WO1988007150A1

WO1988007150A1 - Transmission means, the controlling thereof and its use in trailers

Info

Publication number: WO1988007150A1
Application number: PCT/FI1988/000035
Authority: WO
Inventors: Jouni Tiainen; Sakari Klapuri
Original assignee: Parikkalan Kone- Ja Laitehuolto Ky
Priority date: 1987-03-12
Filing date: 1988-03-11
Publication date: 1988-09-22
Also published as: FI76632B; FI76632C; AU1495088A; EP0357624A1; FI871079A0

Abstract

The invention relates to a fluid driven transmission means comprising a fluid pressure source (1) and a motor (5, 5a) connected thereto for rotating a secondary shaft (9). The invention also relates to a process for the realization and control of a transmission and further to a trailer (32) connectable to a tractor (36) and having wheel boggies (34). According to the invention the differences in pressures and counter pressures depending on and/or indicating stress and/or rotation of the motor (5, 5a) are detected in fluid conduit means (6, 7) extending between the pressure source (1) and the motor (5, 5a) and a detected pressure difference is used for connecting and disconnecting a secondary shaft (9) to and from the motor (5, 5a). Means (13, 14, 15) for controlling the clutching function are connected to conduit means (3, 6, 7) extending between the pressure source (1) and a gearing unit (5-12). The clutching functions are controlled in such a way that secondary shaft means (9) are disconnected and move freely and independently from the gearing (12), or are in a torsion transmissive connection with the motor (5, 5a). The trailer (32) comprises a transmission/clutching arrangement and further means (33) for adjusting the angle of the boggie (34) with respect to the body (35) of the trailer (32).

Description

Transmission means, the controlling thereof and its -use in trailers

The invention relates to a fluid driven transmission means comprising a fluid pressure source, suitably a hydraulic pump, which preferably is of the constant volume type, and one or more motors which suitably are hydrostatic motors and which are powered by said, pressurized fluid and connected to said pres¬ sure source ^"for rotating a secondary shaft, the primary shaft of said motor suitably being connected to a gearing having its secondary shaft and a coupling device respectively, which system additionally suitably comprises one ore more valve and/or control members. The invention also relates to a method for the realization and control of such a transmission, wherein pressurized fluid from a pressure source, suitably from a pump, is conducted in one or more fluid supply conduits to one or more motors powered by said pressurized fluid, whereby, in the case of a load, the working shaft of each individ al motor is used, suitably by the means of a gearing, for rotating the corresponding secondary shaft and any members suitably con¬ nected thereto.

The invention additionally relates to a trailer connectable to a tractor or the like tractive device, the trailer having wheel boggies and one or more fluid powered motors acting on at least two wheels.

Fluid powered transmission systems especially for trailers and the like are known in the prior art. These so-called auxiliary tractive devices are usually provided in connection with such wheels, which are not tractive in the normal driving. The pur¬ pose of the auxiliary tractive device is then to produce auxiliary tractive power in difficult conditions and e. g. at the start. In these cases the speeds are rather low, normally in the range of 0...10 km/h. At greater speeds these auxiliary tractive devices are disconnected from working. The actual auxiliary use is directed to such moments, when e. g. the wheels of the tractive device in the starting moment or in difficult terrain conditions slip, but the the wheels of the trailer, working machine or the like still are on hard ground, or the auxiliary power generated by them would be sufficient to over¬ come the difficulties.

A fluid powered transmission system, e. g. a hydrostatic slow rotating motor, is quite useful for this kind of auxiliary tractive devices, because its auxiliary pushing power provided in difficult conditions is considerable, and it gives a great torque just at low speeds. At higher speeds this kind of motor on the other hand normally cannot keep pace with the speed, and it is even in other ways of no use worth mentioning, since under these conditions the power of the traction device itself should be sufficient to maintain the driving speed.

Prior known auxiliary tractive devices, however, have the dis¬ advantage of complexity in apparatus and control. Because the auxiliary tractive device in a slow rotating hydraulic rea¬ lization cannot keep pace with a shaft rotating at high speed, the actual motor function must be disconnected during the driving. If disconnecting is not done it will result in that the motor 'will have to rotate at a considerably high speed, in which case the motor only generates extra friction, becomes worn out and the fluid gets hot, which results in a great risk of breakdown.

