6904 Bearing Dimensions

Tolerances and fits for 6904

FULI makes 6904 to P0 as standard, with P6 on request. P0 is the Normal tolerance class of ISO 492; P6 limits are tighter. These are the P0 limits for the 20 × 37 × 9 mm size.

Δdmp and ΔDmp: deviation of the mean bore and mean outside diameter in one radial plane. ΔBs: inner ring width; the outer ring width has the same limits.

Shaft and housing fits (general guidance)

For an inner ring that turns with a solid steel shaft under normal load, bearing makers’ fit tables give 20 k5 for the shaft of 6904. The housing bore is usually H7, or J7 where the outer ring must not creep.

Taking the loosest and the tightest combination of limits, a P0 6904 on a 20 mm k5 shaft has 2 to 21 µm interference. Its outer ring has 0 to 36 µm clearance in an H7 bore and 11 µm interference to 25 µm clearance in a J7 bore.

This is general guidance, not a FULI specification. Heavy or shock loads, a hollow shaft, a thin-walled, split or light-alloy housing, or a large temperature difference between shaft and housing call for other classes, and the drawing of your machine takes priority. A tight shaft fit or a hot housing uses up internal clearance, and that is when C3 is chosen instead of C0.

If the outer ring turns and the shaft stands still, as in an idler pulley or a wheel hub, the fits are reversed: the housing gets the interference fit (for example N7 or P7) and the shaft a sliding one (g6 or h6).

Sources: ISO 492 tables as printed in the NTN catalogue CAT. No. 2203/E (Tables 6.4 and 6.5) and the RKB tolerance catalogue; ISO 286-2 for the seats; SKF and NTN fit tables for the shaft and housing classes.

6904 compared with nearby sizes

6904 measures 20 × 37 × 9 mm: bore d, outside diameter D and width B. Other deep groove ball bearings on this site for the same 20 mm shaft are 6704 (20 × 27 × 4 mm), 6804 (20 × 32 × 7 mm), 16004 (20 × 42 × 8 mm), 6004 (20 × 42 × 12 mm), 6204 (20 × 47 × 14 mm), 6304 (20 × 52 × 15 mm) and 6404 (20 × 72 × 19 mm).

16004 is the next heavier choice for a 20 mm shaft, with an outside diameter 5 mm larger (42 mm) and 1 mm less width (8 mm). In FULI's catalog 16004 has a dynamic load rating of 7.11 kN against 5.76 kN and a weight of about 0.048 kg against 0.037 kg for 6904.

6804 is the lighter option on the same shaft, with an outside diameter 5 mm smaller (32 mm) and 2 mm less width (7 mm), for housings with less room. In FULI's catalog 6804 has a dynamic load rating of 3.60 kN against 5.76 kN and a weight of about 0.017 kg against 0.037 kg for 6904.

6805 has the same 37 mm outside diameter with a 25 mm bore, so it fits the same housing bore on a larger shaft.

6301 has the same 37 mm outside diameter with a 12 mm bore, so it fits the same housing bore on a smaller shaft.

In the 69 (thin section) series the sizes either side of 6904 are 6903 for a 17 mm shaft and 6905 for a 25 mm shaft.

Cr and weight from FULI's catalog; — = not listed on this site.

6904 weight, load ratings and speed

FULI's catalog gives the weight of 6904 as about 0.037 kg, so 100 bearings weigh about 3.7 kg and 1,000 about 37 kg before packing. The open, ZZ and 2RS versions share this catalog line.

6904 has a dynamic load rating Cr of 5.76 kN and a static load rating C0r of 3.55 kN. Cr is the load used to work out rating life; C0r is the reference for loads carried while the bearing is stationary or turning slowly. Limiting speeds in the catalog are 18,000 rpm with grease and 22,000 rpm with oil.

Contact seals add friction at the lip, so 6904-2RS usually runs below the speed of the open bearing. Ask us for the limit of the version you order.

Values from FULI's catalog; confirm for your application.

6904 versions: open, ZZ, 2RS and C3

Every version of 6904 has the same 20 × 37 × 9 mm boundary dimensions; only the cover between the rings and the internal clearance change. FULI supplies open, ZZ and 2RS versions, with C0 clearance as standard and C3 on request. Other makers write the same covers as 2Z or ZZ for shields and 2RS1, DDU or LLU for contact seals.

6904 dimensions in inches

The nearest common inch shaft size to the 20 mm bore of 6904 is 13/16 in (20.64 mm), 0.64 mm larger than the bore, so 6904 will not go on it. Inch shafts need an inch bearing, not a metric one with a sleeve or shim.

Converted at 1 in = 25.4 mm and rounded to four decimals. The bearing itself is made to the millimetre sizes.

Grease for 6904

FULI fills sealed 6904-2RS and 6904-ZZ bearings to about 30% of the free internal space as standard. The fill is set on an automatic greasing machine after sample sets are weighed before and after greasing. Sealed bearings are not regreased in service. A different fill level, a low-noise grease or a grease for a stated temperature range can be written on the order; for North America a range of −40 to 120 °C is often asked for.

Relubricating 6904 from the side takes about 1.67 g each time by the formula in our grease calculator: 0.005 × D × B = 0.005 × 37 × 9. This applies to the open 6904 in a housing with a grease supply.

How FULI supplies 6904

The standard 6904 from FULI has GCr15 chrome steel rings and balls, P0 precision, C0 radial clearance and vibration grade Z3V3; P6, C3 and other vibration grades are made on request. Radial clearance is set on automatic assembly equipment and sampled during production. Orders with a noise or vibration requirement are tested set by set before packing.

Rings are marked with a brand you are authorised to use, with the FULI brand, or left neutral, as your order states. The box is printed with the model, precision class and brand.

Inquiries that leave out the prefix or suffix letters are the usual cause of a wrong quote, so write the full designation, for example 6904-2RS C3.

Tools and guides for 6904

• Bearing size finder: look up a size from bore, OD and width
• Bearing weight chart
• Bearing grease fill calculator
• Shaft and housing tolerance chart
• Open, ZZ or 2RS seal selector
• Bearing internal clearance: C0, C3 and when to change
• Bearing lubrication basics
• Noise grade and precision class
• Ball bearing vs roller bearing
• All bearing products and model pages