JP2002249060A - Emergency steering system of hydraulic steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an emergency steering system of a hydraulic steering device capable of further reducing the energy loss in a normal state. SOLUTION: When the pressure generated in a return flow passage 13 is led to one pilot chamber 20a of a suction control valve 20, the suction control valve 20 is changed over to the regulating position, and regulates the suction of a sub pump SP to set the discharge to be substantially zero. On the other hand, if the return flow rate from a hydraulic steering device PS becomes zero and the pressure in the return flow passage 13 is dropped, the suction control valve 20 is changed over to the open position, the sub pump SP feeds the pressure oil to the hydraulic steering device PS via a merging flow passage 18, and maintaining mechanisms 19 and 21 maintain the open position of the suction control valve 20 by the discharge pressure of the sub pump SP.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic steering system capable of maintaining a steering assist force of a hydraulic steering system even when a main pump of the hydraulic steering system is stopped due to, for example, a stall or a cut of a drive belt. The present invention relates to an emergency steering system for a device.

[0002]

2. Description of the Related Art FIG. 5 shows a conventional system. A hydraulic hydraulic steering device PS is connected via a supply flow path 1 to a main pump MP driven by the engine E.
The hydraulic steering device PS is operated by pressure oil discharged from the main pump MP, and exerts a predetermined assist force. The pressure oil supplied to the hydraulic steering device PS is returned to the tank T via the return flow path 2. The supply flow path 1 is provided with a check valve 3 for preventing backflow.

[0003] A steering assist force maintaining mechanism a is connected to the hydraulic steering device PS. The steering assist force maintaining mechanism a includes a variable discharge type sub-pump sp linked to the wheel 4, a control cylinder 5 for controlling a tilt angle of the sub-pump sp, a control valve 6 for controlling the control cylinder 5, A circuit 8 having two check valves 7, a merging flow path 9 connecting the circuit 8 with the supply flow path 1, an orifice 10 provided in the merging flow path 9, and a downstream side of the orifice 10. And a switching valve 11.

[0004] The sub-pump sp rotates according to the rotation direction and the rotation speed of the wheels 4 and discharges the pressure oil sucked from one of the ports through the other port. The pressure oil discharged from the sub-pump sp is guided from the circuit 8 to the merging channel 9, and is guided to the switching valve 11 via the orifice 10. Then, depending on the switching position of the switching valve 11, the oil is supplied to the hydraulic steering device PS or returned to the tank T. Further, when a flow occurs in the merging flow path 9 as described above, a pressure difference occurs before and after the orifice 10. The control valve 6 is switched by the differential pressure, and the control cylinder 5 adjusts the tilt angle of the sub-pump sp according to the switching position of the control valve 6. That is, the discharge amount of the sub-pump sp is optimally controlled.

[0005] In the conventional system as described above, when the engine E is operating normally, the hydraulic oil discharged from the main pump MP is supplied to the hydraulic steering device PS via the supply passage 1 → the check valve 3. Supplied to for that reason,
The hydraulic steering device PS can exert a predetermined assist force. In the above state, when the vehicle is actually running, the rotation of the wheels 4 causes the rotation of the sub-pump sp, and the pressure oil discharged from the sub-pump sp passes through the merging flow path 9. It is led to the switching valve 11. However, at this time, the switching valve 11
Since the pump discharge pressure is guided from the supply flow path 1 to the pilot chamber 11a, the switching valve 11 maintains the switching position shown in the drawing, and connects the merging flow path 9 to the tank T. Therefore, the pressure oil discharged from the sub-pump sp is returned to the tank T after passing through the orifice 10 without being supplied to the hydraulic steering device PS.

[0006] As described above, the pressure oil is supplied to the orifice 10.
, A differential pressure is generated across the orifice 10. The control valve 6 is switched by this differential pressure, and the control cylinder 5 operates. The tilt angle of the sub-pump sp is adjusted by the operation of the control cylinder 5, and the optimal discharge amount is discharged from the sub-pump sp. That is, when the engine E is operating normally, the hydraulic steering device PS is driven by the pressure oil discharged from the main pump MP.
Does not need to be supplied from the sub-pump sp. Therefore, in such a case, the pressure oil discharged from the sub-pump sp is returned to the tank as it is, and the discharge amount is controlled to be small, thereby suppressing the energy loss.