In one known hydrostatic auxiliary transmission a slowly rotating radial piston motor is used, the disconnection being realized thereby, that the working pistons in the disconnected mode are out of contact with the respective cam rings- Such a motor is, however, rather complicated and heavy and it is therefore not applicable for use in normal trailers. In applications wherein faster rotating hydraulic motors are used, e. g. axial piston motors, mechanical disconnection always has to be used, since disconnection cannot structurally be associated with this types of motor. In most cases this kind of motors, however, are located in the wheel hubs, in which case the clutch is both difficult to realize as well as difficult to control.

A further feature of an auxiliary tractive device is, that it should be operable in both directions and moreover so, that the reversal can be achieved very quickly. Known arrangements cannot normally manage this in a satisfactory way. A breakdown of the apparatus often happens for exactly that reason, that the driver of the vehicle makes a reversal so fast, that there is not sufficient time for the auxiliary tractive device to react to the changes. One has tried to solve this problem by using extra automation, but such automation has until now been quite complicated and therefore not applicable in simple devices. On the other hand, an auxiliary tractive device mounted to an agricultural accessory device provides a match¬ less facility when driving e. g. on soft soil.

The object., of the invention is to achieve a transmission arrangement for an auxiliary tractive device or for a similar use of a temporary nature, wherein a simple arrangement can be used as the regular motor. The motor is preferably connected to driving members, such as driving wheels on the secondary shafts, via a gearing. The intention has especially been to achieve a simple solution, so that the auxiliary tractive device is applicable in demanding operations as well as in quite simple conditions. The invention is applicable to any shafts to which temporary auxiliary tractive power is to be applied in any suitable way. Besides the trailer applications shown, stationary applications such as paper machines or the like can be mentioned. The object of the invention is further to provide such a trans¬ mission arrangement, in which an automatic clutching into con¬ nected and disconnected modes, also in motion, as well as an automatic reversal is accomplished in a single way. Further the solution developed should be as endurable against disturbance as possible, and it must not demand any considerable altera¬ tions in the basic machine. For this reason such a solution is pursued, wherein the fluid volumes and pressures are those, that are commonly used in e. g. tractors. The pressures are typically in the range of 15...25 MPa and the flow rates respectively in the range of 15...60 1/min. Preferably the pressures are in the range of 13...21 MPa and the fluid amounts 16...36 1/min respectively. The device developed is suitable for use in connection with trailers and primarily at the speed range of 0. - .10 km/h at the end of which there will occur an automatic disconnection into the free mode.

According to the invention the problem is solved as disclosed in the appended patent claims. Thus, according to the inven¬ tion, the differences in pressures and counter pressures depending on and/or indicating stress and/or rotation of the motor are detected in one or more fluid conduits extending between the pressure source and the motor, and a detected pres¬ sure difference is used for connecting and disconnecting a secondary shaft to and from the motor.

The apparatus according to the invention is characterized in that a member for controlling the clutching function is con¬ nected to one ore more conduits extending between the pressure source/fluid reservoir unit and a respective motor/gearing unit for detecting pressure differences caused by the pressure source and the motors in order to control the clutching functions for the secondary shafts in common or individually according to the pressure differences so, that when predeter¬ mined pressure circumstances prevail one or more secondary shaft is disconnected and moves freely and independently from the gearing,_ and then on the other hand is in a torsion trans- missive connection with the motor, when other predetermined pressure circumstances prevail.

The invention now will be discussed in more detail with refe¬ rence to the accompanying drawings, wherein figure 1 schematically shows the inventive principle in its simplest form, figure 2 shows a solution according to one embodiment of the invention in form of a scheme, figure 3 shows a solution according to another embodiment of the invention in form of a scheme, figure 4 shows a section of a preferred clutch arrangement according to one embodiment, and figure 5 shows a section of the clutch according to figure 4, and wherein figure 6 schematically shows an embodiment of the invention in one application, figure 7 shows the embodiment of figure 6 seen from the side, and figure 8 exemplifies an embodiment of the solution according to figures 6 and 7.

