Unloading Valve
An unloading valve is the valve that sends pump flow to tank at very low pressure when no work is being done, thereby minimizing power wasted at idle. At idle the line pressure typically drops to 3-10 bar. On systems with fixed displacement pumps it noticeably reduces heating and fuel consumption.
It should not be confused with a relief valve. A relief valve holds the pressure at the set value and dumps the excess flow to tank; that is, the flow goes to tank at the set pressure and all that energy turns into heat. An unloading valve, on the other hand, sends the flow at almost no pressure. Dumping 40 l/min to tank at 200 bar produces about 13 kW of heat, while sending it at 5 bar brings this below half a kW.
- Systems with accumulators: it unloads the pump when the accumulator is full and reloads when pressure drops.
- Dual pump circuits: it limits drive power by unloading the large pump at high pressure.
- Symptom: the pump sound dropping at idle and the tank not heating up show the valve is working.
On a machine where the oil temperature runs continuously high, this element is on the checklist. Since electronic lever control can extend the duration of use, low idle loss becomes even more important in practice.
Where this term is used: How Are Manual Hydraulic Levers Controlled Remotely? A Comparison of 6 Methods