The functional technology of replacing plunger pumps with gear pumps for construction machinery

Due to the structural limitation of a fixed displacement, it is generally believed that gear pumps can only be used as constant flow hydraulic sources. However, the accessory threaded connection combination valve scheme is effective in enhancing its functionality, reducing system costs and improving system reliability. Therefore, the performance of the gear pump can approach that of the expensive and complex plunger pump.

For instance, installing a control valve directly on the pump can eliminate the need for pipelines between the pump and the direction, thereby controlling costs. Fewer pipe fittings and connectors can reduce leakage, thereby enhancing the reliability of work. Moreover, the pump itself is equipped with a valve to reduce the circulating pressure of the circuit and improve its working performance. The following are some circuits that can enhance the basic functions of gear pumps. Some of them are proven feasible basic circuits in practice, while others belong to innovative research.

Unloading circuit

The unloading component will be combined with the high-flow pump and the low-power single pump. The liquid is discharged from the outlets of the two pumps until the 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 output flow of the pump to the system, that is, reducing the power of the magnet to a value slightly higher than that required for the high-pressure part to operate. The percentage of flow reduction depends on the ratio of the ununloaded displacement at this time to the total displacement. The combined or threaded connection unloading valve reduces or even eliminates the leakage of pipelines, channels, accessories and other possible leaks.

The simplest unloading of components is operated manually. The spring connects or closes the unloading valve. When a control signal is given to the valve, the on-off state of the valve can be switched. A lever or other mechanical mechanism is the simplest way to operate this kind of valve.

The pilot-controlled (pneumatic or hydraulic) unloading valve is an improvement in the operation method because this valve can be remotely controlled. The most significant advancement is the adoption of solenoid valves controlled by electrical or electronic shutters, which can not only be remotely controlled but also automatically controlled by microcomputers. It is generally believed that this simple unloading technology is the best application scenario.

Manual operation of unloading components is often used in circuits that require large flow rates for rapid operation and those that need to reduce flow rates for precise control, such as the circuits of rapidly extending and retracting cranes. When the unloading of the circuit shown in Figure 1 has no control signal effect (left position), the circuit continuously outputs a large flow rate. For normally open valves, the circuit will output a small flow rate under normal conditions. Pressure sensor unloading is the most common solution. As shown in Figure 2, the spring action keeps the unloading valve in its high-flow position (left position). When the circuit pressure reaches the pre-set value of the relief valve, the relief valve opens, and the unloading valve switches to its small flow position (right position) under the hydraulic pressure and action. The pressure-sensing unloading valve is basically an automatic unloading component that discharges when the system pressure is reached. It is widely used in the splitting of range gauges and hydraulic vise.

The unloading valve in the flow sensing unloading circuit is also pushed towards the high flow position by a spring (left). The size of the fixed throttle hole in this valve is determined by the flow rate required for the optimal engine speed of the equipment. If the engine speed exceeds this optimal range, the pressure drop through the throttle hole will increase, thereby shifting the unloading valve to a lower flow position (right position). Therefore, the adjacent components of the high-flow pump are made in a size that can throttle the maximum flow rate. As a result, this circuit has low energy consumption, operates smoothly and has a lower cost. A typical application of this kind of loop is to limit the loop flow to the optimal range to enhance the performance of the entire system, or to limit the loop pressure during the high-speed operation of the machine. It is often used in garbage transport trucks, etc.

The unloading valve of the pressure-flow sensing unloading circuit is also pushed to the position with a large flow rate (left position) by a spring. No matter whether the preset pressure or flow rate is reached, it will unload. The equipment can perform high-voltage operations both when idling and at normal working speeds. This feature reduces unnecessary flow, thereby lowering the required power. Because this kind of circuit has a wide range of load and speed variations, it is often used in excavation equipment.

