Vehicle

An agricultural tractor comprises lifting devices to which are hitched rotary supports in the form of rollers. The lifting devices push the rollers downwardly relatively to the rest of the tractor. This transfers some or all of the weight of the tractor to the rollers which reduces or eliminates pressure on the underlying surface, and consequent soil compaction, applied by wheels of the tractor. The rotary supports may be drivable and improve traction. If all of the weight of the tractor is placed on the rotary supports, the wheels are raised from the ground. A system is also described for automatically detecting and eliminating or correcting for skidding of the wheels of the tractor.

SUMMARY OF THE INVENTION 
This invention relates to a motor vehicle such as an agricultural tractor. 
According to a first aspect of the present invention there is provided a 
vehicle comprising wheels for supporting the vehicle on the ground and at 
least one lifting device to which a rotary support can be connected, the 
lifting device being capable of exerting a downwardly directed force to 
reduce pressure applied by at least one of the wheels on the ground. 
According to a second aspect of the present invention there is provided a 
motor vehicle comprising at least two driven ground-engaging rotary 
members disposed one rearwardly of the other with respect to the intended 
direction of operative travel, at least one of the rotary members being 
drivable by means of a steplessly variable change-speed transmission, 
monitoring means being provided for monitoring the number of revolutions 
of the two rotary members and for adjusting the change-speed transmissions 
in a manner such that the peripheral speeds of the two rotary members are 
maintained substantially equal. 
According to a third aspect of the present invention there is provided a 
motor vehicle comprising a steplessly variable transmission by which at 
least one wheel axle is drivable, the transmission comprising at least one 
variable pulley transmission and at least one stepwise variable 
transmission which is arranged between the variable pulley transmission 
and the wheel axle. 
For a better understanding of the present invention and to show how it may 
be carried into effect, reference will now be made, by way of example, to 
the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The tractor shown in FIGS. 1 and 2 comprises a frame 1 on which are mounted 
a driving engine 2 and an operator's or driver's cab 3. The frame 1 is 
supported by steerable front wheels 4 and drivable rear wheels 5, which 
can be coupled with the engine 2 by a torque converter (not shown) and a 
clutch. The front wheels 4 are mounted on a front axle 6, which is 
pivotable with respect to the frame 1 about a horizontal pivotal axis 
lying in the longitudinal central plane of the tractor and extending in 
the direction of travel indicated by an arrow A. The front wheels 4 are 
pivotable relatively to the axle 6 about upwardly extending king pins 7, 
which are secured respectively near the two ends of the front axle 6. The 
wheels 4 are rigidly secured to track arms 8, which extend from the wheels 
rearwardly and inwardly. The ends of the track arms away from the wheels 
are pivotally coupled to a track 9, which extends substantially parallel 
to the front axle 6 and which can be displaced from the cab 3 for steering 
the front wheels 4. Viewed in plan, the front end of the engine 2 is 
located near the front axle 6. With respect to the direction of travel A, 
the cab 3 is located substantially behind the engine 2 and comprises a 
front portion 10 having an internal height which is great enough to allow 
a person who is at least 1.80 meters in height to stand upright. An 
entrace door 11 of the cab 3 has the same height. The rear portion of the 
cab 3 accommodates a driver's seat 13, a steering wheel 14 and other 
control levers and pedals for steering and otherwise controlling the 
tractor and any implements hitched to it. The seat 13, the steering wheel 
14 and at least some of the other controls are arranged on a console, 
which is pivotable about a substantially vertical axis 15 with respect to 
the rest of the tractor. 
The console, and the seat, steering wheel and other controls on it can be 
secured with respect to the frame 1 in two positions about the axis 15 
differing by 180.degree.. In one of these positions, the driver faces in 
the direction opposite the direction A. As seen from the side (FIG. 1), 
the console is located above the front part of the rear wheels 5. 
The tractor has a front power take-off shaft 16 and a rear power take-off 
shaft 17, which are driven by the engine 2 with a speed proportional to 
that of the engine 2. In this embodiment provision is made for the power 
take-off shafts 16 and 17 to be driven at a speed which is proportional to 
that of the driven rear wheels 5. For this purpose a change-over gear box 
(not shown) is provided, which has two input shafts, one of which is 
coupled with the engine 2 by a driver-controlled clutch and the other of 
which is coupled, also by a clutch, with the output shaft on the torque 
converter. 
At the rear, the tractor has a three-point lifting device 18 having two 
lower lifting arms 19 attached to the frame 1 for pivotal movement about 
pivotal shafts 20. Substantially midway along their length, the lower 
lifting arms 19 have pivotal shafts 21, to which upwardly extending pull 
rods 22 are fastened, the top ends of which are pivotally coupled by 
shafts 23 to bell cranks 24, which are pivotable about pivotal shafts 25 
with respect to the frame 1. The ends of the bell cranks 24 away from the 
shafts 23 are connected by pivotal shafts 26 to the piston rods of 
hydraulic rams 27, which are pivotably mounted on the frame 1. The 
hydraulic rams 27 can be actuated by the driver so that the pull rods 22 
can turn the lower lifting arms 19 upwards. Otherwise, by opening a 
hydraulic connection, the driver can cause the hydraulic fluid to flow out 
of the hydraulic rams 27 to allow the lower lifting arms 19 to turn 
downwards under the effect of gravity acting, for example on an attached 
machine. The lifting device 18 comprises furthermore an upper arm 28, the 
length of which is adjustable and which is pivotably connected by a 
pivotal shaft 29 to lugs 30 which are fixed to the frame 1. The machine or 
the tool is hitched to the outermost ends of the lifting arms 19 and 28 by 
pins 31 and 32 respectively. 
In addition to the components of the three-point lifting device 18 
described above, there are provided two hydraulic rams 33, which are 
disposed symmetrically with respect to the central vertical longitudinal 
plane of the tractor and the top ends of which are pivotably connected by 
the pivotal shaft 29 to the lugs 30. The lower ends of the hydraulic rams 
33 are pivotably connected by horizontal pivotal shafts 34 to the 
respective lower lifting arms 19 concerned. The orientation of the rams 33 
is such that in the side elevation of FIG. 1 a line of connection between 
the shafts 29 and 34 is, in this embodiment, an angle of about 15.degree. 
to 30.degree. to the vertical, the rams 33 being inclined to the rear from 
top to bottom. The two hydraulic rams 33 can be actuated from the driver's 
seat 13 so that their lengths are increased under pressure so that the 
lower lifting arms 19 are forcibly pressed down. The hydraulic rams 33 are 
provided in addition to the hydraulic three-point lifting device 18 
already provided on the tractor. It is furthermore also possible for the 
rams 27 to be double-acting and for the linkage 21 to 26 to be constructed 
in a manner such that the rams 27 can press the lower lifting arms 19 
forcibly downwards. 
A rotary support for the tractor is connected to the lifting device 18 by 
the pins 31 and 32. This rotary support comprises in this embodiment two 
rollers 35 and 36 having the same dimensions as each other, and having 
aligned rotary shafts 37. The rollers 35 and 36 are arranged end-to-end 
and cover a width which may be 20% to 50% larger than the overall width of 
the tractor itself. The rollers 35 and 36 are disposed symmetrically one 
on each side of the central vertical longitudinal plane of the tractor. As 
an alternative, a single roller or more than two rollers end-to-end may be 
arranged to cover the width covered by the rollers 35 and 36. If more than 
one roller is used, the rollers may both be rigidly secured to a common 
rotary shaft 37, as in the embodiment shown, but embodiments are possible, 
in which each roller is driven independently about its own rotary shaft 
37. 
