Two bearings can share a bore, an outside diameter and a width, fit the same shaft and the same housing, and still behave completely differently in the same spindle. What separates them is not the envelope but the accuracy class - the set of tolerances that govern how round each ring is, how true the rotating ring runs, and how parallel the two faces are.
Choose a coarse class and a precision spindle will vibrate outside its specification. Choose a class tighter than the shaft and housing can preserve, and you pay a premium for accuracy the assembly throws away. This guide explains what P6, P5 and P4 control, gives the real micrometre deltas, maps them to the ABMA system, and ends with a selection sequence you can apply to a drawing.
- The class controls form and runout, not size alone. Bore and outside diameter deviation, radial and axial runout, width variation and side-face runout are each tightened by a different amount at every step.
- P5 is the practical precision default. It covers most machine tool spindles, precision gearboxes and measuring equipment without the cost and lead time of P4.
- P4 and P2 are system decisions. They only pay back when the shaft, housing and mounting are accurate enough to preserve the ring geometry you have paid for.
- ABEC and ISO are not one-to-one. ABEC 7 and ISO P4 look equivalent and are not always identical, so compare the individual parameter limits before you substitute.
Which standards define the classes
Accuracy classes are a standards construct, not a marketing label. ISO 492 defines the boundary and running accuracy tolerances for radial bearings, and ISO 199 does the same job for thrust bearings. The class names - Normal, P6, P6X, P5, P4 and P2 - and the symbols used to write them come from ISO 5 terminology and ISO 5593 vocabulary, so a drawing that says "P5" means the same thing to every supplier that works to the standard.
In the North American world the same idea runs through ABMA 20, using the ABEC grades ABEC 1, 3, 5, 7 and 9. The two systems overlap heavily but they are not the same document: they measure some parameters differently, and where a project is aerospace or automotive the contract usually names the system it wants. If you need the full mapping parameter by parameter, the ABEC versus ISO tolerance guide walks it through class by class.
What each class actually tightens
A class is a bundle of limits, and each step down the alphabet tightens a different subset of them. The four families that matter for a rotating assembly are set out below.
Bore and outside diameter deviation
These are single-plane diameter tolerances. They set how tightly the ring fits the shaft and the housing, and therefore how much of the theoretical interference you actually obtain. Bore deviation is usually the most tightly controlled parameter, because the inner ring is normally the interference fit and fit loss feeds straight into operating clearance.
Radial and axial runout
Radial runout of the inner and outer ring is the parameter that most directly drives vibration and surface finish in a spindle. Axial runout - sometimes called face runout - matters wherever the bearing locates a face or carries a thrust component. Both are tightened aggressively from P6 to P5 and again from P5 to P4.
Width variation and side-face runout
Width variation between the two faces of a ring, and side-face runout measured against a datum, decide how a bearing behaves when two are mounted side by side or when thrust is taken on a shoulder. This is the parameter most often ignored and the one that dominates paired mounting.
Dimensional accuracy versus rotational accuracy
The distinction matters commercially. A class raises both, but runout is the expensive half. If a position only needs a tight bore for a controlled fit, buying a higher class to get it also buys runout the job may not need.
What the numbers look like at a 30 mm bore
Abstract class names become useful the moment you see the micrometre delta. The table gives typical ISO 492 limits for a 30 mm bore deep groove bearing across Normal, P6, P5 and P4.
| Parameter (30 mm bore, series 0 and 2) | Normal | P6 | P5 | P4 |
|---|---|---|---|---|
| Bore deviation, dmp | 0 to -10 µm | 0 to -8 µm | 0 to -6 µm | 0 to -5 µm |
| Outer diameter deviation, Dmp (D = 62 mm) | 0 to -13 µm | 0 to -11 µm | 0 to -9 µm | 0 to -7 µm |
| Inner ring radial runout, Kia | 13 µm | 8 µm | 4 µm | 3 µm |
| Outer ring radial runout, Kea | 25 µm | 13 µm | 8 µm | 5 µm |
| Width variation, VBs | 20 µm | 10 µm | 5 µm | 3 µm |
Published ISO 492 limits for a 30 mm bore single-row deep groove bearing in diameter series 0 and 2, which covers the 6200 and 6300 families. The limits are tabulated by bore and diameter series, so a thin-section series has wider limits at the same bore; for a specific bearing, read the governing table in ISO 492 or the brand catalogue rather than extrapolating from this one.
