North Sea heli ditching: Oct 2012
"Just for others to clerify things please see drawing
there is actualy 3 sets of bearing, one set in the bottom of shaft, one just above where the shaft cracks, and one above the epicyclic gear"
Okay, I stand corrected. The shiny part IS for a third bearing which wasn't in place when I looked up into the bottom of a gearbox. But it seems to be there to support against side loads from the reaction to torque driving the pumps, which was bothering me so much.
Even so, when the fractures occurred and the shaft was no longer driving the pumps, there would have been the main rotor shaft bearings still doing their job and the pump drive shaft would have stopped turning.
It might not even have dropped much at all, but there would not have been a noticable imbalance to the pilots and certainly no bearings and metal chunks thrashing around to do further damage.
Well, I live and learn and many thanks to FBav for the diagram on the previous page.
Pass me another slice of humble pie, please.
there is actualy 3 sets of bearing, one set in the bottom of shaft, one just above where the shaft cracks, and one above the epicyclic gear"
Okay, I stand corrected. The shiny part IS for a third bearing which wasn't in place when I looked up into the bottom of a gearbox. But it seems to be there to support against side loads from the reaction to torque driving the pumps, which was bothering me so much.
Even so, when the fractures occurred and the shaft was no longer driving the pumps, there would have been the main rotor shaft bearings still doing their job and the pump drive shaft would have stopped turning.
It might not even have dropped much at all, but there would not have been a noticable imbalance to the pilots and certainly no bearings and metal chunks thrashing around to do further damage.
Well, I live and learn and many thanks to FBav for the diagram on the previous page.
Pass me another slice of humble pie, please.
Last edited by Colibri49; 26th Nov 2012 at 01:00. Reason: Improved subsequent understanding of components
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Couldn’t agree more especially as the MOD45 MARMs showed a significant upwards trend at a point I can only assume was post shaft fracture, as it appeared to be towards the end of the flight.
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Trying to make this an exchange of information is very difficult with people going back and editing there contributions. It makes the flow of information very disjointed, and in some cases unfathomable. For information only there are 2 bearings above the gear. Non of them have anything to do with the epicyclic.
Referring to FBav's diagram on page 24, and also from a Eurocopter drawing of the 360 fracture, it occurred very close to the horizontal line at the top of the conical shaft, above the hole in the conical section.
I've just removed the second paragraph because of uncertainty as to what I'm looking at, probably mistakenly identifying as bearings.
I've just removed the second paragraph because of uncertainty as to what I'm looking at, probably mistakenly identifying as bearings.
Last edited by Colibri49; 20th Nov 2012 at 15:45.
gearbox design
Just some clarifications about some of the posts of today:
1. There is a manufacturing reason why the weld is in that place, such kind of bevel gears are finished before welding, because, with an "integral shaft", you physically can't grind the teeth (grinding wheel shall impact with the shaft)
2.If you look at the gear mesh, you see that it is above the weld, no significant torque is transferred to the bottom part of the shaft, below the weld, but "flows" to the top, to the epyciclic stage. What is reacted by the two roller bearings are the side loads of the mesh, i.e., the bending effect on shaft due to axial and radial forces of the bevel gear mesh. I will not speculate on actual stress value or margins of safety, but this is why the bottom shaft looks "thin", there is only bending stress, not bending + torque stress.
3. respect to the Main Rotor Torque, the effect of the pumps should be quite marginal (how many KWs for 2 pumps respect to the MCP...?), besides, they are driven by spur gears, that is, no axial or radial forces are introduced, so no significant side forces.
1. There is a manufacturing reason why the weld is in that place, such kind of bevel gears are finished before welding, because, with an "integral shaft", you physically can't grind the teeth (grinding wheel shall impact with the shaft)
2.If you look at the gear mesh, you see that it is above the weld, no significant torque is transferred to the bottom part of the shaft, below the weld, but "flows" to the top, to the epyciclic stage. What is reacted by the two roller bearings are the side loads of the mesh, i.e., the bending effect on shaft due to axial and radial forces of the bevel gear mesh. I will not speculate on actual stress value or margins of safety, but this is why the bottom shaft looks "thin", there is only bending stress, not bending + torque stress.
