How Does a 2-Stage Hydraulic Pump Work?
2-stage hydraulic pumps are used in motor-driven operations wherein a low-pressure, high rate inlet must be transferred to high pressure, low flow-rate outlet. Single-stage pumps are rated to a static max pressure level and have a limited recycle rate.
To achieve high pressure without a 2-stage unit, the drive engine would require significantly higher horsepower and torque capacity but still lack an effective cycle rate. Other hydraulic pump variants exist – such as piston pumps – but are expensive, making 2-stage units more feasible.
Uses of a 2-Stage Hydraulic Pump
Standard 2-stage hydraulic pumps generate positive displacement – a static fluid volume at the outlet – each cycle. The primary benefit
Applications requiring a rapid turnover between pressure ranges (e.g., to propel and retract a rod) will often utilize 2-stage hydraulic pumps.
An efficient log splitter will generate enough pressure to split a log while maintaining sufficient fluid flow to sustain a reliable cycle rate.
For example, a single gear hydraulic pump might be designed to generate a high-pressure output. Still, it will be unable to repeat a cycle rapidly due to a necessarily low flow rate at the intake. A 2-stage unit ensures consistent flow to increase cycle turnover.
Compactors utilize a similar 2-stage process. High-pressure flow drives the compacting rod, while the low-pressure flow retracts the mechanism and feeds the high-pressure chamber for repeated impacts.
How to Turn Off or Alternate the Pump
Alternating the 2-stage hydraulic pump between units is coordinated by the check and unloader valves.
1. Fluid is drawn from a reservoir into a high volume, large-gear unit, elevating the gear housing’s pressure level.
2. Once the first-stage pressure meets a certain pressure threshold, a combiner check valve will open and feed into the second-stage, small-gear unit – joining flows at relatively low pressure.
3. A load sensing pin will trigger the unloading valve to open and the check valve to close. The flow will be directed exclusively to the discharge port.
4. A small amount of fluid may feedback to a load sensing pin to measure the pressure at the outlet and signal lower flow rate in the first unit, lowering the pressure and providing the conditions for a cycle to repeat.
Why Not Use a Piston Pump?
A piston pump operates according to variable displacement. Flow is determined by the angle of an internal slant disk attached to the pump shaft. Pump adjustments – like torque or horsepower limiters – allow piston pumps to emit a max flow rate regardless of pressure level.
Price Per Unit
In most cases, hydraulic piston pumps are an order of magnitude more expensive than gear-based pumps. Potential downtime and part replacement in high volume work conditions exacerbate price disparities further.
Price of Operation
Chiefly: fuel and power consumption. A piston pump operating in high-pressure ranges will regularly demand the full horsepower capabilities of its associated drive engine – increasing the power utilization of the system.
Opportunity cost may also be considered when using a piston pump. Depending on the application (e.g., log splitting), work output can be heavily impacted by the cycle speed of the pump. Not only is a piston pump more expensive to peruse, it is also slower than 2-stage pumps.
The Panagon Systems Difference
Navigating around OEM assemblies and components – often outdated, out-of-service, or limited (and expensive) – can prove challenging.
Panagon Systems has specialized in manufacturing industry-standard and custom hydraulic assemblies for 25 years. Reach out to our team for a consultation on your specific operational and equipment needs.