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

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