“Structural” Bearing Failure

“Structural” Bearing Failure
SR4 at Abbot Point ASPEC Team Meeting Nov 2016 Adam Dunn (& Michael King)

SR4 at Abbot Point Coal Terminal
Rocker Boom Bearing Failure on SR4 at Abbot Point This is SR4. It’s a fairly typical balanced stacker reclaimer, commissioned in 2011 Earlier this year the rocker boom bearing failed. Abbot Point noticed some noise from the bearing and an inspection showed clear signs that something was wrong. The machine didn’t fall down, but it is impacting the operation of the machine. The machine was only 5 years old, so the failure was unexpected. It’s not the boom pivot pin and bearing, which we have looked at for other sites. Not the counterweight pin. Earlier this year ASPEC assisted Abbot Point with reviewing why the bearing failed. And we also looked at what replacement options were available, and how they could be actioned. It’s not an easy bearing to replace. SR4 at Abbot Point Coal Terminal Location of Bearing

SR4 at Abbot Point Coal Terminal
Rocker Boom Bearing Failure on SR4 at Abbot Point This is SR4. It’s a fairly typical balanced stacker reclaimer, built in 2012. Earlier this year the rocker boom bearing failed. Abbot Point noticed some noise from the bearing and an inspection showed clear signs that something was wrong. The machine didn’t fall down, but it is impacting the operation of the machine. The machine was only 5 years old, so the failure was unexpected. It’s not the boom pivot pin and bearing, which we have looked at for other sites. Not the counterweight pin. Earlier this year ASPEC assisted Abbot Point with reviewing why the bearing failed. And we also looked at what replacement options were available, and how they could be actioned. It’s not an easy bearing to replace. SR4 at Abbot Point Coal Terminal Location of Bearing

Why should we talk / care about this failure?
Major failures aren’t common and generally aren’t intended The cause isn’t cut and dry The design “was audited” (sort-of). Other machines might be affected ? We might learn something Theres a couple of reasons. 1) Major failures don’t happen a lot. i.e. we might learn what not to do by studying this failure. 2) No one is really sure why the bearing failed. i.e. it might be interesting to talk about. 3) Machine design was audited by ASPEC, to some extent. And also Aurecon. So this failure might be a good example to show that Standards compliance isn’t necessarily “enough”. Ill talk more about that later. 4) This bearing failure isn’t a one-off. Abbot Point has other machines with a similar design. It’s probably worth pointing out – I think it’s an interesting failure and worth talking about. And ASPEC spent a bit of time reviewing what might have gone wrong and how to fix it. But ROCKFIELD were the one’s engaged by Abbot Point to do most of the analysis.

Why do we think it happened? Loads, fatigue? Other reasons?
Outline / Scope What happened? Why do we think it happened? Loads, fatigue? Other reasons? Did something go wrong? What should Abbot Point do about it? Impact of failure Replacement options. Installation of new bearings What can ASPEC learn? When we say bearing failure, what do we mean? We can’t be sure, but why do we think the bearing failed? Was it overloaded? Was fatigue an issue? Or are there other issues that might be present? I think this point is important. It’s not common to see a failure. And there are systems in place that are supposed to prevent these failures. So why did it happen? Was the machine design flawed? Was it operated outside its design envelope? Was there something else that went wrong? The failure is impacting the machine’s ability to perform, which I’ll discuss later. So what should Abbot Point do about it? It isn’t an easy bearing to replace so I’ll talk about the installation methodologies that we reviewed.

Rocker boom pivot pin is now rotating in clevis.
What happened? Abbot Point noted: Thrust washers have been pushed out One of the bearings has walked out of position Retaining bolts have sheared off. Rocker boom pivot pin is now rotating in clevis.

Dual plain PTFE bearings. Bearings rotate with luff.
What happened? Background SR4 is 5 years old. Dual plain PTFE bearings. Bearings rotate with luff. Machine was audited by ASPEC (structural). Mass was added to the machine. Terrain tracking enabled recently. 1) 2) GGB Bearing Technology (GGB) 3) The rocker pin bearing experiences 1.5 degrees rotation for every 1 degrees of boom luff. Over 24 degrees. 4) Rocker boom pivot pin was audited, but bearing wasn’t part of our scope. Aurecon undertook mechanical checks but it’s clear they didn’t look at this bearing in great detail. 5) 6) Does everyone know what terrain tracking is? Terrain tracking is relevant because it was heavily discussed by ROCKFIELD as a cause for the failure. To quickly explain terrain tracking. A surface map of the terrain below the stockpile is programmed into the PLC. When the machine is reclaiming on the bottom bench, the luffing system tries to follow the grade of the terrain. To explain the impact on the rocker boom pivot bearings I’ve got a few graphs.

