Dynamic Friction Coefficient Testing of Sealing Rings

Dynamic friction coefficient testing of sealing rings measures the sliding resistance of elastomeric and polymeric seals against mating surfaces under controlled load, speed, temperature, and lubrication conditions. This test is essential for evaluating seal performance in hydraulic cylinders, pneumatic actuators, automotive systems, and medical devices, where friction directly affects energy efficiency, wear life, stick-slip behavior, and equipment reliability. Accredited third-party laboratories perform these tests in accordance with recognized standards such as ISO 15113, ASTM D1414, and ASTM G115.

Technical Overview

Sealing rings, including O-rings, X-rings, U-cups, and rotary shaft seals, must maintain reliable sealing contact while allowing smooth relative motion between mating components. The dynamic friction coefficient (also called the kinetic friction coefficient) quantifies the resistance encountered when a seal slides against a counter-surface under a defined normal load. It is defined as the ratio of the friction force to the normal force acting at the contact interface, expressed as μ = F / N, where F is the measured sliding friction force and N is the applied normal load.

For rotary applications, the friction coefficient is typically derived from measured torque using μ = T / (R × N), where T is the friction torque, R is the effective contact radius, and N is the radial or normal load. Because friction in sealing systems depends on material formulation, surface roughness, lubrication, temperature, sliding speed, and compression level, dynamic friction testing must be performed under tightly controlled and reproducible conditions.

Relevant reference standards and guides include ISO 15113 (rubber — determination of frictional properties), ASTM D1414 (standard test methods for rubber O-rings), ASTM G115 (guide for measuring and reporting friction coefficients), ASTM D1894 (static and kinetic coefficients of friction), and applicable national standards in the GB series. Testing is commonly carried out by accredited third-party laboratories using universal tribometers, linear reciprocating friction testers, or rotary friction test rigs.

Testing Samples

O-Rings and X-Rings

Toroidal seals in standard AS568 or ISO 3601 sizes, molded from NBR, FKM, EPDM, silicone (VMQ), or fluoroelastomer compounds. Typical cross-sections range from 1.5 mm to 8.4 mm, and samples are tested in the as-molded condition after standard conditioning in a controlled atmosphere.

Reciprocating Rod and Piston Seals

Y-rings, U-cups, glyd rings, step seals, and composite piston seals used in hydraulic and pneumatic cylinders. These samples are tested against standard honed tubing or ground and hard-chrome-plated rods with defined surface roughness, typically Ra 0.2 µm to 0.8 µm.

Rotary Shaft Seals

Radial lip seals and PTFE-based rotary seals designed for rotating shafts, tested against a calibrated shaft mandrel at controlled speed, interference, and oil immersion level.

PTFE and High-Performance Polymer Seals

Seals produced from virgin PTFE, bronze-filled, carbon-filled, or glass-filled PTFE, polyurethane (PU), and other engineering polymers, frequently selected specifically for their low friction characteristics.

Flat Material Specimens

When finished seals cannot be mounted in the test fixture, flat plaques or molded slabs cut from the same compound are tested against standard counter-surfaces such as polished steel, anodized aluminum, glass, or engineering plastics.

Testing Items

• Dynamic (kinetic) friction coefficient under steady-state sliding conditions
• Static (breakaway) friction coefficient and breakaway force
• Friction force versus sliding distance or time profiles, including stick-slip tendency evaluation
• Friction coefficient as a function of sliding speed (e.g., 0.1 mm/s to 500 mm/s)
• Friction coefficient as a function of normal load or squeeze (compression) ratio
• Temperature-dependence of friction, typically from -40 °C to +200 °C depending on the elastomer class
• Friction behavior under dry, oil-lubricated, water-based, or grease-lubricated conditions
• Effect of fluid exposure, thermal aging, or compression set on frictional performance
• Frictional stability over extended cycle counts, including running-in behavior
• Combined friction and wear assessment, such as mass loss and surface damage after sliding

Testing Methods

Linear Reciprocating Friction Test

The sealing ring is mounted in a fixture that applies a defined normal load or compression squeeze against a flat counter-surface or a rod. The counter-surface is driven in a reciprocating or unidirectional linear motion at a controlled speed while a calibrated load cell continuously records the friction force. The dynamic friction coefficient is calculated from the averaged steady-state friction force divided by the normal load. This method is the most widely used approach for O-rings and reciprocating seals and is consistent with the principles of ISO 15113 and ASTM G115.

