Due to the structure of fixed displacement limit, generally think of gear pump can make only constant flow hydraulic source use. However, accessories and thread connection combination valve scheme to improve its function, reduce the system cost, and improve the system reliability is effective, therefore, the performance of the gear pump can be close to price the, complex plunger pump.

For example, the pump installed directly control valve, it saves the pump and direction valve between line, so as to control the cost. Less pipe and fittings can reduce leakage, which can improve the work reliability. Install the valve and pump itself can reduce loop circulating pressure and improve its working performance. Below are some can improve gear pump basic function of the circuit, and some are proved feasible basic circuit, and some are innovative research.

Discharge circuit unloading element will be in the big flow pump with small power single pump combined. Liquid from two pump outlet, till to the expected pressure and (or) flow. At this moment, big flow pump gave flow from its export cycle to the entrance, so as to reduce the pump to the system output flow, the pump power reduce to slightly higher than high pressure parts of the work required value. The percentage of flow discharge capacity at this time not depends on the ratio of of the total displacement. The combination or thread connection unloading valve to reduce and even eliminate the line, concreted complete auxiliary parts and other possible leakage.

The most simple unloading element by artificial control. Spring makes unloading valve on or off, and when a control signal to the valve, valve on-off state good be switching. Lever or other mechanical mechanism is the manipulation of the valve is the most simple method.

Guide control (pneumatic or hydraulic) unloading valve is a kind of improved control way, because this kind of valve can be remote control. Its biggest progress is to use electrical or electronic switch control of the electromagnetic valve, it not only can be used remote control, and available computer automatic control, usually think that this simple unloading technology is the best application situation.

Artificial control discharge components are commonly used for quick action and need to mass flow and quick action and need to mass flow and for accurate control and reduce the flow of the loop, such as the rapid expansion of the boom loop. Figure 1 shows loop unloading valve without control signal function, loop has been output mass flow. For normally open valve, in normal circuit will output a small flow.

Pressure sensing unloading valve is the most common scheme. As shown in figure 2 shows, spring action make unloading valve in the large flow position. Loop pressure to achieve overflow valve preset value, overflow valve open, unloading valve in hydraulic and under the action of switching to the small flow position. Pressure sensing discharge circuit used to travel to the rapid, at the end of the trip to high pressure low speed hydraulic cylinder for liquid. Pressure sensing unloading valve base is basically a achieve system pressure namely discharge automatic unloading element, commonly used in log splitter and hydraulic pressure in vice.

Flow sensor discharging circuit of unloading valve also by the spring will be the pressure to the big flow position. The valve of the fixed orifice size according to equipment of engine speed required to determine the optimum flow. If the engine speed beyond the best range, the throttle hole pressure drops increase, thus will discharge valve shift to small flow position. Therefore the larger flow pump adjacent element can be made for maximum flow throttling size, therefore less energy consumption circuit, stable and low cost. This circuit is typical application, limit loop flow of the best range in order to improve the performance of the whole system, or limit machine during high speed loop pressure. Often used in garbage carrying trucks, etc.

Pressure and flow sensor discharge loop unloading valve also by the spring pressure to the big flow position, no matter to the expected pressure or flow, can discharge. Equipment in idle or normal working speed under both can complete the work pressure. This characteristic to reduce unnecessary flow, so reduces the required power. Because of this circuit has a wide load and speed range, reason often used for mining equipment.

Figure 5 for a power integrated pressure sensing discharge circuit, it consists of two groups of a little change of pressure sensor unloading pump composition, two groups of pump by the same prime motor drive, each pump accept another unloading pump guide control discharge signal. This kind of sensing approach is called interactive sensor, it can make a group of pump work under high pressure and the other group pump in the big flow next work. Two overflow valve according to each loop special pressure adjustment, in order to make one or two pump discharge. This scheme reduces the power demand, therefore, can use small capacity and low price prime mover.

Figure 6 shows the load sensing discharge circuit. When master valve control cavity (inferior vena) no load sensor signal, the pump flow all the valve 1, valve 2 lines back to the fuel tank, As to the control valve applied load sensing signal, pump to loop feed liquid; When the pump output pressure more than load sensing valve pressure the preset value, pump only to return to provide work flow, and excess flow valve 2 by the throttled position of the bypass back to tank.

On load sensing element of the gear pump and plunger pump, which was compared with low cost, resisting pollution ability and the advantages of low maintenance requirements.

Priority flow control whether pump speed, work pressure or branch need flow size, fixed value a flow control valve can always ensure that equipment for the job flow. Figure 7 shows in the loop, the pump output flow must be more than or equal to a oil circuit required flow, secondary flow can make it or back to the tank. Fixed value a flow valve (proportional valve) will be a control and hydraulic pump combined, save line and to eliminate external leakage, therefore reduced the cost. The typical application of this kind of gear pump loop auto crane often is seen on the steering gear, it save a pump.

Load sensing flow control valve function and fixed value a flow control function were very similar: namely whether pump speed, work pressure or branch smoke should flow size, provide a flow. But only through a oil port to a oil circuit provide the required flow, until its biggest adjustment value. This circuit can replace the standard a flow control loop and obtain maximum output flow. Because of no load loop pressure below the fixed value a flow control scheme, the loop low temperature, no-load power consumption small. Load sensing than column flow control valve and a flow control valve, its typical applications are power steering mechanism.