Steering hydraulic system and forklift

Abstract

This application belongs to the field of industrial vehicles and discloses a steering hydraulic system and a forklift. The steering hydraulic system includes a steering cylinder and a steering gear; a booster cylinder includes a small piston cylinder and a large piston cylinder located at both ends. The large piston cylinder is hydraulically connected to the oil outlet of the steering gear, and the small piston cylinder is hydraulically connected to the cylinder chamber of the steering cylinder. The piston assembly of the booster cylinder includes a piston connecting oil passage for connecting the two cylinder chambers of the booster cylinder, and an oil passage on / off valve is provided in the piston connecting oil passage. This application increases the pressure output of the hydraulic system by setting a booster cylinder, thereby reducing the difficulty of steering operation, making steering lighter and more flexible, improving steering performance, and also improving the system's response speed and stability. The piston assembly of the booster cylinder includes a piston connecting oil passage for connecting the two cylinder chambers of the booster cylinder, which allows for full steering and prevents steering operation from jamming.

Description

Technical Field

[0001] This application belongs to the field of industrial vehicles, and specifically relates to a forklift and its steering hydraulic system. Background Technology

[0002] Forklifts are industrial material handling vehicles primarily used for loading, unloading, stacking, and short-distance transport of goods, widely used in warehouses, ports, factories, and other similar settings. In common forklifts, the forks extend to carry loads. When moving or relocating under load, agile steering is often required. However, under conditions such as idling, high internal leakage, and high steering speeds, heavy steering can easily occur, making operation difficult for the driver, resulting in a poor driving experience and reduced safety. This is especially true for forklifts with a rear-mounted steering axle, where the steering force required is even greater, and in some emergency situations, steering may even be impossible. Utility Model Content

[0003] The purpose of this application is to provide a steering hydraulic system and a forklift to improve the flexibility of steering operations.

[0004] To achieve the above objectives, this application provides a steering hydraulic system, the steering hydraulic system comprising:

[0005] Steering cylinder and steering gear;

[0006] The booster cylinder includes a small piston cylinder and a large piston cylinder located at both ends. The piston area ratio between the large piston cylinder and the small piston cylinder is not less than 2. The large piston cylinder is hydraulically connected to the oil outlet of the steering gear, and the small piston cylinder is hydraulically connected to the oil cylinder chamber of the steering cylinder.

[0007] The piston assembly of the booster cylinder includes a piston connecting oil passage for connecting the two cylinder chambers of the booster cylinder, and an oil passage on / off valve is provided in the piston connecting oil passage.

[0008] In some embodiments, the oil passage on / off valve is a one-way shut-off valve, which is configured to allow hydraulic oil to flow from the large piston cylinder of the booster cylinder to the small piston cylinder and to shut off in the reverse direction.

[0009] In some embodiments, the one-way shut-off valve is an adjustable one-way valve.

[0010] In some embodiments, the oil passage on / off valve is an electromagnetic switch valve.

[0011] In some embodiments, the steering hydraulic system further includes:

[0012] The piston stroke switch includes a first piston stroke switch disposed in the small piston cylinder and a second piston stroke switch disposed in the cylinder chamber of the steering cylinder.

[0013] The controller is configured as follows:

[0014] It is confirmed that the first piston limit switch has been triggered;

[0015] It was determined that the second piston limit switch was not triggered;

[0016] The control opens the oil passage on / off valve to connect the piston to the oil passage.

[0017] In some embodiments, the piston assembly includes:

[0018] The small piston is located in the cylinder cavity of the small piston cylinder;

[0019] The large piston is located in the cylinder chamber of the large piston cylinder;

[0020] A piston linkage shaft rigidly connects the small piston and the large piston;

[0021] The piston connecting oil passage is axially formed in the piston linkage shaft.

[0022] In some embodiments, the steering gear is provided with a first oil outlet and a second oil outlet, a first connecting oil passage is provided between the first end of the steering cylinder and the first oil outlet, and a second connecting oil passage is provided between the second end and the second oil outlet.

[0023] The booster cylinder includes a first booster cylinder disposed in the first connecting oil circuit and a second booster cylinder disposed in the second connecting oil circuit.

[0024] In some embodiments, the first and second booster cylinders have the same structure and dimensions.

[0025] In some embodiments, the steering hydraulic system further includes:

[0026] Hydraulic pump;

[0027] A priority valve is used to distribute the pumped hydraulic oil from the hydraulic pump to the steering gear, which has a manual pump function.

