Bearing Guides
Practical, engineer-checked guides to reading, comparing and choosing rolling bearings. These cover the things that decide whether a "same-size" bearing is actually a valid replacement — the knowledge behind every cross-reference on this site.
How to read a bearing number
Decode any designation — 6204, NU 205, 30204, UC204, 22212 — into bore, series and type. The bore code, the series digit and every prefix explained.
What "interchangeable" really means
Same dimensions is not the same bearing. The five things — clearance, precision, seal, cage, ratings — you must check before you swap one brand for another.
Are SKF, FAG, NSK & NTN interchangeable?
A 6204 is 20×47×14 mm from every brand — but interchangeable in size is not interchangeable in service. What differs between the brands, the suffix trap, premium lines and counterfeits.
How to replace a ball bearing
Remove and fit a bearing without wrecking it: get the right part, pull it without damaging the shaft, and mount through the correct ring — the mistake that quietly brinells a good bearing.
2RS vs ZZ: sealed, shielded or open?
Open, shielded and contact-sealed bearings share one envelope but trade friction against protection. How to choose by environment, speed and lubrication — not just decode the code.
Ball vs roller: which bearing type?
Ball, cylindrical, tapered, spherical and needle compared by load, direction, speed, space and misalignment. Pick the right type first — the cross-reference only works within a type.
Precision classes: P0, P6, P5, P4 & ABEC
What the ISO classes and the ABEC scale actually control, how they map, when a tighter class earns its cost — and the load-and-smoothness myth ABEC does not cover.
Shaft & housing fits (k5, m6, H7)
Why the rotating ring needs a tight fit, what k5/m6/H7 mean, how an interference fit eats internal clearance, and the creep and seizure the wrong fit causes.
Cages: steel, brass & polyamide (TN9, M, J)
The cage carries no load but sets the speed and temperature limits. How pressed steel, machined brass and polyamide differ, and how to read the cage suffix.
Grease vs oil: bearing lubrication
When grease beats oil and vice versa, what NLGI grade and base-oil viscosity mean, which bearings can be relubricated, and why over-greasing runs a bearing hot.
Metric vs inch bearings
How to spot an inch bearing from the bore, why tapered inch and metric series don't interchange, and how to tell which system you're in from a measurement.
Why bearings fail: common failure modes
Most bearings don't wear out — contamination, poor lubrication, misalignment, bad fitting or electrical erosion kills them early. How to read each signature so the replacement lasts.
Radial vs axial load: which bearing?
Radial load pushes across the shaft, axial along it. Which types carry which, why a thrust bearing can't take radial load, and how combined loads are handled in pairs.
Bearing preload explained
Why some bearings are set to a slight negative clearance, how rigid and spring preload differ, and why too much preload runs a bearing hot — and can't be matched off one part number.
Internal clearance: CN, C3, C4 explained
Why the clearance suffix matters more than most people think, when to choose C3, and how the wrong clearance can seize a bearing that fit perfectly.
Seal & shield suffixes across brands
2RS vs ZZ, contact vs non-contact, and the brand code map — SKF 2RSH, NSK DDU, FAG 2RSR, NTN LLU, KOYO 2RS — that trips up every cross-reference.
NU, NJ, N, NUP: cylindrical roller types
The letter is not cosmetic. It says which ring carries the guide ribs — and therefore whether the bearing locates the shaft. Why an NU is not an NJ.
Angular contact angles A, B, C
How the contact-angle suffix sets thrust capacity and stiffness, why single bearings are mounted in pairs (DB/DF/DT), and what "universal matching" means.
Tapered roller: cup, cone and why brands don't mix
The separable cup and cone, the e and Y factors that decide load and life, and why inch and metric tapered series are two different worlds.
Identify an unmarked or worn bearing
A step-by-step way to find the number of a bearing whose markings are gone — measure the bore, OD and width, then work back to the designation.
These guides are dimensional and general engineering reference only. For load ratings, speed limits and application suitability, consult the bearing manufacturer's catalogue.