Writing a Bearing Vibration Acceptance Clause: ISO 15242 Grading vs Dimensional Precision Class

Keep the dimensional precision class out of the vibration clause

The most expensive drafting error is to write P5, ABEC-5 or an ISO 492 class into the vibration clause as if it were a vibration grade. Dimensional tolerance classes regulate allowable deviation in bore and outside diameter, ring width, roundness of rings and raceways, and running accuracy such as radial and axial runout^3,5,17.

None of that is a vibration limit. Vibration acceptance is a separate system.

Manufacturers publish vibration classification levels per ISO 15242 for common series, grouping bearings into quietness grades by band-limited velocity²⁶. A dimensional class and a quietness grade therefore classify different things, and the sources support no mapping from one to the other.

If your drawing says P5 and your clause says nothing about velocity limits, you have specified geometry, not quietness. Write the two as separate lines in the RFQ: the dimensional class you need for fit and running accuracy, and the vibration limit set you need for quietness.

Treat any quotation that answers the vibration question with a precision class alone as unanswered.

Make acceptance a pass/fail against limits agreed before the order

Acceptance under ISO 15242 is described as a comparison exercise against limits the parties agree in advance²⁶. That definition is what makes the clause enforceable: the bearing is not judged against a grade name, but against a written set of limits, band by band and parameter by parameter.

The test report is expected to state which limit set was applied and whether each band and parameter passed²⁶. This is the document you should ask for at goods-in.

A certificate that quotes a quietness grade without the limits behind it cannot be checked against your purchase order, because the grade label alone does not tell you what was measured or where the line was drawn. So build the clause backwards from the report.

Decide now which bands and parameters you will require a pass on, put those limits in the RFQ as an annex, and make the report's statement of the applied limit set a condition of acceptance.

Name the measurement quantity: velocity in bands, with acceleration and peak where noise matters

Band-limited velocity is the quantity the cited source describes as the basis for ISO 15242 quietness grading²⁶. That makes velocity in defined bands the default quantity to name in your clause.

Acceleration and peak are not an alternative to velocity. Purchasers in noise-sensitive sectors often specify a stricter grade or add acceleration and peak requirements on top of velocity limits²⁶.

The two sit together: velocity bands carry the grade, and acceleration or peak criteria tighten it where the application is audibly sensitive. General machine condition monitoring also draws on acceleration, velocity and displacement together, which is consistent with using more than one quantity, though that practice is monitoring rather than bearing acceptance⁷.

If your application is not noise-sensitive, velocity bands alone give you a workable clause. If it is, add the acceleration and peak criteria explicitly rather than assuming a stricter grade covers them.

What to demand in writing before the RFQ closes

The sources establish that limits are agreed in advance and that grading is velocity-based, but they do not supply the part numbers, bands, numeric limits, rig settings, calibration rules or noise conditions. Each item below has to be obtained from the ISO 15242 text or the supplier and verified, not assumed.

• ✓Ask the supplier to state which ISO 15242 part applies to your bearing type and which parameters it covers.
• ✓Require the frequency bands and units, and whether values are RMS, peak or both, in the quotation.
• ✓Require numeric velocity limits for your exact size and series, not a grade label alone.
• ✓Require the rig settings: spindle speed, applied axial load, lubrication condition and mounting method.
• ✓Require sensor type and location, and how many outer-ring angular positions are measured and averaged.
• ✓Require calibration traceability of the instrument and the interval for verifying spindle background against a reference bearing.
• ✓For noise-sensitive duty, require the noise test method, acoustic conditions, microphone position and background-noise handling.

Clearance and surface finish change the measured result

Two things you control outside the test rig also move the number your clause will judge. The first is internal clearance.

Inappropriate operating clearance is stated to generate heat, noise or vibration²⁴, and clearance is listed among the factors that affect noise and vibration¹.

A supplier notes that C3 bearings, with their larger initial clearance, can run slightly noisier or with more vibration than standard-clearance bearings when cold or lightly loaded, improving as temperature converges the clearance¹⁸.

That matters for a cold or lightly loaded duty: a limit set validated on a warm, loaded bearing may not describe what the same bearing does at start-up.

Put the duty cycle into the clause discussion and ask the supplier whether the proposed clearance group suits it, rather than tightening the vibration limit to compensate for a clearance choice. The second lever is the shaft and housing.

Turning or fine boring is acceptable for general conditions, but where minimum vibration and noise are required, a ground finish on the fitting surfaces is stated as required²³. That is an installation condition on your side of the interface, not a bearing grade, and no vibration limit will recover a turned seat.

Where the cited sources diverge, and what settles it

The two sources behind this table are not really in conflict about the physics; they describe different jobs. One sets out bearing acceptance against pre-agreed limits, the other describes machine condition monitoring, where acceleration, velocity and displacement are collected together and acceleration enveloping is used for early damage detection^7,26.

The risk is a buyer lifting the monitoring method into an acceptance clause and grading on a quantity the supplier never quoted. Read the table as a list of questions for the supplier, not as a menu.

Where the two positions touch the same parameter, ask which one the test report will grade on and get the answer in writing before the clause is signed.

Before you request a quote

• The evidence leaves this open — put it to the supplier: iSO 15242 part structure and which part covers which bearing type and parameter — obtain the applicable part from the standard text or the supplier.
• The evidence leaves this open — put it to the supplier: measurement quantity units, frequency bands and the RMS versus peak evaluation rules — require these in the quotation.
• The evidence leaves this open — put it to the supplier: vibration grade definitions and numeric limits by bearing size and type — require numeric limits for your exact size, not a grade label.
• The evidence leaves this open — put it to the supplier: test rig setup: spindle speed, axial load, lubrication, mounting and sensor type and location — require the rig settings in writing.
• The evidence leaves this open — put it to the supplier: instrument calibration, traceability and verification intervals for spindle background — ask for the calibration record and interval.
• The evidence leaves this open — put it to the supplier: noise measurement standards and conditions (sound pressure or power level, background noise, chamber, microphone position) — the sources name no noise standard; obtain the method from the supplier.
• The evidence leaves this open — put it to the supplier: bearing-specific noise test methods and acceptance criteria, including abnormal-noise detection — ask the supplier to state the method and criteria.
• The evidence leaves this open — put it to the supplier: required report data fields and certification or traceability format — agree the report content as an annex to the RFQ.