The object of this invention is a hydrostatic auxiliary device for a vehicle equipped with a hydrodynamic transmission.

It is well-known that hydrostatic transmission is used in vehicles and working machines such as terrain-vehicles and tractors. Further¬ more, such solutions are known in which the main transmission is mechanical and a hydrostatic auxiliary device is attached to it. In this case the hydraulic pump for the auxiliary device can be placed in connection with the engine or the gears and the yield of the pump is directed straight to the auxiliary device's hydraulic motor. Thus the synchronization of the speeds of the front and rear wheels pre¬ sents no problem because the rotational speeds transmitted through both the mechanical and the hydrostatic gears are directly pro¬ portional to the rotational speed of the vehicle's engine.

As far as the technical level of the invention is concerned further reference is made to the applicant's patents numbers 43388 and 46605.

A problem that has recently developed as a result of hydrodynamic transmission is that hydrostatic transmission cannot be used in connection with hydrodynamic transmission so that the auxiliary device's hydraulic pump would be directly dependent on the engine's rotational speed, because the ratio of the rotational speeds trans¬ mitted by the hydrodynamic and hydrostatic transmissions does not remain constant, because of the fluctuation in the hydrodynamic transmission's slip.

The purpose of the present invention is to offer a solution that is an easy realization to the problem defined above. To achieve this aim and others that will be clarified later the main features of the invention are as follows:

the auxiliary device's hydraulic pump is fitted on the input shaft of the hydrodynamic transmission

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a measuring valve or some other measurement and control device, which measures the actual rotational speed of the wheels of the main, shaft system, is arranged in connection with the output shaft of the hydrodynamic transmission; and

the aforementioned measuring valve or such is fitted so as to direct an adequate part of the said pump's yield to the auxiliary device's hydraulic motor and to direct the rest of the lattermetitiσneά yield to the auxiliary device's hydraulic fluid tank.

3y applying the auxiliary device in. accordance with the invention a constant ratio is attained for the rotational speeds transmitted by the hydrodynamic and hydrostatic transmissions and thus it is not dependent on the slip of the hydrodynamic transmission. Furthermore, the invention accomplishes a good efficiency because the hydrostatic transmission's pump is connected directly to the engine and it is thus not increasing the slip of the hydrodynamic transmission. A further advantage is that the auxiliary device can be e sily dis¬ connected when the slip of the main, wheels is quite small. Ana- logously, the auxiliary device can be arranged so as to connect into operation when the slip of the main wheels increases. A further essential advantage that can be mentioned is that the auxiliary device's pump rotates synchronically to the vehicle's main engine also at departures and thus, the additional traction is pressurized, if needed.

In the following text the invention is described in detail by referring to the attached figure of a schematic drawing whose details do not restrict the invention.

The vehicle that is schematically illustrated in the drawing has a driving main shaft system 11 and a front shaft system 27 that is equipped with an auxiliary drive according to the invention. The attached drawing does not^illustrate the realization of the vehicle more specifically and it may be in all respects well-known as such and customary, except as far as the auxiliary device according co the invention is concerned. In accordance with the drawing, the

vehicle has two pairs of main wheels, namely the pairs of wheels 10a and 10b, which are on the main shafts 11 that are fastened with bearings to the bogie frames 13 which are on shafts 26 that are fitted with joints in a customary way to the frame of the vehicle. A hydraulic pump 18 for the auxiliary device is arranged to be run by the shaft 19 of the vehicle's engine 14. The power from the engine 14 is transmitted to the main shaft system 11 through the hydro¬ dynamic transmission's 15 gears that are equipped with, for example, a torque converter or a fluid clutch. The engine 14 also operates the hydrostatic auxiliary transmission's pump 18 which has a suction pipe 21 connected to it and a pressure pipe 20 connecting it to the measuring valve 17. The in itself well-known valve Orbitrol (trade¬ mark), for example, is used as the measuring valve 17. The valve 17 is located on the output shaft of the hydrodynamic transmission in such a way that it measures the actual rotational speed of the wheels, 10a and 10b, of the main shaft system 11. The valve 17 divides the yield of the pump 18 according to the rotational speed of the main wheels 10a _. and 10b so that the right amount of the yield goes through pipe 22 or 24 to the front shaft's 27 hydraulic motor 23 and the rest is directed to the hydraulic oil's return tank 16.

The vehicle's auxiliary device described above performs in the following manner: when applying the auxiliairy device in accordance with the invention, for example to the front-wheel drive of a vehicle such as a tractor, the auxiliary drive is coupled to operate with the smallest gear. The transmission of power to the main shaft 11 is hydrodynamic. At the same time the yield of the pump 18 that is coupled to the engine 14 is directed by the measuring valve 17 that is operated by the output shaft of the hydrodynamic trans- mission 15. Thus the measuring valve 17 measures the actual speed of the vehicle and it performs as a control unit allowing that part of the auxiliary pump's 18 yield corresponding to the actual speed, to go to the wheels' 12 hydraulic motor 23 for front-wheel drive. The rest of the yield is directed by free circulation to the oil tank 16 through a return pipe 25. In this way the ratio that is attained, of the rotational speeds of the front and rear wheels 10a,10b, and 12 remains constant and is thus not affected by the

fluctuation of the slip of the hydrodynamic transmission 15 which is caused by loading.

It is possible to arrange the auxiliary drive in accordance with the invention so that it connects automatically into operation when the slip of the wheels 10a,10b of the main shaft 11 increases while the system is otherwise circulating freely. The most suitable way to accomplish this is to choose that rotational speed for the front wheels 12 at which the auxiliary drive's hydraulic motor 23 could rotate them, that is a little bit slower than the speed of the main shaft system 11. .As a result, as long as the slip of the wheels 10a,10b of the main shaft II remains small, the front wheels 12 rotate faster than the hydraulic motor 18 could rotate them. The pressure in the pressure pipe does not increase at this time, on the contrary, in reality the hydraulic circuit 22,24 is circulating freely and it thus does not waste energy. As the slip of the wheels 10a,10b of the main shaft system 11 increases, the speed decreases, at which time the pressure in the pressure pipe 22,24 of the front- wheel drive increases, which means that the front-wheel drive connects into operation.

Here follow the patent claims which describe the bounds of the invention's idea around which the different details of the invention may vary.