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  #21  
Old 11-13-2022, 08:44 PM
Marc Bourget Marc Bourget is offline
 
Join Date: Apr 2009
Location: Stockton, California
Posts: 384
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If I was given the responsibility for ground testing the new PSRU plate, I'd mount the engine/propeller assembly on a Tilt-Yaw table to check (both the old) and the new plate's response to the gyroscopic loads.

There should be digital measuring "cells"? to check relative motion created by the gyroscopic load testing

Last edited by Marc Bourget : 11-13-2022 at 08:46 PM. Reason: added comment on measuring during test
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  #22  
Old 11-13-2022, 09:09 PM
KeithO KeithO is offline
 
Join Date: Jul 2010
Location: Jackson,MI
Posts: 116
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Marc, there are a few problems with what you are proposing.

The first is that this work is entirely self funded with the goal to prevent people getting killed by the failures and the reputation of experimental aviation being drug further into the gutter. Obviously I would not have access to the kinds of facilities needed to do what you are suggesting.

The second is to consider that in its original configuration the crankshaft side drive hub contains a plastic bushing which serves to center the gearbox when it is mounted to the engine. The attachment holes have quite a lot of slop in them and there are no dowels for alignment. The locations dowels would go in the original bellhousing were cut off by Jan when preparing the block for the conversion. The end of the gearbox input shaft is sized so that it slides into the plastic bushing when you assemble it and you then tighten the mounting bolts without applying lateral pressure to the gearbox, thus it should be decently centered in the unloaded state. Then in service, when the prop loads are applied, because the mounting points are all on one side and the end of the gearbox that is supposed to be aligned with the crank is hanging way off in space, the deflection in the circular section mounting posts can now allow the input end of the gearbox to move away from the crank centerline in the axial and radial directions, proportional to the magnitude of the gyroscopic forces. In other words, how abrupt was the pitch up, down or turn rate left or right. Just imagine a stall/spin...

So, when assembled and when gyroscopic loads are applied to the prop, the gearbox input shaft is going to try to move out of alignment with the crank. So long as the alignment bushing is new and undamaged, it will prevent this from happening by transferring the load into the pilot diameter of the input shaft. However many owners have reported that when they removed their gearbox, they found the guide bushing to be so destroyed that they were confused regarding what it was. If it got destroyed, then clearly it will allow relative movement of the input shaft vs the crank and it is that misalignment which will fatigue the drive flange ears because of the rotating bending load put on the pins which is then transferred into the flange itself.

Thus, when I do my stiffness measurement I will not have a centering bushing installed in the assembly because I want to see the stiffness of the system without a radial load being applied to the gearbox input shaft. If I allow the gearbox input shaft to be loaded in these tests there is a good chance that all I would accomplish is move the failure to where the input shaft is breaking off due to the bending load on it. If we see a big reduction in deflection with the new mount, it also means that the chance of having a big side load applied to the input shaft is greatly reduced, which is what we want.

I attached a picture so you can see where the mount locations are relative to the prop centerline and the input shaft centerline in case you missed the earlier one.
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Last edited by KeithO : 11-13-2022 at 09:12 PM.
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  #23  
Old 11-13-2022, 09:40 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by KeithO View Post
Marc, there are a few problems with what you are proposing.

The first is that this work is entirely self funded with the goal to prevent people getting killed by the failures and the reputation of experimental aviation being drug further into the gutter. Obviously I would not have access to the kinds of facilities needed to do what you are suggesting.

The second is to consider that in its original configuration the crankshaft side drive hub contains a plastic bushing which serves to center the gearbox when it is mounted to the engine. The attachment holes have quite a lot of slop in them and there are no dowels for alignment. The locations dowels would go in the original bellhousing were cut off by Jan when preparing the block for the conversion. The end of the gearbox input shaft is sized so that it slides into the plastic bushing when you assemble it and you then tighten the mounting bolts without applying lateral pressure to the gearbox, thus it should be decently centered in the unloaded state. Then in service, when the prop loads are applied, because the mounting points are all on one side and the end of the gearbox that is supposed to be aligned with the crank is hanging way off in space, the deflection in the circular section mounting posts can now allow the input end of the gearbox to move away from the crank centerline in the axial and radial directions, proportional to the magnitude of the gyroscopic forces. In other words, how abrupt was the pitch up, down or turn rate left or right. Just imagine a stall/spin...

So, when assembled and when gyroscopic loads are applied to the prop, the gearbox input shaft is going to try to move out of alignment with the crank. So long as the alignment bushing is new and undamaged, it will prevent this from happening by transferring the load into the pilot diameter of the input shaft. However many owners have reported that when they removed their gearbox, they found the guide bushing to be so destroyed that they were confused regarding what it was. If it got destroyed, then clearly it will allow relative movement of the input shaft vs the crank and it is that misalignment which will fatigue the drive flange ears because of the rotating bending load put on the pins which is then transferred into the flange itself.

Thus, when I do my stiffness measurement I will not have a centering bushing installed in the assembly because I want to see the stiffness of the system without a radial load being applied to the gearbox input shaft. If I allow the gearbox input shaft to be loaded in these tests there is a good chance that all I would accomplish is move the failure to where the input shaft is breaking off due to the bending load on it. If we see a big reduction in deflection with the new mount, it also means that the chance of having a big side load applied to the input shaft is greatly reduced, which is what we want.