What happened? This graph indicates that the majority of the luffing direction reversals (approx. 80%) occur with 1.5 degrees or less of luffing travel after the direction change. What this shows is that with terrain tracking on, the machine experiences a moderate amount of very small rotations. You can also see a few other rotation angles. Around 6 and 8 degress there are changes between benches. And the 26 is a full luff up or down.

What happened? This graph shows the number of small rotations associated with terrain tracking. The green lines show 1 degree of luffing angle change. The red lines show 1 day’s duration.

Why do we think the bearing failed?
What causes bearing failures? Loads on bearing Fatigue Maintenance Installation So I just touched on what happened with the bearing failure, and showed some of the effects of terrain tracking on luffing rotations. So now the next question. Why do we think the bearing failed?

Loads Dead load = 3,237 kN on bearing. Abnormal digging = 3,464 kN. Fatigue loads = 3,527 kN (higher than UU). Max dynamic = 3,605 kN. Max static = 4,080 kN. Sensitive to counterweight mass - additional mass added since design?. The bearing loading appears to be quasi-static. range of motion of approx. 25 degrees rotates very slowly (0.21 mm / sec), only ~30 times/day. The take home messages for loads on the bearing are: The loads on the bearing are fairly constant. 2) The loading on the rocker boom pivot is sensitive to the counterweight mass. Based on ASPEC correspondence from the original SR4 audit in 2009, it looks like the counterweight was increased by 60t from 290t to 350t. This additional ballast would cause significantly higher pivot bearing loads than originally anticipated.

Static capacity of bearing was adequate.
Why do we think the bearing failed? Loads Safety factor of 1.8 for maximum static loads (failed luffing cylinder). Generally SF over 2 for operation. Static capacity of bearing was adequate. Frequency of the bearing oscillations observed may justify a dynamic analysis of the bearing. We analysed 6 weeks of SR4 luffing cylinder data during stacking and reclaiming with terrain tracking enabled. This data showed approximately 28 cycles per day, or roughly one cycle per hour. Over the life of the bearing, this number of cycles could be significant.

Predicted bearing life using dynamic analysis is low.
Why do we think the bearing failed? Cycles / Dynamic Bearing Analysis This bearing is not a typical dynamic bearing, it is “structural”, so do cycles matter? Predicted bearing life using dynamic analysis is low. Impact of terrain tracking is significant – a large reduction in bearing life predicted with TT enabled. We checked whether these cycles matter. We have a good idea of the loading on the bearings. So we used the predicted life formulas given by the bearing manufacturer. The two ways to approach this are to use the loads to determine what the predicted number of cycles will be. The other way is to pick a bearing life and determine what loads need to be limited to, to reach that number of cycles. What we found was: 3) The bearing life was only predicted to be approximately 3,000 luffing cycles, and even that required some non-conservative assumptions. 4) With the number of cycles expected with terrain tracking enabled, the bearing would need to be loaded to about 40% of dead load for a bearing life to even be calculated. These results seemed a bit strange to me. Statically the bearings are fine. They move very slowly and minimally, and because of this their predicted life is very low. Will terrain tracking really have that big of an effect on the bearing life? We are talking about very small bearing oscillations of around 1 degree or less, occurring over minutes.

Dynamic Bearing Analysis When does a bearing go from being statically loaded to dynamically loaded? Does it matter? Which is our bearing? It matters. Nominal load capacity goes from 250 MPa to 140 MPa if the bearing application is considered dynamic. Bearing suitability calculation methodologies change. There is no good threshold between statics and dynamics. Anecdotal evidence suggests it does matter. Allowable loads given by bearing manufacturers vary massively between a statically and dynamically loaded bearing. For our bearing the difference is 250 MPa vs 140 MPa as soon as the bearing is considered dynamic. And the calculation method used to estimate the suitability of the bearing also changes. This is OK for clear cut cases. For instance: A wheel bearing is obviously dynamically loaded. A spherical plain bearing might be used to take up installation misalignments and isn’t designed to rotate. This is clearly a case for static loading. There is no clear threshold determined by the bearing manufacturer or other relevant source to determine when the dynamics/fatigue approach to calculating bearing life becomes more relevant than static bearing analysis. And I don’t think I’m being too pedantic here. I think there are actually a large number of structural bearings on machines that we work with that may be more suited to the dynamic bearing life calculations given by manufacturers.

