WO2021028427A1

WO2021028427A1 - Overheating protection for hydraulic systems

Info

Publication number: WO2021028427A1
Application number: PCT/EP2020/072475
Authority: WO
Inventors: Magnus Junginger; Laura GRÖZINGER
Original assignee: Voith Patent Gmbh
Priority date: 2019-08-15
Filing date: 2020-08-11
Publication date: 2021-02-18
Also published as: DE102019121974A1

Abstract

The invention relates to a gear pump (1) with a housing (9) and with a suction connection (4) connected to a low-pressure chamber (2) and a pressure connection (5) connected to a high-pressure chamber (3). A bypass line (14) is connected to a high-pressure side of the gear pump (1). Hydraulic medium can be discharged from the high-pressure side by opening a bypass valve (13). A temperature-sensitive element (20) is allocated to the bypass valve (13) tp actuate the bypass valve (13); the temperature-sensitive element (20) preferably being mechanically operatively connected to the bypass valve (13).

Description

Overheating protection for hydraulic systems

A hydrostatic supply device for supplying a hydraulic consumer is disclosed, the supply device having a hydrostatic pump. The pump can be driven by a motor, preferably variable in speed.

When pressure-maintaining speed-controlled hydraulic pumps, the problem arises that the oil in the pump can become very hot. If it is heated too much, on the one hand the operating medium becomes too thin and causes wear in the pump and, on the other hand, the elastomer seals are damaged by the high temperature.

The following solutions are known to avoid excessive temperatures: Continuous bypass:

A continuous bypass ensures that there is a permanent flow of oil through the pump. Excessive thermal energy can be dissipated through this oil flow. This permanent oil flow thus represents a cooling. However, the disadvantage is the high power loss over the entire operating period.

In centrifugal pump technology, a line is called a bypass and plays an important role in the regulation or as a relief device. In the control function, it is possible to operate a centrifugal pump at a higher flow rate than corresponds to the usable flow rate in the pipeline. For this purpose, a bypass flow is branched off, which can either be returned in a tight loop directly from the pump pressure connection to the pump suction connection or via other devices such as a condenser and cooling device it can be returned to the suction-side delivery flow with a time delay. The bypass is used as a relief device to compensate for the axial thrust in boiler feed pumps. Reasons for using bypass are - Operating behavior in order to stop operating a pump in the partial load area

- Control, because there is a curve of the power requirement that drops too much (e.g. propeller pump, peripheral pump)

- Thermodynamics, to avoid heating of the pumped medium in the partial load area. The bypass flow is branched off through a free-running check valve (see fitting) which is attached to the pressure port, preferably of high-pressure and extremely high-pressure pumps.

Switchable bypass:

A bypass is switched depending on the temperature in the pump. If a permissible temperature is exceeded, the bypass is activated. If the temperature falls below a predetermined value, the bypass is closed again. The temperature at which the bypass is closed again is lower than the temperature at which the bypass is opened. The switchable bypass reduces losses and power loss. The disadvantage, however, is that pressure peaks occur when the bypass is switched on and off. These pressure peaks cannot be regulated quickly enough and cause problems in the process.

A hydraulic pump with a multifunction valve is known from US 2018/0010598 A1. This multifunctional valve provides, among other things, a bypass. Memory metal is provided for temperature-dependent switching of the bypass valve. As a result, the valve opens suddenly when a predetermined temperature of the hydraulic medium is exceeded. This creates a pressure surge. Furthermore, the use of memory metal as a trigger for the temperature-dependent switching process causes a hysteresis depending on the temperature.

Hysteresis is a system behavior in which the output variable depends not only on the independently variable input variable, but also on the previous state of the output variable. The system can therefore - depending on the previous history - with the same input variable one of several possible Take states. For example, the degree of opening of a valve that switches with hysteresis as the input variable depends on both the temperature and the temperature profile.

DE 102006039554 A1 also provides a valve arrangement with such a memory metal for triggering a switching of the valve. Here too, switching takes place suddenly and switching is also carried out with a hysteresis.

