Fire Engine Wreck

martininsocal

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Ok, for you mechanical engineers and scholars and anyone else who has pertinent backgrounds. How many revolutions per minute will a straight axle set-up handle in a commercial/industrial application? This is using U-Joints and the axles/driveshafts will be operating well within normal angles. The front driveshaft will be of a spicer type. Will the mounting direction of the driveshaft have any impact of the amount of RPM's the shaft will travel?
Here is why. We had an engine crash up on Cajon Pass last August. Prior to the crash, the front drive shaft broke into 3 pieces and fell off the engine. The engine then lost control and skidded towards the shoulder of the road, then back across 4 lanes and off the hillside. CHP says the accident was the fault of the driver due to over correction after the drive shaft fell off the engine.
Here are my concerns. The engine is a 4x4 International with a part time transfer case. The engine was being run in 2wd, but the front driveline will still be spinning, just not under power. I have driven these types of engines for several years and know they are capable of 75+ mph on flat ground. What I am curios to know is if, while in 2wd and travelling down hill, The engine can approach a speed that will surpass the capabilities of the drive shaft which would cause its self destruction. Also- would the direction of mounting i.e. Driveshaft mounted backwards, make a difference in the capabilities of this assembly. 3 firefighters were injured in the accident, one seriously. The Department is blowing off the driveshaft failure and stating that the resulting crash is driver error. What do you guys think?

Martin

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Kritter

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Let me get this straight.

1. Driving down Cajon Pass at 75 Mph
2. DS Fails
3. Rig goes out of control
4. Rig Crashes

If the engine was in 2wd and the front DS fell off...how would this cause the vehicle to go out of control? Did the driver get scared at the noise and lose it?

Was it a 1-piece drive shaft?
Does the 4wd use manual hubs in the front?
Were the hubs inspected afterwards?

“The engine can approach a speed that will surpass the capabilities of the drive shaft which would cause its self destruction.”
Is that a stament or a question?

“Will the mounting direction of the driveshaft have any impact of the amount of RPM's the shaft will travel?”
If it is a standard DS and is symmetrical enough to mistake one end for the other it should not have had an effect on RPMs although it could be argued it would have an effect on life.

“How many revolutions per minute will a straight axle set-up handle in a commercial/industrial application?”

Shafts are designed to operate in a certain range and the range is calculated to make sure that the shaft is not operated at its critical frequency , which is when catastrophic failures happen…the Tacoma Narrows bridge for an example of something resonating or hitting its critical freq.

The factors that determine that freq. or RPM in this case are relative to the characteristics of the shaft and mounting, which I am sure are unknowns to you.

I do know that the factor of safety with regards to this is probably in the 2.5-3 range(meaning that it could spin 2.5-3 times faster then it is intended under normal operation) so I am sure you were not near the critical frequency unless the shaft had already previously failed (a failure is whenever something comes out of tolerance and cant meet its intended engineering needs) although it still appeared to be ok. It could have had a ding in it or came out of balance which would definitely effect its freq range.

OK I better get back to work!


Kris
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shrek

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Martin,

What was the condition of the steering componets after the crash?? Could the driveshaft have damaged the tie rods or center link and caused the engine to go out of control???

Was the engine oversped??? Do you know what speed he was traveling at the time the drive shaft failed??

I hope everyone recovers well.

Matt

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martininsocal

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Here is some more info- The engine was less than 1 month old(new). The witness reports say the front driveshaft came off the engine in 3 pieces prior to the engine veering to the left, then back to the right. Some speculation is that the front U-joint failed, allowing the shaft to flip back under the transfer case, which could have possibly lifted the front wheels off the ground. I do not know if the drive shaft incorporated a Cardan style joint at one end. It would seem that if it was indeed a symetrical shaft, there should be no affect in regards to mounted direction. The manufacturer says there is no current direction from the factory as to which direction the shaft is suppose to mount, but this one and one other one where mounted in a different direction than the other 18. There are no front hubs, the entire drivetrain always rotates if the vehicle is moving, it just won't be under power unless the Transfer case is engaged to supply power to the front wheels. As to safety ratios, These Chassis are designed for medium commercial use(snow plow, dump trucks, tree trimming, etc...) so They are not operated at high rates of speed nor is that their intended purpose.
What gets me is the department is buying off that this is simply drivers error as if the drive shaft failure had nothing to do with crash. The driver will probably face financial as well as disciplinary actions because the engine crashed after the shaft failure. The kicker here is a state agency is investigating a state agency to see if there is any fault to be found with the state agency that owned the vehicle. Not only that, but the investigation would also clear the supplier of the new piece of equipment of any fault if accepted as it stands.

Martin

If your gonna go, go BIG
 

BlueCoyote

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If the front ujoint failed, the driveline will either flail around and then drop to the ground. Simple accident forensics will solve that - are there marks on the ground (where the driveline dug it) prior to the crash. What are the marks on the yolk / driveline? If they are single crush lines the damage occured from the crash. If the marks are rotational or have dirt (debris) the damage occurred prior to the crash.
As for the driveline being oversped - what is the final drive ratio? I find it hard to believe the driveline could be oversped and fail at 75 mph. Semis exceed that speed all the time - failures are not common.
Have they done any forensic checks on the ujoint - presence of lube, internal cracks, missing c clips, etc?
Sounds like they want 'a resolution' and not the truth....

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Ryno

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Martin-

My grandfather worked for 4 wheel Drive back in wisconsin. He built most of the old seagrave and pierce brush rigs. Being that I did the job for 4 seasons, I know exactly what you're talking about. With the driveshaft going to the ground, the truck most likely hopped...and then at the high rate of speed, couldn't recover. What stations rig was it?? I can't see how it would be driver error, except with the speed. I'll talk to my grandfather and ask him what the shafts are good to. My uncle works for pierce now doing R&R work...I'll give him a call this weekend.

Ryan

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shrek

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If the unit was only one month old, then I would say that you recieved a engine with a non-greased driveshaft from the factory. The pieces of the driveshaft should tell you that...............................

Good luck.

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AZmiik

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A remember a few years back 98-99 the 11th Signal had to drive from sierra vista to El Paso at 35 mph because of drive shaft failures. The new FMTVs(i think) that they had received were prone to drop the front shaft and then it was like a pole-vaulter. Plant and flip. Might want to see if you can find any info on that to see if it helps.

Mike
 
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