Each of the rollers 35 and 36 has near each end an end plate 38 supported 
by the rotary shaft 37. Each end plate 38 supports at its periphery a 
great number of rods or tubes 39 which extend helically around the rotary 
shaft 37, these rods or tubes 39 together providing a substantially 
cylindrical outer surface for each roller 35, 36. This cylindrical outer 
surface may, as an alternative, be formed by flat strips extending 
helically or axially, the faces of which constitute the outer surface of 
the rollers. 
The rollers 35, 36 are supported by the rotary shaft 37 in a frame, 
indicated generally by reference numeral 40, having at its two outer ends 
side plates 41 the top edges of which are interconnected by a supporting 
beam 42 extending transversely of the direction A. The supporting beam 42 
is provided with downwardly extending lugs 43, through which pass the pins 
31 at the rear ends of the lower lifting arms 19. Midway along the length 
of the supporting beam 42 there is a pair of lugs 44 for receiving the 
pins 32 to connect upper arm 28 of the lifting device 18 to the supporting 
beam 42. The frame 40 of the rotary support is provided with rearwardly 
projecting carriers 45 and 46, by means of which a machine or a tool, in 
this embodiment a seed drill 47, is fastened to the frame 40. In this 
embodiment the seed drill 47 covers the whole width of the frame 40. 
In this embodiment one of the wheel axles of the drivable wheels 5 of the 
tractor is extended by a length of tubing 48, the outer ends of which 
carries a chain sprocket 49 which is drivably connected by a chain 50 with 
a further chain sprocket 51 rigidly secured to the rotary shaft 37. The 
diameters of the sprockets 49 and 51 are selected so that the rollers 35 
and 36 are driven by the engine 2 with a peripheral speed which is equal 
to that of the rear wheels 5. As an alternative, the rollers 35 and 36 may 
be driven by means of the power take-off shaft 17, when the latter is 
coupled by the aforesaid change-over gear box with the output shaft of the 
toque converter of the tractor. 
In an analogous manner a rotary support for the tractor is hitched to a 
three-point lifting device, indicated generally by reference numeral 52, 
at the front of the tractor. In this embodiment the lifting device 52 is 
secured only to the front axle 6, but this lifting device may, as an 
alternative, be secured directly to the frame 1 of the tractor. The front 
axle 6 is provided with two pairs of lugs 53 projecting forwardly. Lower, 
lifting arms 54A are pivotably connected to the pairs of lugs 53 by 
aligned pivotal shafts 54. At the center the top face of the front axle 6 
is provided with a pair of lugs 55 which are inclined forwardly from 
bottom to top. A top lifting arm 57 of the lifting device is pivotally 
supported at the front ends of the lugs 55 by a pivotal shaft 56, which 
extends, like each of the pivotal shafts 54, horizontally and transversely 
of the direction A. The length of the top arm 57 is variable. The leading 
end of the top arm 57 is pivotably connected to a rigid frame 59 by a 
pivotal shaft 58. The frame 59 is part of a rotary support designated 
generally by reference numeral 60, carried by the lifting device 52. The 
lower lifting arms 54A are pivotably connected to the frame 59 at their 
front ends by pivotal shafts 61. As seen from the side, the lower lifting 
arms 54A and the top lifting arm 57 are substantially parallel to one 
another and are substantially horizontal in a normal operational position 
of the support 60. A doubleacting hydraulic ram 62 is supported at one end 
by the pivotal shaft 56 and the other end is pivotably connected to the 
frame 59 by a pivotal shaft 63. As seen in FIG. 2, there are two hydraulic 
rams 62 which are in a parallel relationship. 
The pivotal shafts 54, 56, 58, 61 and 63 are all parallel to one another 
and extend horizontally and transversely of the direction A. As seen from 
the side (FIG. 1), the pivotal shafts 54, 56, 58 and 61 are located at the 
corners of a quadrilateral which, as shown, is almost a parallelogram. The 
pivotal shafts 58, 61 and 63 are all fastened to the rigid frame 59 of the 
rotary support 60 and are thus disposed at the corners of a rigid triangle 
regardless of the lengths of the hydraulic rams 62. 
In the frame 59 is journalled a substantially horizontal rotary shaft 64, 
which is parallel to the rotary shaft 37 when the tractor is travelling in 
a straight line. Rotatable rollers 65 and 66 are mounted on the rotary 
shaft 64. The rollers 65 and 66 are arranged end-to-end. They have the 
same dimensions as the rollers 35 and 36 and are constructed in a similar 
manner. The width covered by the pair of rollers 65 and 66 is equal to 
that covered by the rollers 35 and 36. The rollers 65 and 66 are in front 
of the front wheels 4, whereas the rollers 35 and 36 are behind the rear 
wheels 5. 
Because it is mounted on the front axle 6, the support 60 is freely 
pivotable with respect to the frame 1 of the tractor about the same 
horizontal pivotal axis located in the central vertical plane of the 
tractor as front axle 6. 
The front of the frame 59 is provided with two pairs of lugs 67 carrying 
aligned pivotal shafts 68 which pivotably connect arms 69 to the frame 59. 
These arms 69 project forwardly from the frame 59 and are inclined 
slightly downwardly from back to front when in an operational position. 
The front ends of the carrier arms 69 are pivotally connected by aligned 
pivotal shafts 70 with a machine or tool which, in this case, is a rotary 
harrow 71. Near the top of the frame 59 there is a pair of ears 72 
supporting a pivotal shaft 73 which pivotably connects an adjustable 
length rod 74 to the frame 59. The front end of the rod 74 is pivotably 
connected by a pivotal shaft 75 to the top fastening point of a trestle 76 
carrying the machine 71. 
About one or both of the king pins 7 is pivotable a lever 77, which, like 
the track arm 8, is rigidly secured to the adjacent wheel 4. The lever 77 
extends forwardly away from the king pin 7 in the direction towards the 
adjacent lower lifting arm 54A. The lever 77 is connected by a control 
link 78 with the adjacent lower lifting arm 54A, this control link 78 
being pivotally connected both to the lever 77 and to the lower lifting 
arm 54A. The pivotal connection between the control link 78 and the lower 
lifting arm 54A is in front of the pivotal shaft 54. 
It should be noted that the rollers 65 and 66 of FIGS. 1 and 2 are 
rotatable independently of one another about the rotary shaft 64. In an 
alternative construction, the rollers 65 and 66 may be driven by the power 
take-off shaft 16, when the latter is coupled with the output shaft of the 
torque converter of the tractor. In this case there is provided a 
change-over gear box in a manner not shown, the input shaft of which is 
coupled by means of an auxiliary shaft with the power take-off shaft 16 
and the output shaft of which is drivably connected through a gear 
transmission with the rotary shaft 64, to which the rollers 65 and 66 are 
then rigidly secured. 
However, in the embodiment shown the frame 59 of the support 60 has 
fastened to it a bearing housing 79, through which passes an auxiliary 
shaft 80. The shaft 80 is connected at one end by a universal coupling 
with the power take-off shaft 16 and at the other end by a further 
universal coupling to a second auxiliary shaft 81. The second auxiliary 
shaft 81 is connected by a third universal coupling to the input shaft of 
the machine 71. In this way the machine 71 can be driven from the power 
take-off shaft 16. 
