Gear pumps are traditionally viewed as constant-flow hydraulic sources due to their design. However, the integration of a threaded coupling valve significantly enhances their versatility, reduces system costs, and improves reliability, allowing gear pumps to perform almost as effectively as more complex and expensive piston pumps.
For example, mounting the control valve directly on the pump eliminates the need for external piping between the pump and the directional control, which helps keep costs low. Fewer fittings and connections also reduce leakage and improve overall system reliability. Additionally, integrating the valve into the pump itself can lower circuit pressure cycles, enhancing performance. Several circuits have been developed to expand the basic functions of gear pumps, some of which are proven practical solutions, while others are innovative approaches.
One such circuit is the unloading circuit, which combines a high-flow pump with a low-power single pump. The liquid from both pumps is discharged until a set pressure or flow rate is reached. At this point, the high-flow pump recirculates its output, reducing the total flow delivered to the system and slightly lowering the power demand for high-pressure operation. The extent of flow reduction depends on the ratio of the unloaded displacement to the total displacement at that moment. This approach minimizes piping, fittings, and potential leak points.
Manual unloading valves are commonly used in systems requiring fast response and precise control, such as telescopic boom mechanisms. These valves can be operated via a lever or mechanical linkage, making them simple to use. Pneumatic or hydraulic pilot-operated unloaders offer remote control capabilities, while solenoid valves provide automated control through electronic signals, making them ideal for computer-controlled systems.
Pressure-sensing unloaders are widely used, where the spring keeps the valve in a high-flow position until the system pressure reaches the relief valve’s preset level. At that point, the relief valve opens, shifting the unloader to a low-flow state. These valves are commonly found in applications like splitters and hydraulic vices.
In flow-sensing unloaders, the valve is spring-loaded to a high-flow position, and the size of the fixed orifice is based on the machine’s optimal engine speed. If the engine exceeds this range, the orifice causes an increase in pressure drop, triggering the unloader to switch to low flow. This helps maintain smooth operation and energy efficiency, often used in garbage trucks and similar equipment.
The pressure-flow sensing unloader operates regardless of whether the pressure or flow reaches its threshold, enabling high-pressure operation without unnecessary load. This makes it ideal for heavy-duty applications like mining machinery.
A power-sensitive pressure-sensing unloader uses two slightly different pressure-sensing unloaders, each receiving a signal from the other. This interactive sensing allows one pump to operate at high pressure while the other provides high flow, reducing overall power requirements and enabling cost-effective prime movers.
Priority flow control ensures a specific flow is always available for critical operations, regardless of pump speed or pressure. This is particularly useful in steering systems of truck cranes, where eliminating an extra pump reduces complexity and cost.
Load-sensing flow control valves work similarly but direct only the required flow to the main circuit, minimizing no-load power consumption and heat generation. They are commonly used in power steering systems.
Bypass flow control maintains a maximum flow rate by diverting excess fluid back to the tank or pump inlet. This helps optimize system performance and reduce costs, often seen in refuse trucks and stationary machinery.
Dry suction valves are pneumatic devices that regulate pump flow based on hydraulic load. They allow full flow during high-demand periods and minimal flow when idle, eliminating the need for a clutch and reducing noise and power consumption. These are ideal for internal combustion engine-driven hydraulic systems.
Today, gear pumps are capable of handling pressures close to those of piston pumps, and combined load-sensing schemes make variable gear pumps feasible. This blurs the traditional distinction between the two types, making gear pumps a more versatile and cost-effective option. In many applications, their lower cost, simpler circuits, and reduced filtration needs make them a practical choice over piston pumps.
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