Read the table as a set of ratios rather than a list of numbers. From Normal to P5 the inner-ring radial runout collapses from 13 µm to 4 µm, and the outer-ring figure from 25 µm to 8 µm, while the bore deviation only moves from 10 µm to 6 µm. The premium you pay from P5 to P4 is almost entirely a running-accuracy premium: runout tightens again, to 3 µm and 5 µm, but the fit dimension barely changes. If the complaint is vibration or surface finish, a class step is the right answer. If the complaint is that the bearing will not press onto the shaft, the class is the wrong lever - the shaft tolerance is.
Which parameter matters for which job
Matching the parameter to the application is what keeps a class decision honest. A grinding spindle and a gearbox output shaft both run fast, but they are sensitive to different numbers.
| Parameter | What it governs | Applications that live on it |
|---|---|---|
| Radial runout | Vibration and roundness of the finished surface | Machine tool spindles, grinding, measuring equipment |
| Axial runout | Face location and thrust accuracy | Pair-mounted angular contact sets, turntables |
| Width variation | Preload consistency in paired mounting | Duplex and triplex sets, gearbox spindles |
| Bore deviation | Fit and clearance loss | Any interference-mounted inner ring |
| Outer diameter deviation | Housing fit and creep | Rotating outer rings in a housing |
The logic follows the load path. A preloaded angular contact pair is dominated by axial runout and width variation, because those two decide whether the rings share load as designed. A motor with a deep groove bearing in a die-cast housing cares only about radial runout and outer diameter deviation.
The ABMA and ANSI equivalents
North American drawings and legacy equipment often specify an ABEC grade rather than an ISO class. The mapping is close enough for ordering and not close enough to ignore the individual parameter limits.
| ISO class (ISO 492) | ABMA grade (ABMA 20) | Practical note |
|---|---|---|
| Normal | ABEC 1 | The standard catalogue bearing, held in stock everywhere |
| P6 | ABEC 3 | Closest match; some runout limits differ slightly |
| P5 | ABEC 5 | Closest match; the industrial precision default |
| P4 | ABEC 7 | Not identical - compare axial runout and width limits individually |
| P2 | ABEC 9 | Spindle grade; parameter-by-parameter comparison is essential |
The nuance sits at ABEC 7 against P4. The two are widely treated as equivalent, and for radial runout they very nearly are, but the axial runout and width limits are defined with different measurement methods, so a substitution can pass on paper and still show up as a preload error on a duplex set. Treat the mapping as a starting point and confirm the individual parameters.
Where P5 is enough and where P4 or P2 is required
The honest rule is that the required class is set by the accuracy of the finished assembly, not by the bearing alone. If the shaft runout is 20 µm, a P4 bearing will not make the assembly run better than a P5, because the shaft error is by far the larger contributor.
- General machinery, fans, pumps and conveyors. Normal or P6 is the correct class. Boost it only if the fit or the clearance case actually demands it.
- Electric motors and standard gearboxes. P6 covers most positions; P5 is justified where vibration limits are contractual.
- Machine tool spindles and precision gearboxes. P5 is usually enough, provided the shaft and housing are held to comparable geometry. See the angular contact range used in these positions.
- High-speed spindles, grinding heads and measuring equipment. P4, and the surrounding parts must be ground to match it.
- Aerospace and high-speed turbine positions. P4 or P2, with the mounting geometry and preload specified as a system rather than a part.
Ball bearings carry the classes most visibly, so the deep groove ball bearing range and the tapered roller range are where the ordering decision is usually made. In practice the class is chosen once and then held for the life of the design, which is why it belongs on the drawing and not in a supplier's hands.
Cost, lead time and what the step really buys
Precision is manufactured, not selected. Every step up the classes adds grinding passes, controlled-temperature inspection and scrap, and the commercial consequences are worth stating plainly.
- Availability narrows. Normal and P6 sizes sit on shelves worldwide; P4 and P2 are frequently made to order, which moves the lead time from days to weeks.
- Price rises non-linearly. P6 is a modest premium over Normal, P5 is a clear step, and P4 and P2 are separated from P5 by a much larger margin because of the finishing and gauging time involved.
- Substitution gets harder. A P4 position cannot simply be filled with a P6 bearing without re-accepting the assembly, because the runout budget changes.
- The surrounding parts get more expensive too. A P4 bearing demands a housing bore and shaft that can preserve it, which usually means grinding rather than turning.
How to choose the class for a job
Work the sequence in order. Each step either narrows the options or tells you that the class is being asked to compensate for a problem it cannot fix.
- Start from the required running accuracy. State the permitted radial or axial error at the rotating face in micrometres. That number, not the catalogue, sets the ceiling.
