Hydraulic Rod Airtightness Testing: Standards, Methods, and Applications

Hydraulic rod airtightness testing evaluates the sealing integrity of gas-charged struts, gas springs, and hydraulic cylinder rods by quantifying gas leak rates under controlled pressure and temperature conditions. This guide explains the core testing principles, applicable standards such as ISO 20486, EN 1593, ASTM E515 and GB/T 25751, typical sample types, and industry applications. Use it to select the right leak detection method and acceptance criteria for your hydraulic rod products.

Technical Overview

Hydraulic rods—most commonly manufactured as gas springs, gas struts, and damper rods—are precision actuators whose output force depends on a hermetically sealed charge of high-pressure nitrogen, typically between 5 MPa and 20 MPa, supplemented by a small volume of hydraulic oil for end-of-stroke damping and seal lubrication. Because the force characteristic is directly proportional to the internal gas charge, even a microscopic leak causes progressive force decay, resulting in sagging tailgates, slow-lifting hoods, descending chair columns, and eventually complete functional failure.

Airtightness testing, also referred to as gas tightness testing or leak testing, verifies that the sealed pressure chamber, rod seal package, guide bearing, weld or crimp closures, and gas charging valve do not permit gas escape beyond a defined threshold over the expected service life. In line with EN 1779, which provides criteria for the selection of leak testing methods and techniques, the test method sensitivity must be matched to the permissible leak rate of the product, which for gas springs typically falls within the range of 10⁻⁴ to 10⁻⁶ Pa·m³/s depending on the application and charge volume.

Typical leak paths that airtightness testing is designed to detect include:

• The dynamic rod seal and wiper at the piston rod guide.
• The crimped or rolled closure between the pressure tube and the end guide.
• Weld seams on end plugs and eyelet fittings.
• The gas charging valve orifice and its micro-seal.
• Corrosion-induced pinholes in the pressure tube wall.

Testing Samples

Automotive Gas Struts

Tailgate, trunk lid, hood, and sliding door struts from passenger cars, commercial vehicles, and heavy trucks. These samples are usually tested as finished assemblies at nominal charge pressure, both in new condition and after endurance cycling, because automotive OEM specifications often require leak verification after 20,000 to 100,000 stroke cycles.

Office Chair Gas Lift Cylinders

Class 3 and Class 4 seating gas lifts, which operate at internal pressures up to approximately 15 MPa. Airtightness testing of these double-walled cylinders focuses on the slow creep leakage that causes chairs to sink under load, and is frequently combined with load retention measurement at elevated temperature.

Industrial Hydraulic Cylinder Rods and Piston Rods

Sealed rods, cartridges, and accumulators used in fluid power systems, agricultural machinery, and construction equipment. Samples may be tested as bare rods with mounted seals or as complete cylinder assemblies connected to a pressure source through test ports.

Furniture, Medical, and Special-Purpose Gas Springs

Lockable gas springs for hospital beds and rehabilitation equipment, aircraft cargo and access panel struts, railway compartment dampers, and furniture flap stays. These applications impose stricter leak rate limits because slow force loss can create pinch hazards or operational failure in safety-relevant positions.

Sample Preparation Requirements

• Samples must be clean, dry, and free of surface oil film that could temporarily seal micro-leak paths.
• Units should be stabilized at laboratory temperature, typically 23 °C ± 5 °C, before measurement.
• For type testing, a statistically valid sample set is drawn from production batches; for end-of-line screening, 100% of units are tested.
• Returned or field-failed units should be tested before any disassembly to preserve leak evidence.

Testing Items

Depending on the product specification and stage of development, hydraulic rod airtightness evaluation covers the following measurable items:

• Gas leak rate: the quantity of gas escaping per unit time, expressed in Pa·m³/s, mbar·L/s, or sccm.
• Pressure retention / pressure decay: the reduction of internal charge pressure over a defined dwell period, expressed as a percentage drop or in MPa per 24 hours.
• Force retention: the drift of extension force F1 and compression force F2 over time, measured in accordance with GB/T 25751 for gas springs.
• Post-endurance leak performance: leak rate verification after accelerated stroke cycling to simulate wear of the dynamic seal.
• Temperature cycling leak test: leak measurement after exposure to cold and hot environments, commonly −40 °C to +80 °C, to reveal seal contraction and lubrication breakdown effects.
• Oil seepage assessment: visual and gravimetric inspection for hydraulic oil migration along the rod, which indicates seal lip damage even when gas leakage remains within limits.

Testing Methods

Water Immersion Bubble Test

The simplest and most widely used method, standardized in ASTM E515 and EN 1593. The pressurized rod is submerged in a transparent tank of temperature-stabilized water or glycol solution, and trained operators observe the emission of bubble streams from leak sites over a defined observation period. Sensitivity reaches approximately 10⁻⁴ Pa·m³/s, which is adequate for screening gross leaks in gas struts, and the method simultaneously pinpoints the leak location for corrective action.

