ball bearing fits for small/medium horizontal motors 4

[electricpete]

Is there any on-line reference that addresses bearing shaft and housing fits specifically for motors?

For simplicity I am primarily interested in small/medium horizontal motors with ball bearings.

Is a general k5 shaft fit and H6 housing fit reasonable to apply in this case? Any reference to support it?

I realize the best info come from OEM, but OEM does not always make that info readily available. I also realize there is a lot of detailed info from bearing manufacturers (like SKF CD)... but I'm interested in something simpler geared specifically to motors.

 

[jclough]

I can't speak for other motor manufacturers but the two I've worked for both use the Machinery's Handbook as the guide for fits and tolerances. As far as I know , there is nothing geared toward motors specifically. If such a document exists I'd like to see a copy.

 

[SteveKW]

EASA talks about it, but I am sure the tolerances and fits are universal! What I found to be most enlightening is failure analysis literature from the manufacturers!

 

[Hippo41]

For standard electric motors the following is recommended:-

Shaft < 18mm; tolerance js5
Shaft 18-100mm; tolerance k5 or k6
Shaft 100-140mm: tolerance m5 or m6
Shaft 140-200mm; tolerance m6

In all cases bearing manufacturers would prefer the better tolerance, but will accept the lesser standard.

It's best to specify roundness to within 1 IT grade better than above, i.e. if you have k5, that's IT grade 5, check roundness to within IT 4.

Housing tolerance generally J7 if you require axial displacement of the outer ring (e.g. preload spring). Sometimes the outer ring is physically restrained, but J7 still OK.

the above will be OK for most applications, and can easily be set up on charts for workshop people to follow. i expect that some contributors will have preferences for other fits for specific applications.

Extracted from NSK General catalogue - other manufacturers will all be pretty much the same.
Lester Milton
Telford, Shropshire, UK



Lester Milton
Telford, Shropshire, UK

 

[jbartos]

Suggestion: Visit

http://www.misumiamerica.com/technical/pdf/usa_pdf_tech_p1379.pdf

http://www.ntn.ca/indust-fits.htm

http://www.nskaustralia.com.au/page/bearing2/J7/J7-2-1.HTM

http://www.nbcorporation.com/pdf/sr.pdf

http://www.easa.com/indus/ar100-2001.pdf

etc. for more info

 

[electricpete]

Thanks jc. I looked at Machinery's handbook and I see they reference AFBMA7 for housing fits. I got a copy of that standard and it looks useful. You share initials with another famous person.

Thanks Steve. I don't have access to any EASA stuff (it's not available on-line, is it?) but I will look at FAG bearing failure document when I get a chance.

Thanks Lester. I read AFBMA7 and SKF's manual and it looks like they share almost identical recommendations to each other. Your recommendation k5 or k6 in the range of bearings I'm most interested 18-100mm agrees exactly. Seems there is no question there.

The subject of housing fit gets a lot more complicated. (As you said there is not a lot of agreement or standardization).

SKF/AFBMA7 for general stationary outer ring load recommend H6/H7/H8. Comments &quot;outer ring easily displaced&quot; indicates it is loose enough to allow movement.

I would not see from these documents any reason to make it tighter as per your recommendation J7. J7 is recommended only for case when &quot;direction of load is indeterminate&quot; but I don't believe that to be the case for motors where weight is the primary loading. Am I looking at it wrong?... maybe misalignment and other forces should put me in category J7.

But there is another factor that might push the opposite way toward looser housing fit. There is some suggestions in SKF/AFMBA if heat conduction in the shaft or outer ring 10C more than housing it should be G7.

Interested to hear any comments on whether the indeterminate load or heating should be considered to deviate from the H6/7/8 housing tolerance.

By the way, thanks for mentioning roundness. I have never paid much attention before but when I saw your comment I looked closer and saw those tables. We ask our guys to take three different measurements 120 degrees apart but I never had an idea what is ok (!). Now I know where to go to find those specs.

 

[electricpete]

The references that I mentioned don't specifically mention motors. Are those NSK fit specs specifically identified for motors?

 

[electricpete]

Thanks jbartos. I looked at the EASA link. I see they refer to AFBMA7.

They have table 2-14 which shows shaft fits consistent with AFBMA and more importantly housing fits H6. Footnote states &quot;Fits are practical for most standard electric motor applications&quot;

 

[electricpete]

The published shaft and housing fits are more stringent than required in order to allow for unknown variation of the bearing within it's tolerances. Can we allow less stringent tolerances on our shaft and housing fits by taking advantage of actual measurement of bearing dimensions and still be in agreement with the selected tolerance class?

For example:
6313 bearing H6 housing fit

Housing ID limits 5.5118 - 5.5128
Bearing OD Limits 5.5118&quot; (140mm) +0 – 0.0007&quot; = 5.5111-5.5118

Tightest possible: 5.5118-5.5118 = 0.0000&quot;
Loosest Possible: 5.5128 – 5.5111 = 0.0017&quot;

Allowed clearance: 0 Loose to 0.0017&quot; loose.

Now lets say we have actual measurements:

Actual Housing = 5.5130 (exceeds Housing limit 5.5118 - 5.5128)
Actual Replacement Bearing = 5.5115

Looseness = 5.5130 – 5.5115 = 0.0015 (within clearance limit 0-0.0017&quot; loose)

Does it meet H6 requirements or not?
(Please don't ask me to use my judgement based on the application…. I work in nuke industry where we prefer everything black or white).

 

[Hippo41]

Electricpete
My housing recommendation based on the fact that your load was indeterminate. If you have a fixed load (e.g. belt drive) then a different housing tolerance is specified.

Your later post interesting - yes you could match shaft and housings to specific bearings (and indeed in the aerospace/miniature market you generally do). But these are for machines where extreme accuracy is required - and I can't see why you would want to do this in an electric motor.

For most electric motor applications adherence to the recommended standards should suffice.

Lester Milton
Telford, Shropshire, UK

 

[electricpete]

Lester - It appears all the different sources use the same set of &quot;rules&quot; derived from AFBMA7. But there are some different interpretations. Your interpretation is direction of outer ring load indeterminate resulting in J7 housing fit.

From reviewing various sources it seems to me that H6 is very standard housing fit for pumps and motors, in absence of specific info from machine manufacturer or experience.
References citing H6:
- EASA document linked above for motors
- SKF Centrifugaul Pump Bearing Handbook for pumps.

So it appears they are assuming direction of the load is known. I don't know if there is a firm basis or not. One excercize that I go through to ask myself whether unbalance and misalignment loads can exceed machine weight: convert vibration levels to g's. You have to get to 1ips at 3600 or 2 ips at 1800 to equal 1g which is acceleration of gravity. I realize it is not a precise analysis since we have not defined which part of the machine is moving and above/below resonance etc. Also we have housing measurements which are lower than comparable vib on the shaft. In short it is certainly not a clear-cut call. But I believe the big guys (SKF and EASA) make their opinion clear.

The question on using bearing measurements arises from situation that happened to me last Thursday. We mic the housing, shaft, removed bearings and new bearings to be installed. Knowing the actual new bearing measurements why shouldn't we take credit? It is certainly less trouble than boring/sleeving the housing.

Note the example above was our exact measurements. I have already made the call that it is acceptable and machine is now reassembled. Given your recommendation for tighter tolerance it is a little scarier to be on the loose side of the tolerance. Time will tell.

 

[jbartos]

Suggestion: Visit

http://www.techstreet.com/cgi-bin/basket?action=add&item_id=200625

for ABMA Standard 7
Also, ABMA(AFBMA) site

http://www.afcbearings.com/abma.html

shows the context of standards