Crane slewing buffer reversing valve and open-loop crane slewing braking control system

Abstract

The present invention provides a swing buffer reversing valve for a crane; in the neutral position, the first unloading port is respectively communicated with the first reversing port and the second reversing port through throttling passages; in the first working position, the first reversing port is communicated with the oil inlet port, and the second reversing port is communicated with the oil return port; in the second working position, the second reversing port is communicated with the oil inlet port, and the first reversing port is communicated with the oil return port; in all three states, the first unloading port is communicated with the second unloading port. With the progress of commutation and the reduction of the opening of the oil return port, the increase of the load pressure is weakened. The present invention also provides an open swing braking control system for a crane, including a reversing valve, a motor, a brake, and a brake control unit. The oil inlet port of the first shuttle valve of the brake control unit is communicated with a pilot control oil source, the oil outlet port of the first shuttle valve is communicated with a pressure switch control oil port, the pressure switch triggers a time delay relay, and the time delay relay controls an electromagnetic reversing valve; the two reversing ports of the reversing valve are connected to the two working oil ports of the motor.

Description

Technical Field

[0001] The present invention relates to a reversing valve used in a slewing mechanism of a crane.

[0002] The present invention also relates to an open slewing brake control system for a crane. Background Art

[0003] Modern medium and large-tonnage cranes generally use hydraulic drive for crane slewing control. Due to the large mass and inertia of the boom and components on the upper turntable of the crane, there are serious starting and stopping impacts during slewing operation, which seriously affect the slewing operation performance of customers. Therefore, in order to reduce such impacts, improvements are generally made to the slewing opening speed and the closing time of the brake 3a. For example, Figure 1 As shown, for the brake 3a (opening and closing), but since the hydraulic system itself needs to generate a delay by restricting the damping, for example, in the case of fast opening and slow closing in experiments, it is achieved in the form of a one-way damping valve 7a. At the same time, since the reversing valve 1a (slewing valve) itself adopts a closed-center design in principle and is provided with a buffer overload valve, when the reversing valve 1a returns, the A and B chambers are basically locked. When the turntable continues to rotate due to inertia, the reversing valve 1a has already returned, and the hydraulic oil A / B ports are cut off. Then, the huge inertial force is instantly applied to the enclosed oil, resulting in a rapid increase in pressure. If the brake 3a is also locked at this time, for such a large rotational inertia, if the enclosed pressure generated by the inertial force reaches the degree to open the overload valve, the pressure will be released. If the enclosed pressure does not reach the opening pressure of the overload valve, then the force cannot be released by the pressure relief method. The above two situations both cause impacts during slewing.

[0004] However, there are defects in the prior art:

[0005] 1. The damping cannot be made small enough. Generally, due to reasons such as cleanliness, it is difficult to make the damping hole less than 0.4, so it is impossible to delay for a sufficient long time.

[0006] 2. Due to weather reasons, the crane needs to operate in different temperature environments. The hydraulic oil is highly sensitive to temperature, resulting in a large change in the viscosity of the hydraulic oil, which has a great impact on the delay characteristics of the damping. It also seriously affects the braking effect.

[0007] 3. As the cleanliness of the whole machine's hydraulic oil deteriorates, the probability of the damping hole being contaminated and blocked increases, resulting in slow leakage of the pressure oil in the brake 3a until it is completely blocked, leading to slewing of the crane.

[0008] 4. Since the reversing valve adopts a closed-center switch valve design, the slewing valve only communicates the P port with the T port when opening and closing. Once reversing, the pressure oil at the P port directly enters the A port and B port of the motor 2a. Due to the large moment of inertia and large load, the pressure will instantaneously increase and even reach the overload valve pressure, causing an impact. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a crane swing buffer reversing valve with reasonable structure and convenient use.

[0010] The technical problem to be solved by the present invention is to provide a crane open swing braking control system with simple structure, more flexible implementation and lower cost.

