Monday, August 29, 2016

Bearing Design Guide: Chapter Five: Bearing OD, Wall and Length Calculations


 Here is another time where the "think of a doughnut" theory comes in handy.  You have an inside circle and outside circle.  These dimensions are important and the measurement in between the two is crucial to the designing of the part.  The length of the part is important too.  For example, a bearing is not 3" long just because it fits into an empty space in a machine, its part of a calculation that needs to be precise so that each of the dimensions of the part can withstand all the needs for each individual type of service.



            In designing a sleeve bearing, the wall thickness must be taken into consideration to retain sufficient rigidity and strength to support the load without deformation and to offset the weakening effects of such features as grooves,  holes,  notches  or cut-outs that may be required.

Although strength of material tests show that  a thinner wall will sustain a higher compression  load and have a higher fatigue resistance than a heavy wall, consideration must include the housing material for the added support and  strength. The thinner wall also offers a greater economy.

The wall thickness can be calculated as a percentage of the bearing inside diameter or using the following formula:






Light service: wall thickness=.08D + 1/32" 
Medium Service: wall thickness=.08D + 1/*16" 
Heavy-Duty Service: wall thickness=.08D + 1/8" 




 
For standardization purposes, the results should be taken to the nearest 1/16" dimension with the high side of the wall thickness favored when designing with the features of grooves, holds, cut-outs, notches, etc.

For flanged bearings, the flange thickness can usually be taken as the wall thickness or modification to meet the design requirements.

Length calculations: The length of the bearing should be designed to meet the type of service  involved and to meet the projected bearing to maintain the unit load within acceptable limits of the alloy.

However,  rather than to lengthen the bearing to meet  a projected  bearing  area for the unit load  involved, it is more desirable to increase the bearing  diameter which also increases the surface   speed.

For high speed bearings, it is more desirable to stay within a LID ratio of 1/2 or less. This minimizes the frictional heat being generated  and to reduce the problem  of edge  loading.

For general bearing service, the length of the bearing should not be less than 1 to 1 1/2 times the shaft diameter.

For slow and negligible speeds coupled with heavy loads, an LID ratio of about 3 should be satisfactory. Beyond this ratio, misalignment  and edge loading may become  problems.

Permissible loading where speeds are below 30 fpm, loading can approach the yield point of the alloy when divided by a safety factor of 2.


If there is shock and impact loads, the bearing may support 25 to 50% of the permissible static or yield strength. (These are merely suggestions and are not meant to be specific values.)

               Who would've thunk that there were so many important pieces of the puzzle to the designing of one part.  I hope you are learning something new and you are finding this helpful in some way.  I say goodbye for now.  Until next time my metal loving friends...


Next Up: Week Six, Chapter 6:Recommended Assembly & Retention Practices

Friday, August 26, 2016

Bearing Design Guide: Chapter Four: LOAD SPEED CATAGORIES AND USAGES-Day 5



Manganese bronzes: Extreme high-loads, low-speed.

Manganese bronzes are actually brasses since they are an alloy of primarily copper and zinc. They are the strongest and one of the more wear-resistant, non-heat-treatable alloys with high tensile strength, high yield strength, hardness and ductility similar to steel.

The manganese bronze alloys require mating with a hardened shaft, good finishes, good lubrication and larger running clearances than the other bronze alloys.

Since they also exhibit a short freezing range in solidification, they are used for permanent molded castings but not to the extent of the aluminum bronzes


The manganese bronzes have a high-compression strength suitable for vibration and heavy rolling contact but with speeds substantially below 250 fpm.

They are corrosion resistant and can be more economical than the heat-treated aluminum bronzes.
The average physical properties are:
Brinell hardness 180 to 225 BHN Coefficient of friction (dry) .25 to .35 Minimum shaft hardness 500 BHN
Maximum operating temperature 400 degrees F

 

   CDA             Tensile Strength PSI             Yield Strength PSI          Elongation %          BHN
    862                         90000                                    45000                                18                      180
    863                        120000                                   62000                                12                      225
  

Principal uses: Movable bridge bearings and plates, pump bodies, gears, roller-gate bearings, fixed wheel gates, sheaves hoists, crane wheels and off-road construction equipment and machinery, crushing machinery, and  permanent molded  products  as wear plates, pads, hinges, etc.
 
