Crankshaft Failures

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Marc W

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There is a bit of a flap going on over on the Thatcher forum at groups.io. There have now been three crankshaft failures in the last few weeks in CX4's. There was also one about a year ago. The first one is the broken crank in California that resulted in the loss of the airplane that was discussed in the "Crashes" thread. The other two had better outcomes. One was on takeoff where the prop departed the airplane and the pilot had enough runway left to set it back down. The other one happened on climbout and the pilot was close enough to make it back to the runway. All were apparently on 2180 cc engines with Force One prop hubs. There are some good pictures of the last one on this thread: thatcher-aircraft@groups.io | Second crank failure. A cautionary tale.

Glen Bradley, who works with Dave Thatcher, called Scott Casler because Scott had built a couple of the engines. He said that in 2012-2013 some manufacturers used CB cranks because the Scat crank wasn't available. That statement leaves a lot unsaid. Scott built my engine, but it was delivered in 2011 so maybe I have the Scat crank. Anyway, I will keep looking for landing fields whenever I fly!
 

Vigilant1

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Glen Bradley, who works with Dave Thatcher, called Scott Casler because Scott had built a couple of the engines. He said that in 2012-2013 some manufacturers used CB cranks because the Scat crank wasn't available. That statement leaves a lot unsaid.
Yes, it does.
I wonder if the crankshafts that failed came from somewhere else or if they were were sourced from GPAS (where Scott and everyone else gets the Force One prop hub and bearing). They've said for many years that their "Top Bug" 82mm crankshaft has had a history zero failures when used with the Force One prop hub. I don't know who makes their Top Bug crankshaft.

Edited to add: I see that the most recent incident was on a plane that had a "Great Plains 2180cc engine." If that means Great Plains built the engine or sold a complete kit for it, then it would have had the GPAS "Top Bug" 82mm crank. It will be interesting to find out more. The GPAS 82mm crank and Force One hub has had a very good reputation when fitted to a lot of different aircraft.
 
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TFF

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Is there other commonalities like type of prop and RPM run? Other engine specs like compression?

I think if I though I had one of those cranks with time on it, I would have to split the case and have it magnafluxed. Might be only three bad in the batch, but I would want to make sure it’s not four.
 

Pops

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One time I had one of the bad cranks that got out in a Lyc engines. The crank broke in the throw in front of the center main. Made it the 7 miles to an airport.

I have been reading the post on the website. Someone said a crank broke on a 1915 cc engine with the force one hub. That would be a stock 69 mm crank. Was it an after market or a VW crank ?
 

BBerson

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That Force One hub should be extended further in. Perhaps all the way to the crank arm and welded to the crank arm.
That would eliminate the need for the key and make it like a Lycoming.
 

Vigilant1

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Two of the features of the Top Bug crank are a 1/2" fine thread end hole (it is 20mm on stock cranks) and a Woodruff key slot that is only .09" deep. These changes leave a lot more "meat" at the nose of the crank and were made specifically to avoid the types of crank failures we are seeing here. And it worked.
It would be a shame to have to re-learn this lesson.
It will be interesting to learn more about these cranks, including where they came from and when.
 

Theodore959

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OEM Volkswagen. preferably Type 4. Attached Redrive helps alleviate stress on crankshaft. Flywheel end.
Thats my consensus from what i've read here and many other sites.
OEM case & crank
now trying to cut same(two cylinder) for 103 class Legal Eagle XL maybe not possible
Thank you
Theo
take care of your wish!
 

Pops

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Having built several VW aero-engines, I have always stayed at 1914 cc or below using the good German VW stock 69 mm crank with the shrunk on prop hub. It takes heat and a good press to get one off. Believe the center bolt is just insurance. I have never had any problems. I still have 3 good stock 69 mm German cranks that I have been saving. I'll be using one on the 1835 cc engine that I'm building for the mud pipe buggie. Also did the Bob Hoover mods for that engine.
At one time I ask Steve at GP's about going to the force-one prop hub for the smaller VW engines and he said just a waste of money for the 1915 cc engine and below.
I have been building a 2180 aero-engine , but its going to be a flywheel drive engine.
 

fly2kads

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The crank failures pictured in the Thatcher group appear, at first glance, to match the failures discussed in this post on the old Great Plains site:
Prop Hub History by Great Plains Aircraft Supply Co., Inc.

That article says their crank and Force One hub were designed to prevent exactly the kind of failures seen here, and they have had a very good service history. So what's changed? Were these engines not using the GP-supplied crank? Or has GP changed their supplier or their design? Was there a previous prop strike on any of them?
 

Pops

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The crank failures pictured in the Thatcher group appear, at first glance, to match the failures discussed in this post on the old Great Plains site:
Prop Hub History by Great Plains Aircraft Supply Co., Inc.

That article says their crank and Force One hub were designed to prevent exactly the kind of failures seen here, and they have had a very good service history. So what's changed? Were these engines not using the GP-supplied crank? Or has GP changed their supplier or their design? Was there a previous prop strike on any of them?
Or was the orifice not drilled out in the case that restricts the oil flow to the #4 bearing in the stock VW case. The restriction is down to .040 " to supply the oil for the #4 bearing that was designed just to take the Generator belt side load on the crank. I would like to mic the wear on the ID of the Force One bearing.
 
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Vigilant1

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The crank failures pictured in the Thatcher group appear, at first glance, to match the failures discussed in this post on the old Great Plains site:
Prop Hub History by Great Plains Aircraft Supply Co., Inc.

That article says their crank and Force One hub were designed to prevent exactly the kind of failures seen here, and they have had a very good service history. So what's changed?
Yes, thanks for that GPAS info on the background of Type 1 crankshaft breaks I stroker engines. I don't know if the present batch of failed cranks on Thatchers came from GPAS, but I'm sure Steve Bennett wouldn't have knowingly allowed 82mm cranks without the Top Bug features to be sold by GPAS for any pulley-side prop setup. Steve's been gone for a few years now, though.
 
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dino

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Torsional vibration With prop combination should also be considered
 

Daleandee

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Like others I thought this was history. I know that years ago the Aerovee was having some prop departing concerns and the company came up with a different crank. I have read of at least one Aerovee crankshaft break recently with the new crank. Not sure what year the new cranks came out as this video is from 2013:

Corvairs were breaking cranks but the 5th bearing seems to have cured that. The other thing that I do (even though many say it's not required) is to index the prop. But it appears in the photos of the VW prop breaks that they are separating right behind the hub. I don't see how indexing would change any of that unless somehow balance or harmonics are affected.

The other thing learned on Corvairs is that the prop, fly wheel, and ring gear should be on the same end of the crank. If you load both ends of the shaft with weights then it's gonna undergo a lot of twisting.

But I'm the dunce in the group ...
 

Flyguyeddy

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Like others I thought this was history. I know that years ago the Aerovee was having some prop departing concerns and the company came up with a different crank. I have read of at least one Aerovee crankshaft break recently with the new crank. Not sure what year the new cranks came out as this video is from 2013:

Corvairs were breaking cranks but the 5th bearing seems to have cured that. The other thing that I do (even though many say it's not required) is to index the prop. But it appears in the photos of the VW prop breaks that they are separating right behind the hub. I don't see how indexing would change any of that unless somehow balance or harmonics are affected.

The other thing learned on Corvairs is that the prop, fly wheel, and ring gear should be on the same end of the crank. If you load both ends of the shaft with weights then it's gonna undergo a lot of twisting.

But I'm the dunce in the group ...
That crank break is definitely one of the older crankshafts. Still has a distributor gear drive on it. The newer ones arent even cut for it.
 
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