[0011] To solve the above technical problems, the present invention provides a crane swing buffer reversing valve;

[0012] This crane swing buffer reversing valve is a three-position six-way valve. This crane swing buffer reversing valve has a first reversing port, a second reversing port, an oil inlet, an oil return port, a first unloading port and a second unloading port. The oil inlet and the first unloading port are respectively communicated with the swing oil source, and the oil return port and the second unloading port are respectively communicated with the oil tank;

[0013] This crane swing buffer reversing valve has a neutral position, a first working position and a second working position. When in the neutral position, the first unloading port is respectively communicated with the first reversing port, the second reversing port and the second unloading port. The passage between the first unloading port and the first reversing port is a throttle passage, and the passage between the first unloading port and the second reversing port is also a throttle passage. The oil inlet and the oil return port are in an open circuit state;

[0014] When in the first working position, the first reversing port is communicated with the oil inlet, the second reversing port is communicated with the oil return port, the first unloading port and the second unloading port are communicated, and the passage between the first unloading port and the second unloading port is a throttle passage;

[0015] When in the second working position, the second reversing port is communicated with the oil inlet, the first reversing port is communicated with the oil return port, the first unloading port and the second unloading port are communicated, and the passage between the first unloading port and the second unloading port is a throttle passage.

[0016] After adopting such a structure, the function of the reversing valve is changed on the original basis. This crane swing buffer reversing valve has passages with the oil return port to varying degrees throughout the reversing stroke range. When in the neutral position, the communication degree with the oil return port is relatively large, and as the reversing progresses, the opening with the oil return port gradually decreases, greatly weakening the instantaneous increase in the load pressure.

[0017] When this crane swing buffer reversing valve is in the first working position, the passage between the first reversing port and the oil inlet is a one-way passage, restricting the oil flow direction from the oil inlet to the first reversing port. The passage between the second reversing port and the oil return port is a one-way passage, restricting the oil flow direction from the second reversing port to the oil return port;

[0018] When in the second working position, there is a one-way passage between the second reversing port and the oil inlet, restricting the oil flow direction from the oil inlet to the second reversing port. There is also a one-way passage between the first reversing port and the oil return port, restricting the oil flow direction from the first reversing port to the oil return port.

[0019] To solve the above technical problems, the present invention provides an open-type swing braking control system for a crane, including a reversing valve, a motor, and a brake.

[0020] It further includes a brake control unit, which includes a pressure switch, a first shuttle valve, a time-delay relay, and a two-way three-position electromagnetic reversing valve. The two oil inlets of the first shuttle valve are respectively connected to two pilot control oil sources. The oil outlet of the first shuttle valve is connected to the control oil port of the pressure switch. The pressure switch is preset with a preset opening oil pressure value and a preset trigger oil pressure value, and the preset opening oil pressure value is less than the preset trigger oil pressure value. When the oil pressure at the control oil port of the pressure switch is greater than the preset opening oil pressure value and less than the preset trigger oil pressure value, the time-delay relay triggers the two-way three-position electromagnetic reversing valve after a delay. When the oil pressure at the control oil port of the pressure switch is greater than the preset trigger oil pressure value, the time-delay relay immediately triggers the two-way three-position electromagnetic reversing valve. The two-way three-position electromagnetic reversing valve includes an oil return port, an oil inlet, and a working port. The oil return port of the two-way three-position electromagnetic reversing valve is connected to the fuel tank. The oil inlet of the two-way three-position electromagnetic reversing valve is connected to the brake oil source. The working port of the two-way three-position electromagnetic reversing valve is connected to the brake. The two-way three-position electromagnetic reversing valve has an initial position and a working position. In the initial position, the oil return port and the working port are connected. In the working position, the oil inlet and the working port are connected.

[0021] The reversing valve adopts the above-mentioned swing buffer reversing valve for a crane. The first reversing port and the second reversing port of the swing buffer reversing valve for a crane are respectively connected to the first working oil port and the second working oil port of the motor. The two pilot control oil sources control the swing buffer reversing valve for a crane to switch to the first working position or the second working position.

[0022] The open-type swing braking control system for this crane has the following technical effects:

[0023] 1. The swing of the open-type swing braking control system for this crane has two characteristics. Since the swing motion is a rotational motion with a large moment of inertia, it is easy to have problems of difficult starting and stopping, that is, large starting and stopping impacts, which seriously affect the user experience of customers. In an open system, the buffer valve generally plays a reversing function and a buffer function.

[0024] In the prior art, other solutions in terms of buffering function generally focus on regulating the pressure during the start-stop process to achieve buffering. However, through a large number of experiments, it is found that this method can never solve the impact problems during the pressure building and pressure relief processes. Because hydraulic transmission is a coupled transmission method, the simple pressure limitation method only buffers at some discrete points (set pressure values), and there are still large impacts within the non-pressure relief interval.