In agricultural work the tracks made in the ground by the tractor wheels 
may often be undesirable. If, for example, a field is first harrowed 
before manure, fertilizer or seed is applied, the pressure of the tractor 
wheels on the harrowed soil locally packs the soil so that the soil 
structure obtained by harrowing, and in particular the distribution of 
capillaries, is adversely affected. After the growth of the plants this 
adverse effect becomes manifest. In order to obviate this disadvantage the 
pressure of the tractor constructed in the form shown in FIGS. 1 and 2 on 
the ground is reduced to such a low value that the underlying soil 
structure, particularly in the region of the plants roots, is maintained 
wholly or substantially wholly in the form obtained by harrowing. This 
applies not only to harrowing but also to soil treatments by means of 
other soil cultivating machines. It can thus be ensured that the seeds can 
germinate and grow in a soil structure which is not, or is only slightly, 
affected by the weight of the tractor. 
From his seat 13 the driver can lift the rotary support at the front or at 
the rear of the tractor or both together with the tool (the rotary harrow 
71 or the speed drill) by means of the hydraulic ram 27, which is normal 
tractor equipment and acts through the bell cranks 24, the pull rods 22 
and the lower lifting arms 19 and the top arm 28, or by means of the 
double-acting hydraulic ram 62, by means of which the foremost support 60 
and the machine 71 are movable relatively to the front axle 6. When a 
hydraulic communication in the hydraulic feed of the ram 27 is opened, the 
rear support and the seed drill drop to the ground under their own weight, 
whereas the front support and the machine 71 can be lowered to the ground 
by extending the rams 62 by hydraulic pressure. However, the hydraulic ram 
27 which is standard equipment is not able to exert downwards pressure on 
the rear support 35, 36 and on the machine 47. For this purpose the two 
hydraulic rams 33 are provided, which can be actuated from the driver's 
seat to force the lower lifting arms 19 downwardly about the pivotal 
shafts 20 so that, in a first instance, part of the tractor weight is 
taken by the rollers 35, 36. In the same manner the hydraulic rams 62 can 
be extended by hydraulic pressure so that the roller 65, 66 also support, 
in a first instance, part of the tractor weight. Thus the pressure applied 
by the tractor wheels 4 and 5 to the ground is reduced so that compression 
of the soil by the wheels 4 and 5 and the consequent deterioration of the 
structure is reduced because, for a given weight, the pressure exerted by 
the combination of the rollers 35, 36 and 65, 66 and the wheels 4, 5 is an 
order of magnitude lower than the pressure exerted by the wheels 4 and 5 
when they provide the sole support of the tractor. 
In order to improve further this advantageous effect of the reduced ground 
pressure, the pressure in the rams 33 and 62 can be increased by the 
driver to an extent such that the full weight of the tractor, the supports 
and the machines is taken by the rollers 35, 36 and 65, 66 so that the 
tractor wheels 4 and 5 no longer exert any significant pressure on the 
ground and may even be lifted clear of the ground. The overall weight is 
then distributed along the long rollers and the ground pressure is then 
reduced to such a low value that deterioration of the soil structure in 
the region of plant roots is avoided. 
The height of the equipment 47 and 71 above the ground is determined by the 
positions of this equipment with respect to the rollers 35, 36 and 65, 66 
respectively. Despite the fact that the support 60 moves with respect to 
the tractor when the tractor is lifted, the orientation of the frame 59 of 
the support 60 does not change significantly, since the pivotal shafts 54, 
56, 58 and 61 are located, as viewed from the side, at the corners of what 
is substantially a parallelogram so that the attached machine also 
maintains its initial orientation. This is important for machines wherein 
their orientation with respect to the ground influences the quality of the 
treatment, as in the case of a rotary harrow 71. It should be noted that 
beneath each of the two carrier arms 69 of the harrow 71 there is a stop 
82 positioned for abutment with the carrier arms 69 loaded by the weight 
of the tool 71. This stop 82 is rigidly fastened to the frame 59 of the 
support 60 and prevents the tool 71 from shifting too far downwards 
relative to the support 60, when, for example, the support 60 is lifted by 
means of the hydraulic rams 62. 
When the driver wants to make a turn, the track rod 9 is displaced 
approximately parallel to itself so that the track arms 8 and hence also 
the front wheels 4 turn about the king pins 7. At the same time the lever 
77 is turned about the respective king pin 7. The control link 78 
connecting the lever 77 to the adjacent lower lifting arm 54A causes this 
lower lifting arm to turn about the associated pivotal shaft 54. This is 
permitted because the lower lifting arm 54A is mounted on the pivotal 
shaft 54 by a ball-and-socket joint. Therefore, when the front wheels 4 
turn about the shafts 7 the rotary support 60 of the tractor will turn 
likewise so that the support 60 guides the tractor round the bend 
regardless of whether or not the front wheels 4 are in contact with the 
ground. The machine 71 coupled to the front of the support 60 is turned at 
the same time and steered through the bend. During operation, the assembly 
of the support 60 and the machine 71 is pivotable with respect to the 
tractor frame about the pivotal axis about which the front axle 6 is 
pivotable so that the support 60 and the machine 71 can follow 
unevennesses of the ground independently of the rear roller 35, 36. 
The torque converter which transmits power between the engine 2 and the 
driven wheels 5 comprises at least one variable belt transmission and, in 
addition, a step-wise change-speed gearbox, which is arranged between the 
variable belt transmission and the axle of the wheels 5. The construction 
of this torque converter can correspond with that described with reference 
to any one of the following embodiments. With the elements of the torque 
converter disposed as will be described, an advantageous load of the belt 
transmission is obtained with differently adjusted transmission ratios of 
the stepwise change-speed gearbox so that for each transmission ratio set 
in the change-speed gearbox (for example, to provide a range of low speeds 
for exerting high tractive power and a range of higher speeds for 
actuating light machines or for road travel) the pulley speed of the belt 
transmission can be adjusted steplessly to transmit maximum torque and/or 
power. 
The tractor can be used in either direction of travel so that, depending 
upon the circumstances, the driven rotary support can be disposed at the 
front or at the rear of the tractor, with respect to the actual travel 
direction, and also in front of or behind the implement. Apart from the 
nature of the implement (it is sometimes preferable to employ implements 
hitched to the front of the tractor) the necessity that the driver is able 
to supervise the job must also be a consideration. Depending, therefore, 
on whether or not the driven rotary support and the implement to be used 
is to be at the front or at the rear of the tractor, the driver's seat and 
the steering wheel and other controls mounted on the console can be turned 
about the pivotal axis 15 to face the driver in the direction A or in the 
opposite direction. 
FIGS. 3 and 4 show another tractor construction having a rotary support, 
which in this case may serve in addition as a non-skid device. Parts 
having functions which correspond with those of the parts shown in FIGS. 1 
and 2 are designated by the same reference numerals. As in the preceding 
embodiment the front power take-off shaft 16 is coaxial with the pivot 
axis about which the front axle 6 is pivotable with respect to the frame 
1. 
The construction of the front lifting device 52 of the tractor shown in 
FIGS. 3 to 5 slightly differs from that of the preceding embodiment. The 
interior of the hollow front axle 6, which is pivotable about the shaft 16 
with respect to the rest of the tractor, accommodates one or more 
hydraulic devices capable of turning two pivotal shafts 83 disposed above 
the front axle 6 one on each side of the central longitudinal plane of 
symmetry of the tractor. A lever 84 is rigidly secured to each shaft 83. 
The end of each lever 84 away from the pivotal shaft 83 is pivotally 
coupled to a connecting element 85, the lower end of which is connected by 
a pivotal shaft 86 with a lower lifting arm 54A. The connecting element 
85, in this embodiment, is a double acting hydraulic ram actuable from the 
driver's seat independently of the hydraulic device in the front axle 6. 
