Gear pumps are typically considered as constant-flow hydraulic sources due to their fixed displacement design. However, the integration of a threaded coupling valve significantly enhances their functionality, 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 additional piping between the pump and the actuator, which helps control costs. Reducing the number of fittings and connectors also minimizes leakage and increases operational reliability. Additionally, integrating the valve within the pump itself can lower circuit pressure cycles, thereby improving overall performance. Several circuits have been developed to enhance the basic functions of gear pumps, with some being proven practical and others offering innovative solutions.
Unloading Circuits
An unloading element combines a high-flow pump with a low-power single pump. The liquid from both pumps is discharged to reach a set pressure or flow rate. At this point, the high-flow pump recirculates its output back to the inlet, reducing the flow delivered to the system and slightly increasing the magnetic power above the desired level for high-pressure operation. The percentage of flow reduction depends on the ratio of the unloaded displacement to the total displacement at that moment. The combination or threaded unloading valve helps reduce or eliminate piping, holes, and other potential leak points.
Manual unloading components are the simplest to operate. A spring mechanism controls the unloader valve, switching it on or off. Mechanical levers are the easiest way to manipulate the valve.
Pilot-operated (pneumatic or hydraulic) unloader valves offer remote control capabilities. The most advanced form involves electrical or electronic control via solenoid valves, enabling automatic computer control. This simple unloading technology is often considered the best option.
Manually operated unloaders are commonly used in circuits requiring high flow rates for rapid operations, such as fast telescopic boom systems. These circuits can provide large flows without manual intervention, while normally open valves deliver small flows under normal conditions. Pressure-sensing unloading is the most common approach. A spring keeps the unloader valve in the high-flow position. When the system pressure reaches the relief valve's preset value, the relief valve opens, shifting the unloader valve to the low-flow position. Pressure-sensing unloaders are widely used in splitters and hydraulic vices.
In flow-sensing unloading circuits, the unloader valve is also spring-loaded to the high-flow position. The size of the fixed orifice in the valve is based on the optimal engine speed for the machine. If the engine speed exceeds this range, the orifice pressure drop increases, moving the unloader valve to the low-flow position. This ensures that the high-flow pump is properly sized to throttle the maximum flow, resulting in lower energy consumption, smoother operation, and reduced costs. This type of circuit is often used in garbage trucks to limit flow during high-speed operation.
The unloader valve in a pressure-flow sensing unloading circuit is similarly spring-biased to the high-flow position, unloading regardless of whether the pressure or flow reaches the preset levels. This allows equipment to operate under high pressure even when idle or running at normal speeds, reducing unnecessary energy use. Due to its adaptability across various load and speed conditions, this circuit is frequently used in mining equipment.
A power-sensitive pressure-sensing unloader circuit uses two slightly different pressure-sensing unloaders, both driven by the same prime mover. Each pump receives a pilot signal from the other. This interactive sensing method enables one pump to work at high pressure while the other operates at high flow. Each circuit can adjust its relief valves to unload one or both pumps, reducing power requirements and allowing for a cost-effective prime mover.
Priority Flow Control
Regardless of the pump speed or the pressure and flow demand of the branch, the primary flow control valve ensures the necessary flow for equipment operation. Installing a flow valve (proportional valve) combines control with the hydraulic pump, eliminating external piping and leakage, thus lowering costs. A typical application of this circuit is in the steering mechanism of truck cranes, where it eliminates the need for an extra pump.
Load-sensing flow control valves function similarly to standard flow control valves, ensuring the required flow is provided regardless of pump speed, working pressure, or branch flow demand. However, the load-sensing version only provides the necessary flow through one port up to its maximum setting. This circuit replaces the standard primary flow control loop with a more efficient system, resulting in lower no-load pressure, reduced temperature, and lower no-load power consumption. Load-sensing valves are commonly used in power steering systems.
Bypass Flow Control
In bypass flow control, the pump always supplies fluid to the system at a predetermined maximum, regardless of pump speed or operating pressure. Any excess fluid is drained back to the tank or pump inlet. This solution limits system flow and optimizes performance. It offers the advantage of controlling maximum flow through the loop, reducing costs, and combining the pump and valve to minimize circuit pressure and reduce line leakage.
Bypass flow control valves can be designed with medium-duty load-sensing control valves that define the range of operating flows. These types of gear pump circuits are commonly found in refuse trucks, power steering pump circuits, and stationary machinery, where they help optimize engine speed and hydraulic operation.
Dry Suction Valve
A dry suction valve is a pneumatic valve that regulates the pump’s intake. When the hydraulic load is high, only a small amount of fluid passes through the pump; when the load is low, the pump delivers full flow. This circuit eliminates the need for a clutch between the pump and the prime mover, reducing costs and minimizing no-load power consumption by maintaining the prime mover’s power with minimal flow through the circuit. It also reduces pump noise during no-load operation. Dry suction valves are commonly used in internal combustion engine-driven hydraulic systems, such as refuse trucks and industrial equipment.
Hydraulic Pump Program Options
Today, the operating pressure of gear pumps is approaching that of piston pumps, and combined load-sensing schemes make variable gear pumps possible. This blurs the traditional boundaries between gear and piston pumps. Choosing the right hydraulic pump depends largely on the overall system cost. Compared to expensive piston pumps, gear pumps are a practical choice in many applications due to their low cost, simple circuit design, and minimal filtration requirements.
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