Taffy said:
well i had really long chat today with two or three bearing stockists and eventually went on to none other than SKF in the UK.
another thing - we never discussed the revs or the loading as it was felt that the bearings should - all being well - be up to the job.
well that's it for this time.
regards
Taffy
Taffy, thanks for this, it is interesting. I'm not going to knock anything :wink: I''ll try to add to the conversation.
Speed and load factors were discussed a long long time ago but I think those discussions are all but lost. But for reference the limiting spped for the ball bearing 6206 is 15k rpm with a reference speed of 24k rpm with a dynamic load of a bit over 20kN and static load of 11 odd kN. That's just under 2500 or 1100 kilogram force respectively . Reference and limiting speed for the roller bearing will generally be less but the loading will be higher. It is probably enough
As for the comment about misalignment. of course there is a theoretical ideal that the bearings should be perfectly aligned but I refer you to the technical reference by SKF themselves:
This one is for roller bearings:
"Misalignment
Single row deep groove ball bearings have only limited ability to accommodate misalignment. 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
– the bearing size
– the internal design
– the forces and moments acting on the bearing.
Because of the complex relationship between these factors, no generally applicable specific values can be given. However, depending on the various influences of the factors, the permissible angular misalignment lies between 2 and 10 minutes of arc. Any misalignment will result in increased bearing noise and reduced bearing service life."
Now read what it says about the cylindrical roller bearing:
"Misalignment
The ability of single row cylindrical roller bearings to accommodate angular misalignment of the inner ring with respect to the outer ring is limited to a few minutes of arc. The actual values are
– 4 minutes of arc for bearings in the 10, 12, 2, 3 and 4 series
– 3 minutes of arc for bearings in the 20, 22 and 23 series.
These guideline values apply to non-locating bearings, provided the positions of the shaft and housing axes remain constant. Larger misalignments may be possible but may result in shorter bearing service life. In such cases, it is advisable to contact the SKF application engineering service.
When the bearings are used to locate the shaft axially, guideline values must be reduced, as uneven flange loading can lead to increased wear and possibly even to flange fracture.
The maximum values for misalignment do not apply to bearings of the NUP design or bearings of the NJ design with an HJ angle ring. Because these bearings have two inner and two outer ring flanges and the axial internal clearance is relatively small, axial stresses may be induced in the bearing. In case of doubt, it is advisable to contact the SKF application engineering service."
As you will note, generally there is a greater tolerance for misalignment in the ball bearings that in the roller bearings.
You'll also notice that the earlier reference to "flange" is correct in engineering terms :wink:
As for internal clearance, this is usually specified according to ISO 5753:1991 and as you say C3 is greater than normal, then C4 and C5. C2 is less than normal.
To indicate the differences:
A ball bearing of a bore diameter of 30mm would have a normal internal clearance of min 6 micrometers and max 20. C3 would be min. 15 max. 33 micrometers and C4 min. 28 max 44.
Technically speaking we are talking internal clearance not tolerance.
I have seen C4 bearings come out of engines that have apparently not been opened up before.
So lets look at this from the bearing perspective.
Whether roller or ball they most probably have the necessary load tolerance and speed tolerance for our purposes.
They differ in terms of tolerance to misalignment, quite significantly.
With regards to axial load both bearings are able to tolerate this. Ball bearings no more that 50% of the static load. Roller bearings however become more complicated because there needs to be runout within the bearing and other factors, including temperature, actual radial load and
lubrication.
I suspect there may be a problem here. Because of the internal bore fit the rollers will tend to allow some axial shift whereas the roller can't and we must then rely 100% on the internal axial clearance within the bearing and its ability to tolerate axial load and/or crankshaft end float.
Now, it is very interest to note that end float specification has increased in latter years, almost doubling....
I know what I say doesn't answer any questions directly but it does beg some questions.
Now in terms of failures is it most often the left hand bearing that disintegrates? I believe it is. Given that this side also supports the counterbalancer assembly I believe they have to be considered in unison.
Big end pin diameter as I've said before cannot merely be accounted to dealing with crankshaft flex as it usually also relates to conrod loading too. If it is flexing, is it going to flex more than 3 minutes of the arc? Or is it more of a case of whip?
I sincerely believe there is something to do with internal engine harmonics going on - however mad that might seem....whether it is the absolute cause, I don't know but I think it's relevant.
All the best,
Simon