On the other hand, if the engine E stops due to sudden braking or the like while the vehicle is running, the main pump M
P also stops. When the main pump MP stops,
Since no pressure oil is supplied from the main pump MP to the hydraulic steering device PS, the assist force cannot be obtained as it is. If the assisting force of the hydraulic steering device PS cannot be obtained, the steering wheel suddenly becomes heavy. For example, in the case of a vehicle that requires a large steering force, such as a large truck, the steering operation cannot be performed only by human power. Therefore, in some cases, there is a possibility that the vehicle may not be able to turn a corner or the like.

However, in this system, when the main pump MP stops and the pressure in the supply flow path 1 decreases, the switching valve 11 is automatically switched by this, and the sub pump s
p communicates with the hydraulic steering device PS. With this configuration, the pressure oil from the sub pump sp is supplied to the hydraulic steering device PS, so that the assist force of the hydraulic steering device PS is maintained.

When the switching valve 11 is switched as described above and the hydraulic oil from the sub-pump sp is supplied to the hydraulic steering device PS, the control valve 6 switches according to the change in the differential pressure across the orifice 10. The high pressure upstream of the orifice 10 is supplied to the control cylinder 5. The control cylinder 5 controls the tilt angle in a direction to increase the discharge amount of the sub-pump sp. Therefore, a sufficient flow rate is supplied to the hydraulic steering device PS from the sub pump sp.

[0010]

In the above-mentioned prior art, when the engine E is operating normally, the energy loss is suppressed by reducing the discharge amount of the sub-pump sp. was there. That is, in order to control the discharge amount of the sub-pump sp, the orifice 10
Energy loss. In addition, there is also a drawback that the pressure loss there is likely to be increased by the amount of passage through the orifice 10. An object of the present invention is to provide an emergency steering system of a hydraulic steering device that can reduce energy loss during normal operation.

[0011]

SUMMARY OF THE INVENTION The present invention provides a main pump operated by a drive source such as an engine, a hydraulic steering device connected to the main pump, and a steering assist force maintaining mechanism connected to the hydraulic steering device. It is premised on an emergency steering system of a hydraulic steering device provided.

According to a first aspect of the present invention, while assuming the above system, the steering assist force maintaining mechanism includes a sub-pump driven by rotation of a wheel, and a junction connecting the discharge side of the sub-pump and the hydraulic steering device. A return path for guiding the return fluid from the hydraulic steering device to the tank; a pressure generating mechanism for providing a predetermined pressure by the return fluid from the hydraulic steering device; There is provided a suction control valve connected to the suction side, and a holding mechanism for keeping the suction control valve in an open position according to the discharge pressure of the sub-pump. Then, when the pressure generated in the return flow path is guided to one pilot chamber of the suction control valve, the suction control valve switches to the regulation position, regulates the suction amount of the sub-pump, and reduces the discharge amount to almost zero. On the other hand, when the return flow from the hydraulic steering device disappears and the pressure in the return flow path decreases, the suction control valve switches to the open position, and the sub-pump supplies pressure oil to the hydraulic steering device via the merge flow path, Further, at this time, the holding mechanism is configured to maintain the open position of the suction control valve by the discharge pressure of the sub-pump.

According to a second aspect of the present invention, in the first aspect,
The holding mechanism includes a holding valve connected to the other pilot chamber of the suction control valve, and an orifice provided in the merging flow path. When a predetermined differential pressure occurs before and after the orifice, the holding valve operates as a suction control valve. It is characterized in that the pressure upstream of the orifice is led to the other pilot chamber to keep the suction control valve in the open position.

According to a third aspect, in the first aspect,
The holding mechanism is provided in the merging flow path and includes a holding check valve that opens when a predetermined pressure is reached, and a pilot line that guides the pressure upstream of the holding check valve to the other pilot chamber of the suction control valve. When a predetermined pressure is generated upstream of the holding check valve by the pressure oil discharged from the sub-pump, the suction control valve is kept at the open position by the pressure. In a fourth aspect, in the first to third aspects,
A pressure release valve is provided for connecting one pilot chamber of the suction control valve to the tank.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment shown in FIG.
2, a hydraulic steering device PS is connected.
The hydraulic steering device PS is operated by pressure oil discharged from the main pump MP, and exerts a predetermined assist force. Further, a return passage 13 is connected to the hydraulic steering device PS, and the pressure oil supplied to the hydraulic steering device PS via the return passage 13 is supplied to the tank T.
To return to. In addition, in the supply flow path 12,
A check valve 14 for preventing backflow is provided.

