EuroBLECH 2026 (Hannover, 20–23 October) puts presses, press brakes and forming lines in the spotlight. Yet the accumulator is often one of the least deliberately specified parts of a press hydraulic system. This guide gives press builders and plant engineers a short decision path: when a press needs an accumulator, which type fits, how to set precharge by function, and what to specify for safety blocks and PED/CE documents before sending an enquiry.
Key Engineering Takeaways
- Start with the duty profile: Define required flow, usable fluid volume, pressure range and cycle timing before choosing an accumulator. Press tonnage alone is insufficient.
- Choose construction by duty: Compare bladder and piston designs using discharge flow, cycling, usable volume and monitoring needs. Product capacities depend on pressure and configuration.
- Set precharge by function: Energy storage, pulsation damping and shock absorption require different application checks. Use nitrogen and the selected manufacturer’s guidance.
- Specify safety and documents: Define isolation, overpressure protection and controlled depressurization, and request conformity and traceability records for the actual supplied configuration.
Why Do Presses Use Hydraulic Accumulators?
A hydraulic press accumulator stores energy in compressed gas and releases hydraulic fluid when the circuit requires it. Whether a press needs one depends on its duty profile, pump and valve arrangement, cycle time and pressure requirements. Four common duties are worth separating before selecting a vessel.
1. Peak flow during approach or return
A press may need brief flow above the pump’s continuous delivery during a rapid approach, return or auxiliary motion. An accumulator can supply part of that transient demand, subject to the required pressure range, fluid volume and discharge time. The sizing input is the flow-versus-time profile at the actuator, not tonnage alone. SCHWERLL’s high-flow forming system for hydraulic presses uses an accumulator power source on the rapid return and pressing strokes to shorten cycle time; typical applications include SMC/LFT presses, tryout and spotting presses, hybrid-drive hot-forming presses for high-strength steel, servo presses and wire-wound ultra-high-pressure presses.
2. Pressure shock during switching or unloading
Rapid valve switching, deceleration or unloading can create pressure transients. An accumulator may help reduce pulsation or cushion a transient when it is placed and sized for that circuit. It does not replace correct valve selection, line sizing, pressure relief or transient analysis.
3. Pump pulsation
Where pump ripple affects pressure stability, noise or process quality, an accumulator can act as a pulsation damper. The required response depends on pump type, ripple frequency, line layout, fluid and operating pressure; it should be treated as a damping application rather than assumed to be an energy-storage duty.
4. Make-up flow during holding
During a pressure-hold phase, an accumulator can supply limited make-up flow as the circuit compensates for leakage or small volume changes. Confirm the permissible pressure band, hold duration and leakage rate first. It is not a substitute for correcting excessive leakage.
Press Application Map: Start With the Machine Duty
| Machine/application | Typical question for the hydraulic design | Accumulator consideration |
|---|---|---|
| Stamping and hot-forming presses | Is extra flow needed during approach, forming or return, and for how long? | Compare the measured flow-time curve and pressure window with pump and valve capacity. |
| Press brakes | Is the hydraulic axis pump-driven, or is it an electric servo-pump architecture? | Many modern press brakes use servo-driven hydraulic systems; an accumulator is not automatically required. Verify the circuit and the role it would perform. |
| SMC/LFT and tryout presses | Does the cycle need a short high-flow phase, pressure stabilization or reserve volume? | Define each motion and hold phase separately; do not size from press force alone. |
| Die-casting injection | What are the injection-stage flow, pressure and discharge-time requirements? | Shot-end injection is typically powered by a piston accumulator system, and the injection cylinder rod must reach about 8–12 m/s. Integrated (giga) casting is pushing system pressures higher, so piston accumulators with energy monitoring are preferred. See the die-casting accumulator system. |
| Forging and extrusion presses | Is the project a high-flow, short-duration discharge duty? | See sizing a 3,200 L piston accumulator station for a large-flow example; for fast forging, a larger accumulator volume can reduce installed power (fast-forging system). |
Bladder or Piston on a Press? Five Decision Rules
Neither construction is universally better. Begin with port velocity and cycle duty, then consider transient flow, required volume and monitoring. For the general bladder/piston/diaphragm comparison, see our full comparison guide; the rules below are press-specific.
- Oil-port velocity: Oil-port velocity approaching or exceeding 7 m/s is a known factor that shortens bladder life. High-flow press circuits at or above this level should be reviewed for a piston design or a larger port/multiple units.
- Very short cycling: In energy-storage duty, a single charge/discharge cycle of about 10 seconds or less also shortens bladder life – relevant to high-stroke-rate stamping lines.