A pressure sensing unloading circuit with power integration consists of two sets of slightly varying pressure sensing unloading pumps. Both sets of pumps are driven by the same prime mover, and each pump magnetically accepts the pilot-controlled unloading signal from the other unloading pump. This sensing method is called interactive sensing, which enables one group of pumps to operate under high pressure while another pump operates at a large flow rate. Two relief valves can be adjusted according 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 adopted.

The shown is the load sensing unloading circuit. When there is no load sensing signal from the control valve (lower chamber) of the main control valve, all the flow of the pump is discharged back to the oil tank through valve 1 and valve 2. When a load sensing signal is applied to this control valve, the pump supplies liquid to the circuit. When the output pressure of the pump exceeds the preset pressure value of the load sensing valve, the pump only supplies the working flow to the circuit, and the excess flow is bypassed back to the oil tank through the throttling position (upper position) of valve 2. Gear pumps with load sensing elements have the advantages of low cost, strong anti-pollution ability and low maintenance requirements compared with plunger pumps.

Priority flow control

Regardless of the pump's rotational speed, working pressure or the required flow rate of the branch, the fixed primary flow control valve can always ensure the flow rate needed for the equipment's operation. In the circuit shown in Figure 7, the output flow of the pump must be greater than or equal to the flow required by the primary oil circuit, and the secondary flow can be used for other purposes or returned to the oil tank. The fixed-value primary flow valve (proportional valve) combines primary control with the hydraulic pump, eliminating the need for pipelines and external leakage, thus reducing costs. A typical application of this type of gear pump circuit is the steering mechanism often seen 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 primary flow control: that is, it provides primary flow regardless of the pump's rotational speed, working pressure or the required flow rate of the branch. However, the shown scheme only provides the required flow to one oil circuit through one oil port until its maximum adjustment value. This loop can replace the standard primary flow control loop to achieve the maximum output flow. Because the pressure of the no-load circuit is lower than that of the fixed primary flow control scheme, the temperature rise of the circuit is low and the no-load power consumption is small. The load-sensing proportional flow control valve, like the primary flow control valve, is typically applied in power steering mechanisms.

Bypass flow control

For bypass flow control, regardless of the pump's rotational speed or working pressure, the pump always supplies liquid to the system at the predetermined maximum value, and the excess part is discharged back to the oil tank or the pump's inlet. This scheme limits the system's traffic, enabling it to achieve the best performance. Its advantage is that the maximum adjustment flow can be controlled through the loop scale, reducing costs. The pump and valve are combined into one unit, and through the bypass control of the pump, the pressure in the circuit is reduced to the minimum, thereby reducing the pipeline and its leakage.

Bypass flow control valves can be designed together with cluster-type load sensing control valves that limit the working flow (working speed) range. This type of gear pump circuit is often used in garbage trucks or power steering pump circuits that limit hydraulic control to achieve the optimal engine speed, and can also be used in fixed mechanical equipment.

Dry oil suction valve

The dry suction valve is a pneumatic hydraulic valve. It is used for throttling the oil intake of the pump. When the hydraulic load of the equipment is no-load, only a very small flow rate passes through the pump. When there is a load, the full-flow suction pump is used. As shown in Figure 10, this circuit can eliminate the clutch between the pump and the prime mover, thereby reducing costs and also decreasing no-load power consumption, as the extremely small flow through the circuit maintains the prime mover power of the equipment. In addition, the noise of the pump when it is unloaded has also been reduced. The dry suction valve circuit can be used in the on-off hydraulic system of any vehicle driven by an internal combustion engine, such as garbage loading trucks and industrial equipment.

Selection of hydraulic pump schemes

At present, the working pressure of gear pumps has approached that of plunger pumps. The combined load sensing scheme provides the possibility of variable changes for gear pumps, which means that the originally clear boundary between gear pumps and plunger pumps is becoming increasingly blurred. One of the decisive factors for the rational selection of a hydraulic pump solution is the cost of the entire system. Compared with the expensive plunger pump, the gear pump has become a practical and feasible choice for many application scenarios due to its lower cost, simple circuit, and low filtration requirements.