I attached a picture so you can see where the mount locations are relative to the prop centerline and the input shaft centerline in case you missed the earlier one.
I have doubts that the plastic centering bushing was doing much of anything to help maintain alignment ( which is why they got destroyed).
For that reason, testing a modification with a bushing installed may be beneficial in that it may provide a clue whether any movement is taking place.
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  #24  
Old 11-13-2022, 10:22 PM
KeithO KeithO is offline
 
Join Date: Jul 2010
Location: Jackson,MI
Posts: 116
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The plastic bushing is described by Viking as an assembly aid ONLY that has to be replaced every time the gearbox is separated from the engine. They KNOW it is getting severely damaged when the engine is run. Thus it is acknowledged that there is relative movement between the gearbox and crankshaft. Just not in those words.

If the current failure mode is to be addressed there has to be a reduction in relative movement. A significant reduction that is. I can verify that by measuring the deflection without a guide bushing as I described. If the design goals are met the new system should have near to no deflection. If that condition is met then it is safe to use a permanent guide bushing without the risk that we break off the input shaft.

Quote:
Originally Posted by rvbuilder2002 View Post
I have doubts that the plastic centering bushing was doing much of anything to help maintain alignment ( which is why they got destroyed).
For that reason, testing a modification with a bushing installed may be beneficial in that it may provide a clue whether any movement is taking place.
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  #25  
Old 11-14-2022, 12:16 AM
12vaitor 12vaitor is offline
 
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The bushing is for alignment only when installing the gearbox. The original design with the aluminum flywheel used the stock Honda needle bearings in the crankshaft end to carry the end of the drive pinion when installing the gearbox. Those were removed and the plastic bushing was used. I have had my gearbox off twice and had no appreciable wear on the alignment bushing. I suspect it may have more to do with how good you are at installing the gearbox attachment bolts while maintaining the centering with no side loading. Once you torque the mounting bolts, the bushing should do nothing but sit there if it is still centered. There is a fairly simple way to stop the gearbox from moving while the bolts are being tightened.

John Salak
RV-12 N896HS
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  #26  
Old 11-14-2022, 01:07 AM
KeithO KeithO is offline
 
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Location: Jackson,MI
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Mark told me that when he took his gearbox off, his bushing was so badly wallowed out that at first he didnt know what it was.

John, can you confirm whether there is any pilot feature between the flywheel drive flange and the crankshaft itself. In other words, does the plastic bushing center the flywheel side drive flange, as well as centering the gearbox input shaft ? I'm trying to understand whether it is only the flywheel bolts providing run out control on the flywheel side drive flange or whether some other feature is provided to center the flywheel and drive flange.

Remember that in this system the thing that counts is how well all the parts run true to the crankshaft itself. If the flywheel drive flange and possibly the flywheel itself is only located by the flywheel bolts, that would provide a significant amount of run out that could not be fixed later no matter what you did, without replacing those parts with ones that are better designed.
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  #27  
Old 11-14-2022, 01:10 AM
KeithO KeithO is offline
 
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John, this fact alone could be the reason why you have not had any issues with your setup. How many hours on your engine ?

Quote:
Originally Posted by 12vaitor View Post
I have had my gearbox off twice and had no appreciable wear on the alignment bushing.

John Salak
RV-12 N896HS
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  #28  
Old 11-14-2022, 09:37 AM
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rv6ejguy rv6ejguy is offline
 
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I haven't seen an auto flywheel that wasn't located by a central spigot or counter bore.

The Egg Subaru drives also didn't use the factory dowel pins on the engine for location and required an alignment tool.

Some Ross drives were machined incorrectly and had misalignment causing rapid wear.

Lesson: use the factory dowels to locate the drive concentrically with the crankshaft. It's important and makes things easy for maintenance. That's why auto OEMs do it this way.

This would involve a casting or machined billet housing like almost all other commercial PSRUs use. More expensive, slightly heavier but way stiffer.

Stiffness here is critically important to gearbox longevity. Deflection and bending create loadings which can fail components prematurely.
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  #29  
Old 11-14-2022, 11:47 AM
12vaitor 12vaitor is offline
 
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Quote:
Originally Posted by KeithO View Post
John, this fact alone could be the reason why you have not had any issues with your setup. How many hours on your engine ?
I have 350 hours on the engine, average time per flight is about 2 hours.

The flywheel and drive pin plate center on the crankshaft end nipple as you can see in the photo. The steel drive pin plate is CNCed, so I presume the pins centers are concentric to the center hole around the crankshaft nipple. The plastic alignment bushing is inserted in the center pilot hole.

The other photo is the gearbox as it came off the engine with 125 hours from the last removal. The arrow is pointing to the plastic alignment bushing which shows little wear. The bushing has a small step machined on the end to match the step in the crankshaft pilot hole. The part that broke on Mark's drive was one of those three pin ears. The gearbox drive pin plate is pressed on to the splined drive pinion gear shaft. There are two bearings on the drive pinion shaft, one in the mounting case and the other in the removable cover on the other side of the mounting case. It is important to check that the drive pin plate stays parallel to the mounting case as it is pressed on.

John Salak
RV-12 N896HS
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Name:	Honda 110 Gearbox drive with centering bushing installed.jpg
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  #30  
Old 11-14-2022, 12:09 PM
lr172 lr172 is offline
 
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I hate to step into something I know nothing about. However, when I look at that frame structure with the leverage afforded by those long rods, all located on just one side, and compare it to all the bell housings I have seen over the years (always SOLID, half round structure), I am not surprised that it exhibits a lot of flexing. Not an ME, but as I look at it, it seems that meaningfull flexing is all but guaranteed, affecting the area near the crank, that is well offset from the supports.

Larry
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Last edited by lr172 : 11-14-2022 at 12:13 PM.
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