Bearings and circlips

November 3, 2026  ·  Indumor

A conveyor gearbox came into the shop with the output shaft bearing replaced three times in one year. The millwright who brought it was certain all three bearings had been defective. They were ordinary 6308s and had left the factory sound; the mounting was what killed them. Each one had been driven on with a hammer against the outer ring while the inner ring was an m6 press fit on the shaft, so every blow went through the balls. The raceways carried the dents of all eight balls before the shaft had turned once. With an induction heater, 90 °C and a push by hand, the fourth bearing has now run for two years.

Plenty of bearings that die young die like that, or with a fit that used up their clearance, or drowned in grease. This chapter covers what you need to pick one, order it and mount it: what the number stamped on the ring means, the dimensions of the common series, shaft and housing fits, internal clearance, the circlips that hold them in place, the mounting rules and, at the end, the life calculation.

What the number on the ring means

The designation is an ISO 15 code that every maker writes almost the same way (DIN 623-1 is the German version). The first digit, or the letters, give the type. The second is the dimension series: for a given bore, how much bigger and wider the bearing is, and so how much more load it carries. The last two digits are the bore number, which multiplied by 5 gives the shaft diameter. Everything after the dash is a suffix: seals, cage, clearance, precision.

To make sense of the series, look at one bore in all its versions. A 25 mm shaft can carry anything from a 61805, 37 mm outside and 7 mm wide, up to a 6405 at 80 mm and 21 mm wide; the dynamic load rating runs from about 4 kN to more than 35 kN between the two. When there is room for it, going from the 62 to the 63 series is the cheapest way to buy bearing life.

Dimensions of the 60, 62 and 63 series

These three deep groove series cover nearly everything that turns in a jobbing shop. The dimensions come from the ISO 15 boundary plan and are the same in every brand, which is why a 6205 from one maker replaces another without touching the shaft. In the small sizes (604 to 609, 623 to 629) the last digit is the bore in millimeters, no multiplying.

The other families share the bore numbers and often the envelope. An NU 205 or a 7205 B drops into the same place as a 6205; so does a 30205, but it is 1.25 mm wider, because the cone and the cup are fitted separately and the total width T is measured assembled.

For tapered roller bearings T is the total width assembled; B is the cone (inner ring with rollers) and C the cup. For thrust bearings H is the overall height.

Shaft and housing fits

The rule has one idea in it: the ring that rotates relative to the load gets an interference fit, and the ring that stands still relative to the load can have clearance. On a shaft turning under a pulley the load always points the same way and the inner ring turns beneath it, so the inner ring is tight (k5, m6) and the outer ring gets an H7 or J7 that lets it settle. On an idler wheel running on a fixed axle it is the other way round: the outer ring rotates relative to the load and takes the interference (M7, N7), while the inner ring gets g6 or h6. A ring that should have been tight and ended up loose starts creeping slowly in its seat, wears it and leaves a stain of red oxide; once you see that reddish dust on a shaft, the fit is already gone.

The tolerances are the ISO 286 ones explained in chapter 3; here is which letter to pick. The tables summarize what the general catalogs recommend; the catalog of the bearing you are fitting has the last word, above all for hollow shafts, aluminum housings and shock loads.

Light load: under 6 % of the dynamic rating C. On hollow or thin-walled shafts go one grade tighter (k instead of j). The values are a guide: the bearing catalog rules.

Two more things about the seat that matter as much as the letter: the shaft shoulder must be square to the axis and lower than the inner ring, so a puller can get a grip behind it; and the fillet radius has to be smaller than the bearing chamfer (0.6 mm on a 6203, 1 mm on a 6205, 1.1 on a 6208), or the ring sits on the radius and runs cocked.

Internal clearance: when to ask for C3

Radial clearance is how far the inner ring can move relative to the outer one, measured before mounting. Normal (CN) assumes a j/k shaft fit and even temperatures. When the inner ring goes on with interference, the fit eats up about 80 % of that interference; when it also runs hotter than the outer ring (electric motors, hot-air fans, dryers), thermal expansion eats up as much again. If the two together take the clearance to zero, the bearing heats up, tightens further and ends up blue. That is why electric motors come from the factory with C3 and must be replaced with C3; C4 is for vibrating screens, ovens and anywhere the inner ring passes 100 °C.

Circlips DIN 471 and DIN 472

A circlip (retaining ring, snap ring) is the cheapest axial stop there is for a bearing: a groove in the shaft (DIN 471, external) or in the bore (DIN 472, internal) and a spring steel ring that is spread or squeezed with a pair of pliers. What you machine is the groove, and the groove has three dimensions: the bottom diameter d₂, the width m and the minimum distance n to the end; the ring thickness s is a little less than m so it drops in without forcing. A wide groove lets the ring rattle and the part picks up end play; a narrow one means the ring goes in pinched and jumps out under the first load.

The distance n is the one nobody respects. It is the material left between the groove and the end of the shaft, and it is what carries the axial load that reaches the ring; with less than n, that lip shears off. On a 40 mm shaft it is 3.8 mm; on an 80 mm shaft, 5.3. The groove diameter is toleranced h11 (shaft) or H11 (bore) and the width H13, which is what a parting tool of the right width leaves behind.

Groove: d₂ toleranced h11 (h12 in the large sizes), width m H13. The ring is spread with circlip pliers (DIN 5254) just enough to pass over the shaft.

Groove: d₂ toleranced H11 (H12 in the large sizes), width m H13. The 47, 52, 62, 72, 80, 85, 90 and 100 sizes are the outside diameters of the 6204 to 6211.

Mounting, grease and bearing life

Mounting decides half of a bearing's life. The rules are few, and they do not change with the brand.

Temperatures, grease percentages and clearance reduction are indicative values from the general catalogs: the bearing maker's data rules.

Catalog life, L₁₀, is the number of revolutions that 90 % of a batch of identical bearings will exceed before the first sign of fatigue shows on a raceway (ISO 281). It is worked out from the dynamic load rating C in the catalog and the equivalent load P the bearing actually sees. Because the ratio is cubed, halving the load multiplies life by eight; the other way round, an over-tensioned belt or a misaligned shaft divides it by five without anyone noticing until the bearing starts to sing.

Frequently asked questions

What does 2RS mean on a bearing?

Two contact rubber seals, one on each side: the bearing comes greased for life and closed to dust and water. 2Z or ZZ means two metal shields with no contact, which keep dust out but do not seal against liquids and add no drag.

What size is a 6204 bearing?

20 mm bore, 47 mm outside diameter, 14 mm wide. The 04 is the bore number (04 × 5 = 20) and the 2 is the light series. A 6304 has the same bore with 52 mm outside and 15 mm wide.

When should I use a C3 bearing?

When the inner ring runs hotter than the outer ring or goes onto the shaft with a heavy interference fit: electric motors, hot-air fans, dryers, shafts with an m6 fit. C3 clearance is larger than normal and keeps the bearing from running with zero clearance once it warms up.

Sources: ISO 15:2017 (boundary dimensions of radial bearings); DIN 623-1 (designation); ISO 281:2007 (dynamic load ratings and rating life); ISO 5753-1 (radial internal clearance); ISO 355 (tapered roller bearings); ISO 104 (thrust bearings); DIN 471:2011 and DIN 472:2017 (retaining rings for shafts and bores); ISO 286-2 (shaft and hole tolerances); fit and mounting recommendations from the general catalogs of the bearing makers.