According to figure 1 the system comprises a hydraulic pump 1 acting as fluid pressure source, which pump is driven by a motor (not shown). In the shown embodiment pump 1 takes hyd¬ raulic fluid from a reservoir 2 and conveys it under pressure along a conduit 3 to a control valve 4, which is a customary direction control valve for controlling the direction of rota¬ tion of a motor 5. From the direction control valve 4 two conduits 6 and 7 lead to the motor in _*uch a way that,: depending on the direction of rotation of the motor, one of the conduits acts as a feeder line and the other as a return line, whereby the latter via valve 4 and suitably via a filter 8 leads back to reservoir 2. In the shown example the position of valve 4 is such, that conduit 6 works as feeder line and conduit 7 as return line, whereby in an opposite valve position the arrangement is the contrary. According to the invention the movement of a shaft 9 extending from motor 5 is controlled by means of a clutch 11 arranged between said shaft and a primary shaft 10 of the motor or an eventual gearing 12 which is not shown in this figure. The function of clutch 11 is controlled by an actuating cylinder 13. The actuating or clutch cylinder 13 is directly connected (in a way indicated by broken lines) to the feeder line 6 between pump, 1 and motor 5 via a control conduit 14, or preferably via a shuttle valve 15 to both conduits 6 and 7. Thus the clutching arrangement always works in the same way independently from the motor' s direction of rotation, which gives a certain advantage which is discussed in more detail in the following.

According to the shown embodiment the pressure is detected in the respectively used feeder line 6 leading from pump 1 to motor 5 used respectively for the feeding of fluid. This pres¬ sure is either directly 14a or indirectly 15 brought to act on clutching means 11 arranged between motor 5 and/or gearing 12 and the secondary shaft 9. Then the clutching means 11, which are open in the low pressure mode, under sufficient pressure will connect the secondary shaft 9 to gearing 12 and/or the working shaf _s 10 of the motor 5 with a torsional strength required by the utility in question.

The fundamentals for the operation procedure of the apparatus is now discussed in more detail:

The dimensioning of the apparatus is based upon the axial torque needed in the auxiliary tractive device. The hydrostatic pump 1 and one or more hydrostatic motors 5 associated there¬ with are dimensioned according to said aspect. In order to accomplish a proper speed of rotation for the rotated secondary shaft 9 it is often of advantage to arrange a gearing between motor 5 and the rotated secondary shaft 9. A gearing-clutching arrangement according to one embodiment of the invention is shown in detail in the figures 4 and 5.

When the tractive device is functionally engaged by using the operation valve, e.g. in connection with a start up, the pres¬ sure of pump 1 will be conveyed by conduit 6 to motor 5, where¬ by said motor 5 will start rotating. Due to the rotation said motor 5 will generate a counter pressure directed towards said conduit 6. In certain cases a choker 16 can be provided in the return conduit 7 in order to guarantee said counter pressure, as shown in figure 3 in connection with another embodiment of the invention. Usually said counter pressure is sufficient to generate a certain positive pressure in conduits 6 and 7, especially in connection with two motors 5 and 5a connected in series as is shown in figure 2. Said positive pressure is con¬ veyed via the control conduit 14, and said pressure is used in a actuating cylinder 13 of clutch 11 to control one or more clutch discs.

The control function in the embodiment shown in figures 4 and 5 is preferably self-adjusting in such an aspect, that one or more clutch discs 17, 18 press ever harder against friction surfaces 19, 20 in proportion to an increasing control pres¬ sure. Motor 5 then faces an ever increasing resistance as long as said motor 5 still gives additional power to devices attached to said secondary shaft 9, e. g. driving wheels 21 in figure 2. The counter pressure in conduit 6 then increases accordingly, and friction surfaces 17-19 and 18-20 will press ever closer against each other. In this way the clutch assembly 11 will provide a relatively flexible start up, which is essential in cases when the ground under the driving wheels is slippery or has poor bearing quality.