A trestle 87 is hitched to the front ends of the lower lifting arms 54A and 
the top arm 57. This trestle 87 has the shape of an inverted V, as viewed 
in the direction of travel A. Near the places where it is hitched to the 
lifting arms 54A, trestle 87 has two forwardly projecting, substantially 
horizontal carrier arms 88 secured to it, and near the place where it is 
hitched to the top arm 57, there is secured a rod 89 which is inclined 
downwardly from back to front. The carrier arms 88 and the rod 89 support 
at their front ends a sleeve 90, in which is journalled a rotary shaft 91 
extending parallel to the front axle 6. Rotary members 92 in the form of 
wheels or durms are mounted on the portions of the rotary shaft 91 
projecting from the ends of the sleeve 90. In the embodiment illustrated 
the width of each of these rotary members 92 is substantially equal to the 
distance between the lowermost point of the trestle 87 and the lateral 
boundary of the tractor, but this width may be larger. The diameter of 
each rotary member 92 is about 50 to 90% of the diameter of the adjacent 
tractor wheel 4. The rotary members 92 are disposed symmetrically about 
the central longitudinal plane of the tractor. The outer circumference of 
each of the rotary members 92 is constituted by cylindrical sheet material 
provided with a plurality of rows (in this embodiment there are eight 
rows), of pointed projections 93, which extend radially outwardly. Each 
row of projections comprises, for example, three projections 93. As seen 
from the side, each projection 93 is tapered, and as seen in a tangential 
direction, each extension 93 looks, in outline, like a truncated cone, so 
that when a projection 93 has penetrated into the ground, a large driving 
contact surface is obtained between each projection 93 and the ground. 
The sleeve 90 is provided with a gearbox 94 having an input stub shaft 95 
projecting towards the power take-off shaft 16. The input stub shaft 95 
can be drivably connected by an auxiliary shaft to the power take-off 
shaft 16, in which case the auxiliary shaft would pass through the 
V-shaped trestle 87. The pinion transmission in the gearbox 94 provides a 
speed difference between the power take-off shaft 16 and the rotary shaft 
91, the rotary shaft 91 is provided near the gearbox 94 with a pinion 
which is in mesh with the pinion transmission in the gearbox 94. 
The rotary support comprising the elements 92 and the associated frame 
parts 87, 88 and 89 and the driving elements 94 and 95 may also be 
fastened to the rear lifting device 18, the drive then being applied by 
the power take-off shaft 17. In this way the tractor can be provided at 
the front as well as at the rear with a rotary support as in the preceding 
embodiment. In this embodiment both supports are drivable. The support at 
the front can be lifted by means of the hydraulic device accommodated in 
the hollow front axle 6, which device can cause the levers 84 and the 
connecting elements 85 to turn, while the support at the back can be 
raised by turning the pivotal shafts 25 and the associated levers 24 as 
well as the pull rods 22. The pull rods 22 may be replaced by separately 
actuable double-acting hydraulic rams controlled from the driver's seat so 
that when the rotary supports have been lowered to the ground, they can be 
forced downwards by the energization of the hydraulic ram 22 and of the 
double-acting ram 85, so that the pressure of the tractor wheels 4 and 5 
on the ground can be reduced or eliminated at the driver's option as is 
described for the first embodiment. 
Elements 92 hitched to front or rear or both lifting devices of the tractor 
are also effective as non-skid devices. The elements 92 can be pressed to 
the ground by the hydraulic rams 22 and 85, as the case may be, with 
enough force to cause the projections 93 to penetrate the ground, while 
the wheels 4 and 5 continue to bear on the ground with almost the full 
wheel pressure. The projections 93 provide a firm grip on the ground so 
that an appreciable part of the driving torque provided by the driving 
engine 2 is transferred through the non-skid arrangement to the ground and 
therefore, the torque to be transferred by the wheels 4 and 5 may be lower 
when the elements are driven by the power take-off shafts 16 and 17. By 
fastening the supports or non-skid devices 92 to the front and rear 
lifting devices the tractor can be employed on slack or yielding soil 
while the pressure of the tractor can be reduced to avoid deterioration of 
the soil structure by compaction. In the latter case the elements 92 may, 
if desired, be pressed against the ground with considerably heavier force 
than in the first case. Consequently, despite the attached heavy machines, 
a comparatively low ground pressure can be ensured. The non-skid device 92 
has a simple structure and can be readily detached. When the non-skid 
device 92 is raised from the ground, its weight increases the pressure of 
the wheels 4 and 5 on the ground, which is advantageous under normal soil 
conditions for producing strong traction forces. 
A drive arrangement which can be used with either of the tractors of FIGS. 
1 to 4 is shown schematically in FIG. 5. The frame 1 comprises a central, 
hollow frame beam extending away from its junction with the front axle 6 
to behind the axles of the wheels 5. Near its rear end, the frame beam 1, 
which is located in the central vertical longitudinal plane of the 
tractor, is rigidly secured to a beam 96 which extends transversely of the 
direction A and carries the lower lifting arms 19 and a vertical 
supporting beam 97 located in the central longitudinal vertical plane of 
the tractor. The top arm 28 of the lifting device 18 is attached to the 
beam 97 by the pivotal shaft 29. 
At a position behind the rear most points of the front wheels 4 and in 
front of the rear wheels 5, a hollow beam 97A is secured to the central, 
hollow frame beam 1. This beam 97A supports driving engines 98 and 99 
which are arranged so that their output shafts are aligned with and 
project towards one another, the output ends of the engines facing one 
another. In this embodiment the output shafts are at right angles to the 
central vertical plane of the tractor, but they may be inclined to that 
plane. The engines 98 and 99 are disposed one on each side of the central 
vertical plane of the tractor. The adjacent ends of the output shafts of 
the engines are provided with bevel gear wheels 100 and 101 respectively, 
both of which mesh with bevel gear wheels 102 and 103 located one on each 
side of a vertical plane containing the axes of the output shafts of the 
engines 98 and 99. The axis of the bevel gear wheels 102, 103 are aligned 
with one another and are parallel to and above the centerline of the 
hollow frame beam 1. Between the bevel gear wheels 102 and 103 there is a 
coupling piece 104, which is splined to, and therefore axially slidable 
along, a drive shaft 105 of a variable transmission 106 of the tractor. 
Axial displacement of the coupling piece 104 along the drive shaft 105 
causes the shaft 105 to engage either the bevel gear wheel 102 or the 
bevel gear wheel 103 so that the drive shaft 105 can be rotated in either 
direction by one or both of the engines 98 and 99. The gear wheels 102 and 
103, if they are not rotationally coupled with the shaft 105 by the 
coupling piece 104, are, therefore, freely rotatable about the shaft 105 
(overrunning). The shaft 105 projects forwardly beyond the gear wheel 102 
and constitutes an input shaft of a gear box 107. The gear box 107 has two 
output shafts 108 and 110. The output shaft 108 drives a hydraulic pump 
109 and the output shaft 110, which is accommodated in the hollow beam 1 
and projects beyond the front end of the hollow beam 1, constitutes a 
power take-off shaft 16, as well as constituting the pivotal shaft about 
which the front axle 6 is pivotable with respect to the frame beam 1. The 
portion of the drive shaft 105 located behind the gear wheel 103 drives by 
means of two bevel gear wheels 111 and 112 a horizontal shaft 113 
extending at right angles to the vertical central plane of the tractor. 
The shaft 113 has near each end a pulley 114 or 115 respectively, these 
pulleys being disposed symmetrically one on each side of the vertical 
central plane of the tractor. The pulleys 114 and 115 are drivably 
connected by drive belts 116 and 117 respectively with pulleys 118 and 119 
respectively, each of which is mounted on a shaft 120 and 121 
respectively. The shafts 120 and 121 constitute input shafts of stepwise 
change-speed gearboxes 122 and 123 respectively (FIG. 5), the output 
shafts of which drive the rear wheels 5 of the tractor. The gearboxes 122 
and 123 are both adjustable from the driver seat in a manner such that the 
same speed of the shafts 120 and 121, two ranges of speeds of the wheels 5 
can be provided. 
