ENGINEERING SHAFT DESIGNS BEARINGS WITHOUT INNER RINGS When the shaft is used as the inner raceway for needle roller bearings it must have a hardness of 58 HRC or higher and a wave- free fi nish in order to realize the full load-carrying capability of the bearing. 1. Metallurgy – either case-hardening or through-hardening grades of good bearing-quality steel are satisfactory for raceways. To realize full bearing capacity, the raceway area must be at least surface hard with a reasonable core strength. During the carburizing or induction-hardening of case hardened steel, not only must the surface hardness requirement of 58 HRC or higher be met, but the basic concept is that the case depth with a hardness of HV 550 (52.3 HRC) must be 0.4 mm or higher. However, if the roller diameter is smaller than 4 mm, a case depth of (0.1 × Dw) mm or higher is recommended. (Dw:roller diameter) 2. Strength – the shaft must be of suffi cient strength to keep the operating defl ections within the limits outlined. 3. Tolerance – the suggested shaft diameter tolerances for each type of needle roller bearing are indicated in the appropriate section of this catalog. 4. Variation of mean shaft diameter (taper) – within the range of the bearing width, 5 μm or less per 25 mm or one-half the diameter tolerance or less (whichever is smaller). 5. Deviation from circular form – the radial deviation from true circular form of the raceway should not exceed 2.5 μm for diameters up to and including 25 mm. For raceways greater than 25 mm, the allowable radial deviation should not exceed 2.5 μm multiplied by a factor of the raceway diameter divided by 25. 6. High frequency lobing – the lobing that occurs 10 or more times around the circumference of a shaft and exceeds 0.4 μm from peak to valley is called chatter. Chatter usually causes undesirable noise and reduces fatigue life. 7. Shaft slope – Operating conditions which cause misalignment (shaft defl ection, inaccuracy of shaft and housing, mounting errors) can affect bearing performance. For needle roller bearings, Table A-10 shows misalignment limitations based on bearing width. Table A-10. Misalignment limitations Bearing width Maximum slope (mm/mm) mm in. Caged Full complement ＜25.4 ＜1 0.0015 0.0010 25.4 － 50.8 1 － 2 0.0010 0.0005 ＞50.8 ＞2 0.0005 0.0005 8. Surface fi nish – In addition to a wave-free fi nish, the raceway surface roughness of Ra 0.2 μm must be maintained for the bearing to utilize its full load rating. The raceway area also must be free of nicks, burrs, scratches and dents. Oil holes are permissible in the raceway area, but care must be taken to blend the edges gently into the raceway, and if possible, the hole should be located in the unloaded zone of the raceway. Care also must be taken to prevent grind reliefs, fi llets, etc., from extending into the raceway area. If the rollers overhang a grind relief or step on the shaft, there will be high stress concentration with resultant early damage. 9. End chamfer – for the most effective assembly of the shaft into a bearing, the end of the shaft should have a large chamfer or rounding. This should help in preventing damage to the roller complement, scratching of the raceway surface, and nicking of the shaft end. 10. Sealing surface – in some instances, bearings have integral or immediately adjacent seals that operate on the surface ground for the bearing raceway. Here, particular attention should be paid to the pattern of the shaft fi nish. In no instance should there be a “lead,” or spiral effect, as often occurs with through-feed centerless grinding. Such a “lead” may pump lubricant past the seal. BEARINGS WITH INNER RINGS When it is undesirable or impractical to prepare the shaft to be used as a raceway, inner rings are available as listed in the tabular pages. If the shaft is not used directly as a raceway, the following design specifi cations must be met: 1. Strength – the shaft must be of suffi cient strength to keep the operating defl ections within the limits outlined. 2. Tolerance – the suggested shaft diameter tolerances for each type of needle roller bearing are indicated in the appropriate section of the catalog. 3. Variation of mean shaft raceway diameter (taper) and deviation from circular form of the raceway – should not exceed one-half the shaft diameter tolerance. 4. Surface fi nish – the surface fi nish should not exceed a roughness of Ra 0.8 μm. 5. Locating shoulders or steps – locating shoulders or steps in the shaft must be held to close concentricity with the bearing seat to prevent imbalance and resultant vibrations. Table A-11. Shaft designs summary Shaft Raceway surface Fitting surface Out-of-roundness Shaft dia. 25 mm: 2.5 μm or less Shaft dia. > 25 mm: 2.5 μm × (shaft dia./25 mm) or less One-half of shaft dia. tolerance or less Variation of mean dia. (taper) 5 μm or less per 25 mm within the range of bearing width, or one-half of shaft dia. tolerance or less (whichever is smaller) One-half of shaft dia. tolerance or less Surface roughness 0.2a or less 0.8a or less Hardness 58 HRC or harder1) – 1) During the carburizing or induction-hardening of case hardened steel, not only must the surface hardness requirement of 58 HRC or higher be met, but the basic concept is that the case depth with a hardness of HV 550 (52.3 HRC) must be 0.4 mm or higher. However, if DW is smaller than 4 mm, a case depth of (0.1 × Dw) mm or higher is recommended. (Dw: roller dia.) NEEDLE ROLLER BEARINGS A-13 A