Originally Posted by
Lonewolf_50
It is troubling to me, the idea that of two suppliers for a precision component in a drive train/transmission, one had 3x failure rate (or significantly less reliability) and yet was still retained as a supplier/vendor by the manufacturer. While there may be more to that story, on the face of it someone didn't take reliability as seriously as they ought to have done, or, a batch/lot of material of considerable size was produced and distributed and it took a long time for people to discover and try to remedy that.
I think we need to be careful not to conflate spalling with ‘failure’. I am not even sure the term “less reliable” is appropriate either, as Time Since New is not listed against each unit withdrawn, only collective fleet hours fitted with respective bearing types (apart from the Angolan one).
The two catastrophic failures were by fatigue crack propagation beyond the hardened depth. Not all spalling results in deep fatigue cracking, as evidenced by the 20 FAG bearings that have spalled ‘harmlessly’. Remember that 7 SNR bearings have spalled ‘harmlessly’ too. Given the total production numbers of bearings, most of both types would actually appear to have either been still in satisfactory service or have reached design SLL when these statistics were gathered. In that sense the behaviour of all except the REDL and LN-OJF bearings has been ‘reliable’ in that they followed the expected end of life degradation process.
You can look at those same statistics presented in Table 6 a very different way. Look at the total number of units of each type and how many of them DID NOT produce spalling. That would give “reliability” scores of 99.35% and 99.76% for FAG and SNR respectively. Not so clear cut now is it? Remember also the definition of L10 life for bearings (see 1.6.8.2).
You cannot equate that minor difference in reliability simplistically with Hertzian contact stresses of 1800MPa and 1550MPa (86% of 1800MPa) respectively. There is far more to it than that. A lower Hertzian contact pressure (Edit: by widening the path) in a spherical raceway can involve more slippage and, depending on lubrication, generate more surface friction that is more likely to turn a crack inwards.
Further investigation should not only focus on metallurgical differences but the whole tribology of these bearings and their operational conditions.
Last edited by Concentric; 16th May 2017 at 11:13.