            Although CDA 865 Manganese bronze is in this family of brasses, the physical and mechanical properties are somewhat less than the above two, with tensile strength of 65,000 PSI, yield  strength of 45,000 PSI,   elongation of 20%, and a brinell hardness of 100 BHN. It can be used for surface speeds up to 250 fpm, with loads reasonably higher than the leaded bronze  and tin bronze alloys.  Since it has a short-freezing range in solidification in casting, it also can be used for permanent mold castings.

The preceding lists are intended as a working guide to the selection of various bronze alloys to meet certain speed-load categories for the greatest number of desireable properties they offer based on past usage.

Many other factors enter into the final decision and when each factor is refined, they will indicate the most promising alloy to be satisfactory  for that set of conditions.

Well...that's it for the Chapter 4: Load Speed Categories and Usages!  I hope ypu had a fun ride and now feel the need...THE NEED FOR SPEED!  

 Until next time my metal loving friends...

 Next Up: Week Five, Chapter 5:Bearing OD, Wall & Length Calculations

Thursday, August 25, 2016

Bearing Design Guide: Chapter Four: LOAD SPEED CATAGORIES AND USAGES-Day 4


Aluminum Bearings: High-loads, Low-speed

          The aluminum bronze alloys are used more frequently as bushings for their wear resistance, heavy load-carrying ability, and temperature service since they retain their physical and mechanical strengths at higher temperatures than the other bronze alloys. They possess excellent corrosion resistance and are used extensively in marine service.  Because of their short freezing or solidification range in casting, they are preferred for permanent mold castings.


          The aluminum bronzes with about 8% aluminum content will not respond to heat treatment.  Above 10% aluminum content, they can be heat treated.

           The aluminum bronze alloys are considered to be non-sparking and suitable for equipment handling explosives, petroleum, chemical and gas products and in mines.
                 
                            The average physical properties are:
                            Brinell hardness 140 to 230 BHN
                            Coefficient of friction (dry) .30
                            Minimum recommended shaft hardness 400 BHN
                            BHN with heat-treated aluminum bronzes increased to  600 BHN
                            Maximum operating temperatures 750° with intermittent service   through 1200° F.




   CDA             Tensile Strength PSI             Yield Strength PSI          Elongation %          BHN
    952                         68000                                    26000                                20                      125
    953                         70000                                    26000                                25                      140
    953HT                    80000                                    40000                                12                      175           954                         85000                                    32000                                12                      170
    954HT                    95000                                    45000                                10                      195
    955                         95000                                    42000                                10                      195
    955HT                   110000                                   62000                                 8                       230

Principle uses: Marine equipment, pump impellers, acid-resistant pumps, valve seats, guides, worm screws, nuts, gears, mine equipment, chemical, gas and petroleum-related machinery and equipment. Several of these aluminum bronze alloys are used extensively in permanent molded castings such as pole-line hardware and other products replacing sand-cast items.



 Well...that's it for our fourth section. Only one more to go!  Until next time my metal loving friends...



 Next Up: Chapter 4: Load Speed and Usages- Manganese Bronzes: High-loads, Low-speed

Wednesday, August 24, 2016

Bearing Design Guide: Chapter Four: LOAD SPEED CATAGORIES AND USAGES-Day 3




 
Tin bronzes:Low-speed, High-loads

          The tin bronzes are hard bronzes that offer increased load-carrying capacities.  They are capable in accepting shock and impact loads but with reduced surface speeds. They possess excellent corrosion resistance but require mating with hard shafts, good surface finishes and good lubrication since they are more prone to seizure and cold-welding than the bronzes containing lead.

          The working load can be taken at 5000 PSI but with reduced speeds. Surface speeds should not exceed 250 fpm unless positive and ideal lubrication are available.  The permissible static load can be taken at 10,000 PSI. These alloys require accurate alignment and 
hardened polished shafts.