[0028] This application also provides a forklift, including a rear-mounted steering axle and the aforementioned steering hydraulic system, wherein the steering cylinder acts on the steering axle.

[0029] In the technical solution of this application, a booster cylinder is added between the steering gear and the steering cylinder. By setting up the booster cylinder, the pressure output of the hydraulic system is increased, achieving a boosted drive effect, thereby reducing the difficulty of steering operation and making steering lighter and more flexible. The piston assembly of the booster cylinder includes a piston connecting oil passage for connecting the two cylinder chambers of the booster cylinder. This allows the hydraulic oil output from the steering gear outlet to flow sequentially through the large piston cylinder, the oil passage on / off valve, and the small piston cylinder to the cylinder chamber of the steering cylinder, thereby pushing the piston of the steering cylinder to move to the end, ensuring smooth and full steering without jamming. Adding the booster cylinder effectively improves the performance of steering operation, while also increasing the system's response speed and stability, and optimizing the energy utilization efficiency of the hydraulic system.

[0030] Other features and advantages of the embodiments of this application will be described in detail in the following detailed description section. Attached Figure Description

[0031] The accompanying drawings are provided to further illustrate the embodiments of this application and form part of the specification. They are used together with the following detailed description to explain the embodiments of this application, but do not constitute a limitation on the embodiments of this application. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without any inventive effort. In the drawings:

[0032] Figure 1 This is a partial hydraulic schematic diagram of a steering hydraulic system according to the first embodiment of this application;

[0033] Figure 2 for Figure 1 A schematic diagram of the booster cylinder used in the process;

[0034] Figure 3 A complete hydraulic schematic diagram of the steering hydraulic system according to the first embodiment of this application;

[0035] Figure 4 This is a partial hydraulic schematic diagram of the steering hydraulic system according to the second embodiment of this application;

[0036] Figure 5 for Figure 4 A schematic diagram of the booster cylinder used in the process.

[0037] Explanation of reference numerals in the attached figures

[0038] Detailed Implementation

[0039] The specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this application.

[0040] The forklift and its steering hydraulic system according to this application are described below with reference to the accompanying drawings.

[0041] In common forklifts, the steering hydraulic system typically includes a hydraulic pump, a priority valve, a steering gear, and a steering cylinder. The hydraulic pump acts as the power source, supplying oil to the system. The priority valve distributes the hydraulic oil. The steering gear, as the core control component, controls and measures the hydraulic fluid. The steering cylinder, as the actuator, is connected to the steering gear outlet. When internal leakage or excessively fast steering wheel turning leads to insufficient hydraulic fluid, heavy steering can easily occur, causing the driver to struggle and affecting driving experience and safety. Furthermore, while the fully hydraulic steering gear in a forklift's steering hydraulic system has a manual pump function, the high steering pressure required for rear-mounted steering axles often renders this manual pump function ineffective, preventing steering in emergency situations.

[0042] Therefore, this application discloses a novel hydraulic steering system. For example... Figure 1 As shown, in this embodiment, the novel steering hydraulic system includes:

[0043] Steering cylinder 7 and steering gear 5;

[0044] The booster cylinder includes a small piston cylinder and a large piston cylinder located at both ends. The piston area ratio between the large piston cylinder and the small piston cylinder is not less than 2. The large piston cylinder is hydraulically connected to the oil outlet of the steering gear 5, and the small piston cylinder is hydraulically connected to the cylinder chamber of the steering cylinder 7.

[0045] The piston assembly of the booster cylinder includes a piston connecting oil passage for connecting the two cylinder chambers of the booster cylinder, and an oil passage on / off valve is provided in the piston connecting oil passage.

[0046] As can be seen, a booster cylinder is specially added to the steering hydraulic system of this application. As shown in the figure, the large end (i.e., the large piston cylinder) of the booster cylinder is connected to the oil outlet of the steering gear 5, and the small end (i.e., the small piston cylinder) of the booster cylinder is connected to the cylinder chamber of the steering cylinder 7. By setting up the booster cylinder, the pressure output of the hydraulic system can be increased. That is, when the output oil pressure of the steering gear 5 is equal, the oil pressure output to the steering cylinder 7 through the booster cylinder is greatly increased, and the piston driving force of the steering cylinder 7 is stronger, thereby significantly reducing the steering effort and making steering more light and flexible.