Thus one or preferably several clutch discs 17, 18, are used as clutching members, said discs being in regard to the secondary shaft 9 of gearing 12 non-rotating but movable in the direction of said shaft 9. Under the impact of the control pressure each clutch disc 17, 18 will press against one or both of the side surfaces of a cog-wheel 28 rotated by a cog 26 or screw wheel 27 on said primary shaft. Thus a power transmissive and preferably at least to some extent sliding connection between the secondary shaft 9 and cog-wheel 28 is effected.

Said clutching members thus preferably comprise frictional discs 17, 18 which are suitably made of brass or the like material and are torsion resistively movable in the longitudi¬ nal direction of the shaft 9. Under the impact of the pressure in the control conduit 14 and with- the intermediation of a suitably annular piston member 13a said discs are pressed against the side of said motor rotated cog-wheel 28 in said gearing 12.

An especially preferred embodiment of the invention is shown in figure 2. This embodiment comprises two hydrostatic motors 5 and 5a connected in series. Each motor 5 and 5a is connected to its own gearing 12, to which respectively is associated a corresponding clutching arrangement 11, 13 comprising clutch cylinders 13^', 13a, one for each shaft. In the shown embodiment both clutch, cylinders 13, 13a are connected via a common cont¬ rol conduit 14 to a shuttle valve 15, which again itself is connected to said feeder and return conduits 6 and 7. The gearing associated with the second motor 5a is in figure 2 shown without its casing, and thus a cog-pinion 26 and cog¬ wheel 28 arranged on the primary shaft 10 can be seen schemati¬ cally, the clutch discs acting on said cog pinion and -wheel. The direction control valve 4 then defines the mutual distribu¬ tion of the functions of feeder and return conduits 6, 7, according to which motors 5, 5a rotate in either direction, or are totally disconnected in the central position of valve 4.

The shuttle valve 4 in the shown solution automatically ascertains .that the connection function controlling pressure in the control conduit 14 is determined according to the pressure occurring in whichever conduit 6 or 7 being the feeder conduit. Upon a quick change of the rotational direction the pressure will drop preferably momentarily, due to the function of the shuttle valve 15, whereby clutches 11, 11a suitably will be released for a moment. This detachment is not necessarily complete, bub makes it possible for motors 5, 5a to change direction without hitch. The conduit 22 connecting motors 5, 5a comprises in some embodiments a relief valve 23 in order to protect against any possible pressure shock.

The connection system shown makes a certain differential function possible, because each motor 5, 5a operates indepen¬ dently from the other, whereby an eventual rotational speed difference can be controlled with the assistance of clutches 11, 11a on one hand and relief valve 23 on the other hand. Correspondingly multiple driven wheel shaft assemblies can be connected successively, whereby the tractive force can always can be freely distributed between different shafts and even between different wheels. Jn practice the distribution can be effected by connecting and disconnecting subsequent motors either directly or via limiting valves. In this way the motions of e. g. the individual shafts of a multiple axle vehicle can be controlled.

Figure 3 shows an embodiment operating in a manner correspond¬ ing to that of figure 2, but wherein, as was mentioned above, the counter pressure optionally is secured using a special choker 16. Figure 3 shows further an optional choker 24 in control conduit 14, its mission being to achieve a lag in the operation of the clutch cylinders 13, 13a. A pressure reduction valve 25 used in some embodiments for restricting the pressure to the clutch cylinders is also shown.

It is further generally typical for a power transmission system according to the invention, that the system itself automati¬ cally takes care of the disconnection in such circumstances, when the auxiliary tractive force is unnecessary or cannot be utilized. Since the control of clutches 11, 11a will take place in accordance with the pressure difference prevailing in the feeder conduit 6 (or 7, respectively), the secondary shafts 9 are always disconnected or free when motors 5, 5a or pump 1 are non-operating. If e. g. the direction control valve 4 is closed the pressure will fall in both conduits 6, 7, and clutch 11, 11a will disconnect shafts 9 from the connection to motors 5, 5a. Thus e. g. a trailer can always without any further opera¬ tions follow the tractive device if the pressure pump 1 is inoperable or if e. g. some malfunction appears in the appara¬ tus.

