Monday, September 12, 2016

Bearing Design Guide: Chapter Seven: Oil Holes and Oil Grooves

          Oil?....Like vegetable oil?...olive oil??  Nooooo...don't be silly.  In this chapter we will learn about the different types of holes and grooves used to lubricate a bearing.

         Oil holes and oil grooves are important features of a journal bearing to introduce and distribute the lubricant adequately to the bearing surface as needed.

         Oil holes in the sleeve bearing are the simplest and most effective method in introducing the lubricant into the bearing area but must be located in the unloaded area of the bearing. The oil hole also can be located in the shaft. The oil entering through the shaft will be centrifuged into the sleeve-bearing area in the same amount or more.
         Grooving is rarely necessary in short bearings with an LID ratio of .5 or less unless high-surface speeds require a larger volume of oil to pass through to dissipate the frictional heat generated. In those cases, an annular or circular groove will enhance the results.

         For bearings with an L/D ration of 1 or more, oil grooves may be necessary depending upon the speed, load and type of lubricant viscosity.

         Grooves are generally required for grease lubricated bearings since grease does not have the mobility of oil nor does grease dissipate the frictional heat being generated.

         Oil inlet holes should be at least as wide as the groove it is supplying. Any smaller inlet hole can be blocked with sludge or other debris and result in restricting the lubricant from entering the bearing surface and starve the application which will result in failure.

         The oil inlet holes should be chamfered and have all edges rounded and broken to form unrestricted entrances for the lubricant to enter.
         All grooves should be blended and rounded to reduce the effects of sharp edges that interfere and scrape the lubricant and prevent the formation of an oil film.

          In high-speed bearings or pressure-lubricated bearings, a small "V" or vent groove may be added to remove entrapped air or permit dirt or other wear debris to escape and permit slight oil leakage to reduce frictional heat.

 Types of Grooving

Oil hole: A single oil hole without any grooving is commonly used in short bearings with an L/D ratio of .5 or less. Oil will flow axially unaided to each side of the oil inlet hole by as much as 1/2". The oil hole should be centrally located and in the unloaded area to ensure that oil is distributed equally to both sides.

The bearing with a single oil hole can have approximately three times the load bearing capacity then a bearing with an annular or circumferential groove in the same length bearing.

Straight axial groove: is used when the bearing length exceeds an L/D ratio of 1.5 but stops short of
each end by 1/8" to 1/4". The groove must be located in the unloaded area.

Circular or annular groove: is generally used when lubrication is pressure-fed or direction of load
varies and a low-pressure region cannot be located. This type of groove divides the bearing into two shorter bearings which do not carry the same load as a single bearing. When an annular or circumferential groove is used, it is important that it is placed exactly along the center of the bearing. If the groove is placed off center, then half of the bearing will tend to operate with a greater eccentricity than the other.

This groove can be used in combination with a straight axial groove but the axial groove must be located in the unloaded area.

The oil flow of a bearing with a circular groove is about 2.5 times that of a bearing with an oil hole only.

Oval groove: A single- or double-oval groove connected with an oil entry hole will distribute the
lubricant more positively and more copiously.

Although the groove passes angularly through the loaded area, only a small measure of load pressure will be affected. The oval groove also should run short of each end by 1/8" to 1/4" unless the lubricant is introduced from the bearing end, then that groove side should be open into the reservoir.

Figure-S Groove: is a modification of the double-oval groove and is generally preferred in grease
lubricated applications or to offer a greater exposure of graphite in graphited bearings.

The "V-Shaped" Groove: and radiused, cross-sectioned grooves are best suited for oil lubrication since the groove edges, blended or rounded, promote the formation of the oil film.

The Rectangular, Cross-Sectioned Groove: is better suited for grease and graphite or other solid
lubricants since it offers a larger surface area for the grease or graphite to adhere or offer a larger reservoir of grease.

If two bearings are used in line in an oil- or grease-lubricated mode, a central reservoir should be located between the two bearings by at least twice the wall thickness or more.

Any angular groove should be open only on the reservoir side if the lubricant is not introduced through the bearing length. Again, oil grooves or grease grooves should extend to within 1/8" to 1/4" of each bearing end when using a centrally-located inlet hole.

The groove width and depth will depend on the volume of oil which must pass through the bearing to
maintain the viscosity within the range of operating temperatures.

Precision-Groove Applications: The groove width (W) should be taken as .06 times the bearing bore diameter and the depth (D) to (.5W).

Medium-Groove Applications: The groove width (W) should be taken as .08 times the bearing bore
and depth (D) to (.5W).

Loose-Groove Applications: The groove width (W) can be taken as .10 times the bearing bore and
depth (D) to (.5W).

Although the generally-accepted print tolerances are usually given as plus-or-minus .005 to plus-or-minus .010, the width and depth can be specified in less restricted tolerances.

In general, the suggested widths and depths of oil-lubricated grooves can be taken as follows:

       Bearing ID                 Groove Width                (W)        Groove Depth            (D)
         0.5                                  1/16                      0.062              1/32               0.032
           1                                   1/8                        0.125              1/16               0.062
           2                                   5/32                      0.156              5/64               0.078
           3                                   3/16                      0.188              3/32               0.094
           4                                   1/4                        0.25                1/8                 0.125
           5                                   5/16                      0.312              5/32               0.156
           6                                   3/8                        0.375              3/16               0.188
           7                                   7/16                      0.437              7/32               0.219
           8                                   1/2                        0.5                  1/4                 0.25
           9                                   9/16                      0.562              9/32               0.281
         10                                   5/8                        0.625              5/16               0.312
         11                                   11/16                    0.687              11/32             0.344
         12-20                             3/4                        0.75                3/8                 0.375

Well that was GROOVY...Haha, ya see what I did there?!  Anyway, that's it for now.  Until next time my metal loving friends...

Next Up: Week Seven, Chapter 8:Grooves for Grease and Graphite-Filled Bearings

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