[0047] With the addition of a booster cylinder, the higher terminal output pressure helps improve the response speed and stability of the hydraulic system, ensuring greater precision and reliability in steering operations under complex conditions. Furthermore, the booster cylinder can optimize the energy efficiency of the hydraulic system, reduce unnecessary energy loss, extend system lifespan, and lower maintenance costs.

[0048] Specifically, such as Figure 1 As shown, the key to introducing the booster cylinders is to connect two booster cylinders in series between the outlet of the steering gear 5 and the steering cylinder 7, so that the booster cylinder and the steering cylinder form an approximately closed circuit. The hydraulic oil at the outlet of the steering gear 5 first drives the booster cylinder to operate, and then drives the steering cylinder 7 through the booster cylinder, thereby achieving twice the result with half the effort. It can smoothly drive the steering cylinder 7 even at low pressure output, solving the problem of heavy steering.

[0049] More in detail, such as Figure 2 The booster cylinder shown typically includes small piston cylinders located at both ends (i.e., Figure 2 The upper end of the booster cylinder) and the large piston cylinder (i.e. Figure 2 The booster cylinder (located at its lower end) is hydraulically connected to the outlet of the steering gear 5 via a large piston cylinder, and to the steering cylinder 7 via a small piston cylinder. With this configuration, since the cylinder thrust equals the hydraulic pressure multiplied by the piston's working area, and the small piston cylinder of the booster cylinder is connected to the steering cylinder 7, the pressure of the small piston cylinder of the booster cylinder is the same as the pressure of the steering cylinder 7. Furthermore, because the piston area ratio between the large and small piston cylinders is not less than 2 (meaning the piston area of ​​the large piston cylinder of the booster cylinder is more than twice that of the small piston cylinder), the pressure of the large piston cylinder of the booster cylinder is a fraction of the pressure of the steering cylinder. Since the large piston cylinder of the booster cylinder is connected to the steering gear outlet, the pressure output by the steering gear is also a fraction of the pressure of the steering cylinder, thus achieving a power assist effect and reducing the force required to turn the steering wheel. In addition, the booster cylinder can improve the system's response speed and stability, ensuring steering accuracy and reliability under complex operating conditions.

[0050] However, in practical applications, hydraulic cylinders inevitably experience oil leakage after long-term use. This can lead to a mismatch or disconnect between the strokes of the booster cylinder and the steering cylinder. For example, during operation, the piston of the booster cylinder may reach its extreme position, while the piston of the steering cylinder 7 may not have reached its extreme position. This will result in the steering cylinder 7 not performing its steering action correctly.

[0051] Therefore, in the steering hydraulic system of this application, the piston assembly of the booster cylinder includes a piston connecting oil passage for connecting the two cylinder chambers of the booster cylinder, and an oil passage on / off valve is also provided in the piston connecting oil passage. Thus, even if the piston of the booster cylinder reaches its extreme position while the piston of the steering cylinder 7 does not, the hydraulic oil output from the first outlet A or the second outlet B of the steering gear 5 can flow sequentially through the large piston cylinder, the piston connecting oil passage, and the small piston cylinder to the cylinder chamber of the steering cylinder 7, thereby continuing to push the piston of the steering cylinder 7 to move, ensuring smooth steering operation and full steering.

[0052] The piston connecting oil passage cannot be in a normally open state; otherwise, the hydraulic oil from the large piston cylinder will flow directly to the small piston cylinder through the piston connecting oil passage, preventing the booster cylinder from functioning as a booster. Therefore, an on / off valve is installed in the piston connecting oil passage to control its opening and closing as needed based on stroke requirements.

[0053] In this embodiment, see Figure 2 As an example, the oil passage on / off valve is a one-way shut-off valve 8. The one-way shut-off valve 8 is configured to allow hydraulic oil to flow from the large piston cylinder of the booster cylinder to the small piston cylinder and to shut off in the reverse direction. Because the one-way shut-off valve 8 has a certain back pressure, such as the elastic pressure generated by the compression spring that elastically presses the valve core against the one-way valve port, this back pressure can normally resist the hydraulic oil in the large piston cylinder from opening the one-way shut-off valve 8 in the forward direction. When the piston stroke of the booster cylinder reaches its extreme position, the hydraulic oil in the large piston cylinder continuously accumulates, and the pressure increases, thereby resisting the compression spring and opening the one-way shut-off valve 8 in the forward direction. This opens the piston-connecting oil passage, allowing the hydraulic oil to flow sequentially through the large piston cylinder, the piston-connecting oil passage, and the small piston cylinder to the cylinder chamber of the steering cylinder 7, thus continuing to push the piston of the steering cylinder 7 to move.