Dynamic Bearing Analysis Bearing appears inadequate based on strict dynamic analysis Static capacity / one direction quasi-static Dead load This slide is a graph from the bearing manufacturer to show the relationship between bearing pressure and desired life. The red line is the static capacity, or quasi-static with rotation in one direction only. Where a bearing is oscillating, bearing capacity is reduced for a given number of cycles. I’ve also put a blue line on the graph to show dead load only. This is to show that in our case, using the manufacturer’s dynamic analysis, any oscillating or dynamic movement at all will be problematic. Note – this graph doesn’t take the velocity of the movement into consideration.

Dynamic Bearing Analysis PV Analysis Bearing fatigue is based on a combination of bearing pressure (load) and the sliding velocity between the bearing surfaces, V. Theory & practice don’t align at low V. Very high P (static loads equivalent) with very low V can give values similar to moderate P and V. Related to wear of bearings, not crack growth. Oscillations Rotation in one direction only “is more static” ?? Does terrain tracking count? I mentioned that there isn’t a good threshold between dynamic and static analysis. Typically, with a bearing that moves, we could use a PV analysis to determine suitability. Bearing wear is based on a combination of bearing pressure, P, and the sliding velocity between bearing surfaces, V. Theoretically, as V approaches zero (in other words, the bearing isn’t rotating), the allowable pressure becomes the same as the static value (i.e. no rotation). But in practice, for very low values of sliding velocity, and high contact pressure, bearing life calculations don’t give expected results. Generally, using manufacturer’s calculations, if you expect ANY movement, the bearing capacity drops significantly, as I just showed. But is this bearing really acting dynamically? That last graph was independent of velocity – shouldn’t the slow rotation of our bearing increase the bearing capacity? Another issue we had with the analysis of this bearing was the definition of oscillations given by the manufacturer. According to the manufacturer GGB (and they aren’t alone), where a bearing is oscillating, the bearing capacity is reduced, whereas a quasi-static rotation in one direction only does not reduce the bearing capacity. But how much oscillation is required before we count it? Do terrain tracking luffing movements of ½ a degree count? They aren’t strictly sinusoidal oscillations, but they are direction reversals. But they are also very small – is there a threshold below which we don’t count?

Dynamic Bearing Analysis “Effective PV factor”: If oscillations are ignored the low sliding velocity provides a calculated bearing life of 4,840 hours. Compared to active movement time (active luffing time) this life may be considered appropriate. The impact of terrain tracking on bearing life is complex – if we count these movements as true oscillations/reversals, it would be very difficult to get a bearing to work in the rocker boom pivot, and probably the boom pivot too. So I made some assumptions to understand the limitations of the manufacturer’s calculations for bearing life. The bearing life is calculated based on the manufacturer’s “Effective PV Factor”, the low sliding velocity of the bearing provides a calculated bearing life of 4,840 hours. A very big assumption here, is that this analysis requires that cycles are ignored. And that leads me to my next point.

Installation tolerances were outside recommendations.
Why do we think the bearing failed? Installation “Misalignment over pair of bearings should not exceed 0.020mm” Surface finish better than 0.4 micrometers. Installation tolerances were outside recommendations. Deflection of rocker pin under load would exacerbate this misalignment. Installation tolerances for a pair of heavily loaded PTFE bearings like those used on SR4 is critical. The manufacturer lists the following requirements. And they state these are particularly important with DU bearings because there are no lubricants to spread the load. In practice this would be very difficult to achieve in the pivot bearing location with a pair of bearings in tandem. The as built drawings showed misalignment tolerances an order of magnitude higher, around half a mm. This misalignment of the pin and bearing bores during construction may have been a significant contributor to the bearing failure, perhaps as important as the design capacity of the bearing.

Bearing maintenance was OK.
Why do we think the bearing failed? Summary Static capacity of bearing was OK, but significantly higher bearing loads than originally anticipated (extra counterweight). Predicted bearing life using dynamic analysis is low (Impact of terrain tracking significant) – relevent? Bearing maintenance was OK. Bearing was improperly installed. So in summary, why do we think the bearing failed? There are a few reasons: Bearing installation appears to be the most likely candidate.

So, did something go wrong?
Very difficult to identify the single most influential factor in the failure. Bearings may have been undersized? Extra counterweight. Given the small oscillation range, relatively constant load, and very low sliding velocity, ASPEC believes it isn’t clear cut whether the bearing should be analysed using a static or dynamic basis. Isn’t clear whether terrain tracking is a primary cause. Misalignment of the bearing pairs during construction may have been as important to the failure as capacity of the bearings. Spherical plain bearings may have avoided the issues with installation. I guess the answer is YES. The bearing failed, so something did go wrong. And like most failures there is rarely one easy answer for the cause. We think the installation tolerances used were a contributor, but maybe the bearings were undersized?

