DUKW crashed with a bus

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mudflap
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Re: DUKW crashed with a bus

Post by mudflap » Sat Nov 24, 2018 10:18 pm

To All,

Just happened to run across this. It is the NTSB accident report on the DUKW/motor coach accident in Seattle back in 2015. Did not find a follow up on this topic, so I thought it might be worth posting.

https://www.ntsb.gov/investigations/Acc ... AR1602.pdf

The report is lengthy, but worth the read.

Brief history:

A company called RTDI (Ride the Ducks International) manufactures these vehicles using a combination of new and existing parts. While somewhat modified, the front axle looks a lot like the standard Banjo axle used on any of the GMC 2-1/2 ton trucks. [p 14]

Summary of Failure and Analysis:

1) On 9/24/2015, the DUKW lost control, veered left, and struck the side of a motor coach, killing 5 passengers.

2) Loss of control was caused by the separation of the left front axle ball from the axle tube. This occured due to fatigue cracking, which ultimately led to a fracture at the point where the ball section transitions into the tube cross section, which is welded to the axle tube.

3) The cracks originated from regular concentric single point tool marks, probably made during manufacturing (i.e. from a lathe).

4) Once the ball section separated from the axle tube, it temporarily ran on the front axle shaft, until it too failed.

5) RTDI had experienced this same failure several times before , and had unsuccessfully attempted to reinforce the front axle at this point (the "welded tabs").

So, basically what they are saying is that the machine tool marks created a stress concentration. After many load/unload cycles, small cracks developed at these locations. As the number of load/unload cycles increased, one or more of the cracks continued to spread a little bit each time (striations), until the part was weakened to the point where it could no longer support its load.

Other factors:

I think it is worth noting that while it appears that RTDI essentially utilized the stock front axle, their vehicles differed from the original design in several areas

1) Modified with power disc brakes.

2) Added power steering.

3) Extended the frame.

4) Maintained long term operations at a GVW of approximately 26,000 lbs. (Note: TM9-1802 specifies a total weight of 18,800 lbs.)

If I had to bet, I would say #4 is probably the worst on that list by far, but I doubt any of the above loads were anticipated in the original design. Also, the tab fix (along with a subsequent mod that had yet to be installed on this vehicle), as implemented by RTDI was never subjected to any engineering analysis or testing ! That means no CAE, no durability testing, no multiple test samples... no nothing. Very unusual for a safety related component. No surprise that it did not work.


What This Means to Me:

I don't own a DUKW, but my CCKW does have banjo axles with this same design. Even though my truck is not lugging around the added weight of a hull, or the passenger load of a commercial sightseeing company, I am still going to clean the layers of paint off of that area and inspect it very, very closely.

If I did own a DUKW, in addition to a visual inspection, a more sophisticated crack detection method such as magnafluxing, or some alternate process would give me some added peace of mind. (If I can identify a viable process, I may do this on my truck anyway.)

It would also be a good idea to do a good visual inspection of front wheel camber (vertical angle) during the walk around inspection. The report stated that RTD maintenance personnel had found cracked axle housings on the left side of two other vehicles just by using this method.

I understand that most private owners will never subject their DUKWs to anything close to what their commercial counterparts would see. But, as these vehicles are now well into their 7th decade of service, why not take a little extra time and effort to ensure that your vehicle is as safe as possible?

Best Regards.....

p.s. Anybody care to guess why all of the failures are on the left side?


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Re: DUKW crashed with a bus

Post by motto » Sun Nov 25, 2018 11:02 am

mudflap wrote:
Sat Nov 24, 2018 10:18 pm
p.s. Anybody care to guess why all of the failures are on the left side?
More steering loads are applied to the left side as the force required to steer both front wheels pass through the L/H swivel housing.
The R/H side only has to handle steering loads for one wheel.
Good information mudflap. Thanks for posting.

David

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Re: DUKW crashed with a bus

Post by Joe Gopan » Sun Nov 25, 2018 2:19 pm

75 years and still in constant service has a lot to do with it. Metal fatigue and stress corrosion do not take a day off.
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Re: DUKW crashed with a bus

Post by W. Winget » Sun Nov 25, 2018 6:25 pm

Thank You for posting this.
Too many generic statements are typically worthless fluff not related to the facts of the accident.

All vehicles with axle failure were the modified extended versions of original equipment in continual overloaded conditions. It's even possible that the axles were never original DUKW items to begin with, but that's not revealed. (these ends had 2.5 gallons of oil inside each side bellows, that does not sound like a DUKW or CCKW axle end)

Welding was not properly done (penetration) and design of a modification (early type) was not sufficient to make a 2-1/2T axle uprated to basically a 5 Ton truck rating.

Bottom line DUKW owners do not appear to have experienced this axle failure with normal DUKW operations, CCKW owners that have abused their vehicles may have similar issues as the extended DUKWs had, or someone that hauls more weight than rated for cargo.

No need to speculate, the facts are now out there. If running ashore and hitting a hard surface, or repeated hits in large potholes, prudence would dictate an inspection of the front axle ends for cracking, it does not appear to be a normal failure to be concerned about in regular service.
V/R W Winget

mudflap
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Re: DUKW crashed with a bus

Post by mudflap » Sun Nov 25, 2018 6:50 pm

More steering loads are applied to the left side as the force required to steer both front wheels pass through the L/H swivel housing.
You are The Man.. :)

Best Regards...

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Re: DUKW crashed with a bus

Post by uglydukwling » Mon Oct 05, 2020 8:31 am

I'm reading the NTSB report. It looks like this is the final report, so there won't be any other details from the NTSB. I wonder if anyone has any further information, either from other published sources, or from personal knowledge.

The axle separated at the narrow point between the housing proper and the steering knuckle.
The axle was modified :
A "boot was attached with hose clamps to a metal flange welded onto the front axle housing approximately 6–8 inches from the end of the knuckle ball. The opposite end was connected to the inside of the steering knuckle. This design created a waterproof seal, and the area was filled with approximately 2.5 gallons of gear oil".
The report shows a picture of the inboard flange ,but not the boot or its outboard attachment.
The report implies that the axle was a modified DUKW axle, but I don't think it says that definitively. Did it start out as something else?
Did the axle fail because of these or other modifications, or did it fail strictly because of age and overloading? The DUKW was stretched, increasing both length and weight.

Ride The Ducks International recognized the problem and specified a fix. The first version was a 1" x 2" tab welded over the narrow point. The NTSB said that the tab wouldn't have provided any benefit even if it had been properly welded, which it wasn't. RTDI then specified a second modification. Instead of the tab, it consisted of a welded collar covering the entire circumference of the narrow point. The NTSB said there was no engineering evidence that it would have prevented failure. In other words, it might have helped, but RTDI hadn't done any testing to prove it. It's even conceivable that it could have made things worse because of welding stresses. The DUKW involved in the accident had the first modification, but not the second. Has any testing been done since the report, or has any other modification been proposed?

What I'm trying to find out is whether our more-or-less stock DUKWs are at risk of the same type of failure, and whether there are any improvements that might reduce the risk.

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Re: DUKW crashed with a bus

Post by fahtal » Tue Jan 05, 2021 1:47 am

after more then 75 years of service this isn`t a known issue for dukws...

i think making the dukw longer and heigher stessed the axl. if your dukw is stock then i wouldn`t worry. just do your regular maintenace and make sure the water is out after a swim.


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