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 and threaded coupling valve scheme is effective in increasing its functionality, reducing system costs and increasing system reliability, and as a result, gear pump performance can approach that of costly, complex piston pumps. For example, installing a control valve directly on the pump controls the cost by eliminating piping between the pump and the directional valve. Fewer tubing and fittings reduce leakage and improve work 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 both pumps until a predetermined pressure and / or flow rate is achieved. At this moment, 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 pump power slightly higher than the required value of the high pressure part. The percentage of flow reduction depends on the ratio of unloaded displacement to total displacement at this time. The combination or threaded coupling unloader valve reduces or eliminates the need for piping, ducts and fittings, 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. Guided (pneumatic or hydraulic) unloaders are an improvement on the way in which such valves can be remotely controlled. The biggest progress is the use of electrical or electronic switch control of the solenoid valve, which can be used not only remote control, and can be automatically controlled by the computer, usually considered this simple uninstall technology is the best case. Manually operated unloading components are commonly used in circuits requiring high flow rates and rapid movements for rapid actuation, requiring large flow and reducing flow for precise control, such as a telescoping boom circuit. The unloader valve shown in Figure 1 has no signal manipulation (left), the circuit has been a large flow output. For normally open valves, the circuit will output a small flow under normal conditions. Pressure sensing unloading valve is the most common program. The spring action puts the unloader valve in its high flow position (left position). When the circuit pressure reaches the preset value of the relief valve, the relief valve is opened and the relief valve is switched to its low flow position (right position) under hydraulic pressure and action. The pressure sensing unloading circuit is used for the stroke to be fast, and the hydraulic cylinder requiring high pressure and low speed at the end of the stroke is for liquid supply. The pressure sensing unloading valve base is basically an automatic unloading element that achieves system pressure relief, commonly used in range finders and hydraulic vices. The unloading valve in the flow sensing unloading circuit is also urged by the spring towards the large flow position (left position). 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). Therefore, adjacent components of the large flow pump can be made to the size of the maximum flow throttling, so the circuit less energy consumption, stable and low cost. 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 unloading pumps, two pumps driven by the same prime mover, each receiving a pilot unloading signal from another unloading pump. This type of sensing is called interaction sensing, which allows one set of pumps to operate at high pressure and the other to operate at high flow rates. 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. Load sense unloading circuit. When the main control valve cavity (lower chamber) no load sensing signal, the pump all the 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 loop 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. In this circuit, the pump output flow must be greater than or equal to the required flow of the oil circuit, the secondary flow can be used for it or back 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 required flow rate is only provided 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, with the excess drained to the tank or pump inlet. This scenario limits the amount of traffic that enters the system to give it the best performance. This has the advantage of reducing the cost by controlling the maximum flow rate through the loop size; combining the pump and the valve and bypassing the pump to minimize the loop pressure, thereby reducing tubing and leakage. 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 landfill trucks 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 hydraulic valve used to pump into the oil throttling, when the device is under no-load hydraulic flow, only a very small flow (<18.9t / min) through the pump; With load, full flow suction pump. 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. The choice of 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 of variable gear pump. This means that the originally clear boundary between the gear pump and the piston pump More and more fuzzy 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.
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