[0054] It is evident that when using the one-way shut-off valve 8, the setting of its back pressure, i.e., the selection of the compression spring specifications, is crucial and needs to be appropriately chosen based on the specifications of the booster cylinder and the steering cylinder 7. Therefore, the one-way shut-off valve 8 is also designed as an adjustable one-way valve, meaning the back pressure of the one-way valve is adjustable. Those skilled in the art will know that adjustable back pressure one-way valves are common in the market, and will not be elaborated upon further here.

[0055] Furthermore, oil passage on / off valves can have various structures and control methods. For example, an oil passage on / off valve can be a hydraulically controlled valve or a solenoidally controlled valve. Figure 4 , Figure 5 In this embodiment, the oil passage on / off valve is an electromagnetic switch valve 10. Based on this, the steering hydraulic system of this embodiment may further include:

[0056] The piston limit switch includes a first piston limit switch 11 disposed in the small piston cylinder and a second piston limit switch 12 disposed in the cylinder chamber of the steering cylinder 7.

[0057] The controller is configured as follows:

[0058] Confirm that the first piston limit switch 11 has been triggered;

[0059] It has been determined that the second piston limit switch 12 has not been triggered;

[0060] Control the opening of the oil passage on / off valve to allow the piston to connect to the oil passage.

[0061] In this design, limit switches can be installed at the end of the stroke of the small piston cylinder of the booster cylinder and at the end of the piston stroke of the steering cylinder 7. When the piston of the booster cylinder reaches the bottom, but the piston of the steering cylinder does not reach the bottom, the solenoid switch valve 10 can be opened to connect the piston to the oil passage, so that the hydraulic oil flows to the steering cylinder 7 through the piston to continue to drive the steering cylinder 7 to work.

[0062] In this embodiment, limit switches are used to assist the controller in determining the piston position. However, this application is obviously not limited to this; various other methods can be used to determine the piston position, such as laser ranging, pull-string methods, etc., which will not be elaborated here. Furthermore, the number of limit switches and their arrangement are not limited to... Figure 4 , Figure 5 As shown.

[0063] exist Figure 5 In the booster cylinder, the piston assembly may include:

[0064] The small piston is located in the cylinder chamber of the small piston cylinder;

[0065] The large piston is located in the cylinder chamber of the large piston cylinder;

[0066] Piston linkage shaft 6, rigidly connecting the small piston and the large piston;

[0067] The piston connecting oil passage is formed axially through the piston linkage shaft 6.

[0068] Figure 2 , Figure 5 In a typical booster cylinder structure, the hydraulic oil from the outlet of the steering gear 5 first enters the cylinder chamber of the large piston cylinder. The hydraulic oil in the large piston cylinder pushes the large piston to move, which in turn drives the small piston to move synchronously via the piston linkage shaft 6. The piston linkage shaft 6 is a hollow shaft, with both ends of the hollow shaft cavity connecting the small piston cylinder and the large piston cylinder. Thus, the hollow shaft cavity forms a piston-connecting oil passage, connecting the large and small piston cylinders. Furthermore, the hollow shaft cavity of the piston linkage shaft 6 can be equipped with an oil passage on / off valve, such as a one-way shut-off valve 8 or a solenoid switch valve 10.

[0069] Because an oil passage on / off valve is added to the piston of the booster cylinder, when the piston of the booster cylinder reaches its limit position while the piston of the steering cylinder does not reach its limit position, the oil passage on / off valve will be opened, connecting the small piston cylinder and the large piston cylinder to achieve a compensation effect.

[0070] Figure 2 , Figure 5Both examples show a booster cylinder with an on / off valve for the piston passage, where the piston-connecting oil passage is located in the piston linkage shaft 6. Alternatively, the piston-connecting oil passage is not limited to being located in the piston linkage shaft 6; for example, the piston-connecting oil passage could be a separate hydraulic pipe connecting the large and small pistons. In this alternative, a piston-connecting oil passage is also provided between the small and large piston cylinders, such as a hydraulic oil pipe, which may contain a one-way shut-off valve 8. The one-way shut-off valve 8 is configured to allow hydraulic oil to flow from the large piston cylinder to the small piston and to cut off the flow in the reverse direction.

