Angular-contact bearings
Directional load paths, paired arrangements, clearance and preload, with bounded NSK catalogue examples and rating definitions.
Tags: components, bearings, rotating-equipment
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NSK's single 7206 A and 7206 B with standard W cages both measure 30 × 62 × 16 mm. Their contact angles are 30° and 40°, and their basic dynamic ratings are 24.8 kN and 22.6 kN, respectively. The back-to-back effective load-center spacing is 42.6 mm for A and 54.6 mm for B. Use the matched-pair ratings for the chosen arrangement and check preload, cage and lubrication separately. NSK, single/matched mountings, PDF p. 211.
Contact angle and load direction
A single-row angular-contact bearing takes radial load and axial load in one direction. Its radial load also creates an axial component that needs an opposing reaction. NSK, Rolling Bearings for Industrial Machinery, E1103d, 2025 B-10, pp. C072–C073 (PDF p. 202).
NSK's designation key assigns C = 15°, A5 = 25°, A = 30° and B = 40°. These are contact-angle codes, separate from cage, internal design, arrangement and preload/clearance codes. Its example 7208BEAT85SUCNB combines B (40°), EA (high-load-capacity internal design), T85 (polyamide 46 cage), SU (universal single-row arrangement) and CNB (CN-equivalent clearance for universal arrangement). That example describes a clearance designation, rather than an unspecified positive preload. NSK, p. C074 (PDF p. 203)
Opposed pairs and tandem pairs
The effective load centers explain why bearing spacing and arrangement affect moment support. NSK illustrates the following arrangements; the model strings are its examples, not interchangeable specifications for every bearing manufacturer. NSK, Table 2, p. C073 (PDF p. 202)
| Arrangement | NSK example | Load-path distinction |
|---|---|---|
| Back-to-back, DB | 7208 A DB |
Radial load and axial load in either direction; a relatively large effective load-center spacing supports moments. |
| Face-to-face, DF | 7208 B DF |
Radial load and axial load in either direction; a smaller effective spacing gives less moment capacity than DB in NSK's comparison. |
| Tandem, DT | 7208 A DT |
Both bearings share axial load in the same direction; tandem alone does not provide the opposed axial direction. |
Pairing is also a manufacturing designation. NTN distinguishes DB/DF/DT matched pairs from D2, its universal flush-ground pair. “Universal” identifies a particular matching capability; it does not establish matching between arbitrary loose bearings or across brands. NTN also separates tolerance class, internal clearance and shaft/housing fit: they describe different dimensional relationships. NTN Americas, angular-contact and clearance FAQs
A double-row angular-contact bearing is another construction. NSK describes two back-to-back rows with the inner rings integrated into one ring and the outer rings likewise integrated. This supports axial loads in both directions and moments within one bearing; it is not simply a designation for two separate bearings. NSK, p. C075 (PDF p. 203)
Clearance and preload
Clearance describes available relative movement before contact loading; preload introduces internal elastic loading. NSK describes preload as negative internal clearance, with the rings and rolling elements elastically deformed even though measured clearance under that stress is zero. This can increase rigidity and reduce shaft displacement. Position preload and spring-based constant-pressure preload have different mechanical behavior: the latter permits movement while maintaining nearly constant force. NSK, 8—Preload
Preload is not simply the external working load. In NSK's opposed-pair explanation, increasing external axial load loads one bearing while unloading the other; the unloaded bearing can reach preload loss. Excessive preload adds rolling-contact load and can increase heat and friction while reducing fatigue life. Consequently, an initial preload designation alone does not establish operating rigidity, temperature or life. NSK, 8—Preload, “Preload Loss” and “Importance of Proper Preload”
Two bearings with equal boundary dimensions
NSK catalogue E1103d, 2025 B-10, pp. C090–C091 gives separate single and matched-pair ratings for the standard W pressed-steel cage. NSK, single/matched mountings, PDF p. 211
| Catalogue quantity | 7206 A, standard W |
7206 B, standard W |
|---|---|---|
| Single basic dynamic rating, Cᵣ | 24,800 N | 22,600 N |
| Single basic static rating, C₀ᵣ | 14,800 N | 13,500 N |
| Single grease / oil limiting speed | 9,500 / 13,000 min⁻¹ | 7,100 / 9,500 min⁻¹ |
| Matched Cᵣ / C₀ᵣ, DB/DF/DT table columns | 40,000 / 29,500 N | 36,500 / 27,000 N |
| Matched grease / oil limiting speed | 8,000 / 11,000 min⁻¹ | 5,600 / 7,500 min⁻¹ |
| Effective load-center spacing, DB / DF | 42.6 / 10.6 mm | 54.6 / 22.6 mm |
These are source-specific ratings and limiting speeds, not allowable combined service loads or guaranteed operating speeds. NSK states that load ratings depend on cage type and ball count; its speed discussion distinguishes standard and optional cages. Near-limit speed and temperature, marginal lubrication, vibration and moment loading affect applicability. Grease and oil speed columns represent different lubrication conditions, not a conversion between lubricants. NSK, pp. C072–C073 and C076–C077 (PDF pp. 202, 204)
What the ratings mean
NSK defines the basic dynamic rating for stationary outer rings and rotating inner rings with a basic rating life of one million revolutions. For a radial bearing, the reference load is a central radial load of constant magnitude and direction. Basic rating life is statistical: under identical conditions, 10% of the bearing population has failed by rolling-fatigue flaking at that life. Noise, grease deterioration and other functional limits can end service earlier; the rating is not an individual-bearing lifetime promise. NSK, pp. A032–A033 (PDF p. 21)
The static rating describes a different criterion. NSK gives 4,200 MPa calculated maximum contact stress for ball bearings other than self-aligning ball bearings, associated with combined permanent deformation of the rolling element and raceway of approximately 0.0001 times the rolling-element diameter. C₀ᵣ is therefore not a fracture load or a zero-deformation threshold. Dynamic and static equivalent loads also differ; the angular-contact table distinguishes single/DT from DB/DF load factors and contact angle. NSK, pp. A052–A053 (PDF p. 31), angular-contact load-factor table, p. C091 (PDF p. 211)
ISO's public abstract for ISO 281:2007 describes basic rating life at 90% reliability and modified life incorporating reliability, lubrication, contamination and fatigue load. Wear, corrosion and electrical erosion are outside its stated life coverage. This supports a distinction between a rolling-fatigue calculation and total service life; the full standard's calculation clauses are outside this article's source coverage. ISO 281:2007 public scope and status
Source versions and coverage
The cited NSK file is served at an e1103b.pdf URL, but its printed colophon identifies E1103d, 2025 B-10, with first edition January 2016. The URL suffix is not the edition. NSK's July 1, 2024 announcement describes revised dynamic ratings, including angular-contact bearings, and corresponding online catalogue updates; values from older brochures are not automatically the values in this edition. NSK colophon (PDF p. 461), NSK rating revision announcement
Sources were accessed October 31, 2026 PDT; this is a retrieval date. ISO's public record identifies edition 2, February 2007, confirmed in 2021, and lists a replacement draft under development. ISO public record No full ISO standard was read. NTN and NSK educational HTML supplied no publication date in the cited passages. The examples do not establish a particular assembly's fits, operating preload, lubricant, heat balance, alignment, fatigue life or qualification. No universal installation or preload-setting procedure follows from the catalogue rows.