From US 2008/0202450 A1 it is known to provide a bypass circuit in order to achieve more rapid heating of a hydraulic medium. For this, the hydraulic medium is fed to a transmission via a heater. Protection against pressure surges is provided for the heating. Such pressure surges are usually generated by separate bypass valves. To avoid such pressure surges, a double valve arrangement is provided. This double valve arrangement closes the line from the heater and when a predetermined temperature is exceeded, the oil supply from the heater is opened.

A switchable bypass is known from Filuwa, for example, in a Multisafe double hose diaphragm pump with a bypass flow rate control. For a short-term regulation of the flow rate, a throttle device (similar to the pressure relief valve) can be attached to the pump, via which a certain, adjustable part of the reservoir fluid displaced by the piston / plunger is directed into the reserve space with each pressure stroke of the pump. Since the delivery volume displaced from the piston / plunger onto the membrane (hose or flat membrane, depending on the pump type) is smaller than the piston / plunger displacement by the amount that has flowed out into the reserve space, the delivery rate of the pump is reduced accordingly. This bypass control is only required for a relatively low drive power of up to 5.5 kW or for short-term control, e.g. B. when starting the system designed. In addition, opening the bypass valve briefly when starting the pump enables relatively quick venting and design-appropriate positioning of the membrane. Temperature protection is known from DE 10 2014 223 186 A1. With this temperature protection, the power loss of the pump is calculated in a servo controller and the drive of the pump is reduced and switched off in an emergency. The disadvantage is that the calculation is very imprecise due to different pump parameters. For an exact prediction, an individual map would have to be created for each pump and the data stored.

Displacement drives are characterized in that the pressure connection of a pump is connected directly to the active surface of the piston of a hydraulic linear drive or the pressure input of a hydraulic motor. The speed of the drive is determined directly by the volume flow delivered by the pump. In another operating mode, the force of the drive is determined directly by the pressure generated by the pump and the effective area of the piston.

Various topologies of displacement drives are known, for example, from DE 19600650 A1, JP2002039110 A, DE 102016124118 A1, DE102014218887 B3, EP1882534A1, DE102017117595A1. As a rule, the pump is operated as part of a controlled system. The controlled variable is either the speed of the actuator or the force delivered by the actuator. All of the above examples can be simplified for an active operating case. The pump is fed from the tank and driven by the variable-speed motor. The pump outlet is directly connected to the pressure chamber, limited by the working piston of the cylinder. The motor is driven by the power amplifier.

In the first case, a pressure setpoint is compared with the actual pressure value in a difference calculator. The control difference controls the power amplifier, whereby various linear, non-linear or more complex methods can be used to generate the motor setpoint from the control difference. The motor setpoint can be the setpoint for a motor torque or a motor speed. The motor can be equipped with sensors for speed or angle of rotation, which can be used to control the actual motor value. In the second case, a nominal position value is compared with the actual position value generated by the displacement sensor. The difference in position is used to control the motor in the amplifier.

It becomes clear that the pump is part of a control loop. Controls are difficult to master if there are discontinuous transfer elements in the control loop. Such a discontinuity would be, for example, a sudden change in the volumetric efficiency of the pump or, with a similar effect, a volumetric flow branched off abruptly from the pump volumetric flow.

It is known from the prior art to switch a bypass valve to open when a certain fluid temperature is exceeded in order to allow the pump an additional volume flow for cooling. However, this additional volume flow will have a disruptive effect as a discontinuity in the control loop.

However, if the additional volume flow is increased gradually and not abruptly as the temperature rises, a controller can compensate for this well. This would be possible by generating the control signal for an electrically operated proportional valve derived from a temperature sensor. However, temperature sensors and additional controllers and actuators are expensive and contain failure possibilities.

The object of the invention is to provide a reliably working device and method with which a hydraulic pump, in particular a gear pump, can be cooled in a cost-effective manner as required, in particular without pressure peaks or pressure drops.

The object of the invention is achieved by the characterizing features of claim 1.

The invention relates to a bypass device for a gear pump and a gear pump with a bypass device and a method for operating a gear pump with a bypass device. The gear pump with a housing and with a low pressure side with a suction connection and with a pressure connection on a high pressure side, also referred to as the pressure side. A bypass line with a bypass valve is connected to the pressure side. A temperature-sensitive element is provided for continuously actuating the bypass valve. Actuation is understood to mean closing and opening. A temperature can be detected by the temperature-sensitive element and the bypass valve can be continuously operated as a function of the detected temperature. A temperature sensor is not required, which has a beneficial effect on costs.

