Due to the structure of the displacement limit, it is generally considered gear pump can only be used as a constant flow hydraulic source. However, the attachment-threaded coupling valve scheme is effective in increasing its functionality, reducing system costs and increasing system reliability. As a result, gear pump performance can approach the costly and complex piston pump. For example, the direct installation of the control valve on the pump eliminates the need for piping between the pump and the direction, thus controlling the cost. Less fittings and connectors reduce leakage and improve operational reliability. And the pump itself to install the valve can reduce the cycle pressure of the circuit, improve its performance. Here are some of the loops that can enhance the basic functions of a gear pump, some of which are practically proven viable circuits, while others are innovative. Unloading circuit The unloading element will combine the high flow pump with the low power single pump. Liquid is discharged from the outlets of the two pumps to achieve a predetermined pressure and / or flow rate. At this point, the high-flow pump circulates the flow from its outlet to the inlet, reducing the pump's output flow to the system by reducing the magnetic power slightly above the desired value for high-pressure operation. The percentage of flow reduction depends on the ratio of the unloaded displacement to the total displacement at this time, and the combination or threaded unloading valve reduces or even eliminates the piping, apertures and accessories and other possible leaks. The easiest to uninstall components by hand. The spring causes the unloader valve to be switched on or off, and when the valve is manipulated, the on-off condition of the valve is switched. Lever or other mechanical mechanism is the easiest way to manipulate this valve. The pilot (pneumatic or hydraulic) unloader valve is an improvement on the way in which the valve can be remotely controlled. The biggest progress is the use of electrical or electronic control of the solenoid valve, which can be used not only remote control, and can be automatically controlled by the computer, it is generally considered that this simple uninstall technology is the best case. Manually operated unloading components are commonly used in circuits requiring high flow rates and rapid operation for rapid operation, requiring large flow and reducing flow for precise control, such as a telescopic boom circuit. When the loop shown in Figure 1 is unloaded and there is no signal manipulation (left bit), the loop always outputs a large amount of flow. For normally open valves, the circuit will output a small flow under normal conditions. Pressure sensing unloading is the most common program. As shown in Figure 2, the spring causes the unloader valve to be in its high flow position (left position). When the circuit pressure reaches the preset value of the relief valve, the relief valve opens and the relief valve is switched to its low flow position (right position) under hydraulic pressure and under action. Pressure sensing unloading valve is basically an unloading system to achieve the pressure relief unloading components, commonly used in the splitter and hydraulic vise jaw. Engineering machinery gear pump piston pump function to replace the technical analysis of the unloading valve in the load sensing circuit unloading valve is also the spring pressure to the large flow position (left). The size of the fixed orifice in the valve is based on the required flow rate for the machine's optimum engine speed. If engine speed exceeds this optimum range, the orifice pressure drop will increase, displacing the unloader valve to the low flow position (right position). As a result, adjacent components of the high-flow pump are dimensioned to throttle the maximum flow rate. Therefore, the circuit consumes less energy, operates smoothly and costs less. A typical application of this circuit is to limit the loop flow to the optimum range to improve the overall system performance or to limit the circuit pressure during high speed operation of the machine. Commonly used in garbage truck and so on. The unloading valve of the pressure-flow sensing unloading circuit is also spring-biased to the high-flow position (left-hand position) and will be unloaded regardless of the desired pressure or flow rate. Equipment in idle or normal working speed can be completed under high-pressure work. This feature reduces unnecessary traffic and therefore reduces the power required. Because of this circuit has a wide range of load and speed changes, it is often used in mining equipment. A power-sensitive pressure-sensing unloader circuit consisting of two slightly different pressure-sensing off-load pumps, two pumps driven by the same prime mover, each receiving a pilot unload signal from another unload pump. The way we sense it is interactive sensing, which allows one set of pumps to work at high pressure and the other at a high flow rate. Two relief valves can be adjusted for each circuit's special pressure to unload one or two pumps. This program reduces the power requirements, it can be low-cost prime mover. The load sensing off-load circuit is shown. When the main valve control valve (the lower chamber) no load sensing signal, all the pump flow through the valve 1, valve 2 row back to the tank; when the control valve to apply a load sensing signal, the pump to the circuit for liquid; When the pump output pressure exceeds the load sensing valve pressure preset value, the pump only to the circuit to provide working flow, and excess flow through the valve 2 throttling position (upper) bypass back to the tank. Compared with the plunger pump, the gear pump with 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 branch flow required size, the value of a flow control valve can always ensure the required flow of equipment work. In this circuit shown in Figure 7, the output flow of the pump must be greater than or equal to the required flow rate of the primary circuit. The secondary flow rate can be used for it or returned to the tank. Setting a flow valve (proportional valve) will be a combination of control and hydraulic pump, eliminating pipelines and eliminate external leakage, so reduce the cost. A typical application of such a gear pump circuit is the steering mechanism often found on truck cranes, which eliminates the need for a pump. Load sensing flow control valve function and set a value of flow control function is very similar: no matter the pump speed, working pressure or branch pumping demand flow size, provide a flow. However, the solution shown provides only the required flow to the primary circuit through one port up to its maximum adjustment. This circuit replaces the standard primary flow control loop for maximum output flow. Due to no-load circuit pressure is lower than the value of a flow control program, so the loop temperature low, no load power consumption. Load sensing than the flow control valve and a flow control valve, its typical application is power steering. Bypass Flow Control For bypass flow control, the pump always supplies fluid to the system at a predetermined maximum, regardless of pump speed or operating pressure, and the excess is drained to the tank or pump inlet. This solution limits the system's traffic and gives it the best performance. This has the advantage of controlling the maximum flow rate through the loop to reduce costs; combining the pump and the valve and bypassing the pump to minimize circuit pressure and reduce the line and leakage from the surface. The bypass flow control valve can be designed with a medium-duty load-sensing control valve that defines the range of working flows (operating speeds). Gear pump circuits of this type are commonly used in refuse truck or power steering pump circuits that limit the hydraulic operation for optimum engine speed and also for stationary machinery. Dry suction valve Dry suction valve is a pneumatic control valve, it is used to pump into the oil throttling, when the device's hydraulic load, only a very small flow through the pump; and in the load, the full flow inhalation Pump. As shown in Figure 10, this circuit eliminates the need for a clutch between the pump and the prime mover, reducing costs and reducing no-load power consumption by maintaining the plant's prime mover power with minimal flow through the circuit. In addition, it also reduces the pump noise at no load. Dry suction valve circuit can be used in any vehicle driven by the internal combustion engine hydraulic system, such as refuse trucks and industrial equipment. Selection of a hydraulic pump solution At present, the working pressure of the gear pump is close to that of the piston pump, and the combined load sensing scheme offers the possibility for a gear pump, which means that the original clear limits between the gear pump and the piston pump Get more and more blurred. Reasonable choice of one of the decisive factors in the hydraulic pump solution is the cost of the entire system. Compared with the expensive piston pump, the gear pump is feasible in many applications due to its low cost, simple circuit and low filtration requirements The choice of program.