[0025] The crane slewing buffer reversing valve always adopts full-stroke P-A-B-T throttle speed regulation. The method of throttling pressure relief can ensure the maximum weakening of impact and naturally utilize the characteristics of liquid resistance for buffering. The open slewing braking control system of this crane can greatly reduce the manufacturing cost of the entire buffer valve while ensuring the same or even better buffering effect. It is especially suitable for the slewing situation of cranes with large rotational inertia.

[0026] 2. The braking function of the winch of the crane is a safety device. During the slewing process, the brake generally does not participate in the entire movement process, so it needs to be quickly opened and delayed to close. Because the participation of the brake will increase the impact during startup and shutdown, the previous general solution was to use a one-way damping valve to solve the problem. However, due to the dual influence of the cleanliness and temperature of the hydraulic oil, the damping shows great inconsistency during operation, so it often brings various performance problems. For example, the damping of the one-way damping valve will cause different degrees of vehicle creep or even greater safety problems due to pollution blockage. The influence of different seasons and temperatures will also bring inconsistent problems in damping delay, causing discomfort to the operation of customers.

[0027] The open slewing braking control system of this crane avoids the above disadvantages. In terms of delayed braking, it adopts an electric control method through the combination of a pressure switch and a time-delay relay to achieve a better consistency in start-stop time control, which is not affected by the cleanliness of the hydraulic oil and the ambient temperature on the delay time. At the same time, the delay time can be set arbitrarily, and the pressure relief and pressure building speeds are faster, and the reliability is also greatly improved.

[0028] 3. The structure of the open slewing braking control system of this crane is simpler, more flexible to implement, and lower in cost, which is suitable for large-scale promotion.

[0029] In the open - type slewing braking control system of this crane, an overflow valve group is provided between the crane slewing buffer reversing valve and the motor. The overflow valve group includes a second shuttle valve, a bypass overflow valve, and a two - position two - way solenoid valve. The first oil inlet of the second shuttle valve is connected to the pipeline between the first reversing port of the crane slewing buffer reversing valve and the first working oil port of the motor. The second oil inlet of the second shuttle valve is connected to the pipeline between the second reversing port of the crane slewing buffer reversing valve and the second working oil port of the motor. The bypass overflow valve is installed on the pipeline between the oil outlet of the second shuttle valve and the oil inlet of the crane slewing buffer reversing valve. The two - position two - way solenoid valve is connected in parallel beside the bypass overflow valve, and the two - position two - way solenoid valve is in a normally - closed state.

[0030] In the open - type slewing braking control system of this crane, a unloading valve group is also provided between the crane slewing buffer reversing valve and the motor. The unloading valve group includes a first unloading valve, a second unloading valve, a first check valve, and a second check valve. The first unloading valve and the second unloading valve are connected in series, and the oil flow through the first unloading valve and the second unloading valve is in the opposite direction. The series - connected first unloading valve and second unloading valve are respectively connected to the first reversing port and the second reversing port of the crane slewing buffer reversing valve. The pipeline between the first unloading valve and the second unloading valve is connected to the second unloading port;

[0031] The first check valve and the second check valve are connected in series, and the oil flow through the first check valve and the second check valve is in the opposite direction. The series - connected first check valve and second check valve are respectively connected to the first reversing port and the second reversing port of the crane slewing buffer reversing valve. The pipeline between the first check valve and the second check valve is also connected to the second unloading port. Brief Description of the Drawings

[0032] Figure 1 is the hydraulic circuit schematic diagram of the prior - art slewing system.

[0033] Figure 2 is the structural schematic diagram of the embodiment of the crane slewing buffer reversing valve of this crane.

[0034] Figure 3 is the hydraulic circuit schematic diagram of the embodiment of the open - type slewing braking control system of this crane. Detailed Embodiment

[0035] As Figures 2 to 3 shown

[0036] The crane slewing buffer reversing valve 1 of this crane is a three - position six - way valve. The crane slewing buffer reversing valve 1 has a first reversing port A, a second reversing port B, an oil inlet P, an oil return port T, a first unloading port P', and a second unloading port T'. The oil inlet P and the first unloading port P' are respectively connected to the slewing oil source, and the oil return port T and the second unloading port T' are respectively connected to the fuel tank;

[0037] The swing buffer reversing valve 1 of this crane has a neutral position, a first working position, and a second working position. When in the neutral position, the first unloading port P' is respectively communicated with the first reversing port A, the second reversing port B, and the second unloading port T'. The passage between the first unloading port P' and the first reversing port A is a throttling passage (the aperture of the throttling passage is smaller than that of the normal passage, and the flow rate of the throttling passage is less than that of the normal passage). The passage between the first unloading port P' and the second reversing port B is also a throttling passage. The oil inlet P and the oil return port T are in an open circuit state.