In a corresponding manner the secondary shaft 9 will be dis¬ connected when the rotational speed of the shaft exceeds the structural speed of the motor 5, 5a. In such a situation, e. g. when a pulled trailer reaches a sufficient speed and the auxiliary tractive force thus is unnecessary, motor 5, 5a will at excessive speed start operating as a pump. The output from the original pump 1, which usually is driven by the driving motor of the"^""tractive device, being dependent upon the pump rotation speed, will not be sufficient to maintain pressure in conduit 6 or 7, and then the pressure relative to the atmo¬ spheric pressure will drop, because motor 5, 5a now operates as a pump and thus generates a pressure reduction in said conduits instead of a counter pressure. In this situation the control pressure in conduit 14 will drop and the clutch 11, 11a will free the shafts 9 from a connection with motors 5, 5a, whereby the situation will be normalized. The control conduit 14 may comprise a restrictive valve for suppressing any pos¬ sibly occurring alternating clutching.

The connection of auxiliary power itself can in the same way be done under speed, whereby the secondary shafts 9 will be con¬ nected only in such a case, when the motor 5, 5a can provide any auxiliary power. Both the connection and the disconnection takes place without any hitching, because the pressure diffe¬ rence controlling the function of clutch 11 will build up gradually. The connection of the auxiliary power even under speed does for this reason not give any trouble. It is also a special optional advantage of the invention, that the coupling function takes place with great force when the shaft movement is slow, the torsional moment is high and thus the counter pressure is also high. In practice the torque acting upon the secondary shaft 9 can vary flexibly from zero up to a structu¬ ral maximum value. Correspondingl , when the movement is quick and adaptation problems between the driving device and the auxiliary power device speeds could arise, the coupling force is correspondingly low and makes a certain slip possible. If the need for adaptation becomes significant, e. g. because the wrong gear has been selected at the driving device, the above mentioned limited output capacity of the pump 1, with respect to the capacity of the motor 5, 5a now working as pump con¬ nected in series, will be sufficient to keep clutches 11, 11a in a free position.

An apparatus-_^^according to the invention can also, if desired, be used as an auxiliary brake, by providing said control conduit with a supplementary pressure feed means 29, for by¬ passing, via a brake valve 30, the connection to feeder conduit 6 or 7 and for opening a direct connection 29 to the pressure source 1. In this case the torque on the secondary shaft 9, which torque is due to the speed, will act via the frictional surfaces 17, 18 upon motors 5, 5a in order to achieve a braking effect.

Figures 6 and 7 show an especially advantageous optional feature in the embodiments according to the invention. This feature fits excellently to the use of a transmission-clutching system according to invention in connection with a trailer 32 with wheel boggles 21, 31. The invention is then adapted for connecting, 1- a load situation, one or more wheels or suitably pairs of wheels, to a motor 5, 5a and a gearing possibly con¬ nected thereto. In this adaptation the trailer 32 further comprises means 33 for adjusting the angle of the boggie 34 with respect to the body 35 of the trailer 32, so that the load of the trailer 32 substantially can be made to impact upon one selected pair 21 of wheels in the boggie 34, the other pair 31 of wheels remaining suitably in an upraised position. In this way the resultant centre of gravity for the combination of tractive device/trailer can be distributed, between the tractive device 36 and the trailer 32 adapted at need with tractive wheels 21, in a manner which differs from normal driving conditions. This load balancing system, which suitably is realized using the fluid powered actuating cylinders 33, makes it thus possible to move the load to that selected pair of wheels 21 or 37, which at that very moment has the best grip onto the ground.

Even though the load balancing system mentioned . will work satis actorily in trailers equipped with normal auxiliary tractive devices, obtaining full benefit of the system will widely require exactly those features which are offered by the transmission system specified above. With such a combination the centre of gravity for the combination can be transferred during any phase of the driving, so that the main load will shift from wheels 37 of the tractive device 36 to the tractive wheels 21 of the trailer, whereby the auxiliary power device, when the wheels 37 of the tractive device loose their grip, takes care of keeping the combination in motion. The centre of gravity can, in a respective manner, momentarily be transferred completely away from the tractive wheels 21 of trailer 32 towards the "t-ractive device 36 by inclining the boggie 34 in an opposite direction, whereby the stress upon the auxiliary transmission drops, the counter pressure originated from motors 5, 5a drops and motors 5, 5a disconnect from action. Such a load balancing thus requires extensional adaptivity between tractive device 36 and the auxiliary transmission device, and thus the special features of the invention can be utilized at the best.

