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Wave patterns and thrust roller failure.


We have recently failed our uphill thrust roller shaft. It is my opinion that the failure of the thrust roller shaft is ultimately due to extended periods of uphill thrust of the unit. The root cause(s) for that need to be evaluated. The cyclic fatigue followed by brittle fracture evident on the bearing stationary shaft is likely due to steel "waves" forming on the south thrust face (varying @ roughly 1.5 - 3" intervals) compounded by extended time under continuous thrust. Alternatively, any crack that had propagated through the shaft may have caused roller deflection and caused the waves (now I am definitely speculating). In any case, I feel that constant thrust alone would not have failed the roller in the same way; likely the bearing would have failed via high temp mechanisms. These waves have formed relatively recently (5 months) and it is not something that many people have seen before. If the past roller did contribute or directly cause the waves, we may have eliminated further propagation by installing a new one (unlikely - washboard effect on gravel road stop signs?), however I think it may be more than that. Note the period between waves seems to have some common specific interval, but do vary in places and may overlap. The cycles required for this type of failure is easily understandable given the period of the wave and the drum rpm (pretty darn quick.) Any input on the wave formation and how to move forward would be much appreciated. Cliff

Dear Cliff, I think your analysis is pretty much correct. Let me fill in a few more details. Wave patterns, which are not uncommon, have two potential sources; 1) gear and pinion - uneven power transfer from tooth to tooth due either to badly worn teeth or misalignment or 2) high thrust loads due to excessive support roller skew. The difference in symptoms between case 1 and case 2 is the spacing of the "waves". If they are spaced like the gear teeth we have case 1 if the spacing is greater and not necessarily uniform we have case 2. I think we are looking at a case two scenario here as you have indicated. When there is a strong thrust due to excessive roller skew the system loads up like spring. When the stored energy is high enough it breaks the friction between the support rollers and the tires and the drum "jumps back" and the process repeats, causing the wave patterns to develop. The thrust roller support pedestal with its lack of stiffening flanges acts like a tree in the wind and is itself a spring that stores energy and contributes in this process. In due course fatigue sets in and breaks the roller shaft (and/or the bearings) as you have correctly concluded. This is not a theory. I have personally seen this in action. Anecdote: On a much larger unit I have seen a 40" diameter thrust roller repeatedly bend more than 1" after every 5 shell rotations. Enough energy was stored and then instantly released to "jump" the unit (weighing approximately a thousand tons). Needless to say that was nothing short of an earthquake and could not be run in that condition but required immediate support roller adjustment. In your situation the unit is lighter and is running much faster and so produces a very short cycle time, hence the wave patterns. Your solution is just as simple. Adjust the support rollers correctly. You can also consider a more substantial thrust roller assembly. Larger diameter thrust roller, larger diameter shaft and a support that has some attitude (radial support gussets between the plate at the bottom and the barrel) so it does not act like a pole bending in the breeze.



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