MANUFACTURING / JAPAN SOURCING

Same rubber hardness: what must match before changing a seal compound?

Two O-rings can share dimensions and nominal hardness while leaving different questions about the fluid, temperature and recovery after compression. Compare identified compounds in their actual sealing conditions.

Start with the sealed joint, not the hardness number

A buyer seeks an alternative O-ring for a stationary cover on a water/glycol circulation housing. Both offers have the same nominal Shore A hardness and dimensions. That narrows the comparison, but it does not establish that the proposed compound will work in this joint.

Parker’s selector treats operating temperature, polymer group, hardness and fluid compatibility as separate inputs, then considers the hardware configuration.[1] Send the fluid formulation or product reference, concentration, temperature range and excursions, pressure history, cleaning exposure and joint drawing. Mark unknowns rather than substituting a generic “water service” description.

Three records behind a seal-compound comparison

1Seal: compound reference, dimensions and hardness test
2Joint: groove, compression and pressure conditions
3Exposure: process fluid, cleaning, temperature and duration

The ring, joint section and exposure record are illustrative. The drawing gives no groove dimensions, compression recommendation or approved material.

Ask for the compound reference and the evidence it identifies

A polymer name such as NBR or EPDM is a material family, not a complete purchasing reference. Ask for the supplier’s exact compound designation, relevant drawing revision and the data that apply to the offered finished seal. Keep the hardness scale, test method, tolerance and specimen basis with the result.

Hokusei Rubber describes formulation design alongside separate mechanical and physical properties.[3] For this enquiry, ask the supplier which evidence supports exposure to the specified water/glycol product and cleaning fluid. Do not infer compatibility from hardness or choose a substitute solely from a generic polymer chart.

Read compression-set results with their conditions

Compression set describes deformation remaining after a defined compression and recovery process. Parker identifies specimen geometry, deformation, time, temperature and contact medium among the factors needed for a valid comparison.[2] A lower percentage in one data sheet cannot rank two compounds when those conditions differ.

For two candidate reports, request the method/version, finished O-ring or test specimen, dimensions, imposed compression, medium, temperature, duration and recovery/readout conditions. If they differ, ask for comparable evidence or an explanation of the limitation. A laboratory percentage alone does not approve sealing performance or predict service life.

Evidence to compare for the stationary-cover example
Record	Keep with each candidate	Question still to resolve
Identity	Compound code, drawing, lot and hardness method/tolerance	Does the evidence concern the offered seal?
Exposure	Named fluid, concentration, temperature/time and cleaning	Does the evaluation represent the actual exposure?
Compression set	Method, specimen, deformation, time, temperature, medium and recovery	Are the reported percentages comparable?
Joint	Groove, mating surfaces, assembly and pressure history	Has the candidate been assessed in this joint?
Decision	Required trial, criteria, owner and release record	Who accepts the change and what remains open?

Evaluate the changed compound in the intended assembly

Ask the responsible engineering team which material, dimensional and functional checks are needed for this change. For the cover seal, the plan may need to address assembly, leakage under the agreed conditions and the effects of exposure over time. Agree methods, sample numbers and acceptance criteria before the trial.

Keep the joint design and operating history visible even if the old seal worked for years. A different compound can require a fresh assessment of the existing installation. This article does not select a compound or provide a universal test duration, squeeze value or leakage limit.

Carry the accepted reference into repeat orders

The purchase record should identify the accepted compound and part revision, evidence reviewed, any exceptions and who approved the change. Ask how the supplier will identify lots and notify changes to formulation, manufacturing or relevant source materials.

If all you have is a used seal and a hardness reading, begin by identifying the part and its operating conditions. Keep that investigation separate from approving a new compound as interchangeable.

Questions to send your supplier

Open supplier questions

Application: stationary-cover O-ring for a circulation housing
Part/drawing revision, groove and mating surfaces:
Existing and proposed compound references:
Hardness scale, method, tolerance and specimen basis:
Actual fluid/product, concentration, temperature/time, pressure and cleaning:
Please identify evidence for the offered compound under these exposures.
For compression set, provide method/version, specimen, deformation, medium, temperature, duration and recovery conditions.
Which assembly/functional trial remains, with what criteria and decision owner?
Quantity, destination, timing and change-notification arrangement:

Save questions ↓

Sources

Parker — O-Ring Selector
Public selection inputs distinguish temperature, polymer, hardness, fluid and hardware. No candidate compound selected or certified.
Parker — O-Ring Handbook, section 8.6, printed pages 94–95
Compression-set definition and conditions for comparing results. No graph values, design limits or lifetime predictions adopted.
Hokusei Rubber — Investigation items
Development scope includes formulation and distinct mechanical/physical properties. The page has an inconsistent NR/nitrile label, which is not reproduced. No universal material compatibility or temperature claims adopted.