Rotary Friction Test

The seal is installed on a rotating mandrel or shaft with defined interference fit and axial loading. The test rig rotates the shaft at a specified speed while the reaction torque is measured with a torque sensor. The friction coefficient is derived from the torque, the effective sealing lip contact radius, and the applied radial load. This configuration is suitable for rotary shaft lip seals and PTFE rotary seals.

Cylinder Bench Simulation Test

A full-size seal assembly is installed in an instrumented hydraulic or pneumatic cylinder test bench operating under realistic pressure, stroke, speed, and temperature conditions. Friction force is measured directly from actuator force measurements, providing the most representative data for design validation, although with higher scatter than coupon-level tests.

Standard Test Conditions and Procedure

A typical test sequence includes the following steps:

• Conditioning of specimens for at least 16 hours at 23 °C ± 2 °C and 50% ± 5% relative humidity in accordance with ISO 23529 or ASTM D618
• Measurement and recording of specimen dimensions, hardness, and counter-surface roughness before testing
• Application of the specified normal load, squeeze ratio (commonly 8% to 25% for O-rings), and test medium (dry or lubricated)
• Execution of a running-in phase followed by the measurement phase at constant sliding speed
• Continuous acquisition of friction force or torque, with the dynamic friction coefficient calculated from the mean steady-state values
• Post-test examination of the seal and counter-surface for wear, abrasion, and surface damage
• Reporting of the friction coefficient, test conditions, environmental parameters, and any observed anomalies

Typical Test Results

Representative dynamic friction coefficient ranges observed under common conditions include the following:

• NBR or FKM against polished steel, lubricated: approximately 0.05 to 0.20
• NBR or FKM against polished steel, dry: approximately 0.5 to 1.5
• Silicone rubber against steel, dry: approximately 0.4 to 1.0
• Virgin PTFE against steel: approximately 0.05 to 0.15
• Filled PTFE against steel, lubricated: approximately 0.03 to 0.10

Actual values vary significantly with compound formulation, surface finish, lubrication state, and test parameters, which is why standardized comparative testing is essential.

Applications

Dynamic friction coefficient testing supports design, qualification, and quality control across a broad range of industries:

• Hydraulic and pneumatic systems: piston and rod seals for cylinders, valves, and actuators, where low and stable friction improves efficiency and control precision
• Automotive engineering: brake components, shock absorbers, power steering systems, engine and transmission seals, and fluid handling circuits
• Medical and pharmaceutical devices: syringe plungers, stoppers, infusion pump components, and inhaler seals requiring controlled breakaway and gliding forces
• Aerospace: actuator seals, landing gear components, and fuel system seals with stringent friction and wear requirements
• Food and beverage equipment: seals for filling machines and processing equipment requiring low friction and clean running behavior
• Semiconductor and precision equipment: low-outgassing, low-friction seals for vacuum systems and precision motion stages
• Material development: comparison of elastomer compounds, filler systems, surface treatments, and coatings for friction optimization

FAQ

What is the difference between static and dynamic friction coefficient of a sealing ring?

The static (breakaway) friction coefficient describes the resistance at the instant motion begins, while the dynamic (kinetic) friction coefficient describes the resistance during continuous sliding. In sealing systems, static friction is usually higher than dynamic friction, and a large difference between the two promotes stick-slip behavior, which is undesirable in precision motion applications.

Which factors most strongly influence the test results?

The dominant factors are the elastomer compound and hardness, the counter-surface roughness and material, the lubrication condition, temperature, sliding speed, and the applied squeeze or normal load. Rigorous control and reporting of these parameters are required for meaningful and comparable results.

Can tests be performed at elevated or sub-zero temperatures?

Yes. Environmental chambers allow testing across a wide temperature range, commonly from -40 °C to +200 °C, matching service conditions for automotive, aerospace, and industrial sealing applications. Specialized setups can extend this range further depending on the material class.

How many specimens are required for a reliable measurement?

A minimum of three to five specimens per condition is generally recommended to account for the natural scatter of polymeric materials. Each specimen is typically tested over multiple cycles so that both running-in transients and steady-state behavior are captured.

How should seals be prepared before testing?

Specimens should be freshly molded or as supplied, free from defects, release agents, and contamination. They must be conditioned at standard laboratory atmosphere, and their dimensions and hardness should be verified before testing to confirm compliance with the specified requirements.

What deliverables are provided after testing?

A typical test report includes the test method and standards applied, specimen identification, test conditions (load, speed, temperature, lubrication, counter-surface), measured friction force or torque traces, calculated static and dynamic friction coefficients, statistical summary, and observations of wear or surface damage, all issued under the laboratory's accredited reporting framework.