In a preferred embodiment it is provided that the bypass valve is actuated without hysteresis. The position of the bypass valve only depends on the temperature.

A preferred embodiment provides for the position of the bypass valve to be detected and fed to a controller. It has been found to be advantageous to use this position information for a function comparison. If an open bypass is detected and the pump is operated in a mode of operation in which no noticeable heating of the hydraulic medium is to be expected, a malfunction can be deduced from this. A closed bypass valve in an operating situation in which heating and opening of the bypass valve would be expected is an indication of a malfunction.

In a preferred embodiment it is provided that the temperature-sensitive element is mechanically in operative connection with the valve. The temperature-sensitive element detects a temperature. The temperature-sensitive element changes as a function of the temperature. This change acts on the bypass valve via a mechanical connection. This enables the bypass valve to be operated. There can be predetermined limit values from which an actuation of the bypass valve is provided. The mechanical operative connection has the advantage of a simple structure. An electronic control or regulation is not required. A power supply for switching the bypass valve can also be dispensed with. This makes the bypass device particularly simple in structure and also functionally robust.

In one embodiment, an expansion element is provided as the temperature-sensitive element, the expansion element expanding or contracting depending on the temperature acting on the expansion element. The bypass valve is operated as a function of this change.

In a variant of the expansion element it is provided that the expansion element changes its length as a function of the temperature on the expansion element. The expansion element contributes to reliable actuation of the bypass valve.

A bi-metal can also be provided as the temperature-sensitive element. Because bi-metal changes its shape depending on the temperature. This change is recorded. The bypass valve is switched depending on the shape of the bi-metal. Bi-metal here is also to be understood as meaning elements which comprise two different metallic or non-metallic materials and their combinations, the materials used having different coefficients of linear expansion.

It has been found to be advantageous to provide a continuous valve as a bypass valve. With a continuous valve it is possible to provide a steady outflow of hydraulic medium via the bypass valve. This prevents pressure peaks and pressure surges.

In particular, a piston valve has proven to be particularly suitable as a bypass valve. The piston valve is a linear proportional valve. Piston valves are available at low cost as standard components. The drive of a single-stage, direct operated proportional directional valve does not have to differ much from that of a switching directional valve. A linearly movable piston that connects or closes the openings to the inputs and outputs is moved mechanically in one direction or the other. Depending on the movement of the piston of the piston valve, a flow cross section of the piston valve is released or reduced again. Piston valves can also be actuated as a function of an applied voltage or current strength.

In a further embodiment variant, a throttle valve is provided as a bypass valve. Throttle valves are manually operated flow control valves for influencing the volume flow through an adjustable cross-sectional constriction by means of a throttle spindle. The volume flow depends on the pressure difference and viscosity. For example, a rotary knob provided can allow mechanical adjustment. With manually adjustable valves, the adjustable values can be repeated. Compared to other valves such as the piston valve, throttle valves have the advantage that they can be integrated into a smaller installation space and manufactured more cost-effectively.

In one embodiment it is provided that at least the high pressure bypass line is formed in the housing of the gear pump. This has a positive effect on the compactness of the gear pump. If the low pressure bypass line is also formed in the housing of the gear pump, then the bypass line and the gear pump form a closed hydraulic system. The hydraulic medium discharged via the bypass valve can be fed back to the gear pump via the suction connection or the low-pressure chamber on the low-pressure side. Such a design is advantageous in terms of leakage and also in terms of compactness.

In an advantageous embodiment it is provided to assign a cooler to the bypass valve. The hydraulic medium discharged via the bypass valve passes the cooler before it is returned to the suction side. This can further heating of the gear pump can be counteracted particularly effectively.

In a preferred embodiment it is provided that the expansion element comprises a bi-metal. Bi-metal as an expansion element has the advantage that the stroke and the force acting on the valve can be designed and produced with very high repeatability depending on the temperature using different material pairings.