[0071] Since the large and small pistons are rigidly connected by a linkage shaft, meaning the piston assembly is a single moving part, given a fixed steering oil pressure required for the cylinder chamber of steering cylinder 7, the hydraulic oil pressure required for the large piston cylinder is relatively small; that is, a relatively small outlet oil pressure from steering gear 5 is sufficient. As mentioned earlier, the ratio between the steering oil pressure required for steering cylinder 7 and the outlet oil pressure from steering gear 5 should be the ratio of the piston areas of the large and small pistons. Therefore, the larger the piston area ratio between the large and small pistons, the more pronounced the booster effect. As an example, Figure 1 , Figure 2 In the booster cylinder shown, the piston area ratio between the large and small pistons should be no less than 2. However, this application is not limited to this; the piston area ratio can be determined and selected according to specific operating conditions.

[0072] The piston assembly of the booster cylinder in this application is not limited to... Figure 2 The structure of the large and small pistons and piston linkage shaft 6 shown can be optionally integrated, with the piston assembly being a stepped shaft, the small end of the stepped shaft serving as the small piston, and the large end serving as the large piston, which facilitates integral machining.

[0073] It should be noted that in the steering hydraulic system of this embodiment, depending on the working conditions, only one booster cylinder may be installed, that is, only steering operation on one side is assisted. For example... Figure 1 , Figure 4 As shown, it typically has two booster cylinders, namely the first booster cylinder 61 and the second booster cylinder 62. For details, see [link to documentation]. Figure 3 The steering gear 5 is provided with a first oil outlet A and a second oil outlet B. The first end of the steering cylinder 7 is provided with a first connecting oil passage between the first oil outlet A and the second end is provided with a second connecting oil passage between the second end and the second oil outlet B. The booster cylinder includes a first booster cylinder 61 provided in the first connecting oil passage and a second booster cylinder 62 provided in the second connecting oil passage.

[0074] Due to the balance and symmetry of left and right steering operation, the first booster cylinder 61 and the second booster cylinder 62 can be of the same specifications, that is, the same structure and size. Of course, depending on the required amount of assistance, the first and second booster cylinders can also be of different specifications and sizes, but in this case, the hydraulic system design must take into account the overflow and oil replenishment requirements.

[0075] See Figure 3 The steering hydraulic system of this embodiment includes not only the steering gear 5, the steering cylinder 7, and the booster cylinder, but may also include:

[0076] Hydraulic pump 2;

[0077] Priority valve 4 is used to distribute the pumped hydraulic oil from hydraulic pump 2 to steering gear 5, which has a manual pump function.

[0078] Hydraulic pump 2 supplies oil to the system, priority valve 4 distributes the pumped hydraulic oil, steering gear 5, as the core control component, plays a role in control and metering, and steering cylinder 7 is the steering actuator; the outlet of steering gear 5 is connected to steering cylinder 7. Figure 5 In this diagram, the priority valve 4 and the steering gear 5 both use commonly available valve components, so they will not be described in detail here. As can be seen from the diagram, the steering gear 5 has a manual pump function.

[0079] visible, Figure 3 The embodiment discloses an improved power steering system, comprising a filter 1, a hydraulic pump 2, a pump oil check valve 3, a priority valve 4, a steering gear 5, a booster cylinder, and a steering cylinder 7. The hydraulic oil pumped by the hydraulic pump 2 first passes through the priority valve 4 for flow distribution, then enters the P port of the steering gear 5. After being metered by the steering gear 5, the oil exits from the steering gear 5 and connects to the large piston chamber of the booster cylinder. The large piston and small piston of the booster cylinder are rigidly connected, and their areas are proportional. The small piston cylinder is connected to the steering cylinder 7. A connecting oil passage with a check valve 8 is added between the large and small piston cylinders of the booster cylinder to compensate for oil leakage during operation.

[0080] In addition, this application also protects a forklift including a rear-mounted steering axle (not shown) and the aforementioned steering hydraulic system, wherein the steering cylinder 7 acts on the steering axle.