3. respect to the Main Rotor Torque, the effect of the pumps should be quite marginal (how many KWs for 2 pumps respect to the MCP...?), besides, they are driven by spur gears, that is, no axial or radial forces are introduced, so no significant side forces.
Last edited by dascanio; 20th Nov 2012 at 16:54.
Just some clarifications about some of the posts of today:
1. There is a manufacturing reason why the weld is in that place, such kind of bevel gears are finished before welding, because, with an "integral shaft", you physically can't grind the teeth (grinding wheel shall impact with the shaft)
2.If you look at the gear mesh, you see that it is above the weld, no significant torque is transferred to the bottom part of the shaft, below the weld, but "flows" to the top, to the epyciclic stage. What is reacted by the two roller bearings are the side loads of the mesh, i.e., the bending effect on shaft due to axial and radial forces of the bevel gear mesh. I will not speculate on actual stress value or margins of safety, but this is why the bottom shaft looks "thin", there is only bending stress, not bending + torque stress.
3. respect to the Main Rotor Torque, the effect of the pumps should be quite marginal (how many KWs for 2 pumps respect to the MCP...?), besides, they are driven by spur gears, that is, no axial or radial forces are introduced, so no significant side forces.
1. There is a manufacturing reason why the weld is in that place, such kind of bevel gears are finished before welding, because, with an "integral shaft", you physically can't grind the teeth (grinding wheel shall impact with the shaft)
2.If you look at the gear mesh, you see that it is above the weld, no significant torque is transferred to the bottom part of the shaft, below the weld, but "flows" to the top, to the epyciclic stage. What is reacted by the two roller bearings are the side loads of the mesh, i.e., the bending effect on shaft due to axial and radial forces of the bevel gear mesh. I will not speculate on actual stress value or margins of safety, but this is why the bottom shaft looks "thin", there is only bending stress, not bending + torque stress.
3. respect to the Main Rotor Torque, the effect of the pumps should be quite marginal (how many KWs for 2 pumps respect to the MCP...?), besides, they are driven by spur gears, that is, no axial or radial forces are introduced, so no significant side forces.
If we consider a point near to the weld, there is limited torque because that part of the shaft drives only the pumps. This limited torque results in a small shear stress.
The separating force of the spiral bevel (caused by the pressure angle of typically 20 degrees) is that which acts upwards and inwards from the centreline of the pinion towards the centreline of the rotor shaft above the epicyclic. This results in a moment that tries to tip the spiral bevel wheel away from the pinion. The reaction to this moment at the lower roller bearing (near the pumps) is of a similar scale to the gear separation force. However, at the weld, because it is significantly closer to the centre of the moment, the forces are higher. This results in bending stress at the point we are considering.
As the spiral bevel wheel turns, the bending stress oscillates from positive to negative. This oscillating stress component is far higher than the shear stress, or any stress due to the weight of the shaft or the reaction of the pump gears, and is the dominating stress in the area of the weld.
The splines for the epicyclic input gear are a small distance above the spiral bevel wheel. Since the spiral bevel wheel is a very substantial component and the input gear is a thin-walled component, it seems unlikely that there is any force from the driving of the epicyclic influencing the weld area.
Thin-walled shaft with holes in it subjected to oscillating bending stress. Interesting.
Last edited by jimf671; 20th Nov 2012 at 22:54.
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1. There is a manufacturing reason why the weld is in that place, such kind of bevel gears are finished before welding, because, with an "integral shaft", you physically can't grind the teeth (grinding wheel shall impact with the shaft)
Another interesting feature shown in the cross section drawing is that the oil pump drive gear is separate from the shaft. It appears to be attached with some sort of internal threaded faster and possibly a narrow curvic face coupling(?). The joint seems to be just below the lower roller bearing. There is also a separate inner bearing race clamped in place by the pump gear.
