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  #61  
Old 11-30-2022, 11:57 AM
Lufthans's Avatar
Lufthans Lufthans is offline
 
Join Date: Dec 2017
Location: Hilversum, The Netherlands
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Quote:
Originally Posted by rv6ejguy View Post
Yup. I can only think of two companies who actually engineered and tested their redrives properly for the Experimental market. All the others were designed with TLAR and most of these have had issues to varying degrees (some catastrophic and deadly).

TLAR can work if you deal with each failed part as they break but it's time consuming and potentially dangerous. Certainly not the way to do it for commercial products.

All redrives really need to be validated regardless of how or who designed them though. Even the experts can overlook something and have failures. Simulation and computer modeling isn't a substitute for actual testing on the engine with the prop in place, preferably in flight.

These small companies rarely have the resources to do this and most won't even bother consulting an engineer, figuring they are quite capable of designing a reliable drive. We see the results here, yet again.
The redrives that I am using on my Jodel and now RV3 have been designed by someone who is doing this for a living. I believe he also has the Robinson helicopter and several Formula 1 teams on his resume. They have then been built by people who know their stuff. The company was preparing to race their engines at Reno (in Lancairs). Several millions of dollars of R&D money have been invested. And in the end, even these guys ran into a brick wall with their 6 cylinder turbocharged engines and Hartzell propellers. So much so that they have discontinued their operation. Leaving me with two of their very few redrives in existence.

My 4 cylinder engine and lighter, wood-core MT prop run free of torsionals, as long as I stay away from the 1100-1500 rpm range with driven motor. This means per flight two times - during startup and after landing when leaving the runway for a brief half a second. I can live with this, but it would not look good on a production engine.

What I am saying is - this stuff is hard.

Jan doesn't have millions of dollars to his disposal to perform proper research and product development. He didn't when he was doing the Subarus and most likely he still hasn't.

It's sad to see that some customers have to suffer the consequences, apparently.

(And I'd be interested to learn which are these two companies with proper R&D Ross!)
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Hans Teijgeler
Hilversum, The Netherlands


PH-KLT Resurrecting an RV-3B, with Subaru EJ25 STi
PH-BRR Bowers Fly Baby; Imported and upgraded
PH-MGA Jodel DR1050; Built, with Subaru EJ25
PH-SUM RV-4; Imported and upgraded. Sold my share :-(
PH-EIL RV-4; Imported and upgraded for friend. Sadly crashed
PH-ERD Robin DR300; Built with Subaru EZ30 for friend.
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  #62  
Old 11-30-2022, 12:00 PM
bobnoffs bobnoffs is offline
 
Join Date: Jul 2012
Location: n. wi
Posts: 979
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if the 3 pieces of the guibo slip on and off of the pins then i don't see how there can be a strain on one pin. seems like this test would make the focus more on the dimensions of the metal piece
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Bob Noffs
n. wi.
dakota hawk/jab 3300 built and flying. sold 6/18.getting serious about the 12. in the hangar now as of 10/15/19
RV-12 kit as of 9/13
RV-12 airworthiness cert. 11/30/2021
first flight 7/25/2022
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  #63  
Old 11-30-2022, 12:22 PM
KeithO KeithO is offline
 
Join Date: Jul 2010
Location: Jackson,MI
Posts: 149
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Bob, given that the broken drive ear went out the side of the cowling at high speed in both cases, there is now no way to determine the geometry of the overloaded pin position. But I think that everyone agrees that if both drive flanges had been symmetrical and if the guibo links were the same length, then either all pins should have been loaded up or none. In which case all 3 drive ears would either be cracked or not.

To have total failure on 1 drive ear to the point that it breaks off completely and then have the 2 remaining drive ears look nearly perfect suggests that something has to be way off, either in the geometry of the drive flange itself or the guibo. If the spline features in the hub of the drive flange were to be non concentric to the axis of the hub, that could be another potential problem because that could put 1 drive pin out at the longest radius and it would always be taking the load.

