A feasible circuit to improve the basic functions of the gear pump

Due to the structural limitations of the fixed displacement, it is generally considered that the gear pump can only be used as a constant flow hydraulic source. However, the accessory and threaded combination valve solution is effective for improving its function, reducing system cost and improving system reliability. Therefore, the performance of the gear pump can be close to the expensive and complicated plunger pump.

For example, installing a control valve directly on the pump eliminates the need for piping between the pump and the directional valve, thus controlling costs. Fewer tubes and fittings reduce leakage and increase job reliability. Moreover, the pump itself can be installed with a valve to reduce the circulating pressure of the circuit and improve its working performance. Below are some of the circuits that improve the basic functions of gear pumps, some of which are basic circuits that have proven to be viable, while others are innovative.

Unloading circuit

The unloading element will combine a high flow pump with a low power single pump. The liquid is discharged from the outlets of the two pumps until a predetermined pressure and/or flow rate is reached. At this point, the high flow pump circulates the flow from its outlet to the inlet, thereby reducing the pump's output flow to the system, ie, reducing the pump's power to slightly higher than the high pressure portion requires. The percentage of flow reduction depends on the ratio of undischarged displacement to total displacement at this time. Combination or threaded unloading valves reduce or even eliminate piping, tunnels and accessories and other possible leaks.

The simplest unloading element is manually manipulated. The spring turns the unloading valve on or off. When the valve is actuated, the on/off state of the valve is switched. Lever or other mechanical mechanisms are the easiest way to manipulate such valves.

Pilot-controlled (pneumatic or hydraulic) unloading valves are an improvement in the way they are operated, as they are remotely controlled. The biggest advancement is the use of electrical or electronic switch-controlled solenoid valves, which can be used not only for remote control, but also for automatic control by a microcomputer. This simple unloading technique is generally considered to be the best application.

Manually operated unloading elements are often used in circuits that require high flow and fast action for fast motion and require large flow and reduced flow for precise control, such as a fast-expanding boom circuit. When the unloading valve of the circuit shown in Figure 1 has no steering signal, the circuit always outputs a large flow. For normally open valves, the circuit will output a small flow rate under normal conditions.

Pressure sensing unloading valves are the most common solution. As shown in Figure 2, the spring action causes the unloading valve to be in its large flow position. When the circuit pressure reaches the relief valve preset value, the relief valve opens and the unloading valve switches to its low flow position under hydraulic pressure. The pressure sensing unloading circuit is mostly used for the hydraulic cylinder that needs to be fast in the stroke and needs high pressure and low speed at the end of the stroke. The pressure sensing unloading valve base is basically an automatic unloading element that reaches the system pressure and is unloaded, and is commonly used in odometer splitters and hydraulic vise.

The unloading valve in the flow sensing unloading circuit is also pressed by the spring to a high flow position. The fixed orifice size in the valve is determined by the flow rate required for the optimum engine speed of the equipment. If the engine speed exceeds this optimum range, the orifice orifice pressure drop will increase, shifting the unloading valve to the low flow position. Therefore, the adjacent components of the large flow pump are made to be capable of throttling the maximum flow rate, so the circuit consumes less energy, works smoothly, and has low cost. A typical application of such a loop is to limit the loop flow to an optimum range to improve the performance of the overall system or to limit the loop pressure during high speed travel of the machine. Often used in garbage trucks, etc.

The unloading valve of the pressure flow sensing unloading circuit is also pressed by the spring to a large flow position and will be unloaded regardless of the predetermined pressure or flow rate. The equipment can perform high pressure operation at idle or normal working speed. This feature reduces unnecessary traffic and therefore reduces the power required. Because such circuits have a wide range of load and speed variations, they are commonly used in excavation equipment.

Figure 5 is a pressure-sensing unloading circuit with power integration consisting of two sets of slightly variable pressure-sensing unloading pumps. The two sets of pumps are driven by the same prime mover, and each pump receives a pilot-controlled unloading signal from another unloading pump. . This type of sensing is called interactive sensing, which allows one set of pumps to operate at high pressure and the other set to operate at high flow rates. Two relief valves can be adjusted to the specific pressure of each circuit to unload one or two pumps. This solution reduces power requirements, so a small-capacity, low-cost prime mover can be used.

Figure 6 shows the load sensing unloading circuit. When the control chamber (lower chamber) of the main control valve has no load sensing signal, all the flow rate of the pump is discharged back to the oil tank through the valve 1 and the valve 2; when the load sensing signal is applied to the control valve, the pump supplies the 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 bypasses the return to the tank through the throttle position of the valve 2.

Compared with a plunger pump, a gear pump with a load sensing element has the advantages of low cost, high pollution resistance and low maintenance requirements.

Priority flow control

Regardless of the pump's speed, operating pressure or the amount of flow required by the branch, a fixed value flow control valve always guarantees the flow required for the equipment to operate. In the circuit shown in Figure 7, the output flow of the pump must be greater than or equal to the flow required for the primary line, and the secondary flow can be used for other purposes or return to the tank. The fixed-rate primary 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 found on truck cranes, which eliminates the need for a pump.

The function of the load sensing flow control valve is very similar to that of the fixed value flow control: that is, the flow rate is provided regardless of the pump speed, working pressure or branch pumping flow rate. However, the required flow rate is supplied to the primary oil passage only through the primary oil port until its maximum adjustment value. This circuit replaces the standard primary flow control loop for maximum output flow. Since the pressure of the no-load circuit is lower than the fixed-rate one-time flow control scheme, the loop temperature rise is low and the no-load power consumption is small. The load sensing ratio flow control valve is the same as the primary flow control valve, and its typical application is the power steering mechanism.

Bypass flow control

For bypass flow control, regardless of the pump speed or operating pressure, the pump always supplies the system with a predetermined maximum value, and the excess is drained back to the tank or pump inlet. This scenario limits traffic entering the system for optimal performance. The advantage is that the maximum adjustment flow rate is controlled by the loop scale, and the cost is reduced; the pump and the valve are combined into one body, and the bypass pressure of the pump is used to minimize the loop pressure, thereby reducing the pipeline and its leakage.

The bypass flow control valve can be designed with a mid-group load sensing control valve that defines the range of operating flow (operating speed). This type of gear pump circuit is often used in garbage trucks or power steering pump circuits that limit hydraulic operation to achieve optimum engine speed, as well as stationary machinery.

Dry suction valve

The dry suction valve is a pneumatically controlled hydraulic valve used for pumping oil throttling. When the hydraulic pressure of the equipment is no load, only a very small flow (< 18.9t/min) is passed through the pump; Full flow suction pump. As shown in Figure 10, this circuit eliminates the need for a clutch between the pump and the prime mover, thereby reducing costs and reducing no-load power consumption, as the prime mover power of the device is maintained by the minimum flow through the loop. In addition, the noise of the pump at no load is also reduced. The dry suction valve circuit can be used in switching hydraulic systems in any vehicle driven by internal combustion engines, such as garbage trucks and industrial equipment.

Selection of hydraulic pump solutions

At present, the working pressure of the gear pump is close to the plunger pump, and the combined load sensing scheme provides the possibility of variable variables for the gear pump, which means that the originally clear boundary variation between the gear pump and the plunger pump is increasingly blurred. It is.

One of the decisive factors for a reasonable choice of hydraulic pump solution is the cost of the entire system. Compared with the expensive plunger pump, the gear pump is practical and feasible for many applications due to its low cost, simple circuit and low filtration requirements. The choice of options.

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