Deep Groove Ball Bearings: Selection Guide

Deep groove ball bearings are the single largest bearing family in the world — and in our own catalogue they account for 31,521 listed SKUs, second only to cylindrical roller bearings. That popularity is not accidental: a deep groove ball bearing carries radial load, carries axial load in both directions, tolerates moderate speed, is cheap to make to tight tolerance, and is available in more size and sealing variants than any other design. It is also the family where the most selection mistakes are made, because the part number looks simple and the consequences of choosing the wrong suffix are not.

This guide is written for engineers, buyers and maintenance planners who need to go from a shaft diameter and a duty description to a correctly specified part number — and to understand exactly what each digit and suffix commits them to. Every section is backed by the governing standard, and every recommendation is one we apply when quoting from the catalogue.

The geometry: why the groove is deep

A deep groove ball bearing has an inner ring, an outer ring, a cage, and a complement of balls. The distinguishing feature is the depth of the raceway groove in both rings — the groove radius is typically around 0.515 of the ball diameter, so the ball is held in a channel rather than resting on a shallow track. That wrapping is what lets the bearing transmit axial load in both directions with only a small contact angle, while still running as a predominantly radial bearing.

Point contact and its consequences

Load passes from ring to ball at a nominally elliptical contact patch. The small contact area is the reason deep groove ball bearings run cool and fast: low rolling resistance, low heat generation, high limiting speed. It is also the reason their load capacity is modest compared with roller bearings — the same load has to be carried through a much smaller area.

The practical trade is worth stating plainly: for the same envelope, swapping a 6205 deep groove ball bearing for a cylindrical roller bearing of similar size raises radial capacity substantially, but removes axial capability entirely and reduces the speed ceiling. There is no free upgrade — only a different set of compromises.

What the groove depth gives you

The deep groove permits an axial load of roughly 25–30 % of the basic dynamic radial rating on a standard bearing, with a contact angle of around 8–12° under thrust. That number is not a manufacturer opinion; it follows from the raceway conformity and is the basis of the axial-load guidance in ISO 76 and ISO 281. Beyond that ratio the contact angle grows, the elliptical patch becomes unfavourable, and life falls away faster than the load increase would suggest.

Decoding the part number: 6205-2RSH/C3

A bearing designation is a compressed specification. Reading it left to right gives you the size, the sealing arrangement, the internal clearance and sometimes the cage, tolerance and special features.

The bore code rule

Bore codes 04 to 96 multiply by 5: 04 = 20 mm, 05 = 25 mm, 06 = 30 mm, 10 = 50 mm. Codes 00, 01, 02 and 03 are exceptions — 10, 12, 15 and 17 mm respectively. Bores below 10 mm are written as the actual millimetre value (e.g. 608 = 8 mm bore, 6 series, extra-light). This rule is universal across ISO 15, so it works identically for SKF, FAG, NSK and our own house brand.

The series digit is a capacity decision

At the same 25 mm bore, the series digit changes everything except the shaft fit. Moving from 6005 to 6205 to 6305 buys capacity and costs weight and speed — a trade many specs get wrong because the part number "looks equivalent".

ISO 15 boundary dimensions: the three series at 25 mm bore

ISO 15 fixes the envelope for each bore and dimension series. Because it is a standard, these numbers are identical from every manufacturer, and they are the first thing to check when a drawing shows a bearing seat. The ISO 15 boundary dimension reference extends the table to the full bore range.

Published ratings vary by a few percent between manufacturers because cage design, ball complement, heat treatment and internal geometry differ even when the envelope does not. Values above are typical published ratings for the ISO 15 dimension series and are adequate for first-pass selection; confirm against the chosen manufacturer's catalogue before release. Real, orderable examples at this bore in our deep groove ball bearing range include 6205-2RSH and 6205-2RSR.

C and C0: two ratings, two different questions

Confusion between these two numbers causes more premature failures than any other single misunderstanding.

