Liters/Minute (l/min, Flow Rate)
The flow rate unit expressing the volume of oil passing per unit time. It directly determines actuator speed: the speed of a cylinder is found by dividing the flow going to it by the piston area. The flow rate of mobile equipment pumps typically falls in the 20-120 l/min range.
The link between flow rate and speed is very clear once you line up the units. For example, if 30 liters of oil per minute goes to a piston with an area of 50 cm²: 30 liters is 30,000 cm³, and dividing that by 50 cm² gives 600 cm per minute, that is a speed of about 10 cm/s.
- Pump flow is directly proportional to rpm; when the engine drops to idle all motions slow down.
- When more than one axis is operated at the same time the flow is shared; in an uncompensated system the lighter one takes most of the oil.
- A throttle valve and a narrow hose limit the flow, and the excess oil returns to tank through the relief valve and produces heat.
That is why the reason a motion is slow is not always the pump; a clogged filter, a throttled valve or an engine at idle gives the same symptom. An electronic remote control can drive the lever more precisely but cannot increase the vehicle's flow rate; even after the kit is installed the speed limit is set by the vehicle's own hydraulics.
Where this term is used: The Importance of Hydraulic Control: Why Should It Be Commanded by an Electronic Signal?