[0038] When in the first working position, the first reversing port A is communicated with the oil inlet P, the second reversing port B is communicated with the oil return port T, the first unloading port P' and the second unloading port T' are communicated, and the passage between the first unloading port P' and the second unloading port T' is a throttling passage.

[0039] The passage between the first reversing port A and the oil inlet P is a one-way passage, restricting the oil flow direction from the oil inlet P to the first reversing port A. The passage between the second reversing port B and the oil return port T is a one-way passage, restricting the oil flow direction from the second reversing port B to the oil return port T.

[0040] When in the second working position, the second reversing port B is communicated with the oil inlet P, the first reversing port A is communicated with the oil return port T, the first unloading port P' and the second unloading port T' are communicated, and the passage between the first unloading port P' and the second unloading port T' is a throttling passage.

[0041] The passage between the second reversing port B and the oil inlet P is a one-way passage, restricting the oil flow direction from the oil inlet P to the second reversing port B. The passage between the first reversing port A and the oil return port T is a one-way passage, restricting the oil flow direction from the first reversing port A to the oil return port T.

[0042] Combined with the above specific introduction of the swing buffer reversing valve 1 of this crane, the open swing braking control system of the crane is introduced.

[0043] The open swing braking control system of the crane includes the swing buffer reversing valve 1 of the crane, a motor 2, a brake 3, and a brake control unit.

[0044] The brake control unit includes a pressure switch 42, a first shuttle valve 41, a time-delay relay 43, and a two-position three-way electromagnetic directional control valve 44. The two inlet ports A2 and A3 of the first shuttle valve 41 are respectively connected to two pilot control oil sources Pm and Pn. The outlet port B2 of the first shuttle valve 41 is connected to the control oil port of the pressure switch 42. A preset opening oil pressure value and a preset trigger oil pressure value are preset in the pressure switch 42. The preset opening oil pressure value is 0.1 bar, and the preset trigger oil pressure value is 2 bar. When the oil pressure at the control oil port of the pressure switch 42 is greater than the preset opening oil pressure value and less than the preset trigger oil pressure value, the time-delay relay 43 triggers the two-position three-way electromagnetic directional control valve 44 after a 3-second delay. When the oil pressure at the control oil port of the pressure switch 42 is greater than the preset trigger oil pressure value, the time-delay relay 43 immediately triggers the two-position three-way electromagnetic directional control valve 44. The two-position three-way electromagnetic directional control valve 44 includes an oil return port T1, an oil inlet port P1, and a working port C1. The oil return port T1 of the two-position three-way electromagnetic directional control valve 44 is connected to the fuel tank. The oil inlet port P1 of the two-position three-way electromagnetic directional control valve 44 is connected to the brake oil source Pi. The working port C1 of the two-position three-way electromagnetic directional control valve 44 is connected to the brake 3. The two-position three-way electromagnetic directional control valve 44 has an initial position and a working position. In the initial position, the oil return port T1 and the working port C1 are connected. In the working position, the oil inlet port P1 and the working port C1 are connected;

[0045] The first switching port A and the second switching port B of the crane slewing buffer reversing valve 1 are respectively connected to the first working oil port A1 and the second working oil port B1 of the motor 2. The two pilot control oil sources Pm and Pn control the crane slewing buffer reversing valve 1 to switch to the first working position or the second working position.

[0046] An overflow valve group is provided between the crane slewing buffer reversing valve 1 and the motor 2. The overflow valve group includes a second shuttle valve 51, a bypass overflow valve 52, and a two-position two-way solenoid valve 53. The first inlet port A4 of the second shuttle valve 51 is connected to the pipeline between the first switching port A of the crane slewing buffer reversing valve 1 and the first working oil port A1 of the motor 2. The second inlet port A5 of the second shuttle valve 51 is connected to the pipeline between the second switching port B of the crane slewing buffer reversing valve 1 and the second working oil port B1 of the motor 2. The bypass overflow valve 52 is installed on the pipeline between the outlet port B3 of the second shuttle valve 51 and the inlet port P of the crane slewing buffer reversing valve 1. The two-position two-way solenoid valve 53 is connected in parallel beside the bypass overflow valve 52. The two-position two-way solenoid valve 53 is in a normally closed state.