The inboard flange of each pulley 118 and 119 is biased by a spring 124 and 
125 respectively towards the other flange of the pulley concerned, and so 
the associated belt 116 or 117 respectively is clamped between the conical 
faces of the flanges. 
The outboard flange of each pulley 114 and 115 can be urged by means of 
hydraulic means in a housing 126 or 127 respectively towards the inboard 
flange of the respective pulley. Such hydraulic means, known in the art, 
are supplied with hydraulic fluid through hydraulic conduits 128 and 129 
respectively, both of which lead from a control valve 130, which can be 
adjusted by the driver or by a control device. 
The drive shaft 105 is coupled through one or more pinions (located in the 
hollow beam 1) drivably engaging the pinion 101 with the rear power 
take-off shaft 17; which take-off shaft extends to the rear inside the 
hollow beam 1. 
In this embodiment the front wheels 4 are driven by hydraulic motors 131 
mounted on the front axle 6, the output shafts of which drive, via a gear 
transmission a ring gear which is rigidly secured to the wheel rim. The 
hydraulic pump 109 is connected to the hydraulic motors 131 by two pairs 
of lines 132. 
A fuel tank 133 is arranged behind the two engines 98 and 99 and between 
the pairs of wheels 4 and 5. The tank 133 is thus located between the two 
engines and driver's seat 13. Viewed on plan, the fuel tank is elongate 
and its length is at right angles to the central vertical plane of the 
tractor. The tank covers the whole width of the tractor. Seen from the 
side, the fuel tank 133 is substantially the same height as the engines 98 
and 99. This has the advantage that the noise produced by the engines will 
not readily penetrate into the cab owing to the screening effect of the 
fuel tank and its contents. 
As stated above, the elements 92 can be driven from the power take-off 
shaft 16 by means of an auxiliary shaft and the gearbox 94. FIG. 5 shows 
an alternative drive in which each of the elements 92 can be driven by a 
hydraulic motor 134, the housing of which is fastened to the sleeve 90. 
The hydraulic motors 134 communicate through pairs of hydraulic lines 135 
with the pump 109. 
Inlets 136 for air intended for cooling the two engines 98 and 99 are 
provided (see FIG. 3) in the sides of the engine cover of the tractor. 
These air inlets 136 in the sheet cover are larger than the radiator of 
the engine concerned located behind them. 
The stepwise change-speed gearbox 123 (FIG. 5) is shown in section in FIG. 
7. The other gearbox 122 is the mirror-image of this one. The output shaft 
121 of the pulley 119 is supported in a housing 137 of the gearbox 123 by 
bearings 138 and 139. The bearings 138 and 139 are spaced apart by a 
comparatively large distance because of the load applied to the shaft 121 
by the belt 117. The shaft 121 is drivably connected by means of two 
meshing pinions 140 and 141 with a shaft 142 which is parallel to the 
shaft 121. The transmission ratio between the pinions 140 and 141 is such 
that the shaft 142 rotates more slowly than the shaft 121. An internally 
toothed annulus 143 is rigidly secured to the shaft 142. The teeth are 
near the outer periphery of the annulus 143, and they mesh with the teeth 
of a plurality of planet wheels 144, which are mounted on shafts 
journalled in the planet carrier 145 and in a supporting ring 146. The 
planet carrier 145 is splined to an auxiliary shaft 147 which is coaxial 
with the shaft 142. The planet wheels 144 mesh with a sun wheel 148 which 
is keyed to a sleeve 149. The sleeve 149 is freely rotatable about the 
auxiliary shaft 147. A ring 150 is bolted to the sun wheel 148, the outer 
periphery of this ring being provided with teeth 151. A second ring 152 is 
mounted on bearings 153 on a spigot like portion of the sun wheel 148. The 
ring 152 has the same diameter as the ring 150 and is also provided with 
teeth 154 on its outer periphery. The ring 152 serves as a further planet 
carrier, and rotatably supports shafts of planet wheels 155. These shafts 
are also supported by a supporting ring 156A. The planet wheels 155 mesh 
with teeth on the outer periphery of the sleeve 149, which thus serves as 
a sun wheel. The planet wheels 155 also mesh with internal teeth at the 
outer periphery of an annulus 156, which is splined to the auxiliary shaft 
147. The outboard end of the auxiliary shaft 147 is splined to a sun wheel 
157 surrounding the shaft and meshing with planet wheels 158. The planet 
wheels 158 also mesh with internal teeth provided in an end portion of the 
housing 137. The planet wheels 158 are supported on shafts mounted in a 
planet carrier 159, which is splined to a wheel axle 160 which is coaxial 
with the shafts 142 and 147. The shaft 160 is provided with one of the 
wheels 5. 
In operation, the tractor can be powered by both of the two engines 98 and 
99, or by only one of them. There are advantages in providing a tractor 
with more than one engine of comparatively low power rather than with a 
single large engine. One is that, should one of the engines become 
defective, the tractor can continue to operate normally with the other 
engine, if the circumstances do not require high power, or the tractor can 
reach a repair shop under its own power. A further advantage is that 
certain jobs require the power of only one engine so that the tractor will 
operate more enconomically for these jobs. A third advantage is that it is 
generally cheaper to provide, for example, two engines of comparatively 
low power than one large engine having twice the power of one of the 
smaller engines, because smaller engines tend to be produced in larger 
quantities than large ones and are, therefore, appreciably cheaper. In 
this embodiment each of the engines 98 and 99 has a power of about seventy 
HP. Owing to the back-to-back disposition of the engines 98 and 99 a 
simple pinion coupling with the main shaft 105 is sufficient, in which 
each engine drives one of the pinions 100 and 101 respectively, which in 
turn, dependent upon the position of the coupling piece 104 connecting one 
or the other of the pinions 102 or 103 with the shaft 105, drives the 
shaft 105 through the pinion 102 or 103. By moving the coupling piece 104 
along the shaft 105 the direction of travel of the tractor can be changed 
simply. On the sides of the engines 98 and 99 facing the shaft there are 
provided coupling members (not shown) for connecting or disconnecting the 
engines to or from the pinions 100 and 101 respectively. These coupling 
members as well as the coupling piece 104 can be operated from the 
driver's seat 13. If the direction of travel is changed by displacement of 
the coupling piece 104, the driver's seat 13 together with the steering 
wheel and other controls can be turned about the axis 15 so that the 
driver can face in the actual direction of travel. 
The shaft 105 is connected by the gearbox 107 to drive the hydraulic pump 
109 as well as the power take-off shaft 16 and hence, if the 
above-mentioned auxiliary shaft is provided, the two elements 92. 
The hydraulic pump 109 pressurises fluid to drive the hydraulic motors 131 
which drive the front wheels 4. 
The front axle 6, with the front wheels 4 and the two elements 92, is 
freely pivotable about the shaft 16 with respect to the rest of the 
tractor so that the front wheels and the elements 92 can follow 
unevennesses of the ground independently of the ground conditions at the 
rear wheels 5. This precaution helps to keep the driven front wheels 4 and 
the elements 92 always on the ground so that, if wheel tracks have to be 
avoided, the elements 92 have a uniform low ground pressure, while the 
extensions 93 of the two elements 92 penetrate into the ground in an 
identical manner in order to ensure a good grip and a consequent high 
driving torque for the tractor. These advantages apply equally to an 
identical rotatable support which may be hitched to the rear lifting 
device 18. 