Basic dynamic load rating, C

C is the constant radial load at which 90 % of an identical group survives one million revolutions under the rating conditions of ISO 281. It answers the fatigue question: how long can this bearing run before metal fatigue begins to pit a raceway. When you compute an L10 life, C is the number you plug in.

Basic static load rating, C0

C0 is the load that produces a calculated contact stress of 4,200 MPa at the most heavily loaded ball in the most heavily loaded position of a stationary bearing, as defined in ISO 76. It answers a different question: will the raceway take a permanent dent during a shock, a press fit, a stalled rotor or a handling accident. Note that C0 is usually about half of C in a deep groove ball bearing — for the 6205 above, 7.8 versus 14.0 kN.

The failure modes are distinct, so the selection checks are distinct:

• Rotating, steady load → compare the equivalent dynamic load against C, compute L10 life, compare with the required service interval.
• Shock, stall, static hold, or press-fit assembly → compare peak load against C0 and keep the static safety factor S0 comfortably above 1.
• Combined radial and axial load → convert to an equivalent load P using the X and Y factors for the contact angle, then compare against C.
• Oscillating or very slow motion → the fatigue model degrades; weight the static and lubrication checks more heavily.

Internal clearance: C2, CN, C3, C4

Internal clearance is the total radial gap between the balls and the raceways before the bearing is mounted. It is defined numerically in ISO 5753 and DIN 620, and it is the single most frequently mis-specified feature we see in incoming enquiries. The summary below is enough for selection; the dedicated clearance article carries the full numeric reduction tables.

The mechanism people miss

When a bearing is fitted with an interference fit on a rotating inner ring, the ring is stretched and the clearance is reduced. When the shaft runs hotter than the housing — the normal case, because the inner ring is closest to the heat source — the inner ring expands further and clearance falls again. A CN bearing that measures correctly on the bench before assembly can be running with zero or negative clearance at temperature. That is why C3 is the correct default for most electric motor and gearbox positions, and why replacing a specified C3 with a CN equivalent because "they look the same" is a documented cause of overheating and early failure.

How to choose a clearance class

• Estimate the temperature differential between inner and outer ring in service.
• Estimate the interference the specified fit produces, and its effect on ring diameter.
• Subtract both from the bench clearance.
• Require a small positive operating clearance — enough to avoid preload, not so much that the balls lose guidance, load zone grows and noise rises.
• If your arithmetic lands close to zero, step up one class rather than accepting the risk.

Seals and shields: 2RS, 2RZ, ZZ, open

The closure choice sets friction, speed ceiling, contamination resistance, grease retention and whether relubrication is possible. It is a design decision, not a preference.

Choosing between a shield and a seal

A shield is a static steel plate with a small labyrinth gap. It creates almost no friction, so it barely affects the speed rating, but it only excludes coarse particles. A contact seal touches the inner ring land, which gives real protection against moisture and fine dust at the cost of friction, heat and a reduced speed limit. If the bearing is greased for life in a dirty environment, 2RS is correct. If it runs hot and fast in a clean gearbox, 2Z or open with oil lubrication is correct.

Note the manufacturer-specific suffix conventions: 2RSH, 2RS1 and 2RSR all denote a contact seal on both sides, while 2RSL and 2RZ denote light-contact designs. This is exactly the kind of substitution that matters when cross-referencing brands, and it is the reason our interchange lookup returns sealing variants as a separate attribute rather than assuming equivalence.

Speed limits and what actually derates them

Catalogue speed figures are reference values, not guarantees. Several factors move them, and the smallest one wins.