[0047] An unloading valve group is also provided between the crane slewing buffer reversing valve 1 and the motor 2. The unloading valve group includes a first unloading valve 61, a second unloading valve 62, a first check valve 63 and a second check valve 64. The working pressure of the first unloading valve 61 and the second unloading valve 62 is 21.5 MPa. The first unloading valve 61 and the second unloading valve 62 are connected in series. The oil flow through the first unloading valve 61 and the second unloading valve 62 is in the opposite direction. The series-connected first unloading valve 61 and second unloading valve 62 are respectively connected to the first reversing port A and the second reversing port B of the crane slewing buffer reversing valve 1. The pipeline between the first unloading valve 61 and the second unloading valve 62 is connected to the second unloading port T'.

[0048] The first check valve 63 and the second check valve 64 are connected in series. The oil flow through the first check valve 63 and the second check valve 64 is in the opposite direction. The series-connected first check valve 63 and second check valve 64 are respectively connected to the first reversing port A and the second reversing port B of the crane slewing buffer reversing valve 1. The pipeline between the first check valve 63 and the second check valve 64 is also connected to the second unloading port T'.

[0049] The working process of the crane open-type slewing braking control system is as follows:

[0050] (1) When there is no action on the crane pilot handle, there is no pressure in the pilot control oil circuit, the crane slewing buffer reversing valve 1 does not reverse, and there is no pressure at the oil inlet, oil return port of the crane slewing buffer reversing valve 1, and the first working oil port A1 and the second working oil port B1 of the motor 2. Moreover, the pressure switch 42 does not detect a pressure increase, and the oil pressure at the control oil port of the pressure switch 42 is less than 0.1 bar of the preset opening oil pressure value. The two-way three-position electromagnetic reversing valve 44 does not reverse, and the brake 3 is in the braking state.

[0051] (2) When the handle moves (assuming a left movement), the pilot control oil starts to build pressure. When the oil pressure at the control oil port of the pressure switch 42 reaches 2 bar, the pressure switch 42 is electrically closed. When the pressure rises, the time-delay relay 43 does not play a delay role. The two-way three-position electromagnetic reversing valve 44 is instantaneously energized and opened. The pressure oil enters the brake 3 and the brake 3 is opened. At the same time, the side of the crane slewing buffer reversing valve 1 with the first reversing port A and the second reversing port B (assuming port A) starts to build pressure until the pressure reaches the value that can push the turntable and the turntable starts to move.

[0052] (3) When the handle returns to its original position, the pilot oil is depressurized, and the crane slewing buffer reversing valve 1 starts to return to its neutral position. During this process, since the oil supply from the oil inlet P of the crane slewing buffer reversing valve 1 to the first reversing port A and the second reversing port B decreases and the oil discharge from the oil return port T of the crane slewing buffer reversing valve 1 increases, the motor 2 starts to decelerate until the speed of the turntable has dropped to a very low level after the crane slewing buffer reversing valve 1 returns to its neutral position. The oil inlet P of the crane slewing buffer reversing valve 1 presents a damping state to the first reversing port A, the second reversing port B, and the oil return port T until it stops slowly. At this time, since the pilot control pressure has dropped below 2 bar, the signal is transmitted to the delay relay, and the delay relay 43 will perform a delay process at this time. Approximately 3 seconds after the valve returns to its neutral position, the solenoid reversing valve is de-energized, and the brake 3 is depressurized to start braking.

[0053] The above is only one implementation mode of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several variations and improvements can be made, and these should also be regarded as belonging to the protection scope of the present invention.

Claims

1. A crane slewing buffer reversing valve, characterized in that: This crane slewing buffer reversing valve is a three-position six-way valve. This crane slewing buffer reversing valve has a first reversing port, a second reversing port, an oil inlet, an oil return port, a first unloading port and a second unloading port. The oil inlet and the first unloading port are respectively connected to the slewing oil source, and the oil return port and the second unloading port are respectively connected to the fuel tank; This crane slewing buffer reversing valve has a neutral position, a first working position and a second working position. When in the neutral position, the first unloading port is respectively connected to the first reversing port, the second reversing port and the second unloading port. The passage between the first unloading port and the first reversing port is a throttling passage, and the passage between the first unloading port and the second reversing port is also a throttling passage. The oil inlet and the oil return port are in an open circuit state; When in the first working position, the first reversing port is connected to the oil inlet, the second reversing port is connected to the oil return port, the first unloading port and the second unloading port are connected, and the passage between the first unloading port and the second unloading port is a throttling passage; When in the second working position, the second reversing port is connected to the oil inlet, the first reversing port is connected to the oil return port, the first unloading port and the second unloading port are connected, and the passage between the first unloading port and the second unloading port is a throttling passage; The aperture of the throttling passage is smaller than that of the normal passage, and the flow rate of the throttling passage is smaller than that of the normal passage.