Design Bearings weren’t part of an audit. Would an audit have revealed anything? Static or dynamic bearings? Dynamic analysis might be relevant but may not be accurate. Other manufacturer’s would suggest this bearing is operating in the quasi-static range. Terrain tracking was enabled. Isn’t clear whether this was part of machine spec. Do these small oscillations really reduce life of bearing so much? Installation Was problematic. Dual plain bearings is probably not a good design. The two identified issues to talk about here are: Did Something Go Wrong in The Design Did Something Go Wrong in The Installation. It’s hard to say whether something went wrong. The two key issues I think are that the dual plain bearings design is probably not a good design. But it doesn’t appear to be non-compliant.

What could have prevented this failure? Should we do anything differently in the future? If a better design exists, what is our role in recommending it? Another way to look at whether something went wrong, is to ask a few questions like: I don’t have good answers for these questions, except to say, maybe spherical plain bearings would have been a better design. Which leads me to the next question. In which circumstances should we say, “look this design appears to be adequate, and is compliant with Australian Standards, but there may be a lower risk profile associated with a different design”. Or should we always strive for compliance only? FAM is reluctant to change the design on their new machines, despite this failure, so it may not have been very useful identifying this bearing arrangement during the audit anyway.

What should Abbot Point do about the failure?
Impacts of the bearing failure Seized bearing = additional friction torque (~ 1.3 MN) Torque in pin Issues with AS3990 & fatigue Cracks have been detected Higher luffing loads Equivalent fatigue based on bucketwheel torque SCADA Additional loads appear lower than AS fatigue (0.7 * U). So lets look at some impact of the failure. The major impact of the failure is the additional friction torque that the bearing is causing. The failed bearing is seized. The rocker boom pivot pin is rotating inside the clevises at the ends of the pin. (show next slide). This is causing a significant extra friction torque on the pin between the seized bearing and the clevises. This was estimated to be 1.3 MN, which is significant. This has a couple of effects on the machine. Firstly, there’s the actual torque being put into the rocker boom pin, A pin check to AS3990 shows the pin is undersized to handle this torque in addition to the design loads. Interestingly, cracks have been detected in the pin on SR4. Suspected to be fatigue cracks. They could be due to the friction torque reversal due to the bearing failure. The other main effect is the extra loads put into the machine structure from the additional luffing loads to overcome the friction. However, we found the additional loading due to the bearing failure friction does not look like it is causing global fatigue loads higher than the original design fatigue loads.

This is a graph of luffing cylinder force, plotted versus luffing angle, after the failure. This graph shows 2 interesting things. First there is the two distinct lines of luffing force at each luff angle. Secondly, at 5 degrees luff the tension limit appears to be exceeded. This may be a significant issue as it may compromise the stability of the machine. For instance, this is equivalent to a large upward force on boom when the machine is already at its most counterweight heavy. We aren’t really sure whether this is due to the bearing failure (which may not be a stability issue), or whether this is something completely separate.

This graph shows how we estimated the additional friction torque at the bearing. As I just mentioned, the luffing force graph showed two distinct lines for force versus luffing angle. This is a zoomed in version. Normally you would expect a reasonably consistent luffing force at each luffing angle. For SR4 we assumed that the two lines are present because the luffing force each luffing angle is different when the machine luffs up versus luffs down, due to the friction in the bearing. The machine is counterweight heavy, which puts the cylinders naturally in tension. When the machine luffs up, the friction actually helps the cylinder, lowering the amount of tension. When the machine luffs down, the cylinder has to overcome bearing friction, increasing the amount of tension required. We measured the average difference compared to the design cylinder force, and we estimated a torque force of 1.3 MN, which was similar to the number ROCKFIELD got using a different technique. We also reviewed strain gauge data to assess ROCKFIELD’s findings on the magnitude of additional loading of the machine due to operating with the failed bearing still in place.

Large friction torque & increased luffing loads. The pin does not comply with AS3990 strength checks due to the significant additional friction torque. Fatigue also appears to be an issue. Fracture mechanics assessment of the pin appears to indicate there is no immediate risk of total failure. Global forces on clevises hasn’t increased BUT the clevises weren’t designed for rotation. This slide presents a summary of the impacts of the failure.