In a preferred embodiment, it is provided that the expansion element consists of a material that increases in axial extent when the temperature increases. As a result of the enlargement in the axial direction and with the bypass valve adjoining it with axial alignment, direct actuation of the bypass valve can be realized by the expansion element. An expansion element can also be provided as the expansion element, as is known, for example, from the ACS GmbH company. The expansion element refers to an actuator filled with expansion material and usually consists of a housing, a working piston and the expansion material. The housing is filled with the expansion material, the filling consists of oil, wax, hard paraffin or metal, depending on the working temperature. A material-dependent phase transition occurs due to a change in temperature. The expansion material experiences a significant change in volume. By using a mixture of oils or waxes with different melting points, a switching range is defined over the melting interval. The same applies to metal alloys. Corresponding intermediate positions of the actuator are established for intermediate temperatures, which helps to avoid undesirable oscillations in the control loop and thus also mechanical wear on the piston.

By providing these known expansion elements, the structure is particularly simple and can be provided at low cost.

In one embodiment it is provided that the expansion element is converted into a deflection movement of the valve via a guide. This allows an axial movement of the expansion element in a rotation for actuating the bypass valve be convicted. In addition, a guide has an advantageous effect on the reliability of the system, even with linear actuation.

In one embodiment it is provided that the expansion element and the valve are arranged in a bypass device which forms an independent unit. This means that if these components are defective, they can be exchanged particularly easily. A gear pump can also be provided with a bypass particularly easily at a later date.

In one embodiment it is provided that the temperature-sensitive element is arranged in the area of the leakage flow, preferably on a leakage flow path for recording the temperature in the leakage flow path. This enables the bypass valve to be actuated particularly quickly. Hydraulic medium flowing in the leakage gap reacts sensitively to the operating situation of the gear pump.

In one embodiment, the expansion element is provided with a restoring element, preferably a restoring spring. The switching points and the switching behavior of the bypass valve can be adjusted using the reset element. Resetting of the bypass valve can also be ensured by the resetting element.

In one embodiment it is provided that the expansion element is firmly coupled to the valve, preferably the piston valve, in the axial direction. The valve is then returned to its starting position directly by contracting the expansion element. A reset element is not required. By friction of the expansion element and friction in the valve, a hysteresis can be achieved so that the valve is opened again at a higher temperature than when it was just closed again.

The method for operating a gear pump with a bypass comprises the following method steps: - Recording of the temperature of the hydraulic medium on the high pressure side or in a leakage flow path in the gear pump through the temperature-sensitive element

- Change of shape, in particular length extension, of the temperature-sensitive element as a function of the temperature

- From a predetermined recorded temperature of the temperature-sensitive element, the valve is actuated by the temperature-sensitive element, the temperature-sensitive element preferably being mechanically operatively connected to the valve, and hydraulic medium can flow out via the valve

- If the temperature falls below the temperature recorded by the temperature-sensitive element, the valve is reset again in the direction of a closed starting position by the temperature-sensitive element and / or by a reset element, the valve being completely closed at a lower temperature than the temperature at which it was opened the bypass line takes place through the valve.

This method offers the possibility of mechanical actuation of the valve, electrical control is not required and therefore it is not necessary to provide a power supply to the valve. An electronic control for the valve is also not required.

Operating the pump in the continuous operating zone is harmless with regard to an impermissible increase in temperature and is therefore permanently permissible. Temperature monitoring is therefore not absolutely necessary, but can still be carried out. With the solution presented here, monitoring is always carried out and thus the risk of an unusual incident is also monitored. If the temperature-sensitive element and valve are mechanically coupled, permanent monitoring takes place, which provides additional security with regard to overheating. The continuous operating zone is preferably defined in that at one measured pump pressure a dependent minimum speed - that is, a pump speed that is above a first limit speed - is present, so that sufficient heat is dissipated from the pump via the pressure medium.

In the pressure maintenance mode, the pump speed is sufficient to compensate for the internal leakage of the pump, but the delivery rate of the pressure medium flowing out of the pump is too low to dissipate the heat generated in the pump to a sufficient extent via the pressure medium. The bypass line must be opened to ensure that the heat is dissipated.