2. The crane slewing buffer reversing valve according to claim 1, characterized in that: When in the first working position, the passage between the first reversing port and the oil inlet is a one-way passage, restricting the oil flow direction to be from the oil inlet to the first reversing port. The passage between the second reversing port and the oil return port is a one-way passage, restricting the oil flow direction to be from the second reversing port to the oil return port; When in the second working position, the passage between the second reversing port and the oil inlet is a one-way passage, restricting the oil flow direction to be from the oil inlet to the second reversing port. The passage between the first reversing port and the oil return port is a one-way passage, restricting the oil flow direction to be from the first reversing port to the oil return port.

3. An open-type crane slewing braking control system, including a reversing valve, a motor and a brake, characterized in that: It further includes a brake control unit. The brake control unit includes a pressure switch, a first shuttle valve, a time delay relay and a two-way three-way electromagnetic reversing valve. The two oil inlets of the first shuttle valve are respectively connected to two pilot control oil sources. The oil outlet of the first shuttle valve is connected to the control oil port of the pressure switch. The pressure switch is preset with a preset opening oil pressure value and a preset triggering oil pressure value. The preset opening oil pressure value is less than the preset triggering oil pressure value. When the oil pressure at the control oil port of the pressure switch is greater than the preset opening oil pressure value and less than the preset triggering oil pressure value, the time delay relay delays to trigger the two-way three-way electromagnetic reversing valve. When the oil pressure at the control oil port of the pressure switch is greater than the preset triggering oil pressure value, the time delay relay immediately triggers the two-way three-way electromagnetic reversing valve. The two-way three-way electromagnetic reversing valve includes an oil return port, an oil inlet and a working port. The oil return port of the two-way three-way electromagnetic reversing valve is connected to the fuel tank. The oil inlet of the two-way three-way electromagnetic reversing valve is connected to the brake oil source. The working port of the two-way three-way electromagnetic reversing valve is connected to the brake. The two-way three-way electromagnetic reversing valve has an initial position and a working position. When in the initial position, the oil return port and the working port are connected. When in the working position, the oil inlet and the working port are connected; The reversing valve adopts the crane slewing buffer reversing valve as described in Claim 1 or 2. The first reversing port and the second reversing port of the crane slewing buffer reversing valve are respectively connected to the first working oil port and the second working oil port of the motor. Two pilot control oil sources control the crane slewing buffer reversing valve to switch to the first working position or the second working position.

4. The open-type slewing braking control system of a crane according to Claim 3, characterized in that: An overflow valve group is provided between the crane slewing buffer reversing valve and the motor. The overflow valve group includes a second shuttle valve, a bypass overflow valve and a two-way two-position solenoid valve. The first oil inlet of the second shuttle valve is communicated with the pipeline between the first reversing port of the crane slewing buffer reversing valve and the first working oil port of the motor. The second oil inlet of the second shuttle valve is communicated with the pipeline between the second reversing port of the crane slewing buffer reversing valve and the second working oil port of the motor. The bypass overflow valve is installed on the pipeline between the oil outlet of the second shuttle valve and the oil inlet of the crane slewing buffer reversing valve. The two-way two-position solenoid valve is connected in parallel beside the bypass overflow valve, and the two-way two-position solenoid valve is in a normally closed state.

5. The open-type slewing braking control system of a crane according to Claim 3, characterized in that: A unloading valve group is further provided between the crane slewing buffer reversing valve and the motor. The unloading valve group includes a first unloading valve, a second unloading valve, a first check valve and a second check valve. The first unloading valve and the second unloading valve are connected in series, and the oil flow through the first unloading valve and the second unloading valve is in the opposite direction. The series-connected first unloading valve and second unloading valve are respectively communicated with the first reversing port and the second reversing port of the crane slewing buffer reversing valve. The pipeline between the first unloading valve and the second unloading valve is communicated with the second unloading port; The first check valve and the second check valve are connected in series, and the oil flow through the first check valve and the second check valve is in the opposite direction. The series-connected first check valve and second check valve are respectively communicated with the first reversing port and the second reversing port of the crane slewing buffer reversing valve. The pipeline between the first check valve and the second check valve is also communicated with the second unloading port.

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