An Introduction to Different Hydraulic Circuits

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An Introduction to Different Hydraulic Circuits

In the Hydraulic industry, the term “hydraulic circuit” will be quite familiar to everyone. When anyone asks about hydraulic circuit types to a beginner, most of them will reply with open circuit and closed circuit. But, there are many types of hydraulic circuits and some are mentioned here.

First of all, what is a hydraulic circuit? A hydraulic circuit will contain different components like reservoirs, pumps, cylinders, valves, actuators, accumulators, filter elements, sealing solutions, etc… interconnected with hoses/tubes to facilitate pressurized fluid flow through the system. This system will function in order to convert mechanical energy to hydraulic energy by arranging each component according to objectives and specifications in order to achieve energy efficiency, ease of maintenance, safe and optimal performance, smooth movement capability, etc…

How is hydraulic power calculated? Hydraulic power generated is directly proportional to pressure and flow rate. I.e; expressed as:

L(kW) = PQ÷60 ; P is the Pressure(Mpa) and Q is the flow rate(L/min)

The different hydraulic circuits are:

  1. Unload Circuits: The primary aim of an unloading circuit is to maintain the pump in a state of no-load operation. This is achieved by making the oil flow back to the reservoir from the pump under very low pressure when the actuator works intermittently or stops working. The important benefits of unloading circuits include efficiency, improved product life, low power consumption, minimal heat generation, etc… The different types of unload circuits are unload circuit with a three-position change-over valve, unload circuit with accumulator, unload circuit with solenoid relief valve, unload circuit with pressure compensation variable pump, etc…
  2. Pressure Control Circuits: A pressure control circuit in hydraulics is used for providing the required pressure to system components by protecting the system from overpressure or underpressure. There are hydraulic systems that contain components requiring varying pressure for operation. Two-pressure circuit with decompression and check valves, decompression circuit, and weight balancing circuit are the common pressure control circuits in hydraulics.
  3. Speed Control Circuits: Speed change circuit(1), circuit with proportional electro-hydraulic directional and flow control valve(2), differential circuit(3), and pre-fill valve circuit(4) are commonly available types under this category. The speed change circuit will include two flow control valves to change the speed of cylinder motion. While in circuit(2) optimal speed is generated by controlling the amount of spool shifted in the proportional electro-hydraulic valve. The differential circuit will produce faster cylinder-forward motion compared to circuits having a single incoming flow channel. The pre-fill valve circuit will employ subsidiary cylinders and a pre-fill valve to produce the pumping function.
  4. Filter Circuits: The key objective of the filter circuit is filtering the contaminants to maintain the fluid flow at a certain rate. But, when the filter element is clogged, there will be adverse impacts. The different circuits are the pump filter circuit, pressure line filter circuit, return line filter circuit, pressure-line bleed off filter circuit, and off-line filter circuit.
  5. Synchronizing Circuits: This circuit is used for synchronizing the movement of multiple actuators. Here, the synchronization errors are minimized by correcting every error at the end of the operation with a cylinder stroke instead of repeated motion. Synchronizing circuits can be again sub-divided into synchronizing circuits with mechanical combinations, synchronizing circuits with flow control valves, circuits with flow dividers, circuits with synchronized hydraulic motors, circuits with synchronized cylinders, and circuits with servo valves.
  6. Intensifying Circuits: Hydraulic circuits that use a cylinder as an intensifier will intensify pressure using the difference between the cap and the head area of the cylinder.

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