The hydraulic steering device P as described above
A steering assist force maintaining mechanism A is connected to S. The steering assist force maintaining mechanism A includes a circuit 17 including a sub-pump SP linked to the wheel 15, four check valves 16 connected to the sub-pump SP, and a junction connecting the circuit 17 and the supply flow path 12. A path 18, an orifice 19 provided in the merging flow path 18, a suction control valve 20 provided between the circuit 17 and the tank T, and a holding valve 21 connected to a pilot chamber 20b of the suction control valve 20. It is composed of

The sub-pump SP rotates according to the rotation direction and speed of the wheel 15, and discharges the pressure oil sucked from one of the ports through the other port. Then, the pressure oil discharged from the sub-pump SP is guided in the order of the circuit 17 → the converging flow path 18 → the orifice 19 → the supply flow path 12, and is supplied from the supply flow path 12 to the hydraulic steering device PS. Since the supply flow path 12 has the check valve 14, the discharge oil of the sub-pump SP does not flow into the main pump MP.

The pressure oil supplied to the hydraulic steering device PS is discharged to the tank T via a return passage 13, and the return passage 13 is connected to the suction control valve 20. This suction control valve 20
Makes the suction side of the sub-pump SP and the tank T communicate with each other at the maximum opening when in the illustrated open position. Further, by interposing a pressure generating mechanism 22 between the return flow path 13 and the tank T, the return flow path 1
3, a predetermined pressure is generated.

On the other hand, when the suction control valve 20 is switched to the restriction position on the upper side in the drawing, the opening of the communication passage between the sub-pump SP and the tank T is reduced by the throttle 23, and almost the pressure oil is sucked into the sub-pump SP. Not to be. That is, when the suction control valve 20 is switched to the regulation position, the discharge amount of the sub-pump SP becomes almost zero. It should be noted that the pressure generating mechanism 22 is set to generate a predetermined pressure in the return flow path 13 even when the suction control valve 20 is at the regulated position.

The pressure generated in the return passage 13 by the pressure generating mechanism 22 is guided to one pilot chamber 20 a of the suction control valve 20. Further, the holding valve 21 is connected to the other pilot chamber 20b of the suction control valve 20 provided with the spring 24 as described above. The holding valve 21 is switched by a differential pressure generated before and after the orifice 19, and when in a normal position shown in the drawing, the pilot chamber 2 of the suction control valve 20
0b is communicated with the tank T, and when switched, the pressure on the upstream side of the orifice 19 is guided to the pilot chamber 20b.

Next, the operation of the first embodiment will be described.
First, when the engine E is operating normally, pressure oil is supplied from the main pump MP to the hydraulic steering device PS via the check valve 14. Therefore, the hydraulic steering device PS can exert a predetermined assisting force by the discharge oil of the main pump MP. The pressure oil supplied to the hydraulic steering device PS is discharged to the tank T via the return passage 13. At this time, the pressure generating mechanism 22 provided in the suction control valve 20
As a result, a predetermined pressure is generated in the return channel 13.
Then, when this pressure is guided to one pilot chamber 20a of the suction control valve 20, the suction control valve 20 is switched to the regulation position, and the suction side of the sub pump SP is throttled.

In this state, when the vehicle travels and the sub-pump SP rotates accordingly, the sub-pump SP tries to suck the pressure oil from the tank T. However, since the opening degree of communication with the tank T is reduced to a very small value by the suction control valve 20, the sub-pump S
P is supplied with only a small flow rate for lubrication. Therefore, the discharge amount of the sub-pump SP becomes substantially zero, and no waste flow is supplied from the sub-pump SP to the hydraulic steering device SP. In addition, sub pump SP
, The pressure difference does not occur even at the orifice 19. That is, no energy loss occurs due to the pipe resistance.

As described above, since no differential pressure is generated before and after the orifice 19, the holding valve 21 switched by the differential pressure maintains the state shown in the figure by the elastic force of the spring 25. Therefore, the suction control valve 20
The other pilot chamber 20b is kept open to the tank T, and the suction control valve 20 keeps the regulation position.

On the other hand, if the engine stops for some reason or the drive belt of the main pump MP is damaged while the vehicle is running, and the main pump MP stops, the flow of the return flow path 13 will stop. Disappears. As a result, the pressure in the return flow path 13 decreases, whereby the suction control valve 20 is switched to the open position by the elastic force of the spring 24.