• Closure type. Contact seals are usually the binding constraint, often halving the permissible speed relative to an open bearing.
• Lubricant. Grease has a base-oil viscosity and a thickener that limit how fast the ball can sweep through it without churning; oil lubrication generally permits higher speed.
• Cage material. Polyamide cages have a lower temperature and speed envelope than machined brass.
• Load magnitude. Heavy load increases contact stress and heat, so running near C at high speed is a poor combination.
• Cooling and fit. A hot environment raises ring temperature, reduces lubricant viscosity and adds thermal preload.
• Precision class. Higher classes reduce runout-induced vibration, which is often the real limiter well below the nominal speed figure.

Tolerance classes and what they buy

Tolerance classes are defined in ISO 492 and, historically, in ABMA/ANSI Standard 20. They control bore and OD deviation, ring runout and width variation — the geometry of the rotating assembly rather than its capacity.

The rule is simple and frequently ignored: system accuracy is limited by the worst component. Fitting a P5 bearing into a housing machined to P0 tolerances returns P0 behaviour and pays a P5 premium. If you genuinely need P5, specify the housing with equal rigour. Our comparison of ABMA and ISO tolerance classes and the P6/P5/P4 tolerance breakdown carry the numeric deviation limits if you need them.

Materials, cages and heat treatment

Ring and ball steel

The industry standard is a through-hardening high-carbon chromium steel, designated 100Cr6 in the European system and GCr15 in the Chinese system. It is hardened and tempered to roughly 58–65 HRC with a controlled retained-austenite fraction. Martensitic stainless variants such as 440C are used where corrosion resistance matters more than fatigue life, and ceramic balls (silicon nitride) appear in hybrid bearings for high speed and electrical insulation.

Standard materials you will see in a real catalogue

• Through-hardened chromium steel (100Cr6 / GCr15) — the default for almost all industrial deep groove ball bearings.
• Stainless steel (440C / AISI 440C, or 304 for low-load) — food processing, medical, wet environments.
• Case-hardened steel — used for larger rings and shock-loaded designs.
• Silicon nitride (Si3N4) balls — hybrid bearings for high speed, high temperature and where a non-conductive ball path is needed.
• Polyamide PA66 cages — standard for high-volume small bearings, limited to roughly 120 °C continuous.
• Stamped steel cages — the workhorse: cheap, temperature-tolerant, adequate speed ceiling.
• Machined brass cages — for high speed, high temperature and heavy duty.

Heat treatment: what it changes

Standard bearings are dimensionally stabilised for continuous service up to approximately 120 °C. Above that, a dedicated stabilising treatment (often designated with a suffix such as S1 or S2) is required; without it, dimensional growth from retained-austenite transformation can consume the entire internal clearance. If your application runs hot, raise it in the enquiry — it is a manufacturing decision, not an afterthought.

Lubrication in practice

Roughly half of all premature bearing failures we see in returned samples are lubrication-related, not manufacturing-related. The three variables that matter are base-oil viscosity, grease consistency and fill quantity.

• Base-oil viscosity must be sufficient to maintain an elastohydrodynamic film at operating temperature. A mineral oil that works at 40 °C may be too thin at 90 °C.
• Consistency (NLGI grade) governs whether the grease stays put or is thrown out of the bearing at speed. Higher speed generally calls for a softer, smoother grease with better channeling.
• Fill quantity for sealed bearings is set at manufacturing. Overfilling causes churning and overheating; underfilling starves the raceway.
• Compatibility matters when relubricating: mixing an incompatible thickener with the factory fill can destroy the structure of both greases.

For a deeper treatment of grease selection, thickener chemistry and relubrication intervals, see our grease selection guide.

The seven-step selection framework

Working these steps in order prevents the common failure of optimising a secondary parameter before the primary ones are settled.

