Tuesday, April 25, 2017

Bearing Design Guide: Chapters Eight through Eleven: REVISITED - Graphite Plugged and Grooved Bearings

Bearing Design Guide: Chapters Eight through Eleven:  REVISITED
Graphite Plugged & Grooved Bearings

Bearing Design Guide: Chapter Eight: Grooves for Grease and Graphite Filled Bearings

         The groove width (W) for grease lubricated bearings should be increased by 1/32" of that of oil grooves with depth (D) remaining the same or slightly deeper by 1/64".
          The wider grooves permit the shaft a longer contact period with the less mobile grease supply and permit a greater surface coverage of graphite filled grooves.

The suggested groove width (W) and depth (D) as shown below:

          Bearing ID           Groove Width              (W)       Groove Depth            (D)
             0.5                         3/32                     0.094           3/64                    0.046
             1                            5/32                     0.156           5/64                    0.078
             2                            3/16                     0.188           3/32                    0.094
             3                            1/4                       0.25             1/8                      0.125
             4                            9/32                     0.281           9/64                    0.14
             5                            5/16                     0.312           5/32                    0.156
             6                           13/32                    0.406           13/64                  0.205
             7                           15/32                    0.469           15/64                  0.234
             8                           17/32                    0.531           17/64                  0.265
             9                           19/32                    0.593           19/64                  0.296
            10                          21/32                    0.656           21/64                  0.328
            11                          23/32                    0.719           23/64                  0.36
            12-20                     25/32                    0.781           25/64                  0.39
         Since these grooves are not as critical as oil grooves, they can be toleranced looser to plus-or-minus 1/64 to plus-or-minus 1/32.

          In grease lubricated bearings where exposed to contaminants such as dirt, ash or other debris, it is advisable to provide an annular or circular groove near the end of the bearing within 1/8" to 1/411 to create a dam effect. Such an effect will act as a reasonable seal, preventing contaminants from entering the bearing surface.

Bearing Design Guide: Chapter Nine: Graphited and Solid Lubricated Bearings

          Solid lubricated bearings such as graphite or molybdenum disulfides are used in severe environmental situations where normal fluid or grease lubricants cannot be used because of abnormal temperatures which would tend to carbonize or freeze the lubricant to brittle solids.
         They find usage constantly in chemically-reactive environments, in nuclear radiation and vacuum environments and where normal lubricants cannot be tolerated. They also are used where there is limited access to resupplying the lubricant or where it can be neglected.
        These solid lubricants can be used in form of colloidal powders in suspension of a grease carrier or held and bonded by various binders.
        Graphite itself, although one of the most popular solid lubricants, requires some absorbable gases, moisture or hydrocarbon vapors to develop low-shear strength.
         The gases and water vapor in the normal atmosphere are usually sufficient to ensure an adequate supply of absorbable medium but a brief immersion in a heated oil eliminates chance.
          Graphite in excellent through temperatures ranges through 1000 degrees F but are generally not
satisfactory in high altitudes nor in a vacuum conditions since desorption occurs.
          Molybdenum disulfide is an excellent solid lubricant below 750 degrees For in a vacuum since it does not require presence of adsorbable vapors. However, above 750 degrees Fin the presence of air or oxygen, it deteriorates and becomes an abrasive. It is also more expensive than graphite so it has limited usage.

          As with graphite, molybdenum disulfide and the other solid lubricants require a binder to make it adhere to the bearing surface. There are various binders available that can be formulated.

         The thermoplastic resins such as cellulesic or acrylic resins are easily sprayed, fast drying, requiring no baking but are limited to 150 degrees F.

         The thermosetting phenolics have a service temperature of 400 degrees F with good adherence.

         The epoxy resins adhere well and are safer than phenolics being stable through 600 degrees F but requiring heat-curing.
         Some inorganic binders such as sodium silicate would exceed the 750 degrees F and would be suitable for graphite but limited to a lower temperature for molybdenum disulfide.

         For an economical solid lubricant containing bearings, the standard available sintered powdered metal bearing with 18% porosity offers an excellent surface for retaining graphite or molybdenum disulfide formulation that would require a burnish operation of the lubricant into the surface.

Molybdenum Disulfide-Natural State

Molybdenum Disulfide Powder
Molybdenum Disulfide Grease

Bearing Design Guide: Chapter Ten: Graphited Wall Thickness Calculation
Graphite Plugged Bearing

            Since the solid lubricant must be held in form of grooved configurations, plugs or sticks in a series of drilled holes, they require a substantial depth to be retained properly.

