6908 Bearing Dimensions

Tolerances and fits for 6908

FULI makes 6908 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 40 × 62 × 12 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 40 k5 for the shaft of 6908. 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 6908 on a 40 mm k5 shaft has 2 to 25 µm interference. Its outer ring has 0 to 43 µm clearance in an H7 bore and 12 µm interference to 31 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.

6908 compared with nearby sizes

6908 measures 40 × 62 × 12 mm: bore d, outside diameter D and width B. Other deep groove ball bearings on this site for the same 40 mm shaft are 6808 (40 × 52 × 7 mm), 16008 (40 × 68 × 9 mm), 6008 (40 × 68 × 15 mm), 6208 (40 × 80 × 18 mm), 6308 (40 × 90 × 23 mm) and 6408 (40 × 110 × 27 mm).

16008 is the next heavier choice for a 40 mm shaft, with an outside diameter 6 mm larger (68 mm) and 3 mm less width (9 mm). In FULI's catalog 16008 has a dynamic load rating of 11.34 kN against 13.14 kN and a weight of about 0.130 kg against 0.112 kg for 6908.

6808 is the lighter option on the same shaft, with an outside diameter 10 mm smaller (52 mm) and 5 mm less width (7 mm), for housings with less room. In FULI's catalog 6808 has a dynamic load rating of 4.59 kN against 13.14 kN and a weight of about 0.031 kg against 0.112 kg for 6908.

6007 has the same 62 mm outside diameter with a 35 mm bore, so it fits the same housing bore on a smaller shaft.

16007 has the same 62 mm outside diameter with a 35 mm bore, so it fits the same housing bore on a smaller shaft.

In the 69 (thin section) series the sizes either side of 6908 are 6907 for a 35 mm shaft and 6909 for a 45 mm shaft.

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

6908 weight, load ratings and speed

FULI's catalog gives the weight of 6908 as about 0.112 kg, so 100 bearings weigh about 11.2 kg and 1,000 about 112 kg before packing. The open, ZZ and 2RS versions share this catalog line.

6908 has a dynamic load rating Cr of 13.14 kN and a static load rating C0r of 9.79 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 10,000 rpm with grease and 12,000 rpm with oil.

Contact seals add friction at the lip, so 6908-2RS 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.

6908 versions: open, ZZ, 2RS and C3

Every version of 6908 has the same 40 × 62 × 12 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.

6908 dimensions in inches

The nearest common inch shaft size to the 40 mm bore of 6908 is 1-9/16 in (39.69 mm), 0.31 mm smaller than the bore, so 6908 would be loose 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 6908

FULI fills sealed 6908-2RS and 6908-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 6908 from the side takes about 3.72 g each time by the formula in our grease calculator: 0.005 × D × B = 0.005 × 62 × 12. This applies to the open 6908 in a housing with a grease supply.

How FULI supplies 6908

The standard 6908 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 6908-2RS C3.

Tools and guides for 6908

• 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