This graph is a rainflow count of bucketwheel torque. We plotted this to get an idea of how hard the machine is working. This shows the equivalent fatigue digging load (using bucketwheel loads collected after failure) is less than the design fatigue load. Now we wouldn’t necessarily assume the digging loads would go up due to the rocker bearing failure. But we plotted this to get an idea of how hard the machine is working, to see whether the machine operation would have had an affect on the bearing life. It appears that the machine is working about as hard as would have been expected in the original design. The other reason we plotted this is to put into context the AS3990 non-compliance of the pin. The thinking was, if the pin is already hard-working, and we have a feel for the additional loads caused by the bearing failure, we could assess whether the rocker pin could handle the additional load. There are some other interesting points on this graph: The 1st overload protection has been exceeded a number of times The 2nd overload protection seems to be performing correctly The number of cycles exceeding the design fatigue is more than usual for this type of machine.

Pin and clevis should be inspected regularly New bearing, new pin Like for like? Could installation tolerances be achieved? Is bearing capacity adequate? Confidence is low. Spherical plain bearing Comparatively easy to install Single bearing Increased load capacity requires larger diameter Audit remaining machines with a similar bearing Additional failures have been noticed. Disable terrain tracking? Ensure cylinder force is calculated in PLC. Check protection system settings in the short term. So, what should Abbot Point do about the failure? Well a few things have been recommended by both ROCKFIELD and ASPEC. Particularly since a crack in the pin has been found. Secondly, the most obvious, they need a new pin and bearing. But should they go like-for-like? Like-for-like might not be the best option. (next slide shows spherical plain bearing). 3) Accurate assessment of remnant bearing life may be difficult because there are factors other than fatigue that are relevant to the failure (e.g. construction tolerances that may be specific to SR4). 4) Should they disable it? This was recommended by Rockfield. It’s hard to say, but at this point the conservative approach would be to disable it until the effect on bearing life can be better established (i.e. bearing manufacturer sign off).

Construction methodologies Just quickly I’ll run through the construction methodologies for retrofitting a new bearing. And then I’ll close out with some lessons that I thin we can take away. The machine has a storm cradle, but ROCKFIELD

Why should we talk / care about this failure?
Major failures aren’t common and generally aren’t intended The cause isn’t cut and dry The design “was audited” (sort-of). Other machines might be are affected ? We might learn something So going back to one of my first slides. Why should we care about this failure? So the final slide I have is what I think ASPEC can learn from this failure.

Installation tolerances matter for plain bearings in tandem
What can ASPEC learn? Specific Terrain tracking can significantly reduce theoretical design life of “structural” bearings that move as the machine luffs. (Actual damage questionable). Installation tolerances matter for plain bearings in tandem Go with our gut – a spherical plain bearing may have been a better design General Maybe we should be auditing these bearings? Dynamic bearing analysis threshold. Consider whether dynamic analysis is more appropriate. Design audits / compliance aren’t necessarily enough. These are all pretty obvious, but I think some of them are more complex than they appear. Terrain tracking has significant implications on the theoretical life of bearings such as the rocker boom bearings. The same can probably be said for the life of other mechanical equipment. Which raises a few questions: Are terrain tracking oscillations relevant to bearing life calculations as presented by manufacturers? Are there other control systems advances that may have unintended effects on the lifespan of machines and their mechanical equipment? This is obvious, but it slipped through the cracks of the design audit. These tolerances are critical and should be checked. Most other machines have spherical bearings in this location. This may be another one of those cases where, even though plain bearings appeared to work, the design could have been flagged as it “just didn’t look right” with plain bearings. That’s certainly the feedback I got when I showed the design around ASPEC engineers. 4) In this case we didn’t get a mechanical audit budget. So arguably it was Aurecon’s responsibility to audit the bearing with other mechanical components. They may not have had any grounds to say the bearings were inadequate anyway. But I guess my point is that, to be conservative, maybe we should be checking the life of these structural bearings, and maybe as if they are dynamic components. Which leads me to the next point. 5) I think another big take home message for me was the lack of a dynamic analysis threshold for bearings. As I talked about earlier, some of the bearings we come across are not obviously one way or the other, structural or mechanical. For bearings like the rocker boom pivot bearing, how do we reconcile the very slow but dynamic movement of the bearing with the reductions in bearing capacity provided by manufacturers? Should quasi-static just be analysed as static? 6) This last point is a bit fuzzy, but the point is, this machine was audited. On the surface the design appears to be compliant. And yet the bearing still failed.

“Structural” Bearing Failure

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