- Measure the shaft and housing geometry. If shaft runout or housing roundness is several times the bearing runout, a higher class will not help. The measuring bearing accuracy guide covers the gauging method.
- Identify the dominant parameter. Use the table above: radial runout for spindles, axial runout and width variation for paired sets, bore deviation for clearance-critical positions.
- Set the class to the dominant parameter, not the whole bundle. Buying P4 merely to tighten a bore is the most common over-specification in the enquiry inbox.
- Check the fit and clearance case. A tighter bore deviation changes the interference and therefore the operating clearance, which the shaft and housing fits guide and the internal clearance guide both quantify.
- Confirm against the speed and preload requirement. High DN positions may force a class for cage and ball quality reasons before runout is even considered; see the speed limits and DN guide.
- Record the decision. Write the standard, the class symbol and the parameters you actually care about onto the drawing, and pair it with the preload requirement described in the preload in bearing sets guide.
Specifying the class and the mistakes that follow
A drawing that says "precision bearing" is not a specification. Write the standard and the class together, for example "ISO 492, tolerance class P5", and add the parameters that drive the design - typically radial runout of the rotating ring. If the position is a duplex set, add the width variation limit. When ABMA is the contractual system, name it explicitly and note that the interchange against ISO is approximate. Our glossary defines each tolerance term, the bearing selector filters by class across the catalogue, and any SKF listing can be checked against the ISO 492 limits above before a substitution is released.
- Paying for P4 while the shaft runout is 20 µm. The assembly cannot be better than its largest error, and the bearing is rarely the largest error.
- Ignoring the housing. A precision inner ring in an oval or rough housing runs to housing accuracy, not bearing accuracy.
- Treating ABEC 7 and P4 as interchangeable without checking. The axial runout and width limits are defined differently.
- Buying class to fix a fit problem. Bore deviation moves only a few micrometres between classes; if the fit is wrong, it is the shaft tolerance that is wrong.
- Forgetting the clearance link. A tighter fit consumes radial clearance, so a class change silently shifts the operating clearance unless it is rechecked.
- Mixing classes inside one set. Two rings of different classes in a paired mounting preload unpredictably and wear unevenly.
The classes in this guide come from ISO 492 for radial bearings and ISO 199 for thrust bearings, with terminology from ISO 5 and ISO 5593 and the ABEC grades from ABMA 20. When a workload mixes systems, check both the class and the individual parameter limits rather than assuming the mapping is exact. Send us the drawing and the duty, and we will state which class the position actually needs. The related ISO 492 standard overview puts the system in its wider context.
Frequently asked questions
What does P5 mean on a bearing?
P5 is an ISO 492 accuracy class, one step tighter than P6 and one step looser than P4. It halves the radial and axial runout of a P6 bearing and tightens the bore and outside diameter deviation. P5 is the usual specification for machine tool spindles, precision gearboxes and measuring equipment, where vibration and surface finish are controlled parameters.
Is ABEC 7 the same as ISO P4?
They are close but not identical. ABEC 7 and P4 both sit between P5 and P2 and share similar radial runout limits, but the axial runout and width variation limits are defined with different measurement methods. Treat the mapping as approximate and compare the individual parameters you care about before substituting one for the other.
Do I need P4 bearings for a spindle?
Only if the shaft, housing and mounting can preserve P4 accuracy. If the shaft runout is 20 micrometres, a P4 bearing cannot make the assembly better than that, and a P5 bearing at lower cost will perform the same. P4 pays back on high-speed spindles, grinding heads and measuring equipment where the surrounding geometry is held to a few micrometres.
What is the difference between P6 and P5?
P6 mainly tightens bore and outside diameter deviation and draws in the runout limits. P5 goes further and roughly halves the inner and outer ring radial runout, the axial runout and the width variation. For most industrial positions P6 is adequate, while P5 is the entry point for positions where vibration or surface finish is measured.
Can I replace a P5 bearing with a Normal class bearing?
Not without re-accepting the assembly. A Normal class bearing has larger radial and axial runout, so the runout budget of the whole assembly changes. In a position that was specified for P5 because vibration or surface finish was controlled, the substitution will usually push the machine outside its acceptance limits even though the dimensions and the fit are unchanged.
Which tolerance parameter matters most for vibration?
Radial runout of the rotating ring, because it directly adds to the eccentricity of the shaft centreline once per revolution. Axial runout matters where the bearing carries thrust or locates a face. Width variation matters most in paired mounting, where it decides how evenly the two bearings share the preload. Match the class to the parameter the application actually measures.