                    
                    The average physical properties are:
                    Brinell hardness 60 to 80 BHN
                    Coefficient of friction (dry) .25 to .30
                    Minimum recommended shaft hardness 350 BHN
                    Maximum operating temperature 550 ° F

   CDA             Tensile Strength PSI             Yield Strength PSI          Elongation %          BHN
    905                         44000                                    25000                                10                       75
    907                         40000                                    25000                                10                       80
    927                         38000                                    20000                                 8                        77


Principle uses: Pump impellers, valve components, wrist-pin bushings, bridge bearings and wear plates, coal-crushing machinery, press bearings, pump and paper mill bearings, worm wheels and gears.



 Well...that's it for our third section.  Until next time my metal loving friends...



 Next Up: Chapter 4: Load Speed and Usages-Aluminum Bearings: High-loads, Low-speed

Tuesday, August 23, 2016

Bearing Design Guide: Chapter Four: LOAD SPEED CATAGORIES AND USAGES-Day 2





Leaded tin bronzes: Medium-load, Medium-speeds

          This group of alloys contain 10%or less of tin content and are generally used for the greatest number of applications.  Their chemical balance combines a good tin to copper ratio for load-carrying ability and an ample amount of free lead for anti-frictional or surface action properties.  They offer good wear resistance and can be used for impact and shock loading.

          The design working load can be taken at 3000 PSI and a permissible static load of 8600 PSI. The surface speeds through 500 fpm or less are possible.

                    The average physical properties are:
                    Brinell hardness 50 to 70 BHN
                    Coefficient of friction (dry) .25
                    Minimum recommended shaft hardness 250 BHN
                    Maximum operating temperature 500 ° F

   CDA             Tensile Strength PSI             Yield Strength PSI          Elongation %          BHN
    937                         35000                                    20000                                 8                        60
    932                         35000                                    20000                                10                       65
    935                         32000                                    16000                                18                       60


Principle uses: Bearings for most industrial and general machinery bearings, roll neck, mill and pulley bearings


Well...that's it for our second section.  Until next time my metal loving friends...


 Next Up: Chapter 4: Load Speed and Usages-Tin Bronzes: Low-speeds, High loads

Monday, August 22, 2016

Bearing Design Guide: Chapter Four: LOAD SPEED CATAGORIES AND USAGES-Day 1


          This chapter will be broken down into 5 different sections and I will be posting a new Blog post every day this week. The sections are as follows:

  1. High-Lead Tin Bronzes: High-speed, low-loads
  2. Leaded tin bronzes: Medium-load, medium-speeds
  3. Tin bronzes: Low-speed, high loads
  4. Aluminum bearings: High-loads, low-speed
  5. Manganese bronzes: Extreme high-loads, low-speed
So....sit back, fasten your seat belts and enjoy the ride!
  


GO!


  Chapter Four: LOAD SPEED CATEGORIES AND USAGES

The bronze alloys are categorized by their most desireable properties they offer for specific applications.

High-lead tin bronzes: High-speed, low-loads

          The lead content is above 12% and higher and are best suited to meet high surface speeds but with low loads since they are considered to be more plastic in nature, conform readily to imperfections of mated parts, do not require a hardened shaft and have the least galling tendencies.
          The design working load of these alloys can be taken at 1500 PSI and can accommodate surface speeds through 1000 fpm.

                    The permissable static load can be taken at 6000 PSI.

                    The average physical properties are:
                    Brinell hardness 50 to 60 BHN
                    Coefficient of friction (dry) .15
                    Minimum recommended shaft hardness 165 BHN
                    Maximum operating temperature 450 ° F

   CDA             Tensile Strength PSI             Yield Strength PSI          Elongation %          BHN
    943                         21000                                    15000                                 7                         42
    941                         25000                                    17000                                10                        53
    938                         25000                                    16000                                 8                         53


Principle uses: Turbine bearings, compressors, pumps, rotors, seals and where loads are free from  shock and impacts, where lubrication is less than ideal and speeds well over 70 fpm.


Well...that's it for our first section.  Until next time my metal loving friends...


 Next Up: Chapter 4: Load Speed and Usages-Leaded Tin Bronzes: Medium-load, Medium-speeds