 41 
can be omitted. 
FIG. 5 shows an embodiment of the measurement transducer which could be 
used in electrical control circuits. In this case--as in the case 
described above--the housing 40 accomodates a plunger 44' which is pressed 
against the bending bar 31 by means of a compression spring 46. The upper 
section 44a of the plunger 44' is surrounded by a coil 48 which is 
connected to an electric control loop by the connections 49, 50 of its 
winding and which generates a corresponding electric signal depending on 
the position or movement of the plunger (which is conditional on the 
deflection of the bending bar).