Bearing Internal clearance and misalignment 1

[gearman1234]

I am evaluating a gearbox design received from a manufacturer and I notice that the mislaignment is about 8 minutes and the bearings used are deep groove ball bearings with C3 clearance. Manufacturer recommends max 10 minutes of mislaignment. The design is within that but it is usually advisable to work within lower misalignment angles.
Is C3 sufficient for taking up that value of mislaignmemt? As per manufacturer's catalogue the increased clearance bearings are used only for compensating higher temerature differential between inner and outer race and for compensating the expansion of inner race after shrink fit. It does not mention anything about compensating mislaignment by increasing clearance.

I would like to know what is the purpose of increased clearance bearings and will it eat up that kind of misalignment or is it inteneded to eat up misalignment at all?

 

[fwc]

Hi

Check this link;

http://www.skf.com/portal/skf/home/products?paf_dm=shared&maincatalogue=1&lang=en&newlink=1_1_10

 

[diamondjim]

Normally a clearance bearing will have
lower turning torques and allow more
misalignment. The trouble with angular
measurements without knowing the size
range is that it is difficult to translate
into a dimensional range. I assume you
will have to pay a premium for reduced
clearances.

 

[jmoritz]

The extra clearance will help a bit but the dramatic drop in bearing service life curve starts at 2 minutes.

 

[gearman1234]

Hello jmoritz

Is there any evidence of that happening. I tried to search SKF website but could not find any info to corroborate your view.

 

[electricpete]

I can see the logic that change in internal clearance would affect the ability to withstand misalignment. However all the literature I see identifies limits for bearing mislignment for different types of bearings without any mention of internal clearance.

One note, all of the industrial machinery I work with uses C3 clearance, so I tend to think of it as normal clearance, in spite of the AFBMA terminology.

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[gearman1234]

The limits of mesalignment mentione dare general. The claearnce is not only supposed to take misalignment into account but also the reduction in clearance due to rise in temeperature (in some cases), stresses acting on bearing also need to be considered. Each case will be different for mislaignment analysis.

 

[jmoritz]

I've plotted many misalignment vs. life curves and they all come out to show a dramatic difference between 2 & 4 minutes. Put up some app details and, although I can't guarantee good/fast timing, I'll show you what can happen.

 

[electricpete]

Can you clarify what data or calcs you use to generate the misalignment vs. life curves? It is intriguing to me.

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[gearman1234]

jmortitz,

Do you have actual data or calculations which we can have a look at? The application, size of the gear box, gear design, gear quality, material,heat treatment, lubrication, horse power etc. are some of the factors which will also affect the life. I assume that the other factors were same while comparing the mislaignment.

 

[electricpete]

An excerpt from SKF Interactive Engineering Catalogue:

"Single row deep groove ball bearings have only limited ability to accommodate errors of alignment. The permissible angular misalignment between inner and outer rings, which will not produce inadmissibly high additional stresses in the bearing, depends on the radial internal clearance of the bearing in operation, bearing size, the internal design and the forces and moments acting on the bearing. Because of the complex relationship between these factors, no generally applicable specific values can be given. Depending on the various influences of the factors, the permissible angular misalignment lies between 2 and 10 minutes of arc. Any greater misalignment will result in a considerable increase in bearing noise in operation."

Still interested to hear from jmoritz


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[Hippo41]

They may have put C3 bearings in to allow room for inner ring expansion at start-up. The inner ring will heat up and expand at a higher rate than the outer ring (which is bigger and can transfer heat to the exterior more efficiently).

just a thought.

Lester Milton
Telford, Shropshire, UK

 

[brgguy22]

The short answer to your question is "yes" a larger internal clearance will handle more misalignment but the trade off is in the radial load. Hence, the "C0" clearance will handle more load. The reason for different clearances is to accomodate various shaft/housing fits, temps and application (which ring has the press fit). Another question to ask is what type of gearing is it (i.e. the resulting loads). Angular contact bearings are better suited for high axial loads. "C3" clearance bearings may be more commercially available but you have to make sure when you call out the bearing to specify what you want. "C0" or "standard" clearance bearings are still available and should not cost any extra.