          The minimum wall thickness of groove-filled solid lubricated bearings can follow the width (Y'/) and depth (D) described in Chapter VIII. Plug lubricated bearings with a .5 diameter should be held to 3/16" minimum wall thickness and then increased 1/32" for each nominal half-size above. That is, for a 1" ID bearing, a wall thickness of 7/32" should be considered for plug graphited bearings.
Graphite Plugs

                     A general rule of thumb: wall thickness= .08D + 1/8'' (where Dis the bearing ID.)

          The overall length on solid lubricated sleeve bearings can range in various lengths depending on the load, speed, and type of the application. The normal recommended LID ratio is 1. 5. This is to minimize possible shaft deflection and to offer greater stability and surface area. The maximum LID ratio recommended should rarely exceed a ratio of 3 since misalignment edge-loading and frictional heat can be appreciably increased.

The accepted standard solid lubricant coverage should average 30 to 3 5% of the surface area. However, the calculation of plug size and lubricant coverage is shown below. The plug diameter and drill size should be no larger than the wall thickness but no less than half.
Graphite Sticks

                                   Percentage of Graphite or Solid Lube Coverage

Drill Size        10%        15%       20%       25%         30%        35%        40%        45%        50%         3/16              8.95        13.43     17.9        22.38        26.85       31.33       36.81        40.28       44.5  
    1/4                5.03          7.54     10.05      12.57        15.08       17.6         20.11        22.62       25
    5/16              3.22          4.83       6.44        8.05          9.66       11.27       12.88        14.49      16.1
    3/8                2.23          3.35       4.47        5.59          6.7         7.82         8.94          10.06      11.1     
    7/16              1.64          2.46       3.28        4.1            4.92       5.75         6.57          7.39         8.2
    1/2                1.26          1.89       2.51        3.14          3.77       4.4           5.03          5.66         6.28

1. Choose the appropriate drill or plug size.
2. Locate the desired solid lubricant coverage
     Use factor number opposite drill or plug size
3. Multiply bushing ID x bushing length.
    Multiply factor number to obtain the number of holes or plugs.
Example: 2" ID x 2 112 OD x 2" length
1. 1/4 plug diameter
2. 35% coverage from chart 17.6
3. 2 x 2" length x 17.6 = 70 drilled holes or plugs.


                 Bearing Design Guide: Chapter Ten: Graphited Wall Thickness Calculation

           The running clearances for solid lubricated bearings must be substantially greater than in oil and grease lubricated bearings because the frictional heat generated is not dissipated.
          An allowance of .002 minimum per inch of diameter should be considered. Further, since solid lubricated bearings are not generally machined or bored after assembly 11close in11 of the ID must be allowed for. This allowance must be increased further if the bearing is to be used in abnormal temperature service to allow for expansion or contraction of the shaft.
          For more specific clearance allowances, use the attached calculation sheet either for normal temperatures or for abnormal temperature service. (Reference Chapter 6.)
Surfaces Finishes: Solid lubricated sleeve bearings do not require the high degree of surface finishes of oil lubricated bearings. The slightly rougher bearing ID finish is desirable to permit the solid lubricant to coat its surface. Therefore, a bearing surface ID finish of 63 RMS to 125 RMS should be satisfactory. The shaft finish, however, should be ground or polished smooth to approximately 32 RMS.

          Upon installation of the solid lubricated bearing, there will be instant wear until the shaft and bearing become coated with the solid lubricant. It would be advisable to submerge the graphited bearing in an oil bath of slightly heated oil to penetrate the plugs or filled graphite grooves to enhance its initial "running in" or "bedding in" to reduce this initial wear to an acceptable minimum.
          Note that the drilled and plugged holes do reduce the strength and structure of the bearing while adding to the cost.
          Many times it would be more economical to utilize filled groove configurations since they do not weaken the bearing to any noticeable degree.

         Although a solid lubricant can be retained in a serrated ID which can be broached for economy, tests have shown that such a structure weakens the lands in the bearing ID and limits the load and speed capabilities to less than 50 PSI and 30 fms.

General Information: Some commercially available solid lubricants containing compounds of graphite and molybdenum disulphide are sold by Lubriplate and Emhart Companies as Never-Seize and Dri-Slide.