The drive shaft 105 provides the input to the transmission 106, which 
comprises the variable pulley drives and the planetary change-speed 
arrangement shown in detail in FIG. 7. The shaft 105 directly drives the 
two front pulleys 114 and 115 through the bevel pinions 111 and 112. The 
rear pulleys 118 and 119 are driven by the belts 116 and 117. The 
transmission ratio between the front and the rear pulleys is determined by 
the positions of the belts with respect to the pulleys. This position is 
established by the hydraulic devices arranged in the housings 126 and 127 
respectively, which can be supplied with pressurised fluid through the 
conduits 128 and 129 from the control valve 130. The springs 124 and 125 
urge the inner flanges of the rear pulley towards the associated fixed 
flanges which are fastened to the shafts 120 and 121 respectively and so 
ensure that the belts 116 and 117 have the required tension for 
transmitting torque to the planetary gear arrangements 122 and 123 
respectively. 
The output shafts 120 and 121 of the rear pulleys constitute input shafts 
for the planetary gear arrangements (FIG. 7). The shaft 121 drives the 
shaft 142 and hence the annulus 143 through the pinions 140 and 141. When 
the ring 150 is held stationary with respect to the housing 137, in a 
manner to be described more fully later in this description, the sun wheel 
148 and hence also the sleeve 149 are also held stationary with respect to 
the housing 137. The annulus 143 then drives the planet wheels 144 and 
consequently also the planet carrier 145 and the auxiliary shaft 147. 
Rotation of the shaft 147 is transmitted with a speed reduction by means 
of the sun wheel 157 and the planet wheels 158, which run around the teeth 
formed in the housing 137, the planet carrier 159 and hence to the wheel 
axle 160 and the wheel 5. In this condition the ring 152, the planet 
wheels 155 and the annulus 156 simultaneously rotate idly. 
When the ring 150 is allowed to rotate and instead the planet carrier 152 
is held stationary with respect to the housing 137, rotation of the shaft 
142 is transferred in the following manner to the wheel axle 160. The 
driven annulus 143 causes the planet wheels 144, the planet carrier 145, 
the sun wheel 148 and the sleeve 149 to rotate. The sleeve 149 drives the 
satellite wheels 155 and hence the annulus 156, because the planet carrier 
152, as stated above, is stationary. Relative coupling of these planetary 
systems is established, since the planet carrier 145 and the annulus 156 
are both splined to the auxiliary shaft 147 and will, therefore, rotate 
with the same speed. By this conditions the speed of the auxiliary shaft 
147 is fixed. Rotation of the auxiliary shaft 147 is again transferred as 
before by the pinions 157 and 158 and the satellite carrier 159 to the 
wheel axle 160 and hence to the wheel 5. 
Selective locking of the ring 150 or the planet carrier 152 is achieved by 
the arrangement shown in FIG. 8. Beyond the outer peripheries of the ring 
150 and the planet carrier 152, a shaft 161 is journalled in the housing 
137. The centerline of the shaft 161 lies in a plane which is 
perpendicular to the centerline of the auxiliary shaft 147 and is midway 
between the ring 150 and the planet carrier 152. Two arms 162 and 163 are 
keyed to the shaft 161 and constitute a pawl mechanism. Near their ends 
away from the shaft 161, the arms 162 and 163 are each provided with a 
tooth 164 and 165 respectively, which can engage the teeth 154 and 151 
respectively. An arm 166 is rigidly secured to the arms 162 and 163. The 
arm 166 is disposed so that it is directed away from arms 162 and 163, its 
center line bisecting the angle between the two arms 162 and 163. The end 
of the arm 166 away from the shaft 161 is connected by a pivotal shaft 167 
to one end of a rod 168 which, towards the other end, is slidable in a 
sleeve 169. This sleeve 169 is pivotable with respect to the housing 137 
about a rocking shaft (not shown), which is parallel to the shafts 161 and 
167. A compression spring 169A acts between the sleeve 169 and a shoulder 
on the rod 168 near the shaft 167 to bias the rod 168 out of the sleeve 
169. The pivotal shaft 167 projects from the housing 137 and is 
displaceable from the driver's seat 13 in a direction substantially 
parallel to the auxiliary shaft 147. 
The dimensions are such that in the position shown in FIG. 8, the tooth 164 
engages the teeth 154, while when the arm 162 is turned away from the 
teeth 154, the arm 163 and its tooth 165 engages the teeth 151 of the ring 
150. Each of these two positions is stable because, in either position, 
the pivotal shaft 167 is located outside a plane containing the 
centerlines of the shaft 161, the shaft about which the sleeve 169 is 
pivotable, and hence in each of the two operative positions the pivotal 
shaft 167 is on one or the other side of the dead point in which the three 
axes are located in the same plane. The spring 169A thus provides and 
overcenter action and stable locking of the ring 150 or the planet carrier 
152 is therefore achieved. 
By thus locking the ring 150 of the planet carrier 152, either of two 
different transmission ratios can be set between each rear pulley 118 or 
119 respectively and the associated wheel axle. Both transmission ratios 
allow the speed of the pulleys in each of these two positions to be 
comparatively high in order to avoid high belt tensions. By means of the 
two coupled planetary gear arrangements (FIG. 7) two ranges of speed of 
the tractor can be selected, one range covering about 2.5 to 10 kms/hour 
intended for operations requiring a high tractive force and a second range 
of speed covering about 7.5 to 30 kms/hour intended, for example, for hay 
making, fertilizer distribution and the like and for road transport. 
Each of the housings 137 (FIG. 7) is provided with an electric pick-up 170 
(FIG. 6), in this case an inductive pick-up connected by leads 171 to a 
control circuit 194 (FIG. 5) which is shown schematically in FIG. 6. In 
order that the pick-up 170 operates as a counter, it is arranged in the 
direct proximity of the inner edge of the rim of the adjacent wheel 5 or 
of two wheels 4 and 5 succeeding one another in the direction of travel. 
In this embodiment the circumference of the inner edge of each wheel rim 
has one or more notches which move past the pick-up 170 when the tractor 
is moving. Each of the leads 171 is connected to a separate binary counter 
172 and 173 respectively. In this embodiment each of the counters 172 and 
173 respectively consists of, for example, three dividers-by-ten, each of 
which returns to the zero position at the tenth input pulse (emanating 
from the associated pick-up 170). The three coupled dividers indicate in 
known manner hundreds, tens and units and in this case counting can be 
continued up to one thousand, after which the counting process restarts. 
This sequence of dividers is provided with a reset device to that in any 
position the counters can be set to zero manually. 
The outputs of the counters 172 and 173 of the control circuit 194 are 
connected with a comparator 174, which compares at any instant the values 
of the counters 172 and 173. The comparator 174 has three outputs 175, 176 
and 177. The output 175 receives the logic "YES" signal, when the value of 
the counter 172 is higher than that of the counter 173, the output 176 
received the "YES" signal when the value of the counter 173 is higher than 
that of the counter 172 and the output 177 receives the "YES" signal when 
the values of the counters 172 and 173 are equal. When one of the outputs 
175, 176 or 177 has the "YES" signal, the two other outputs have a "NO" 
signal. The outputs 175 and 176 are connected to subtractors 178 and 179 
respectively, comprising, for example, matching "full adders". The 
subtractors 178 and 179 are supplied with the binary data of the values of 
the counters 172 and 173 through leads 180 and 181. The outputs 175 and 
176 of the comparator 174 are also connected to unequality comparators 180 
and 181 respectively, each of which receives the binary data of the 
subtractors 178 and 179 respectively and a binary signal emanating from a 
setting unit 182. Each comparator 180 and 181 is coupled with an amplifier 
183 and 184 respectively, each of which opens a hydraulic valve 185 and 
186 respectively (these valves being coupled with the valve 130 of FIG. 
