Even basic hydraulic systems usually have more than one possible flow path.
That is to say, once the pump has drawn in fluid from the reservoir, there is more than one potential path through valves and actuators for fluid flow back to the reservoir.
The flow paths or branches are said to be in parallel with each other.
Consider the most basic log splitter.
If there is a flow path from one component to another those components are considered to be connected in series.
Each component can exert a restriction on the circuit and each restriction, (essentially a load) in series with the next, has a cumulative effect on the total pressure at the beginning of the branch.
An example of a series circuit would be a directional control valve which leads to a cylinder. The noticeable loads in series would be the cylinder pushing a log and the directional/flow control valve.
If you were to take a pressure reading of an active hydraulic circuit immediately before a load...
And a second pressure reading immediately after the load...
The pressure drop is equivalent to the resistance to flow imposed by the load.
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