         The coefficient of friction for solid lubricated bearings are appreciably higher than those for oil and grease ranging from .15 to .35 initially then reduced to a lower acceptable level.

Ok...I'm done.  I know that was a lot of information all at once, but I thought they should go together.  I have watched the plugging process a few times here in our warehouse and I gotta say it was pretty cool.  From sticking them in, to watching them get ground down to become one with the bearing or wear plate is a neat thing to watch.

I hope you find this as interesting as I do, that's it for now.  Until next time my metal loving friends...

Tuesday, April 18, 2017

SAE 841 - Bar Stock vs Bearings

Day in and day out we have customers who contact us for SAE 841, Sintered Bronze material.  The call will start off by the customer giving an order for pieces of 841 Bar Stock or Solid Bar material.

Then somewhere in the conversation the customer will say that they need a Signed C of C per ASTM B438 and then the dreaded statement comes....."I will be making Bushings".  This is where we have to stop the customer and go into the usual spiel about the ASTM B438 specification.  Now before I continue, here is our disclaimer**:  We are not expertsOur sales team has just been burned enough times that it has forced us to learn and understand this crazy spec. 

 The sales person then proceeds to explain to the customer that the ASTM B438 specification is for Bronze-Base Materials used in Powdered Metal Bearings.  The key word being -- Bearings.  This covers Bearings / Bushings or Thrust Washers. 

841 Facts

Here is the deal.  Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering). The powder metallurgy process generally consists of five steps: 

(1) Blending - The part-specific powder is created by mixing the correct amounts of metals and lubricants to produce the physical and mechanical properties of the finished product.

(2) Molding - the powder is compacted into the desired shape using compressive forces. The part shape is created by compressing the correct blend of materials inside of the part-specific tools.

(3) Sintering - Through the application of heat, sintering permanently bonds the individual metal particles that have been compacted together through the molding stage.  This process is instrumental in providing the majority of the mechanical properties of the final product.

(4) Sizing - Sizing, sometimes referred to as "coining," is the final pressing of the fully sintered part.

(5) Oil Impregnation - Under vacuum, customer-specific oil products are impregnated into the remaining porosity of the completed part.

The Problem

Notice in Step #3 that the Sintering process is the instrumental phase in providing the majority of the mechanical properties of the final product.  The ASTM B438 (05, 08 & 13) specification covering Sintered products applies to Bearing/Bushings/Thrust Washers.  These shapes have specific radial crushing strengths (minimum and maximum's), Impregnation Efficiencies and Impregnation Density's that are unique to the shapes dimensions and sizing.  

So why does that matter to the guy buying Solid Bar Stock and machining it into a Bushing? Consider this, let's say that I buy a piece of 841 Bar Stock, 1" ID x 3" OD x 6-1/2" long.  This piece of bar stock has a specific radial crushing strength, impregnation efficiency & density.  Now when you take that piece of bar stock back to the shop and machine it down to say 1-1/4" OD x 2-1/8" OD, you have now changed the radial crushing strength, impregnation efficiency & density.  Once the customer makes that "structural" change we, Atlas Bronze, can no longer say that that material is certified to the ASTM B438 specification.  It is believed that when you "ream" or "bore out" the ID of a piece of 841 Bar Stock you close the pores and alter the impregnation of that piece of bar.  At that point, Atlas can no longer say that the Signed C of C that lists specific physical & mechanical properties apply.  


Buy what you need.  If you need a Bushing, order a Bushing.  If you need a Flanged Bushing, get a Flanged Bushing.  Yes, you might pay a little more if the parts are not standard sizes...but in the long run, you know that the Physical & Mechanical Properties as outlined in the ASTM B438 spec will be in conformance to what you purchased. And most importantly read the specifications, ASTM B438-05,  ASTM B438-08 & ASTM B438-13.

 If you need a copy of one or all of these specs you can download them by visiting:  ASTM.org and type in ASTM B438 in the search bar.  

**DISCLAIMER:  While Maranatha Now, Inc. dba Atlas Bronze strives to make the content on this blog as informational and educational as possible,  Maranatha Now, Inc. dba Atlas Bronze makes no claims, promises, or guarantees about the accuracy, completeness, or adequacy of the contents of this blog, and expressly disclaims liability for errors and omissions in the contents of this site.