Thanks to FBav for the nice drawing. Very interesting discussion.
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It would appear from the latest revision to Emergency Alert Service Bulletin 04A008 that EC share my concerns about the viability of half a shaft and a reduction in supporting bearings.
Text size edited.
Text size edited.
Last edited by Pablo332; 21st Nov 2012 at 12:30.
"EC share my concerns about the viability of half a shaft and a reduction in supporting bearings."
Looking at the diagram and considering that it is very unlikely that the high torque and load bearing section of a MR shaft would be designed with a welded join, I must agree with the quote from Helicomparator.
"It definitely cracked above the bottom bearing, leaving the 2 other bearings to support the main shaft, one being just above the bevel gear (and hence taking most of the bevel gear side loads). I thikn the intention is that the bottom bearing doesn't take much except the pump gear side loads"
Looking at the diagram and considering that it is very unlikely that the high torque and load bearing section of a MR shaft would be designed with a welded join, I must agree with the quote from Helicomparator.
"It definitely cracked above the bottom bearing, leaving the 2 other bearings to support the main shaft, one being just above the bevel gear (and hence taking most of the bevel gear side loads). I thikn the intention is that the bottom bearing doesn't take much except the pump gear side loads"
Colibrì, the power required to drive the two pumps is minimal, respect to, say, MCP,( less than 10 HP, I imagine), their contribution to the Bearings reaction is truly minor, the side force of the two spur gears being very small, respect to the balance of the bevel gear. The bottom roller bearing is there to support the Shaft, its main loading IS due to the bevel gear mesh, and the bearing is smaller than the top roller one because (like in a lever...) its load is inferior since its distance from the gear mesh is bigger.
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From Emergency Alert Service Bulletin 04A009 Rev 2
In case of bevel gear failure, the top part of the bevel gear continues transmitting the power to the main
rotor, but all the meshing loads are transferred to the upper bearing.
Following the investigations conducted on the 2 bevel gears, it was found that this bearing and the meshing
part of the bevel gear were in good condition after having operated during
almost 10 minutes after failure. Supported by the calculations performed, EUROCOPTER is therefore
confident that in similar cases, the 30 flying minutes permitted by the Flight Manual after activation of the
emergency lubrication can be performed without worsening of the scenario.
Nevertheless, EUROCOPTER will launch tests to demonstrate the latter. Pending the results of these tests,
a new procedure requiring immediate landing is defined for this specific case.
The whole SB is worth a good read. Bottom line at the moment EC thinks the shaft can last 30 mins in this condition, but untill they can prove it an immediate landing is require.
rotor, but all the meshing loads are transferred to the upper bearing.
Following the investigations conducted on the 2 bevel gears, it was found that this bearing and the meshing
part of the bevel gear were in good condition after having operated during
almost 10 minutes after failure. Supported by the calculations performed, EUROCOPTER is therefore
confident that in similar cases, the 30 flying minutes permitted by the Flight Manual after activation of the
emergency lubrication can be performed without worsening of the scenario.
Nevertheless, EUROCOPTER will launch tests to demonstrate the latter. Pending the results of these tests,
a new procedure requiring immediate landing is defined for this specific case.
The whole SB is worth a good read. Bottom line at the moment EC thinks the shaft can last 30 mins in this condition, but untill they can prove it an immediate landing is require.
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I hope by now the queries on how the main gearbox is constructed with regard to the shaft and the purpose of the various bearings has been established to everybody’s satisfaction.
I would now like to move on to another aspect of the current situation.
6,4.5,X what is X
X being the time in hours between a MOD45 MARMs acquisition showing the start of a crack and the next shaft failing.
EC predict this figure to be 4.5hrs that’s why you are allowed to fly for 3 hours after a successful in limits MOD45 acquisition, this allows a 50% safety margin.
Up until yesterday the measures in place were flawed. The measures at that time allowed an aircraft that had just completed a 3hr flight with a single MOD45 acquisition at the beginning of the flight to depart on another 3 hr flight. With the times between detection and failure being 6hrs and 4.5hrs for the first 2 incidences it was possible to lose another shaft, despite MARMs monitoring.