Throttle valve

The directly operated proportional directional control valve has a position-controlled slide. It converts an analog input signal such as voltage or current strength or a mechanical displacement path into a corresponding opening cross-section at the valve outputs.

A proportional valve is a continuous valve which, with the help of a proportional solenoid, not only allows discrete switching positions, but also a constant transition of the valve opening. Proportional valves are used in hydraulics and pneumatics particularly where variable volume flows are required. They usually have a non-linear volume flow characteristic and are therefore less suitable for control tasks than servo valves. The constant transition of the valve opening prevents pressure surges.

A control valve, more precisely a directional control valve, is a proportional valve. The design of the control valve is comparable to a proportional directional control valve. However, the control valve has a linear volume flow characteristic and zero overlap of the valve control edges. The valve is therefore particularly suitable for hydraulic control tasks (pressure, force, torque control, position control). A servo valve is a variant of the proportional valve. It allows any intermediate position of the valve opening and thus the fluid flow. It differs from the simple proportional valve in the design of the piston control edges, with the help of which a greater bandwidth of the frequency response is achieved. However, the precision required to manufacture them makes them expensive.

On the basis of exemplary embodiments, further advantageous features of the invention are explained with reference to the drawings. The features mentioned can not only be implemented advantageously in the combination shown, but can also be combined individually with one another.

The invention is described below with the aid of some exemplary embodiments.

Fig. 1 Internal gear pump with bypass device

Fig. 2 bypass device as an independent unit

Fig. 3 Hydrostatic device with integrated bypass device

Fig. 4 Gear pump with flanged bypass device

A gear pump 1 with a pump housing 9 is shown in FIG. A ring gear 7 with internal teeth is rotatably mounted in the housing 9. A pinion 6 with external teeth is in engagement with the ring gear 7. The pinion 6 is also rotatably mounted in the housing 9. A filler piece 8 is arranged radially between the ring gear 7 and the pinion 6. The gear pump 1 has a low-pressure chamber 2 with a suction connection 4 and a high-pressure chamber 3 with a pressure connection 5 for a consumer, not shown. Hydraulic medium is conveyed from the low-pressure chamber 2 into the high-pressure chamber 3 by means of the meshing gears 6, 7.

The bypass device 10 is located in the pump housing 9, as shown in FIG. 3, or in an independent connection block 29, as in FIGS. 1, 2 and 4 shown. The connection block in FIG. 4 is screwed or flanged onto the pump housing 9. In the bypass device shown in FIG. 2, a stop 24 is provided for the working piston 12. In the bypass device 10 shown in FIG. 2, a sensor 35 is provided which detects the switching position of the bypass valve 13 and forwards this information to a controller, not shown. This information can be used for a plausibility comparison. If, according to the control, the pump 1 is in an operating mode in which heating of the hydraulic medium is not to be expected, but if the position of the bypass is open, a malfunction or a defect can be detected.

The bypass device 13 comprises an expansion element 20 and a valve 13, here in particular a piston valve 21. The valve 21 is actuated by the expansion element 20. The expansion element 20 has a boundary surface 31 which comes into contact with hydraulic medium on the pressure side. As a result, the expansion element, as a temperature-sensitive element 20, picks up the temperature on the high pressure side 3. This boundary surface 31 is on a working piston

12 designated element of the temperature-sensitive element formed. The position of the boundary surface 31 is fixedly arranged in the pump housing 9 or, as shown in FIG. 2, in the connection block 29 and does not change its position.

A bypass line 14 is formed from the high pressure side 3 of the pump 1 to the valve 13, here piston valve 21. A low pressure bypass line 19 is from the valve

13 formed into an external connection 15. The piston valve has a return spring for providing a return force. An expansion element cannot be withdrawn in a defined manner. An expansion element can only extend in a defined manner. For retraction, a certain restoring force is required in order to counteract the flow force in the valve.

When using a bi-metal, the valve 13 is withdrawn. No spring is required in this solution. The different designs shown in the figures differ in that in the variant shown in FIG. 3, the bypass line - pressure side 14 is formed in the housing 9. In the variant shown in FIG. 4, the bypass line pressure side 14 is formed in the connection block 29. There are no functional differences.

