What problems will be caused by an excessively soft sealing ring?

An excessively soft sealing ring can lead to sealing failure, equipment damage, shortened lifespan and increased maintenance costs, and these issues are particularly prominent in high-pressure, high-speed or special working conditions. Here is a detailed analysis:

I. Core issue of low hardness of sealing rings

Sealing performance failure

Increased leakage risk: Insufficient hardness leads to weak rebound force of the sealing ring after compression, unable to effectively fill the gap between the sealing surfaces (such as grooves, shaft hole mating areas), especially prone to medium leakage (such as hydraulic oil, gas) in high-pressure or dynamic sealing conditions.

Poor resistance to extrusion: In high-pressure conditions, low-hardness sealing rings may be squeezed into the sealing gap (such as between the piston and the cylinder), resulting in "shear damage" and damage to the sealing surface.

Low-temperature adhesion: In low-temperature environments, low-hardness rubber may adhere to the mating parts due to increased viscoelasticity, causing tearing (such as in the sealing of refrigeration equipment).

Equipment wear and damage

Component wear: When the hardness of the sealing ring is lower than that of the mating component (such as a metal shaft), the sealing ring material is prone to wear during dynamic friction, resulting in particle contamination of the system (such as oil contamination in the hydraulic system).

Self-tearing of the sealing ring: In reciprocating or rotating seals, the low-hardness sealing ring may cause stress concentration due to excessive deformation, leading to crack propagation (such as "screw failure" of an O-ring on a high-pressure rotating shaft).

Installation damage: Excessive compression or distortion of the low-hardness sealing ring during assembly may cause permanent deformation or structural damage (such as the flange of a lip seal).

Shortened lifespan and increased maintenance costs

Accelerated aging: Low-hardness sealing rings usually contain more plasticizers, which are prone to precipitate and harden under long-term high-temperature or chemical medium action, leading to rapid attenuation of sealing performance.

Frequent replacement: Sealing failure due to leakage or wear requires frequent shutdown for sealing ring replacement, increasing maintenance time and costs (such as sealing in chemical pipelines).

Associated equipment damage: Sealing failure may cause medium leakage, corrosion of surrounding components or contamination of products (such as hygiene problems caused by sealing failure in food processing equipment).

II. Common causes of low hardness

Material selection error

Incorrect selection of low-hardness rubber (such as natural rubber NR with hardness ranging from 40 to 60 Shore A) or excessive addition of plasticizers (such as phthalate esters), resulting in too low material elastic modulus.

Failure to consider working conditions: For example, using ordinary nitrile rubber (NBR) instead of high-hardness models (such as HNBR with hardness of 90 Shore A) in high-pressure hydraulic systems.

Sulfurization process defects

Insufficient sulfurization (short time or low temperature) leads to low rubber crosslinking density and unmet hardness requirements.

Deviation in sulfurizing agent or accelerator dosage, causing uneven hardness (such as surface hardness of an O-ring being lower than the interior).

Post-treatment and storage issues

Post-sulfurization (such as secondary sulfurization of fluorine rubber) with insufficient temperature or time not reaching the target hardness.

High-temperature and high-humidity storage environment causes rubber to absorb moisture or migration of plasticizers, resulting in hardness reduction (such as fluorine rubber becoming soft after long-term storage).

III. Solutions and optimization suggestions

Material and formulation adjustment

Increase hardness: Use high-hardness rubber (such as fluorine rubber FPM with hardness ranging from 75 to 90 Shore A) or reduce the amount of plasticizers (evaluating low-temperature performance is required).

Composite materials: Use rubber-metal composite structures (such as spring energy storage sealing rings) or surface coatings (such as polytetrafluoroethylene PTFE) to enhance resistance to extrusion.

Self-compensation design: Select structures that can automatically adjust sealing force through medium pressure (such as pre-tightening spring of a lip seal).

Process optimization

Sulfurization control: Determine the optimal sulfurization curve through Melt Index viscosity testing, monitor crosslinking density with infrared spectroscopy to ensure uniform hardness.

Post-treatment improvement: Perform sufficient secondary sulfurization (such as 230°C × 16 hours) for fluorine rubber and other materials to eliminate internal stress and stabilize hardness.

Storage management: Sealing rings should be stored in a dry, cool environment (temperature < 25°C, humidity < 60%), avoiding contact with solvents and oils.

Structural and size design Increase the cross-sectional size: If space permits, increase the diameter or thickness of the sealing ring to enhance the resistance to extrusion (for example, change the O-ring from Φ3mm to Φ5mm).

Optimize the groove design: Adopt anti-extrusion structures (such as retaining rings, trapezoidal grooves) or reduce the sealing gap (such as the hydraulic cylinder mating gap ≤ 0.1mm).

Dynamic sealing improvement: For rotational shaft sealing, replace with a combined seal (such as lip seal + dust cap) or magnetic fluid seal.

Operating condition adaptation

Pressure management: For high-pressure conditions, select pressure-resistant sealing rings (such as crescent-shaped rings) or add support rings (such as PTFE retaining rings).

Temperature control: For high-temperature conditions, use heat-resistant rubber (such as fluororubber FFKM); for low-temperature conditions, use low-temperature rubber (such as silicone rubber).

Media compatibility: Select corrosion-resistant materials based on the type of liquid/gas (such as ethylene propylene rubber EPDM for alkaline media).

IV. Case References

Seal failure of hydraulic cylinder: The original NBR O-ring (hardness 65 Shore A) in a certain construction machinery hydraulic cylinder was frequently extruded and damaged under high pressure (35 MPa). After replacing with a HNBR + PTFE retaining ring combination, the service life increased by 5 times.

Seal of automotive air conditioning compressor: The original silicone rubber sealing ring (hardness 40 Shore A) adhered and leaked at low temperature (-40℃). After replacing with fluororubber (hardness 75 Shore A), the low-temperature performance was significantly improved.

Seal of chemical pipeline: The sealing ring of an acid-base pipeline was corroded and softened due to insufficient hardness (50 Shore A). After replacing with a PTFE-coated rubber sealing ring, the corrosion resistance was improved.

V. Verification and Testing

Hardness test: Regularly inspect with a Shore hardness tester to ensure the hardness meets the standard (such as O-rings typically 60-90 Shore A).

Performance test: Conduct high-pressure, high-speed, and medium-temperature tests (such as ASTM D1414 standard test for O-ring performance).

Life assessment: Predict the service life of the sealing ring through accelerated aging tests (such as high-temperature and high-pressure cycling) to optimize the maintenance cycle.

Through comprehensive optimization of materials, processes, and design, the problem of low hardness of the sealing ring can be effectively solved, balancing sealing performance, wear resistance, and adaptability to operating conditions, reducing equipment failure rate and maintenance costs.

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