engineering fundamentals Linear Bearings: Lubrication
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Rotary Bearing Lubrication

Linear bearings often require lubrication in order to reduce rolling resistance and to minimize sliding friction between rolling elements, raceways, and cage. Since linear bearings are often fabricated as self-contained slides, grease is a common lubricant, and in the case of point-contact ball slides, often no lubricant is used. For clean room use, obviously lubricant containing volatiles is not acceptable. Grease is convenient since it is viscous enough to not drip out of the bearing, but it does not flow well enough for it to be used as a continuous-flow coolant for high-speed robotic applications. Oil is used when a cooling fluid is needed for high speed bearings. Circulated oil also serves to clear away particulate debris. The following table summarizes the relative advantages between oil and grease.

Oil Grease
Cooling function: removes heat away from the bearing. Viscous so seals are simplified, plus it acts as its own sealant.
Cleaning function: removes moisture and particulate matter. Permits prelubrication of sealed or shielded bearings.
Lubrication is easily controlled. Requires less frequent lubrication.

Oil mist lubrication systems are used for high speed, continuous operation applications. This system permits close control of the amount of lubricant reaching the bearings. The oil may be metered, atomized by compressed air and mixed with air, or it may be picked up from a reservoir using a venturi effect.

Shaft Race Surface Finish

Most of the following specifications for shafts apply to other types of raceways as well. Race surface finish becomes an issue when the implementation of the bearing involves providing one of the races, as when a shaft is used as the inner raceway for needle or roller bearings. In this instance, the following specifications should be met:

  1. Metallurgy — Case hardening or through hardening grades of bearing-quality steel are satisfactory for raceways. Steels which are modified for free machining, such as those high in sulfur content and particularly those containing lead, are seldom satisfactory for raceways.

To realize full bearing capacity, the raceway area must be at least surface hard with a reasonable core strength. The preferred surface hardness is equivalent to 58 Rockwell C (ref. ASTM E18).

Shaft raceways for all needle roller bearings, in diameters up to 3.5 inches or 90mm should have an effective case depth of 0.030 inch or 0.8mm. (Effective case depth is defined as the distance from the surface, after final grinding, to the 50 HRC hardness level.) For raceways larger than 3.5 inches or 90mm in diameter the effective case depth should be 0.050 inch or 1.3 mm.

  2. Strength — the shaft must be of sufficient size to keep the operating deflections within the limits outlined in the following table.

Bearing Width Maximum Slope
Caged Full Complement
> 50 mm (> 2 in) 5 × 10-4 5 × 10-4
25 - 50 mm (1 -2 in) 10 × 10-4 5 × 10-4
< 25 mm (< 1 in) 15 × 10-4 10 × 10-4

  3. Taper — the taper within the length of the bearing raceway should not exceed 0.0003 inch (0.008 mm), or one-half the diameter tolerance, whichever is smaller.
  4. Out-of-Roundness — the radial deviation from true circular form of the raceway should not exceed .0001 inch (0.0025mm) for diameters up to and including 1.0 inch (25mm). For raceways greater than 1.0 inch or 25mm the allowable radial deviation may be greater than .0001 inch (0.0025mm) by a factor of raceway diameter (in inches) divided by 1.0 or a factor of raceway diameter (in mm) divided by 25.
  5. Surface Finish — the raceway finish should not exceed 8 microinches aa (arithmetic average) or 0.2 µm (on the Ra scale). In addition, the raceway area must be free of nicks, scratches and dents. Oil holes are permissible in the raceway area but care must be taken to blend the edges gently into the raceway. Care must be taken to prevent grind reliefs, fillets, 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 failure.
  6. End Chamfer — for most effective assembly of the shaft into a bearing, the end of the shaft should have a larger chamfer or rounding. This should help in preventing damage to the roller complement, scratching of the raceway surface and nicking of the shaft end.
  7. 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 finish. 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.

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