Claims

1. A process for the realization and control of a fluid driven transmission, wherein pressurized fluid from a pressure source, suitably from a pump (1), is conducted in one or several fluid supply conduits (6, 7) to one or several motors (5, 5a) powered by said pressurized fluid, whereby, in the case of a load, the working shaft (10) of of each individual motor (5, 5a) is used, suitably by the means of a gearing (12) for rotating a cor¬ responding secondary shaft (9) and any means (21) connected thereto, characterized in that the differences in pressures and counter pressures depending on and/or indicating stress and/or rotation of the motor (5, 5a) are detected in one or more fluid conduits (6, 7) extending between the pressure source (1) and the motor (5, 5a) and a detected pressure difference is used for connecting and disconnecting a secondary shaft (9) to and from the motor (5, 5a).

2. A process according to claim 1, characterized in that the pressure in the conduit (6) leading from the pump (1) to the motor (5, 5a) and used respectively for feeding fluid is detected and is brought to act directly or indirectly on clutch means (11, 13) arranged between the motor (5, 5a) and/or the gearing (12) and the secondary shaft (9) so that said clutch means (11) being in an open position under low pressure con¬ ditions will connect the secondary shaft (9) at a sufficiently high pressure to the gearing (12) and/or the working shaft (10) of the fluid powered motor (5, 5a) with a torsion strength required by the utility in question.

3. A process according to claim 1 or 2, characterized in that the choking or counter pressure caused by the fluid powered motor (5, 5a) and/or a separate choker element (16) in the supply conduit (6) from the pump (1) is used respectively in the actuating cylinder (13) of the disc clutch (11) for directing one or several clutch discs (17, 18) into a torsion intermediating condition between the secondary shaft (9) and a gearing (12) suitably connected to the working shaft (10) of the motor (5, 5a).

4. A process according to claims 1 to 3, characterized in that a valve arrangement (15) connected in the pressure system in parallel and/or in series with the motors (5, 5a) is used for controlling the clutching functions, said arrangement suitably comprising one or several choking, control, directional control, and/or shuttle valves (4, 15, 16, 24, 25), respective¬ ly, in the control conduit (14) coupled to the fluid supply conduit (6) for adjusting the pressure, direction, moment and/or amount of the fluid flow to a suitable level for the clutching function.

5. A process according to any of the preceding claims 1 to 4, characterized in that one or several clutch discs (17, 18), are used as clutching members, said discs being non-rotating in regard to the secondary shaft (9) of the gearing (12) but movable in the direction of said shaft (9), each clutch disσ (17, 18) under the impact of the control pressure being pressed against one or both of the side surfaces of a cog-wheel (28) suitably rotated by a cog (26) or screw wheel (27) on said primary shaft (10) of said motor (5, 5a) thus providing a power transmissive and preferably at least to some extent sliding connection between said secondary shaft (9) and said cog-wheel (28).

6. A fluid driven transmission means comprising a fluid pres¬ sure source, suitably a hydraulic pump (1), which preferably is of the constant volume type, and one or more motors (5, • 5a), which suitably are hydrostatic motors and which are powered by said pressurized fluid and connected to said pressure source (1) for rotating a secondary shaft (9), the working shaft (10) of said motor (5, 5a) suitably being connected to a gearing (12) having its secondary shaft (9) and a clutching device (11) respectively, which system additionally suitably comprises one ore more valve and/or control members (4), characterized in that means (13, 14, 15) for controlling the clutching function are connected to one ore more conduits (3, 6, 7) extending between the pressure source/fluid reservoir unit (1-2) and a respective motor/gearing unit (5-12) for detecting pressure differences caused by the pressure source (1) and the motors (5, 5a) in order to control the clutching functions for the secondary shafts (9) in common or individually according to the pressure differences so, that when predetermined pressure circumstance^'s-, prevail one or more secondary shafts (9) is dis¬ connected and moves freely and independently from the gearing (12), and then on the other hand is in a torsion transmissive connection with the motor (5, 5a), when other predetermined pressure circumstances prevail.