5), when the associated comparator 180 and 181 respectively supplies a 
"YES" signal, so that pressurized hydraulic fluid in ducts 187 and 188 
respectively is admitted to the conduit 128 and to the hydraulic device in 
the housing 126 or to a conduit 129 to the device in the housing 127 
respectively (FIG. 5). The valves 185 and 186 are also provided with 
return conduits 189 and 190 respectively leading to a reservoir 191 for 
hydraulic fluid. 
The counters 172 and 173 are coupled with a reset device 192. A signal from 
the reset device 192 is simultaneously applied to the two counters 172 and 
173 and sets both counters to zero. The reset device 192 can be actuated 
by the driver by means of a switch 193. 
When a notch in the wheel rims passes the adjacent pick-up 170, a pulse is 
applied to the counter 172 or 173 respectively. If the inner edge of the 
wheel rim has one notch, the counter will receive one pulse per revolution 
of the wheel. The pulses supplied to the counters 172 and 173 through the 
leads 171 are counted from a zero value established by the reset device 
192. When the tractor moves straight ahead and if the two wheels 5 do not 
skid on the ground, the numbers of pulses counted by the counters 172 and 
173 will be equal. 
It should be noted that the reset device 192 is constructed so that closing 
of the switch 193 causes reset signal to be emitted only once and after 
re-opening and re-closing of the switch 193 it is again emitted only once, 
and so on. 
If one of the wheels 5 skids, the number of revolutions counted by the 
counter 172 will differ from that counted by the counter 173. The 
comparator 174 assesses which of the counter 172 or 173 has the higher 
value and emits the appropriate signal to the output 175 or the output 
176. If, for example, the number of revolutions counted by the counter 172 
is higher than that of the counter 173, the output 175 receives a "YES" 
signal which is applied to the subtractor 178, which subtracts from that 
instant the binary values from the counted numbers of the two counters. 
The signal of the output 175 is also applied to the comparator 180, which 
constantly compares the binary difference applied by the subtractor 178 to 
the comparator 180 from the same instant with a predetermined binary 
number fixed in the setting unit 182. This number may be predetermined by 
the driver once for all and entered in the unit 182, but it may also be 
set by the driver at any instant during operation. The binary number fixed 
in the unit 182 is a limit value for the difference between the numbers of 
revolutions of the two wheels 5 and when this limit value is exceeded, 
means are automatically actuated for ensuring that the difference between 
the numbers of revolutions of the two wheels 5 will not increase or 
decrease. The binary number entered in the unit 182 may, for example, 
correspond to three wheel revolutions. As soon as the difference between 
the numbers of revolution of the two wheels 5 assessed in the subtractor 
178 becomes equal to the binary number fixed in the unit 182 (for example 
the three revolutions), the comparator 180 applies a signal to the 
amplifier 183, which actuates the hydraulic valve 185 in a manner such 
that the hydraulic pressure in the conduit 187 connected with the 
hydraulic pump 109 is transferred to the fluid in the conduit 128 (FIG. 
5). The pressure of the fluid in the conduit 128 increases the pressure by 
which the pulley flange located near the housing 26 is urged towards the 
other flange so that the belt 116 on the pulley 114 moves to a larger 
diameter position so that the transmission ratio to the corresponding 
wheel 5 is reduced and the wheel 5 will rotate more slowly, as a result of 
which skidding is eliminated. Each of the two conduits 128 and 129 
includes a pressure switch 195 and 196 respectively, these switches being 
connected by leads 197 and 198 with the switch 193. As soon as the 
hydraulic valve 185 raises the hydraulic pressure already prevailing in 
the conduit 128 as a result of skidding detected by the circuit 
arrangement 194, a signal is transmitted along the lead 197 after a time 
delay of, for example, 15 seconds, which signal causes the switch 193 to 
close so that the reset device 192 sets the two counters 172 and 173 to 
zero. Then the counters restart counting. If the counters 172 and 173 were 
not returned to zero after a pressure increase in the conduit 128, the 
circuit arrangement would continue to operate in response to the 
difference already recorded (the said three revolutions) so that the newly 
adjusted transmission ratio of the pulley 114 would be maintained, even 
though this may no longer be necessary. Since the counters 172 and 173 
restart counting after a given lapse of time, the signals of the 
comparator 174 disappear and the subtractor 178 supplies the zero number. 
Therefore, the comparator 180 applies a "NO" signal to the amplifier 183 
so that the valve 185 returns to its initial position (for example, by 
spring pressure) and the hydraulic conduit 128 is connected with the 
conduit 189, after which hydraulic fluid can flow back out of the 
hydraulic device in the housing 126 into the reservoir 191 so that the 
pulley 114 can return to its initial position. 
The sequence of actions described above, it being assumed that one of the 
wheels 5 rotates with a higher speed than the other wheel 5 due to 
skidding, occurs in an analoguous manner when the latter wheel 5 performs 
more revolutions that the first-mentioned wheel due to skidding. 
It is emphasized that this circuit arrangement is responsive to the number 
of revolutions of the driven wheels under consideration and hence with 
absolute, dimension-less quantities, as a result of which a high 
sensitivity of the control can be obtained. When the capacity of the 
counters 172 and 173 is full (for example, after one thousand 
revolutions), they automatically return to the zero position. When either 
one of the counters 172 or 173 is full, a feedback to the reset device 192 
is preferred, through leads 199 and 200, so that both counters 
simultaneously restart counting at zero. It is thus ensured that 
accidental small differences are constantly corrected. 
In the embodiments shown in FIG. 5, the front wheels 4 are driven by the 
hydraulic motors 131, which receive their hydraulic energy through pairs 
of hydraulic conduits 132 from the pump 109. The matching of the speed of 
the front wheels 4 obtained by the hydraulic drive with that of the rear 
wheels 5 is analogous with the coupling of the wheels 4 and 5 
schematically illustrated in FIG. 9 and to be discussed with reference to 
FIG. 6. Parts shown in FIG. 9 and already described above are designated 
by the same reference numerals. The tractor shown in FIG. 9 is powered by 
a single engine 201, the output shaft of which is located in the central 
vertical plane of the tractor. As an alternative, the engine 201 may be 
replaced by two driving engines coupled together one behind the other and 
having their output shafts also located in the central vertical plane of 
the tractor. The output shaft of the rear engine or the output shaft 202 
of the single engine 201 is provided with a bevel pinion 203, which meshes 
with bevel pinions 204 and 205 mounted rotatably on a horizontal shaft 206 
extending transversely of the tractor. The portion of the shaft 206 
located between the pinions 204 and 205 is surrounded by a coupling piece 
207 which is rotationally secured to the shaft 206 but is axially 
displaceable along it. The driver can couple the coupling piece 207 by 
means of a claw joint with either one of the pinions 204 and 205, the 
tractor then moving forwards in one of said positions and backwards in the 
other position, as corresponds to the parts 100 to 105 described with 
reference to FIG. 5. The ends of the shaft 206 are again provided with 
pulleys 114 and 115, which are coupled by the belts 116 and 117 
respectively to the pulleys 118 and 119 respectively, which are coupled by 
the planetary gear arrangements 122 and 123 with the rear wheels 5. The 
front wheels 4 are driven by hydraulic motors 208 and 209 respectively, 
each of which is provided with an adjusting device 210 and 211 
respectively, by means of which, for example, swashplates associated with 
the motors 208 and 209 can be adjusted for regulating the output speed of 
the motors 208 and 209. These motors are supplied with fluid through 
hydraulic conduits 212 and 213 respectively from a hydraulic pump 214, 
which is driven by a drive shaft projecting from the front of the engine 
201, which shaft furthermore rotates the front power take-off shaft 16 
through a reduction gear. 