Usually it is quite difficult to get good concentricity on broached features. A lot of gear makers would do the broaching operation first and then locate off that feature when cutting the gears so that they run true. Unfortunately I have no idea on what the manufacturing process is for both the shaft and the drive flange itself, although seeing the shaft slice chips off the drive flange as it is pressed on is not very re-assuring about the fit up of the 2 parts.
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  #64  
Old 11-30-2022, 07:44 PM
KeithO KeithO is offline
 
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Location: Jackson,MI
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This evening had a little help from my neighbor with his 20 ton arbor press. Pressing out the input shaft from the flange proved to be uneventful. I just used the tube spacer that was a close fit to the gear OD and it came out with no drama or strain.

There appeared to be no visible cracks on the 2 remaining drive ears. Dye penetrant inspection will be tomorrow. Then it goes to the metallurgist for analysis. Based on the totally asymmetric damage, I am concerned that its possible that the splined connection may not be concentric. So if you own one of these, remove the guibo and attach a dial indicator to the gearbox housing with the plunger perpendicular to the input shaft centerline. Then adjust the dial indicator so that when one drive pin on the input shaft is at maximum radial dimension the meter is zeroed. Now check the other 2 drive pins. Are they at the same radial distance ? If not that is a problem. The pin that has the largest radial distance will be the hero pin that takes all the load and ultimately breaks off...
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  #65  
Old 12-01-2022, 05:31 AM
KeithO KeithO is offline
 
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Did the dye penetrant inspection on the drive flange. Confirmed that the 2 other ears are not cracked. There were cracks on the welds holding the pins in place but it is quite likely that because of the high carbon of the pins themselves that these cracks formed when the weld solidified, in other words on the day the part was made.

So we are still dealing with a scenario where somehow 1 pin was carrying all the dynamic load until it failed.
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  #66  
Old 12-01-2022, 07:48 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by KeithO View Post
There were cracks on the welds holding the pins in place but it is quite likely that because of the high carbon of the pins themselves that these cracks formed when the weld solidified, in other words on the day the part was made.
Perhaps, but maybe not. I've seen homebuilt axle assemblies with the same cracks, a function of poor design.

Easy to check. Grind off the weld and see if the axle will come out of the flange without application of significant force.

See the sketch. The assembly will not survive unless the axle is a shrink fit in the flange. Even if designed for it, production tolerances can make it iffy.
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Last edited by DanH : 12-01-2022 at 07:50 AM.
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  #67  
Old 12-01-2022, 07:59 AM
KeithO KeithO is offline
 
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Dan, the pins are pressed in, then welded. There was no incidence of pins falling out prior to welding, but then apparently it was considered "safer" to weld them.

I was technically responsible for urea injector production where a hardened 440C orifice plate was laser welded to the injector body following a press fit. The annual volume was approximately 50k/yr from 2012 to the present so 10 years at least. I can assure you that welding high carbon steel successfully is a very difficult proposition and required a very narrow beam (200um) with very accurate position of the beam relative to the joint and very high weld speed (600rpm on the collet chuck) with only 5um total runout of the part.
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  #68  
Old 12-01-2022, 08:25 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by KeithO View Post
Dan, the pins are pressed in, then welded. There was no incidence of pins falling out prior to welding, but then apparently it was considered "safer" to weld them.
Works for me. Apparently some were flown without welds?
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  #69  
Old 12-01-2022, 08:43 AM
KeithO KeithO is offline
 
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Some are being flown without welds. Jan knows who has which parts in what state. The point its that the SB version of the system failed at 134 hours whereas the original input shaft drive flange with the bolts went 800 hours before it failed.

So I hesitate to accept that the SB version is in fact an improvement on the original version. The main issue with the original design was the aluminum flywheel which would crack and some people are still flying that...
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  #70  
Old 12-01-2022, 08:47 AM
KeithO KeithO is offline
 
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Quote:
Originally Posted by DanH View Post
Perhaps, but maybe not. I've seen homebuilt axle assemblies with the same cracks, a function of poor design.

Easy to check. Grind off the weld and see if the axle will come out of the flange without application of significant force.

See the sketch. The assembly will not survive unless the axle is a shrink fit in the flange. Even if designed for it, production tolerances can make it iffy.
.
Dan if you look closely around the drive pin you will see the slightest witness mark from the dye. That was just a tiny bit of seepage from that joint, you can see it hardly bled out at all compared to the bleed out from the end of the crack. So that also indicates a tight fit of the pins to the hole in the flange.
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