7. A fluid transmission means in accordance with claim 6, characterized in that the detecting means, suitably a control pressure conduit (14) with its valves (15, 16, 24, 25), con¬ trolling the clutching function is connected to the pressure and/or return side (6, 7) of the fluid conduits (3) and/or between said conduits in front of and/or behind the motor units (5, 5a) which are suitably connected in series.

8. A fluid transmission means in accordance with claim 6 or 7, characterized- in that the control conduit (14) connected to the fluid supply conduit arrangement (3, 6, 7) comprises one or several choking, control, directional control, and/or shuttle valves (4, 15, 16, 24, 25) in order to adjust the pressure, direction, moment and/or amount of the fluid flow to a suitable level for the clutching function.

9. A fluid transmission means in accordance with any of the preceding claims 6 to 8, characterized in that at least one fluid supply conduit (6) leading from the pump (1) has con¬ nected thereto one or several control conduits (14) for controlling the clutching means (11) between the fluid actuated motor (5, 5a) and the secondary shaft (9) so that said clutching means (11) in the pressurized condition prevailing in the fluid supply conduit (6) provides a torsion intermediating coupling between the secondary shaft (9) and preferably gearing means (12) connected to the working shaft (10) of the fluid actuated motor (5, 5a), but disconnects the said coupling when the pressure is below a specific limit.

10. A fluid transmission means in accordance with any of the preceding claims 6 to 9, characterized in that the control conduit (14) of one or more clutches (11) is connected, suitably via a shuttle valve (15) to^" the supply and return conduits (6, 7) of one or preferably several fluid powered motors (5, 5a coupled in series, said conduits (6, 7) suitably being exchangeable to their supply and return functions for the changing of the direction of rotation of the motors (5, 5a), whereby the total pressure in one conduit (6 or 7), respective¬ ly, deriving from the counter pressure which is caused by the pressure of the pump (1) and the load and/or rotation of the motor (5, 5a), affects the function of one or more clutches (11, 11a) for maintaining a torsion resistant connection between each motor (5, 5a) and the corresponding secondary shaft (9).

11. A fluid transmission means in accordance with any of the preceding claims 6 to 10, characterized in that the gearing means (12) comprise a cog wheel (28) cooperating with cog means (26, 27) of the primary shaft (10) of the motor (5, 5a), said cog wheel (28) being connected to the secondary shaft (9) via one ore more clutch discs (17, 18) moving in axial direction on the secondary shaft (9) and actuated by a working cylinder (13) in rerponse to the control pressure.

12. A fluid transmission means in accordance with any of the preceding claims 6 to 11, characterized in that said clutching means (11) comprise frictional discs (17, 18) which are suitably made of brass or the like material and are torsion resist!vely movable in the axial direction on the secondary shaft (9), said discs being pressed under the impact of the pressure in the control conduit (14) with the intermediation o a suitably annular piston member (13a) against the side (19, 20) of said motor rotated cog-wheel (28) in said gearing (12).

13. A trailer (32) connectable to a tractor (36) or the lik tractive device, the trailer having wheel boggies (34) and on or more fluid powered motors (5, 5a) acting on at least tw wheels (21), characterized in that the transmission-clutchin system of any of the preceding claims 1 to 12 is used for con¬ necting, in a load situation, one or more wheels (21) o suitably pairs of wheels, to a motor (5, 5a) and a gearing (12) possibly connected thereto, and in that the trailer (32) further comprises means (33) for adjusting the angle of the boggie (34) with respect to the body (35) of the trailer (32), so that the load of the trailer (32) substantially can be made to impact upon one selected pair (21 or 31) of wheels in the boggie (34), the other pair (31 or 21, respectively) of wheels remaining suitably in an upraised position, whereby the resultant centre of gravity for the combination of tractive device/trailer (36-32) can be distributed, between the tractive device (36) and the trailer (32), adapted at need with tractive wheels (21), in a manner which differs from normal driving conditions.