The circuit arrangement 194 and the pick-ups 170 arranged near the wheels 5 
operate to compare the number of revolutions of the wheels 5 and to exert 
control by means of the variable pulley transmissions as discussed above. 
Further pick-ups 215 (see FIG. 6) are disposed near the front wheels 4, 
and they are connected by conductors 216 to the circuit arrangement 194. 
Consequently, the number of wheels to be monitored and controlled is 
increased to four. The circuit arrangement 194 is extended by two further 
counters indicated in FIG. 6 by broken lines, these counters being 
additional to the counters 172 and 173. Corresponding additions are made 
to the rest of the circuit arrangement 194, but for the sake of clarity 
this is not shown in FIG. 6. In this case four hydraulic valves (similar 
to valves 185 and 186 in FIG. 6) are provided, each of which is connected 
in an analogous manner with the speed control of a wheel. Hydraulic 
connections 217 and 218 (FIG. 9) similar to the hydraulic connections 128 
and 129 couple corresponding control valves with the adjusting devices 210 
and 211 so that when the circuit arrangement 194 detects the limit value 
of a difference between the numbers of revolutions of one of the four 
wheels and of the further three wheels, the drive train for the wheel 
concerned (the pulleys 114, 115 or the hydraulic motors 208, 209) is 
corrected so that the number of revolutions is matched to the other three 
wheels. The hydraulic valve similar to each of the valves 185 or 186 
coupled with one of the front wheels communicates therewith by means of 
hydraulic conduits 217 and 218 respectively, by means of which the 
swashplate concerned can be displaced in known hydraulic manner. 
This arrangement ensures that, when travelling straight ahead, the 
peripheral speed of the four wheels is always the same. It should be noted 
that the pick-up arrangement has to be adapted to take account of the 
wheels 4 and 5 having different diameters. As stated above, each wheel rim 
has one or more notches which move past the pick-up 170 or 215 
respectively during travel. In order to ensure a measurement of the real 
path covered by the periphery of the wheel concerned, the number of 
notches in the rim of the wheel of smaller diameter is proportionally 
smaller than the number of notches in the rim of the wheel of larger 
diameter so that the same number of pulses from each of the wheels 
represents the same distance covered. The circuit arrangement 194 of FIG. 
6 can be readily adapted to operate when the tractor is negotiating bends. 
In a bend, the wheels on one side of the tractor rotate more rapidly than 
the wheels on the other side so that the difference between the numbers of 
revolution of the wheels on the two sides of the tractor constantly 
increases. 
the circuit arrangement 194 automatically provides for straight-line travel 
of the tractor, and skidding of the wheels on either side of the tractor 
is then invariably eliminated. During straight-ahead travel, the steering 
wheel 14 is in a central position inclusive of a small region of tolerance 
on either side of that position. As soon as the steering wheel leaves this 
region of tolerance it depresses a switches in each direction of 
deflection, this switch being connected by leads 222 and 223 respectively 
(FIG. 6) to a pulse generator 219 which in turn is connected by leads 220 
and 221 respectively to the unequality comparators 180 and 181 
respectively, while at the same time the setting unit 182 is taken out of 
the circuit (broken lines 222A and 223A in FIG. 6). The pulse generator 
219 comprises two parts, each supplying as a function of time a number of 
pulses depending on the degree of steering wheel deflection and the time 
for which this deflection is maintained. Each part of the pulse generator 
219 has a time base, the frequency of which is fixed inter alia by a 
potentiometer which is adjusted by the deflecting steering wheel. The 
number of pulses rate from each part of the pulse generator 219 is also 
proportional to the travelling speed. If, for example, a bend has to be 
covered in which the wheels located on the side of the comparator 180 
rotate with a lower speed than those on the other side of the tractor, the 
time bases of the parts of the pulse generator 219 are adjusted by the 
potentiometer so that the number of pulses per unit time applied through 
the line 220 to the comparator 180 is smaller than the number of pulses 
per unit time applied through the line 221 to the comparator 181. These 
applied pulse sequences are again limit values representative of the 
desired maximum difference between the numbers of revolutions of the 
wheels on both sides of the tractor. 
When the pulse generator 219 is switched on by deflection of the steering 
wheel, a "YES" signal is automatically applied to the two outputs 175 and 
176 of the comparator 174 (broken lines 222B and 223B in FIG. 6) so that 
the two subtractors 178 and 179 become operative. The measured difference 
applied to the comparators 180 and 181 respectively and to the added 
comparator (the signal emanating from the pick-ups 170 and 215 constantly 
increases as a function of time) is compared with the relatively different 
pulse sequences applied through the lines 220 and 221 respectively to said 
comparators. If skidding occurs on one side of the tractor, the number of 
revolutions in a bend assessed by one of the pick-ups 170 will abruptly 
increase and exceed the limit value fixed by the part concerned of the 
pulse generator 219 so that the amplifier 183 causes the valve 185 to 
become operative as a result of which the variable pulley transmission for 
the rear wheel 5 or the hydraulic motor for the front wheel 4 reduces the 
speed of the skidding wheel. It should be noted that the connection 
between the steering wheel and the steerable wheels 4 may be established 
by known mechanical or hydraulic agency. 
FIG. 10 shows an alternative embodiment in which parts corresponding with 
those of the preceding embodiments are designated by the same reference 
numerals. The tractor shown in FIG. 10 is powered by two aligned engines 
98 and 99 which are at right angles to the central vertical plane of the 
tractor. They can be coupled to the drive shaft 105 to rotate it in either 
direction by means of the gear wheel system 100 to 103 and the coupling 
piece 104 described above. The shaft 105 drives the pulleys 114 and 115, 
and 119 respectively, which are connected by hydraulic conduits 229 and 
230 to the control valve 231 controlled by the circuit arrangement 194 of 
FIG. 6. 
In FIG. 10, the shaft provided with the rear pulleys 118 and 119 is 
provided with a bevel pinion 224 which meshes with a bevel pinion 225 on a 
drive shaft 226 constituting the input shaft of a planetary gear 
arrangement 227 which is identical to one of the planetary gear 
arrangements shown in FIG. 5 for the construction illustrated in FIGS. 7 
and 8. The output shaft 160 (FIG. 7) constitutes in this case the input 
shaft of a differential 228 for balancing the torque applied to the wheels 
5. Since the gear arrangement 227 provides two overlapping ranges of 
speed, as described above, and the variable pulley transmission operates 
at a high enough speed to transfer the driving torque effectively, an 
embodiment of the tractor as shown in FIG. 10 can be obtained with a 
reliable, steplessly variable transmission, the disadvantages of the brake 
belts of the conventional automatic transmissions are avoided. The 
construction of FIG. 10 can be produced at low cost. The front pulleys 114 
and 115 are controlled by the above-mentioned hydraulic devices in the 
housings 126 and 127, which communicate through hydraulic conduits 229 and 
230 respectively with the control valve 231, which can be adjusted from 
the driver's seat. By adjusting the control valve 231, the distance 
between the flanges of the pulleys 114 and 115 and hence the transmission 
ratio of the variable pulley transmission can be steplessly varied. 
With regard to the embodiments described above it should be noted that 
those features such as control devices which are described in connection 
with one of the embodiments and are suitable for use in one or more others 
of the described embodiments should be considered to form also part of 
such embodiments. 
Although various features of the tractors, described and illustrated in the 
drawings, will be set forth in the following claims as inventive features, 
the invention is not necessarily limited to these features and encompasses 
all of the features described both individually and in various 
combinations.