1. Define the function. Radial load, axial load, or both? Continuous or intermittent? Is the load stationary relative to the ring, or rotating?
2. Fix the envelope. Bore from the shaft, OD and width from the housing. This usually selects the series before any load calculation starts.
3. Compute equivalent load. Use the X and Y factors for the contact angle to combine radial and axial components into a single P.
4. Check static capacity. Compare peak and shock loads against C0 with an appropriate static safety factor, especially for stall, hold or press-fit conditions.
5. Check dynamic life. Compute L10 against the required service hours at the duty cycle, then compare with the design interval.
6. Set clearance and fit. Estimate temperature differential and interference, then choose the clearance class that leaves a small positive operating gap.
7. Choose closure, cage, tolerance and lubrication — and re-check speed with all of them applied, because speed is usually where the combination binds.

Worked example: 6205 on a 25 mm shaft

Consider a 1.5 kW electric motor driving a belt through a 25 mm shaft at 1,450 rpm, with a measured radial load of 2.2 kN at the drive-end bearing and a belt tension component contributing about 0.4 kN of axial load.

The resulting designation is a 6205-2Z/C3 or 6205-2RS/C3 depending on environment. Orderable equivalents in our catalogue include 6205-2RSH, 6205-2RSR and the ANDGE 6205-2RS. Bearing variants at this bore are listed alongside their full boundary dimensions in the deep groove ball bearing category.

Common selection mistakes

Choosing C3 because it sounds stronger

Clearance classes are not quality grades. C3 in a cool, lightly fitted application gives a larger load zone, more noise and no benefit. Match the class to the thermal and fit budget, not to a preference.

Sizing on shock load only

A bearing chosen purely to survive a one-off shock will usually be oversized for its running duty, which raises friction, mass and cost and can push the operating point below the minimum load needed to keep the balls in positive traction.

Ignoring the axial component

A deep groove ball bearing handles roughly a quarter to a third of its C rating as axial load. Beyond that, life falls faster than the load ratio suggests and the contact angle grows. If the axial component is large, the correct answer is a different family — angular contact or tapered roller — not a larger ball bearing.

Substituting a seal for a shield without re-checking speed

Sealing variants are often treated as interchangeable in procurement. They are not: a 2RS designation can cut the permissible speed roughly in half, and in a high-speed motor that substitution alone can overheat the bearing.

Assuming cross-referenced part numbers are equivalent

Two manufacturers' 6205 series bearings share the ISO 15 envelope and nothing else. Cage material, ball complement, seal design, clearance tolerance band, heat treatment and grease fill can all differ. Treat an interchange result as a starting point for verification, which is exactly how our interchange tool presents it.

Overspecifying tolerance class

P5 or P4 in a P0 housing is money spent on accuracy that the assembly cannot express. Tighten the housing first, or accept P0.

What to include in an RFQ

Complete enquiries get quoted faster and more accurately, because they remove the assumptions a quotation engineer would otherwise have to make.

• Designation and quantity, including any suffix — 6205-2Z/C3, 500 pcs.
• Brand preference or acceptance of equivalents, and whether an interchange substitution is allowed.
• Application and operating conditions: load direction and magnitude, speed, temperature range, environment (dust, water, washdown), duty cycle.
• Required clearance and tolerance class, if specified by drawing.
• Lubrication and closure requirement, including grease type if it must match an existing fill.
• Documentation: material certificates, dimensional inspection reports, traceability to batch.
• Packaging and delivery destination, especially for export consolidation.

If you are unsure of a parameter, say so in the enquiry. Our engineering desk will propose a specification and explain the reasoning — which is faster than quoting a part that will not survive its duty.

Explore the range

Our deep groove ball bearing category lists 31,521 orderable SKUs, while cylindrical roller bearings (38,259 SKUs) and tapered roller bearings (34,808 SKUs) cover higher radial and combined-load duties. For compact high-load positions, see needle roller bearings; for pure thrust duty, thrust bearings; and for shaft misalignment, self-aligning ball bearings or spherical plain bearings.

If you would rather search by size than by designation, the bearing selector filters the whole 323,474-SKU catalogue by bore, outside diameter, width and brand, and the size-indexed deep groove pages group listings by dimension. Our bearing glossary defines the terminology used throughout this guide.