- Fast discharge and poppet closure: During rapid oil discharge, a bladder accumulator’s poppet valve can close prematurely, so the accumulator temporarily stops delivering. Assess the actual flow rate, port geometry and anti-extrusion/poppet arrangement with the manufacturer; do not apply this as a blanket rule to every model.
- Single-unit volume: For large-volume duties, assess bladder service life together with discharge flow, cycle timing and the selected model’s approved operating envelope. A 60 L value should not be treated as a universal bladder-accumulator capacity limit. SCHWERLL’s published bladder-accumulator range includes BAKM units up to 250 L and BAKH units up to 350 L for high-pressure service or 570 L in low-pressure configurations. Confirm the allowable pressure-volume combination for the selected model. Where a press needs more stored volume, compare several bladder units in parallel, a large-volume bladder series, or a piston accumulator with gas bottles.
- Position or energy feedback: Where the control system must monitor stored fluid volume or piston position, a piston accumulator is the better fit: SCHWERLL piston accumulators can be configured with gas-energy and fluid-level monitoring. Confirm the sensor option for the selected model.
| Selection input | Bladder accumulator may fit when… | Consider a piston accumulator when… |
|---|---|---|
| Duty | The cycle and discharge rate are within the selected model’s approved envelope. | The duty is highly dynamic or requires a defined displacement profile. |
| Required volume | One vessel or a bank fits the available space and approved product range. | Required usable volume, monitoring or configuration points toward a piston design. |
| Monitoring | Pressure indication is sufficient for the control objective. | Piston position or stored-volume feedback is required and supported. |
| Final check | Confirm fluid, temperature, pressure, precharge and flow limits against the data sheet. | Confirm seal, sensor, pressure, stroke and mounting limits against the data sheet. |
Set Precharge for the Accumulator’s Function
Precharge is not a single number that can be copied from one press to another. First classify the duty as energy storage, pulsation damping or shock absorption, then use the accumulator manufacturer’s guidance and the system’s actual operating envelope.
Energy storage
For bladder accumulators in energy-storage duty, SCHWERLL’s published standard bladder accumulator data sets these limits:
- Precharge at maximum operating temperature: P₀ ≤ 0.9 × P₁ (minimum working pressure).
- Pressure ratio P₂ : P₀ ≤ 4 : 1 (P₂ = maximum working pressure).
- Precharge with nitrogen only.
These are selection constraints, not a substitute for calculating usable fluid volume and checking the selected model.
Pulsation damping and shock absorption
For damping duties, precharge depends on the mean/system pressure, ripple or transient profile, accumulator location, gas volume and response frequency. Use the selected manufacturer’s application method and validate the circuit response. Do not reuse an energy-storage ratio as a universal pulsation or shock rule.
Common precharge mistakes
- Setting precharge from nominal system pressure without checking minimum operating pressure.
- Measuring precharge while the oil side is still pressurized.
- Ignoring gas temperature and the effect of operating temperature on measured precharge.
- Applying the same precharge method to energy storage, pulsation damping and shock absorption.
- Using an unverified pressure basis or an unsuitable gauge/charging adapter.
- Treating a calculated preliminary value as an approved commissioning value.
For calculation, checking and charging procedures, see SCHWERLL’s guides to nitrogen precharge calculation and checking, nitrogen charging kit and procedure, and common precharge mistakes. Follow the applicable product manual and site safety procedure.
Safety and Shut-Off Blocks: Specify the Functions
An accumulator circuit needs a defined means of isolation, pressure relief, controlled depressurization and pressure checking. Select the safety and shut-off block for the accumulator, circuit pressure, flow, porting and applicable machine risk assessment. SCHWERLL’s accumulator safety and shut-off valve block page should be used for the current product-specific configuration.
SCHWERLL’s SA-type oil-side block combines the block body, an integrated relief valve, a main shut-off valve, a manual or solenoid-operated unloading valve and a gauge port. It is offered in DN10/20/32/50, with relief settings such as 220, 250, 315, 330, 350 and 400 bar (22–40 MPa); the available setting depends on size and configuration.
For gas-side service, identify the charging and pressure-checking block, compatible gas valve and specified adapters. SCHWERLL’s charging and inspection block is rated 350/400 bar (DN10/20/32, G3/4 charging valve), and gas safety valves and rupture discs can be supplied with third-party test certificates (e.g. TÜV, BV, ASME). A gas charging/checking component is not automatically a substitute for an overpressure safety device; confirm each component’s certified function and pressure rating from its own documentation.