Therefore, the suction side of the sub-pump SP communicates with the tank T at the maximum opening degree, the sub-pump SP sucks a sufficient flow rate from the tank T, and discharges the discharged oil to the circuit 1.
7 → supply channel 18 → orifice 19 → supply to hydraulic steering device PS via supply channel 12. By supplying the oil discharged from the sub-pump SP to the hydraulic steering device PS in this manner, the hydraulic steering device PS can exert a predetermined assist force even when the main pump MP stops.

When the discharge oil of the sub-pump SP is supplied to the hydraulic steering device PS as described above, the return flow is guided to the suction control valve 20 via the return passage 13. Therefore, pressure is generated again in the return passage 13 by the pressure generating mechanism 22, and the pressure is guided to one pilot chamber 20 a of the suction control valve 20. On the other hand, at this time, the pressure on the upstream side of the orifice 19 is guided to the other pilot chamber 20 b of the suction control valve 20. This is because, at the same time as the discharge oil of the sub-pump SP is supplied to the hydraulic steering device PS, a pressure difference occurs across the orifice 19, and the pressure difference causes the holding valve 21 to switch to the right position in the drawing.

Therefore, the suction control valve 20 is
The pressure on the upstream side of the orifice 19 is guided to the other pilot chamber 20b, and the pressure on the downstream side of the orifice 19 is guided to one of the pilot chambers 20a. keep.
Since the suction control valve 20 maintains the open position in this manner, the sub pump SP operates the hydraulic steering device PS
And the hydraulic steering device PS can maintain the assisting force. The orifice 19 and the holding valve 21 constitute the holding mechanism of the present invention.

If the vehicle is stopped while the engine is stopped, the differential pressure across the orifice 19 disappears.
If the differential pressure disappears, the holding valve 21
Is switched to the state shown in FIG.
Is released to the tank T. When the pilot chamber 20b reaches the tank pressure in this way, the suction control valve 20 is also switched to the regulation position. Therefore, the sub-pump SP does not discharge the pressure oil again, and returns to the energy saving state.

According to the first embodiment, the main pump M
When P is operating normally, the discharge amount of the sub-pump SP can be made substantially zero, so that energy loss due to pipe resistance can be almost prevented. In addition, since the emergency steering function is mechanically activated by using the return fluid from the hydraulic steering device PS, the function can be surely exerted even if the electric system is stopped.

In the first embodiment, the holding mechanism for holding the suction control valve 20 at the open position is provided by the orifice 19.
And the holding valve 21. The second embodiment shown in FIG. 2 simplifies the structure of the holding mechanism. In the second embodiment, the orifice 19 shown in FIG.
, A holding check valve 47 is provided in the merging flow path 18. The holding check valve 47 is provided with a sub-pump SP
From the flow path 12 to the supply flow path 12 only. The holding check valve 47 has a spring 4
5, a predetermined pressure is generated on the upstream side by the elastic force of the spring 45. Further, a pilot line 48 is connected to the upstream side of the holding check valve 47 so that the pressure in the merging flow path 18 is sucked and directly led to the other pilot chamber 20 b of the control valve 20. The rest of the configuration is exactly the same as in the first embodiment.

According to the second embodiment, there is a case where the engine is stopped while the vehicle is running or the drive belt of the main pump MP is damaged, so that the discharge oil of the main pump MP cannot be obtained. Also, as long as the vehicle continues the inertial running, the pressure oil is discharged from the sub-pump SP driven by the wheels. Then, the discharge oil flows in the order of the merging flow path 18 → the holding check valve 48 → the supply flow path 12 → the hydraulic steering device PS → the return flow path 13.
A predetermined pressure is generated upstream of the holding check valve 48. The predetermined pressure is applied to the pilot line 48.
Through the pilot chamber 20a of the suction control valve 20
It is led to. When the pressure on the upstream side of the holding check valve 47 is guided to the pilot chamber 20a of the suction control valve 20, the suction control valve 20 maintains its open position.

According to the second embodiment, the function of the holding mechanism of the present invention can be obtained by the holding check valve 47 and the pilot line 48. Therefore, the cost can be reduced by eliminating the need for the holding valve 21 and the like as compared with the first embodiment.