[0081] Since forklift steering axles are generally rear-mounted, the steering pressure requirement is high. When the system is without power, such as when pumping hydraulic oil to other work attachments via a priority valve, the fully hydraulic steering gear loses its function as a manual pump, making steering impossible in emergencies. In this situation, the steering hydraulic system of this application, even with low hydraulic oil pressure through the steering gear 5, can still achieve a large pressure output, proportionally reducing the output pressure of the steering gear. The areas of the large and small piston cylinders in the booster cylinder are proportional; according to F=PA, the pressure of the large piston cylinder is less than the pressure of the small piston cylinder. The small piston cylinder is connected to the steering cylinder 7, and the large piston cylinder is connected to the outlet of the steering gear 5. Therefore, it reduces the effort required to turn the steering wheel and extends the service life of the steering gear 5.

[0082] The steering hydraulic system described in this application also improves system accuracy. Because the diameters of the large and small piston cylinders in the booster cylinder are proportional, the flow input to the large piston cylinder is proportionally amplified, thus improving steering precision. Furthermore, it enables the steering gear to function as a manual pump, allowing for steering in emergencies and improving safety. The booster cylinder reduces the steering gear outlet pressure, resulting in a smaller force acting on the steering gear rotor, thereby enabling manual steering without power. Based on this, it solves the problem of large-tonnage forklifts being unable to directly use the steering gear due to excessively high pressure.

[0083] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0084] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between components; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0085] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

Claims

1. A steering hydraulic system characterized by, The steering hydraulic system includes: Steering cylinder (7) and steering gear (5); The booster cylinder includes a small piston cylinder and a large piston cylinder located at both ends. The piston area ratio between the large piston cylinder and the small piston cylinder is not less than 2. The large piston cylinder is hydraulically connected to the oil outlet of the steering gear (5), and the small piston cylinder is hydraulically connected to the cylinder chamber of the steering cylinder (7). The piston assembly of the booster cylinder includes a piston connecting oil passage for connecting the two cylinder chambers of the booster cylinder, and an oil passage on / off valve is provided in the piston connecting oil passage.

2. The steering hydraulic system according to claim 1, characterized by, The oil passage on / off valve is a one-way shut-off valve (8), which is configured to allow hydraulic oil to flow from the large piston cylinder of the booster cylinder to the small piston cylinder and to shut off in the reverse direction.

3. The steering hydraulic system according to claim 2, characterized by The one-way shut-off valve (8) is an adjustable one-way valve.

4. The steering hydraulic system according to claim 1, characterized by The oil passage on / off valve is an electromagnetic switch valve (10).

5. The steering hydraulic system according to claim 4, characterized by The steering hydraulic system also includes: The piston stroke switch includes a first piston stroke switch (11) disposed in the small piston cylinder and a second piston stroke switch (12) disposed in the cylinder chamber of the steering cylinder (7). The controller is configured as follows: It is determined that the first piston limit switch (11) is triggered; It was determined that the second piston limit switch (12) was not triggered; The control opens the oil passage on / off valve to connect the piston to the oil passage.

6. The steering hydraulic system of claim 1, wherein, The piston assembly includes: The small piston is located in the cylinder cavity of the small piston cylinder; The large piston is located in the cylinder chamber of the large piston cylinder; Piston linkage shaft (6) rigidly connects the small piston and the large piston; The piston connecting oil passage is axially formed in the piston linkage shaft (6).

7. The steering hydraulic system according to any one of claims 1 to 6, characterized by The steering gear (5) is provided with a first oil outlet (A) and a second oil outlet (B). The first end of the steering cylinder (7) is provided with a first connecting oil passage between the first oil outlet (A) and the second end is provided with a second connecting oil passage between the second end and the second oil outlet (B). The booster cylinder includes a first booster cylinder (61) disposed in the first connecting oil circuit and a second booster cylinder (62) disposed in the second connecting oil circuit.

8. The steering hydraulic system according to claim 7, characterized in that, The first booster cylinder (61) and the second booster cylinder (62) have the same structure and size.

9. The steering hydraulic system of claim 1, wherein, The steering hydraulic system also includes: Hydraulic pump (2); Priority valve (4) is used to distribute the pumping hydraulic oil of the hydraulic pump (2) to the steering gear (5) which has a manual pump function.

10. A fork lift truck characterised in that, The forklift includes a rear-mounted steering axle and a steering hydraulic system according to any one of claims 1 to 9, wherein the steering cylinder (7) acts on the steering axle.

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