In FIGS. 1 and 3, the expansion element 20 is arranged in the pump housing 9, the end of the expansion element 20 being heated by the flydraulic medium located on the pressure side. This can be hydraulic medium from a leakage gap or hydraulic medium conveyed to the pressure side of the pump.

Depending on the heating, a working piston 12 of the expansion element 20 performs a stroke and moves the piston of the valve 13 in the housing 9 or in the connection block 29. The restoring force of the restoring element 16, here restoring spring 17, is dimensioned so that the restoring spring 17 of the Can counteract the flow force of the valve and ensure a defined return stroke when the thermocouple cools down. By moving the working piston 12, the valve 13 is opened and hot hydraulic medium flows off.

A flow opening of the valve 13 is preferably changed as a function of the degree of opening of the valve 13. If the expansion element 20 has reached a maximum expansion, the flow opening of the valve 13 is opened to the maximum. If the temperature falls again, the piston is pushed back by a restoring element 16, here designed as a restoring spring 17, until the working piston 11 is back in its starting position and the valve 13 is closed.

A guide 22 is preferably provided. As a result of the guide, a rotational movement can result from a longitudinal expansion of the working piston 11. The valve can be opened by the rotational movement. A guide 22 can also be provided through which components connected to the working piston or the working piston 11 itself are guided in the axial direction.

FIG. 4 shows a bypass device which is provided in a separate connection block 29. The connection block is screwed to the pump housing. A working piston 12 is located in this connection block 29. The working piston 12 is operatively connected to a valve. A bypass line from the pressure side of the pump 1 is connected to the connection block 29. The pump is a gear pump with a first pinion 6a and a second pinion 6b. The two pinions 6a and 6b are rotatably mounted and mesh with one another.

The expansion element 20 is arranged in the pump housing 9 or in the connection block 29. The boundary surface 31 of the expansion element 20, which is designed as a working piston, and which faces the hydraulic medium, is itself heated by the hydraulic medium. As a result of the heating, the temperature-sensitive element 20 extends its length in the axial direction. 23. The working piston 12 executes a stroke and displaces the piston 25. This opens the bypass valve 13 and hot hydraulic medium flows off. If the temperature in the pump falls again, the bypass valve 13 is pushed back by the return spring 17 until the expansion element 20 is again in the starting position. The bypass line 14 is closed.

This mechanical solution reacts directly to the temperature in the pump 1. A needs-based leakage flow through the bypass valve 13 for cooling the pump 1 is set. Only as much power loss is generated as is absolutely necessary.

The opening of the valve 13 is continuous, preferably linear, for heating. This will not cause any pressure peaks. The valve regulates itself. No external control effort is required. This ensures this safety function even in the event of a defect in a control system of the pump. The valve automatically compensates for influences such as a change in the efficiency due to wear or a change in viscosity due to the temperature-dependent automatic switching of the valve 13.

Alternatively, actuation of the bypass valve 13 by means of material pairing with different coefficients of linear expansion can be provided as an expansion element 20 depending on the temperature of the hydraulic medium.

In Figure 3, a throttle valve is provided as a bypass valve.

Reference list

1 gear pump

2 low pressure space / low pressure side

3 high pressure chamber / high pressure side

4 suction connection

5 pressure connection

6 gear with external toothing / pinion

7 Gear with internal teeth / ring gear

8 filler piece

9 pump housing

10 bypass device / integrated or self-sufficient 10a housing bypass device 11 piston guide 12 working piston

13 bypass valve

14 Bypass line pressure side

15 External connection

16 reset element

17 return spring

18 Leakage Flow Path

19 Bypass line - low pressure side

20 Temperature-sensitive element / expansion element 21 Piston valve 22 Guide

23 axial direction

24 stop

25 pistons

29 Terminal Block

30 throttle valve

31 Limiting surface 35 Sensor (bypass valve)

Claims

1. Gear pump (1) with a housing (9) and with a suction connection (4) connected to a low-pressure chamber (2) and a pressure connection (5) connected to a high-pressure chamber (3) and with a pressure connection (5) / high-pressure chamber (3) connected bypass line (14) and with a bypass valve (13) for opening the bypass line (14), characterized in that the bypass valve (13) is assigned a temperature-sensitive element (20) for continuously actuating the bypass valve (13).

