Due to the structural limitations of a given displacement, it is generally considered that the gear pump can only be used as a constant flow hydraulic source. However, the attachment-in-thread-coupled combination valve scheme is effective for improving its function, reducing the system cost, and improving the system reliability. Therefore, the performance of the gear pump can be approached by the expensive and complicated plunger pump.
For example, installing the control valve directly on the pump can eliminate the need for piping between the pump and the direction, thereby controlling the cost. Less fittings and fittings reduce leakage and increase operational reliability. And the pump itself to install the valve can reduce the circulating pressure of the circuit and improve its working performance. Here are some loops that can improve the basic functions of the gear pump. Some of them are basic loops that are proven to be practical, while others are innovative research.
Unload circuit
The unloading element will combine the large flow pump with the low power single pump. Liquid is discharged from the outlet of the two pumps and reaches a predetermined pressure and/or flow rate. At this time, the large flow pump circulates the flow from its outlet to the inlet, thereby reducing the output flow of the pump to the system, that is, reducing the magnetic power to a value slightly higher than that required for operation of the high pressure section. The percentage of flow reduction depends on the ratio of ununloaded displacement to total displacement at this time, and the combination or threaded coupling unloader valve reduces or even eliminates piping, bores, and accessories and other possible leaks.
The simplest offloading element is manipulated manually. The spring turns the unload valve on or off. When the valve is given a control signal, the on-off state of the valve is switched. Lever or other mechanical mechanism is the simplest way to operate this valve.
The pilot-controlled (pneumatic or hydraulic) unloader valve is an improvement in the operating mode because it allows the valve to be controlled remotely. The biggest advancement is the use of solenoid valves that are controlled electrically or electronically. They can be used not only for remote control but also for automatic control of microcomputers. This simple offloading technology is generally considered to be the best case for the application.
Manually operated unloading elements are often used in circuits that require high flow and fast operation for fast operation, high flow rates, and reduced control for precise control, such as rapid telescoping boom circuits. When the unloading of the circuit shown in Fig. 1 has no manipulation signal (left position), the circuit always outputs a large flow. For normally open valves, the circuit will output a small flow under normal conditions. Pressure sensing offloading is the most common solution. As shown in Figure 2, the spring acts to place the unload valve in its large flow position (left position). When the circuit pressure reaches the preset value of the relief valve, the relief valve opens and the unloader valve switches to its low flow position (right position) under the action of hydraulic pressure. The pressure sensing unloading valve is basically an automatic unloading element that reaches the system pressure and is unloaded. It is commonly used in range splitting and hydraulic vise.
The unloading valve in the flow sensor unloading circuit is also biased by a spring to a large flow position (left). The fixed orifice size in the valve is determined by the flow rate required for the optimum engine speed of the device. If the engine speed exceeds this optimal range, the orifice pressure drop will increase, displacing the unload valve to the low flow position (right position). Therefore, the adjacent components of the large-flow pump are made to be capable of throttling the maximum flow. Therefore, the circuit consumes less energy, operates stably, and has a low cost. A typical application of such a circuit is to limit the loop flow to the optimum range to improve the performance of the entire system, or to limit the loop pressure during high speed driving of the machine. Commonly used for garbage trucks.
The unloading valve of the pressure flow sensor unloading circuit is also pressed by the spring to the large flow position (left position), and is unloaded regardless of the predetermined pressure or the flow rate. The equipment can complete high pressure work at idle or normal working speed. This feature reduces unnecessary traffic and therefore reduces the power required. Because this type of circuit has a wide range of load and speed variation, it is often used for excavating equipment.
With a power integrated pressure sensing unloading circuit, it consists of two sets of slightly changing pressure sensing unloading pumps. The two pumps are driven by the same prime mover, and each magnetic receiver receives the pilot unloading signal from another unloading pump. This helium sensing method is called interactive sensing. It allows a group of pumps to work under high pressure while the other pump operates under large flow. Two relief valves can be adjusted to the specific pressure of each circuit to unload one or two pumps. This scheme reduces the power demand, so a small capacity and low cost prime mover can be used.
Load sensing offload loop is shown. When the control valve (lower chamber) of the main control valve has no load sensing signal, all the pump flow is returned to the tank via valve 1 and valve 2; when a load sensing signal is applied to the control valve, the pump supplies liquid to the circuit; When the output pressure of the pump exceeds the predetermined value of the pressure of the load sensing valve, the pump only supplies the working flow to the circuit, and the excess flow is bypassed to the tank through the throttle position of the valve 2 (upper position). Compared with the plunger pump, the gear pump with the load sensing element has the advantages of low cost, strong anti-pollution ability and low maintenance requirements.
Priority flow control
Regardless of pump speed, working pressure, or the amount of flow required by the branch, a fixed-rate flow control valve will always ensure the flow required for the operation of the equipment. In the circuit shown in Figure 7, the output flow of the pump must be greater than or equal to the required flow of the primary oil circuit, and the secondary flow can be used as it is or returned to the tank. The fixed one-time flow valve (proportional valve) combines the primary control with the hydraulic pump, eliminating piping and eliminating external leakage, thus reducing costs. A typical application of such a gear pump circuit is a steering mechanism that is often seen on truck cranes. It dispenses with a pump.
The function of the load-sensing flow control valve is very similar to the one-time flow control function: that is, regardless of the pump speed, working pressure, or the size of the branch pumping flow, a flow rate is provided. However, as shown in the figure, the required flow is provided to the oil path only once through the oil port until its maximum adjustment value. This circuit can replace the standard primary flow control loop for maximum output flow. Since the pressure of the no-load circuit is lower than the fixed one-time flow control scheme, the loop temperature rise is low and the no-load power consumption is small. Load sensing is the same as the column flow control valve and the primary flow control valve. Its typical application is the power steering mechanism.
Bypass flow control
For bypass flow control, regardless of pump speed or working pressure level, the pump always supplies the system with a predetermined maximum amount, and the excess part is drained back to the tank or pump inlet. This solution limits the system's traffic to its best performance. The advantage is that the maximum adjustment flow is controlled by the loop size to reduce the cost; the pump and the valve are combined into one, and through the pump bypass control, the loop pressure is minimized and the pipeline and its leakage are reduced from the surface.
The bypass flow control valve can be designed together with a medium mass load sensing control valve that limits the range of working flow (working speed). This type of gear pump circuit is often used to limit the hydraulic operation to achieve the best speed of the engine in the garbage truck or power steering pump circuit, but also can be used for fixed mechanical equipment.
Dry suction valve
The dry suction valve is a pneumatic control hydraulic valve which is used to throttle the pump in. When the hydraulic pressure of the equipment is unloaded, only a very small flow rate is passed through the pump. When there is a load, the full flow suction pump is used. As shown in Figure 10, this circuit saves the clutch between the pump and the prime mover, which reduces costs and also reduces no-load power consumption because the minimal flow through the circuit maintains the prime mover power of the device. In addition, the noise of the pump at no load is also reduced. The dry suction valve circuit can be used in switch-hydraulic systems in any vehicle driven by an internal combustion engine, such as garbage trucks and industrial equipment.
Hydraulic pump options
At present, the working pressure of the gear pump is close to that of the piston pump, and the combined load sensing scheme provides a possibility for the gear pump to be variable, which means that the clear boundary between the gear pump and the piston pump becomes more and more blurred. Now. One of the decisive factors in the rational choice of hydraulic pump scheme is the cost of the entire system. Compared with the expensive ram pump, the gear pump has become a practical application for many applications because of its low cost, simple circuit, and low filtration requirements. The choice of options.