In the third embodiment shown in FIG. 3, a pressure release valve 26 is provided in the return passage 13 of the first embodiment, and the other structure is exactly the same as that of the first embodiment. . The pressure release valve 26 switches when the solenoid is excited from the state shown in the figure, and discharges the pressure oil in the return passage 13 to the tank T. If the pressure oil in the return flow path 13 is discharged to the tank T in this manner, the state becomes the same as when the function of the main pump MP is stopped due to engine stall or other reasons, and the suction control valve 20 is switched to the open position. Instead, an emergency steering function can be exhibited.

That is, while the vehicle is running, the main pump M
Since it is extremely rare for P to stop, such a state is simulated to check whether the emergency steering function operates reliably. Although not shown, the sub-pump S is used to actually confirm that pressure oil is being discharged from the sub-pump SP.
Although not shown, a pressure sensor, a flow rate sensor, and the like are provided between P and the supply channel 12. Note that the emergency steering function check may be periodically performed by automatically outputting an electric signal to the solenoid.

In the third embodiment, the pressure release valve 26
Is electrically switched by a solenoid, but may be switched by mechanical means such as hydraulic pressure or a link. It should be noted that the pressure release valve 26 may be provided anywhere to generate the engine stall state in a simulated manner. For example, the pressure release valve 26 may be provided in a pilot line that guides pressure to the pilot chamber 20a.

FIG. 4 shows a specific structure of the suction control valve 20 according to the first to third embodiments.
The same components as those of Nos. 1 to 3 are denoted by the same reference numerals.
A spool hole 31 is formed in the valve body 30, and a main spool 32 is slidably incorporated in the spool hole 31. A pilot chamber 20b is provided on the left side of the main spool 32 in the drawing, and a spring 24 is incorporated in the pilot chamber 20b. The right end of the main spool 32 is pressed against the step 34 by the elastic force of the spring 24,
In this state, the tank port 35 formed in the valve body 30 and the suction port 36 communicate with each other at the maximum opening degree. The holding valve 21 is connected to the pilot chamber 20b via a pilot port 50. The circuit 18 shown in FIGS. 1 and 2 is connected to the suction port 36.

A chamber 51 is formed in the main spool 32, and the sub spool 37 and the sub spring 38 are incorporated in the chamber 51. And the sub-spring 38
The sub-spool 37 is pressed against the right side surface of the main spool 32 by the elastic force of. This sub spool 37
A plurality of holes 39 are formed on the right side surface of the main spool 32 against which is pressed, and a hole 49 for communicating the chamber 51 with the ports 35 and 36 is formed on the opposite side surface.

The right side surface of the main spool 32 is desired to be the return port 40. The return flow passage 13 is connected to the return port 40. A notch 46 is provided at a part of the edge of the tank port 35.
Is formed. A communication hole 41 is formed in the sub spool 37.

For example, when the engine E is operating normally, the main spool 32 resists the spring 24 before the sub-spool 37 due to the pressure of the fluid guided from the return passage 13 to the return port 40. To the left of the drawing. Therefore, communication between the tank port 35 and the suction port 36 is cut off, but the notch 43 formed on the outer periphery of the main spool 32 allows the tank port 35 and the suction port 36 to slightly communicate with each other. Therefore, in this state, when the sub-pump SP rotates together with the rotation of the wheels, the mist-like fluid is slightly supplied from the notch 43 to the suction port 36 side. The atomized fluid is used as lubricating oil for the sub-pump SP.

When the main spool 32 further moves leftward due to the pressure in the return port 40 from the above state, the return port 40 and the tank port 35 communicate with each other through the notch 46. Therefore, the return fluid from the hydraulic steering device PS is discharged to the tank port 35 via the throttle passage 47. Since the holding valve 21 connected to the pilot port 50 connects the pilot port 50 to the tank T, the pilot chamber 20b has a tank pressure.

On the other hand, the main pump M
When P stops, the pressure in the return port 40 decreases, and the main spool 32 returns to the illustrated position by the elastic force of the spring 24. When the main spool 32 returns to the position shown in the drawing, the communication opening between the tank port 35 and the suction port 36 is fully opened, and sufficient pressure oil is supplied to the sub pump SP.

The hydraulic oil supplied from the sub-pump SP to the hydraulic steering device PS returns to the return port 40 through the return passage 13, so that the main spool 32 and the sub-spool 37 have a thrust in the left direction in the drawing. Works. However, at this time, since the high pressure on the upstream side of the orifice 19 is guided to the pilot chamber 20b through the pilot port 50, the main spool 32 maintains the illustrated state. Therefore, only the sub-spool 37 moves leftward in the drawing due to the pressure of the fluid in the return port 40, and returns the return fluid to the hole 39 → the communication hole 41 → the chamber 51 →
Return to the tank port 35 side through the hole 49.