Pressure Decay Test

The internal pressure of the rod is measured directly, or monitored through a temporary charging connection, over periods from several hours to 28 days. The measured pressure drop is converted into a leak rate using the known charge volume. This method directly reflects the functional failure mode of gas springs—loss of charge—and underpins the force retention requirements of GB/T 25751.

Differential Pressure Leak Test

A highly sensitive indirect method in which the test part and a reference volume are pressurized simultaneously, and a differential pressure transducer measures the divergence between them. With temperature compensation, this technique detects leak rates down to approximately 10⁻³ Pa·m³/s within cycle times of under one minute, making it suitable for automated 100% end-of-line inspection.

Helium Mass Spectrometry Leak Detection

The most sensitive technique available, capable of detecting leak rates down to 10⁻⁹ Pa·m³/s. Two configurations are applied: in the tracer-probe (sniffing) mode, helium charged into the rod during manufacture is detected at the exterior surface to localize leaks; in the vacuum chamber (inside-out) mode, the entire rod is enclosed in an evacuated chamber and the total helium leakage is integrated, enabling rapid whole-part screening. For fully sealed units without a charging valve, the helium bombing method—pressurizing the part in a helium atmosphere before testing—is used. Reference leaks used for instrument calibration are traceable to ISO 20486.

Force Retention and Load Decay Test

As a functional complement to direct leak measurement, the output force of the strut is recorded at defined intervals while the unit is stored at ambient or elevated temperature. A force loss exceeding typically 5% to 10% of the initial value indicates unacceptable charge loss even when the leak is too small to observe by bubble testing.

Method selection follows the guidance of EN 1779, ASTM E432, and EN 13625, balancing required sensitivity, cycle time, access to the internal pressure volume, and destructive versus non-destructive evaluation.

Applications

Hydraulic rod airtightness testing supports quality assurance and product development across multiple industries:

• Automotive OEM and tier suppliers: end-of-line screening of tailgate and hood struts, durability validation, and PPAP-type approval testing.
• Office and contract furniture: verification of gas lift columns for seating, including import compliance inspections.
• Medical and care equipment: leak testing of lockable gas springs for hospital beds, patient lifts, and operating table actuators.
• Railway, marine, and aerospace: certification of access panel struts and dampers exposed to wide temperature and pressure ranges.
• Industrial machinery: incoming inspection of cylinder rods, accumulators, and gas-charged cartridges.
• Failure analysis: accredited third-party laboratories apply helium tracer and pressure decay methods to diagnose field failures, support warranty claims, and resolve supplier disputes.

FAQ

What leak rate is acceptable for a hydraulic rod or gas spring?

Acceptance limits depend on charge volume and service life targets. As a general guideline, automotive gas struts are commonly limited to leak rates below 10⁻⁴ to 10⁻⁵ Pa·m³/s, corresponding to a pressure loss of no more than a few percent per year, while safety-critical applications may require helium testing to the 10⁻⁶ Pa·m³/s range. The limit should always be derived from the functional force retention requirement specified by the customer.

How can fully sealed struts without a charging valve be leak tested?

The helium bombing method is used: the finished rod is pressurized in a helium atmosphere so that helium penetrates any leak path, after which the part is placed in a vacuum chamber connected to a mass spectrometer. Escaping helium indicates a leak, allowing sealed production parts to be screened without drilling or disassembly.

What is the difference between airtightness testing and hydraulic pressure testing?

Airtightness testing quantifies gas leakage through seals and joints under normal or slightly elevated charge pressure and is evaluated in terms of leak rate. Hydraulic pressure testing, including proof and burst tests, verifies structural strength and plastic deformation resistance at pressures of 1.5 to 4 times the working pressure. Both are complementary: a rod can be structurally sound yet fail airtightness due to a defective dynamic seal.

At what pressure is the leak test performed?

End-of-line bubble and pressure decay tests are normally performed at the nominal nitrogen charge pressure of the rod. For structural verification in combination with leak detection, a proof pressure of approximately 1.5 times the working pressure may be applied before the leak measurement.

How does temperature affect airtightness results?

Cold temperatures reduce seal elasticity and can open micro-gaps, while elevated temperatures lower gas viscosity and increase seal permeation. Temperature-stabilized test enclosures and differential pressure compensation are therefore essential, and type testing should always include leak measurement after thermal cycling between −40 °C and +80 °C.

How long does a typical test take?

Bubble tests require 1 to 5 minutes of observation, automated differential pressure tests are completed in under one minute per part, and helium vacuum chamber tests take only a few minutes. Pressure retention and force decay tests for type approval extend from 24 hours to several weeks depending on the specification.