Circuits and Accumulators
Basic circuits, meter-in and meter-out, regeneration, sequencing and sizing an accumulator.
The basic circuit
The smallest useful circuit is the one in the picture: a motor-driven pump draws oil from the tank, a relief valve sets the maximum pressure, a 4/3 directional valve sends oil to either side of a cylinder, and the return oil goes back to the tank through a filter. Everything else is a variation on this, adding speed control, load holding, sequencing and energy storage.
Meter-in, meter-out and bleed-off
| Method | How | Good for | Weakness |
|---|---|---|---|
| Meter-in | throttle in the supply to the actuator | loads that always resist (a press, a lift going up) | a running-away load (gravity, inertia) is not controlled |
| Meter-out | throttle in the return line | overrunning loads, smooth speed both ways | raises the pressure on the rod side (see below); heat and seal load |
| Bleed-off | throttle sends part of the flow straight to the tank | efficient: the pump works only at load pressure | poor speed accuracy: speed varies with the load and the pump leakage |
| Compensated flow control | constant pressure drop across the throttle | speed independent of load | extra pressure loss (about 5 to 8 bar) |
Meter-out has a trap. If the cap side is at 160 bar and the rod side is being throttled, the oil trapped on the rod side is squeezed by the area ratio of 1.46: the pressure on the rod side can reach 234 bar (160 × 1.46), more than the relief setting, and burst the seals or hoses. Always check the rod-side pressure for throttled extension against a push load.
Holding and lowering a load
A vertical cylinder needs a way to stop the load falling when the valve is centered and to lower it at a controlled speed. Spool valves leak, so use a pilot-operated check valve (to hold with no drift) or a counterbalance valve on the rod side, which holds the load at a pressure setting 1.3 times the load pressure and opens by pilot pressure from the cap side to let it down without running away. The setting for the 1 000 kg load of Cylinders and Actuators is 1.3 × 29 bar = 37 bar. A brake valve does the same for a motor.
Regeneration and fast approach
Regeneration joins the rod-side return flow to the pump flow to speed the approach (see Cylinders and Actuators). A fast-approach circuit for a press often adds a small high-speed cylinder, a prefill valve from an overhead tank for a big press, and a pressure switch that changes to a slow, high-force stroke. A large pump flow can then be avoided: a two-pump unloading circuit has a small high-pressure pump and a large low-pressure pump that unloads when the pressure rises.
Sequencing and synchronizing
Sequencing makes one actuator move after another: with sequence valves (pressure-operated), limit switches and solenoid valves under PLC control (see PLC and Logic Control), or by position sensors. Synchronizing two cylinders to move together is harder, because leakage and loads differ: use a flow divider (a pair of gear motors on one shaft), cylinders with equal areas in series, or electronic control with position feedback and proportional valves.
Accumulators
An accumulator stores oil under pressure against a gas cushion (nitrogen) in a bladder, piston or diaphragm. It lets a small pump meet a high peak demand, keeps the pressure steady during a long hold, absorbs pulsation and shock, makes up for leakage and provides emergency power. The gas is pre-charged to a pressure p₀ that is about 90 percent of the minimum working pressure; as oil enters the gas is compressed (p Vn = constant, n = 1 for a slow isothermal process and 1.4 for a fast adiabatic one):
ΔV = V₀ [ (p₀ ÷ p₁)1/n − (p₀ ÷ p₂)1/n ]
Worked example: sizing an accumulator
Worked example: 2 liters between 161 and 201 bar
- The press cycle needs 2 L of oil for a quick movement at a minimum pressure of 160 bar gauge and a maximum of 200 bar gauge. Take the pre-charge at 90 percent of the minimum: p₀ = 0.9 × 160 = 144 bar gauge, 145 bar absolute.
- Fast (adiabatic, n = 1.4): V₀ = 2 ÷ [(145 ÷ 161)0.714 − (145 ÷ 201)0.714] = 2 ÷ 0.1360 = 14.7 L. Choose the standard 15 L accumulator.
- Slow (isothermal, n = 1) the same oil needs only 11.2 L: a fast discharge needs a larger accumulator, because the gas heats and the pressure rises more.
- The gas volume goes from 13.6 L at 161 bar to 11.6 L at 201 bar, a difference of 2.00 L. The stored energy released is 35.9 kJ (the work of the gas expansion).
- Use as a pump booster: a peak of 90 L/min for 4 s with a pump of 30.2 L/min means the accumulator must supply (90 − 30.2) × 4 ÷ 60 = 4.0 L, and a pump that is a third of the peak-flow size saves a large motor.
Safety: an accumulator stores a great deal of energy, so every one needs a safety block (a relief valve, an isolating valve and a manual or solenoid discharge valve); check the pre-charge regularly with a gauge and charge only with nitrogen, never air or oxygen (an explosion hazard). Always discharge and vent before work on a circuit (Troubleshooting and Safety). A shock of 45 bar can appear when a valve slams shut on a 4 m/s flow (Joukowsky: Δp = ρ c v; see Water Hammer), and a small accumulator near the valve cuts it.
Related
- Previous: Valves and Flow Control. Next: Fluids, Filtration and Reservoirs.
- Pneumatic and Hydraulic Actuators.
- Hydraulics and Pneumatics: the whole guide.
FAQ
Why pre-charge at 90 percent of the minimum pressure?
So the bladder or piston does not bottom out at the lowest working pressure, and the gas does most of its work in the working band. Too low a pre-charge wastes volume; too high leaves little delivery.
Can I use compressed air as the gas?
No. Hot compressed air with oil vapor can explode. Use dry nitrogen only.
What is the difference between sequence and counterbalance valves?
A sequence valve passes flow to the next actuator once a pressure is reached; a counterbalance valve holds a load against gravity and lets it down under pilot control.
How do I synchronize two cylinders?
Mechanically if you can (a rigid beam or a rack-and-pinion), with a flow divider for moderate accuracy, or with position sensors and proportional valves for precision.
References
- Esposito, A., Fluid Power with Applications, Pearson (circuits, accumulators); Parr, A., Hydraulics and Pneumatics, Butterworth-Heinemann.
- ISO 10100 (hydraulic accumulators: acceptance tests) and ISO 4413 (hydraulic safety requirements); EN 14359 (gas-loaded accumulators).
- Hydac, Parker and Bosch Rexroth accumulator catalogs and sizing guides.
Last reviewed: October 2026. Pre-charge and gas constants are typical.