2. Device according to claim 1, characterized in that the temperature-sensitive element (20) is mechanically in operative connection with the bypass valve (13).

3. Apparatus according to claim 1 or 2, characterized in that the temperature-sensitive element (20) is an expansion element (20), the expansion element (20) continuously changing its axial length depending on the temperature and with the bypass valve (13) in Operational connection is.

4. Device according to one of the preceding claims, characterized in that a continuous valve is provided as the bypass valve (13), with the bypass valve preferably opening and closing continuously without hysteresis.

5. Device according to one of the preceding claims 1 to 3, characterized in that a continuous valve, in particular a throttle valve (30) or piston valve (21), is provided as the bypass valve (13).

6. Device according to one of the preceding claims, characterized in that the bypass line (14) for connecting the high pressure side to the bypass valve (13) is formed in the housing (9) of the gear pump (1).

7. Device according to one of the preceding claims, characterized in that the hydraulic medium is fed back to the suction connection (4) or the low-pressure side (2) via the bypass valve (13), preferably the bypass line-low-pressure side (19) from the bypass valve (13) to the suction side, preferably at least partially in the housing (9).

8. Device according to one of the preceding claims, characterized in that the bypass valve (13) is assigned a cooler and the hydraulic medium conducted via the bypass valve (13) passes the cooler before being fed to the suction side.

9. Device according to one of the preceding claims 3 to 8, characterized in that the expansion element (20) comprises a bi-metal.

10. Device according to one of the preceding claims 3 to 9, characterized in that the expansion element (20) is converted into a deflection movement of the bypass valve (13) via a guide (22).

11. Device according to one of the preceding claims 3 to 10, characterized in that that the expansion element (20) is in contact with the hydraulic medium on the high pressure side (3) of the pump (1), preferably having a boundary surface (31) in direct contact with the hydraulic medium on the high pressure side (3).

12. Device according to one of the preceding claims 3 to 11, characterized in that the expansion element (20) and the bypass valve (13) are arranged in a bypass device (10) which forms an independent unit.

13. Device according to one of the preceding claims, characterized in that the temperature-sensitive element (20) is arranged in the area of the leakage flow, preferably on a leakage flow path (18) for recording the temperature in the leakage flow path (18).

14. Bypass device (10) for a gear pump as overheating protection with a housing (10a) and with a bypass line low pressure side (19) and a bypass line pressure side (14) and with a bypass valve (13) for connecting the bypass line pressure side (14) with the low-pressure bypass line (19), characterized in that a temperature-sensitive element (20) is assigned to the bypass valve (13) and, depending on the temperature of the hydraulic medium on the pressure side detected by the temperature-sensitive element (20), the bypass valve switches the connection from the bypass line high pressure side (14) to the bypass line low pressure side (19), whereby a cooler is preferably assigned to the bypass valve and the hydraulic medium passed through the bypass valve (13) passes the cooler before it is fed to the suction side.

15. A method for operating a gear pump (1) with an integrated or self-sufficient bypass device according to one of the preceding claims, characterized by the following method steps: - Recording of the temperature of the hydraulic medium on the high pressure side (3) or in a leakage flow path (18) of the gear pump (1) by the temperature-sensitive element (20)

- Depending on the temperature, the temperature-sensitive element (20) changes its shape, in particular its length

- From a predetermined recorded temperature of the temperature-sensitive element (20), the bypass valve (13) through the temperature-sensitive element (20), wherein the temperature-sensitive element (20) is preferably mechanically in operative connection with the bypass valve (13), depending on the measure of the

Heating of the temperature-sensitive element (20) actuated and hydraulic medium can flow out via the bypass valve (13)

- From a fall below the temperature recorded by the temperature-sensitive element (20), the bypass valve (13) is again in the direction of a closed starting position by the temperature-sensitive

Element (20) and / or reset by a resetting element (16), the bypass valve (13) being completely closed at a lower temperature than the temperature at which the bypass line (14, 19) is opened by the bypass valve (13) he follows.