Today we have to consider the available time for a flight is dependent on when the last MOD45 acquisition took place.
All of this is dependent on accurately predicting X.
EC predict X to be 4.5. On the 25 July after the first incident EC issued a statement saying The probability for another component to be equally affected is extremely low. We all know how that prediction went.
I hope x is 4.5
I would now like to move on to another aspect of the current situation.
6,4.5,X what is X
X being the time in hours between a MOD45 MARMs acquisition showing the start of a crack and the next shaft failing.
EC predict this figure to be 4.5hrs that’s why you are allowed to fly for 3 hours after a successful in limits MOD45 acquisition, this allows a 50% safety margin.
Up until yesterday the measures in place were flawed. The measures at that time allowed an aircraft that had just completed a 3hr flight with a single MOD45 acquisition at the beginning of the flight to depart on another 3 hr flight. With the times between detection and failure being 6hrs and 4.5hrs for the first 2 incidences it was possible to lose another shaft, despite MARMs monitoring.
Today we have to consider the available time for a flight is dependent on when the last MOD45 acquisition took place.
All of this is dependent on accurately predicting X.
EC predict X to be 4.5. On the 25 July after the first incident EC issued a statement saying The probability for another component to be equally affected is extremely low. We all know how that prediction went.
I hope x is 4.5
So if Iunderstand your post Pablo you are saying that the clock starts at data downloadtime and not from the actual time of the alarm?? I take that from your comment:-
“The measuresat that time allowed an aircraft that had just completed a 3hr flight with asingle MOD45 acquisition at the beginning of the flight to depart on another 3hr flight.”
Is thatbecause there is no way to look at the whole of the data in detail - forinstance like this:-
http://us1.webpublications.com.au/static/images/articles/i1082/108255_4lo.jpg
This is asimple example of some race car data which in this case looks like a MagnetiMarelli system. The X axis being time, theY axis displaying the range of a variety of inputs. So here you can see clearly if a thresholdwas breeched you could see exactly when that might be.
Otherwiseyou are left with a system that says during this period the threshold wasbreeched and if that is the case the safe option would be to assume it happenedat the start rather than at the end!!
Beinteresting to here how it works.
In any event the message the current bulletin gives is terrible and reliesentirely on the emergency lubrication system and a punt that the hunches arecorrect from a bunch of people who have been proved to have not that good at predictingthe future.
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Pittsextra
The MARMs information is displayed on download as separate acquisitions. For an acquisitions to be made in the first place the aircraft needs to be in a stable state. If the aircraft is not within certain parameters no acquisition is taken. If you are flying to rigs with a lot of straight and level you would probably get a couple of acquisitions per hour. If you are logging or constantly training you may never get an acquisition.
You can tell from the download at what point in the flight time wise the acquisition was made.
Previous to yesterday the requirement was to check the download at the end of the flight and if the MOD45 acquisition was below the limits and no significant upward trend was noted the aircraft could go for another 3 hrs. This was reasonably fine for people who do a lot of straight and level with lots of acquisitions. However it didn’t cater for the people with a low number of acquisitions, the change is to tighten up on the foreseeable possibilities of operation.
The MARMs information is displayed on download as separate acquisitions. For an acquisitions to be made in the first place the aircraft needs to be in a stable state. If the aircraft is not within certain parameters no acquisition is taken. If you are flying to rigs with a lot of straight and level you would probably get a couple of acquisitions per hour. If you are logging or constantly training you may never get an acquisition.
You can tell from the download at what point in the flight time wise the acquisition was made.
Previous to yesterday the requirement was to check the download at the end of the flight and if the MOD45 acquisition was below the limits and no significant upward trend was noted the aircraft could go for another 3 hrs. This was reasonably fine for people who do a lot of straight and level with lots of acquisitions. However it didn’t cater for the people with a low number of acquisitions, the change is to tighten up on the foreseeable possibilities of operation.