The design review should also cover the gas-side safety device (for example, a certified relief device or bursting disc where required), safe discharge routing, isolation and lockout, controlled pressure release, and electrical interlocks. Define the emergency-stop sequence at machine level: how the pump, directional valves and stored hydraulic energy behave must be assessed together. Do not assume an emergency stop automatically makes an accumulator circuit pressure-free.
For the selected accumulator system, identify the applicable design code and conformity route. Specify the required shut-off and pressure-relief functions, test connection, pressure-release device and suitable pressure gauge according to the selected configuration and applicable project requirements.
For inspection and maintenance procedure detail, use the existing metal-forming accumulator maintenance and safety guide; this article focuses on selection and procurement.
PED and CE Documents: What European Buyers Should Request
The applicable conformity route depends on the equipment classification, design and intended market. For a European press project, ask the supplier to identify the applicable legislation and conformity scope for the actual accumulator and assembly, then provide the records required for that configuration. A supplier’s general statement of “PED/CE compliant” is not enough to establish the status of a particular vessel or system.
For reference, SCHWERLL’s high-parameter piston accumulators include a PED EU CE series (CE-stamped, up to 690 bar / 1,500 L as series limits), an ASME series and an SAK series to TSG 21; the conformity module and notified-body details are confirmed per order.
A procurement file may include, as applicable:
- EU Declaration of Conformity for the supplied item or assembly, identifying the applicable legislation and standards.
- PED classification rationale, fluid group, category and conformity-assessment module, with the notified body details where applicable.
- Product nameplate and traceability details tied to the delivered serial number.
- Material certificates (for example, EN 10204 type 3.1) for the specified pressure-retaining materials, where required by contract and conformity route.
- Pressure or hydrostatic test records and final inspection documentation for the supplied configuration.
- Safety-valve or other protective-device certificates and calibration/test records, where applicable.
- Assembly drawings, operating instructions, safe isolation/depressurization procedure and maintenance information.
This is a buyer’s request list, not a legal determination that every listed record applies to every accumulator. Confirm the applicable PED route, standards, module and documentation with the responsible conformity-assessment body and the machine integrator.
RFQ Checklist: Send Us Your Press Duty Profile
To assess a hydraulic press accumulator, send the following information with your enquiry:
- Machine type and press tonnage.
- Accumulator function: peak-flow supply, energy storage, pulsation damping, shock absorption or pressure-hold make-up.
- Minimum, normal and maximum working pressure, plus any relevant transient pressure.
- Required fluid volume change (ΔV) or peak flow, including the pressure range over which it is needed.
- Cycle timing: stroke/phase durations, discharge time, cycles per hour and hold time.
- Hydraulic fluid and minimum/normal/maximum operating temperature.
- Mounting arrangement, orientation, available space and hydraulic/gas connection interfaces.
- Safety-block and instrumentation requirements, including isolation, relief, pressure monitoring and position feedback.
- PED/CE or other market-specific conformity and documentation requirements.
- Quantity and project status: new machine, retrofit or replacement of an existing unit.
Send the duty profile through SCHWERLL Contact. We will review your data and reply with a technical proposal; commercial terms are confirmed in the formal quotation. You can also run a first estimate with our online volume calculator.
Frequently Asked Questions
Does every hydraulic press need an accumulator?
No. The need depends on the machine’s circuit and duty profile. Define the required flow, pressure window, cycle timing and accumulator function, then compare them with pump, valve and actuator capability. For a SCHWERLL selection enquiry, provide these operating inputs rather than press tonnage alone.
Is a press brake accumulator always required?
No. Some press brakes use servo-pump hydraulic architectures and may not need an accumulator for the intended duty. Check the specific circuit and determine whether it needs stored energy, damping or reserve flow.
Should I choose a piston accumulator for a press?
Consider a piston accumulator when usable volume, dynamic discharge or piston-position feedback makes it a suitable choice. SCHWERLL piston accumulators can be configured with gas-energy and fluid-level monitoring; confirm availability for the selected model. Bladder designs remain an option within their approved flow, cycle, pressure and temperature limits.
Bladder or piston for a die-casting shot end?
Shot ends are typically powered by piston accumulator systems because they need high, controlled discharge and energy monitoring. For a SCHWERLL die-casting system enquiry, provide the injection-stage flow, pressure, usable fluid volume and discharge-time profile so the proposed configuration can be reviewed.
Which documents should I request for a PED accumulator?
Request the conformity and traceability documents applicable to the actual vessel or assembly, including its PED category and assessment route where relevant, plus material, test and protective-device records specified for the project. Ask SCHWERLL to confirm the documentation scope for the supplied configuration with the responsible integrator; a general PED/CE statement does not establish the status of an individual vessel.