Hydraulic engineers and designers are continuously trying to get more for less, and as such, hydraulic circuits are becoming more compact, efficient and flexible. To be competitive, machines are being produced with as much functionality as possible, which can increase complexity. Hydraulic circuits can be packed with control valves to achieve this increased functionality, but this can create challenges in today’s market.
Compact and efficient machines trying to “do more with less” are seeing a reduction in power requirements, partly due to stringent emissions standards making smaller engines a more lucrative and sensible option. Smaller engines with less output are also not capable of the same hydraulic power output, and the result is the use of smaller pumps with less flow. And less flow can mean there are compromises in operations and functions, as not all actuators can see full pump flow all the time.
Post pressure compensation is a method of flow sharing that allows all functions in the circuit to operate seamlessly regardless of demand from actuators, even if pump flow is not enough to satiate the demands of each sub-circuit. Each part of the system will compensate for changes in flow demand by reducing flow proportionally to other parts of the circuit.
For example, let’s say you have 10 gpm of flow from a fixed-displacement available to two circuits in parallel, both of which are using flow controls set to 7 gpm each. With one valve open, the 10 gpm is enough to satisfy the function in its entirety. But once both valves are opened simultaneously, flow will be reduced to 5 gpm for each function. In a standard pre-compensation circuit, 7 gpm would go to the part of the circuit with lowest pressure, leaving only 3 gpm for the other function, and this is regardless if the flow controls are using their own compensators.
Standard pre-compensation uses the compensators before the metering valve, which can be a flow control, lever valve or proportional valve. Pre-compensation works fine to maintain flow at a single valve regardless of differences in upstream or downstream pressure, and only as long as inlet flow is higher than the metering valve is asking. If you have only a single valve in your system, pre-compensation is all you need.
Post-compensation uses a load-sense network, usually with a master compensator at the inlet of the valve package. Instead of a single compensator before the metering valve(s), we use individual compensators after the metering and control valves and for each work port of the directional valve. So each directional valve will have compensators on both the A and B port, after the valve. With the load sense network, master compensator and individual compensators, this hydraulic circuit is very complex. I’ll spare you more detail than this, as the circuitry details are less important than the overall concept.
Post-compensation allows you to provide many functions with metered flow, and most importantly, with predictable performance. Even a standard load-sensing system can only be so advanced to provide enough pressure and flow to meet total system demands. With flow sharing (post-compensation), all flow channels are monitored simultaneously for pressure drop, and maintaining as close as possible the flow ratios of each metered valve. This system is the future of compact, efficient and complex mobile hydraulic systems, and provides optimum control of actuators regardless of inlet flow or the number of functions in use.