[0043]

According to the first aspect of the invention, the suction amount of the sub-pump is regulated by the suction control valve when the main pump is operating normally, and the discharge amount is made substantially zero. Therefore, energy loss due to pipe resistance or the like can be completely prevented. Also, using the return flow rate from the hydraulic steering device,
Since the suction control valve is configured to be switched, the emergency steering function can be reliably exhibited even when the electric system is stopped.

According to the second aspect, the holding valve is switched by the differential pressure generated before and after the orifice, and the pressure upstream of the orifice is positively guided to the other pilot chamber of the suction control valve. The open position of the control valve can be reliably maintained.

According to the third aspect of the present invention, the holding mechanism is constituted by the holding check valve and the pilot line without using components such as a switching valve having a complicated structure. System can be provided. According to the fourth invention, it is possible to check whether or not the emergency steering function can be reliably performed even when the main pump does not stop its function due to a failure or the like while the vehicle is running.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment.

FIG. 2 is a circuit diagram of a second embodiment.

FIG. 3 is a circuit diagram of a third embodiment.

FIG. 4 is a specific structural diagram of the suction control valve 20;

FIG. 5 is a circuit diagram of a conventional example.

[Explanation of symbols]

 E Engine MP Main pump SP Sub-pump A Steering assist force maintaining mechanism 13 Return flow path 15 Wheel 18 Merging flow path 19 Orifice constituting the holding mechanism of the present invention 20 Suction control valve 20a One pilot chamber of suction control valve 20b Suction control valve One of the pilot chambers 21 A holding valve constituting a holding mechanism of the present invention 22 A pressure generating mechanism 26 A pressure releasing valve 47 A holding check valve constituting a holding mechanism of the present invention 48 A pilot line constituting a holding mechanism of the present invention

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3D033 EB04 EB06 EB08 GB02 GB05 MA04 3H056 AA05 BB50 CA02 CB02 CD06 EE01 EE06 GG12 3H067 AA17 AA33 BB03 BB13 CC60 DD05 DD12 DD33 FF11 GG22

Claims (4)

[Claims] An emergency steering of a hydraulic steering system including a main pump operated by a drive source such as an engine, a hydraulic steering device connected to the main pump, and a steering assist force maintaining mechanism connected to the hydraulic steering device. In the system, the steering assist force maintaining mechanism includes a sub-pump that is driven with rotation of wheels, a merging flow path that connects a discharge side of the sub-pump and the hydraulic steering device, and a return fluid from the hydraulic steering device. A return flow path leading to the tank, a pressure generation mechanism provided in the return flow path and generating a predetermined pressure by return fluid from the hydraulic steering device, a suction control valve connected to a suction side of the sub pump, and the sub pump Suction control valve according to the discharge pressure of And a holding mechanism for maintaining the open position.When the pressure generated in the return flow path is guided to one of the pilot chambers of the suction control valve, the suction control valve is switched to the regulation position to regulate the suction amount of the sub pump. When the return flow from the hydraulic steering device is reduced and the pressure in the return passage decreases, the suction control valve is switched to the open position, and the sub-pump is operated by the hydraulic steering device via the merge passage. An emergency steering system for a hydraulic steering system, wherein pressure oil is supplied to the device, and at this time, the holding mechanism maintains the open position of the suction control valve by the discharge pressure of the sub-pump. 2. The holding mechanism includes a holding valve connected to the other pilot chamber of the suction control valve and an orifice provided in the merging flow passage. The holding valve is configured to generate a predetermined differential pressure across the orifice. 2. The emergency steering system according to claim 1, wherein a pressure upstream of the orifice is guided to the other pilot chamber of the suction control valve to keep the suction control valve in the open position. 3. A holding mechanism, which is provided in the merging flow path and opens when a predetermined pressure is reached, and guides pressure upstream of the holding check valve to the other pilot chamber of the suction control valve. When a predetermined pressure is generated on the upstream side of the holding check valve by the pressure oil discharged from the sub-pump, the suction control valve maintains the open position by the pressure. An emergency steering system for the hydraulic steering device according to claim 1. 4. The emergency steering system for a hydraulic steering system according to claim 1, further comprising a pressure release valve for connecting one pilot chamber of the suction control valve to the tank.