Deep-groove ball bearings

For a 25 mm bore such as NTN 6205, NTN lists 5–20 μm CN and 13–28 μm C3 initial radial clearance. These are unmounted ranges. Shaft and housing fits and temperature change the running clearance. The 6205 catalog's 15.5 kN dynamic and 7.85 kN static ratings answer separate fatigue and deformation questions. Verify fits, temperature, load and lubricant on the assembly. NTN, A-93, NTN, B-24–B-25, 25 mm bore row.

Construction and load paths

Grooved inner and outer raceways carry load through the balls. The cage spaces and guides them. NTN, Classification and characteristics, printed A-4–A-5.

The grooves support radial and bidirectional axial loads. Axial restraint of the shaft still depends on the assembly. NTN, Deep groove ball bearings, B-17.

Shields, seals and lubricant

NTN offers metal non-contact shields (ZZ), non-contact rubber seals (LLB), contact seals (LLU), and low-torque seals (LLH). Contamination resistance trades against torque and speed. Open bearings are unlubricated; enclosed versions are grease prelubricated. These suffixes are manufacturer-specific. NTN, B-17, Table 1.

Lubricant forms a separating film at rolling/sliding contacts, reducing wear and friction; circulating oil can also carry away heat. Grease behavior depends on base oil, thickener and additives. A seal does not establish suitable viscosity, grease compatibility, fill amount or replenishment interval for a particular duty. NTN cautions against mixing greases of different brands; the catalog does not establish interchangeability from a shared consistency grade. NTN, Lubrication, A-111–A-113.

Internal clearance, preload and support stiffness

Internal clearance is relative ring movement before mounting. Radial clearance and axial clearance are different measurements. Shaft interference can expand the inner ring, while housing interference can contract the outer ring; both reduce available clearance. Ring temperature differences change it further. NTN expresses operating clearance as initial clearance minus fit-induced and temperature-induced reductions. NTN, Bearing internal clearance and preload, A-88–A-89.

NTN Table 8.8 gives unmounted clearance classes. Check how the fits and temperatures change the running gap. NTN, A-93.

Preload introduces initial elastic contact loading. It can increase rigidity, but excessive preload raises torque and temperature and reduces life. There is no universal preload value here. As engineering synthesis, bearing contact stiffness describes load versus ring displacement; assembled support stiffness also includes shaft bending, housing deformation, fits, axial retention and bearing spacing. A catalog load rating does not specify that assembly's deflection. NTN, A-100–A-101.

Bounded catalog example: NTN 6205

For standard NTN 6205, CAT.No.2203-3/E gives these values, separately from EC/AC designs. Allowable speed is its heading; no separate reference-speed rating is declared. NTN, B-24–B-25, 25 mm bore row. NTN describes the tabulated grease and oil allowable speeds as approximate. NTN, A-106.

LLF appears in the column heading, not the 6205 availability row. The oil column establishes no conversion procedure for grease-filled bearings. NTN, B-24–B-25.

NTN separately illustrates the complete designation 6205ZZC3/2AS: double-sided steel shields, C3 radial clearance and Alvania Grease S2. That named catalog example identifies a lubricant; it is not evidence of the grease in every current 6205ZZ shipment or of its suitability for every application. NTN, A-48–A-49.

Conditions behind the speed figures

NTN's tabulated speeds assume correct mounting and prescribed internal clearance, suitable lubricant maintained as needed, normal operating temperature, and ordinary loads P ≤ 0.08Cr and Fa/Fr ≤ 0.3. Here P is dynamic equivalent load, Fa axial load, and Fr radial load. Speed also depends on cooling and cage design; LLU/LLH speeds are constrained by seal circumferential speed. Heavier loading requires adjustment. These are conditional catalog values rather than machine operating approvals. NTN, Allowable speed, A-106.

For radial bearings on vertical shafts, NTN recommends reducing the listed allowable speed to approximately 80%, because lubricant retention and cage guidance are less favorable than with horizontal mounting. The other load, lubrication, clearance and thermal conditions still apply; this orientation adjustment is not a measured speed or an application approval. NTN, A-106.

For this Cr, the catalog's ordinary-load condition corresponds arithmetically to P ≤ 1.24 kN. This derived threshold belongs to the speed assumptions; it is not a universal permissible bearing load. Low load, rapid acceleration and oscillation introduce additional sliding, cage and lubrication concerns. NTN treats those duties separately. NTN, A-106–A-107.

Fatigue life and static deformation are different criteria

Cr anchors statistical rolling-contact fatigue life: for ball bearings, L10 = (Cr/P)³, in millions of revolutions. L10 is the life attained or exceeded by 90% of an identical bearing population under identical conditions. It does not predict each bearing's service life or include every failure mechanism. C0r instead references permanent contact deformation: NTN uses a combined deformation of 0.0001 times rolling-element diameter at the most heavily loaded contact. Neither number alone is a permissible assembly load. NTN, A-20–A-21 and A-30–A-31.

For illustration only, constant equivalent load P = 1.00 kN gives (15.5/1.00)³ = 3,723.875 million revolutions. At a constant 1,000 min⁻¹ this is about 62,065 hours by L10h = L10 × 10⁶/(60n). This is arithmetic under an assumed equivalent load and constant speed, not a qualified 6205 service-life prediction. Contamination, lubrication failure and mounting damage can dominate actual life. NTN, A-20–A-21.

Axial contact geometry and misalignment impose further limits. NTN explains that ring misalignment from shaft/housing flexibility can act like a moment load, requiring consideration of internal design and clearance beyond the simple life equation. Its effect has no single universal life-reduction factor. NTN, A-29.

Evidence and applicability limits

Sources were retrieved on 2026-10-31 PDT. The full PDF cover prints CAT.No.2203-3/E, © NTN Corporation 2024, and 24.09.00 NI/NI. The site's index and filenames use “2203/E”; those URL labels are not the printed edition. The full PDF metadata records September 6, 2024 creation/modification timestamps; metadata is distinct from the printed imprint. The split chapter PDFs lack a standalone dated cover; the cited 6205 spread was checked against the full catalog. NTN full catalog, cover.

The initial direct catalog request returned HTTP 403; a subsequent request with a browser user-agent succeeded. An alternate NTN-SNR download failed TLS certificate verification and supplies no additional evidence. Full ISO/JIS standards were not read; references to their methods here are limited to NTN's explanation, without a standards-compliance claim.

No customer machine, purchased bearing lot or assembled support was tested. Actual fits, thermal gradients, lubricant supply, contamination, shaft/housing stiffness, alignment, retention, duty cycle, noise and required reliability remain application-specific. This reference establishes no universal fit, clearance, preload, lubrication interval, interchangeability or customer qualification.

Production planning references.