Broken drive shaft

[jdub]

Jay - I ran the calculations for a single piece shaft based on Shaft Master's specs for an A340:

Material - 6161-T6
Tube OD - 3.00"
Tube Wall Thickness - 0.125"
Tube Length - 39"
Tube Weight - 4.66 lbs (calculator said 4.31 lbs)
Modulus of Elasticity - 10.0
Density - 0.098
Poly Foam Sound Deadener - Density: 1.0 - 1.3 lb/ft3

This yielded:
Critical speed at 12,480 shaft rpm
1/2 Critical Speed at 6,240 rpm
Torsional Yield at 1990 ft/lbs.

1/2 critical speed is important because a harmonic vibration at or near that speed can induce metal fatigue over time...you want to avoid it. On my tranny (A340), differential (3.91:1), and tire diameter (265x35/18) combo, I will not hit 1/2 critical speed until 120 mph. Assuming the shaft is balanced properly (which it is), the other reasons for a harmonic vibration are listed below...in any case, my normal operating speed is no where near 120 mph

It also appears the DS critical speed is well above the max the car is capable of...the biggest factor on a MkIII is the u-joint angle on a single piece shaft...will the increased angle induce vibration to the shaft? Reality is often quite different than theory...I'm going to ops test one and find out.

Copied from the calculator posted by Piratetip:

Important Critical Speed Information from Dana.com

1. A driveshaft that has the potential of operating above this "safe operating speed" could fail and could be thrown from under the vehicle or machine, which could cause serious injury or death to persons nearby.

2. The twice per revolution vibration characteristics of a cardan U-joint, operating at an angle, can produce a minor vibration if normal operating speed of the driveshaft is near 1/2 of its true critical speed.
A. Re-check the balance and runout of the driveshaft, because unbalance is the major factor in 1/2 critical vibrations. Reducing unbalance may cure this type of vibration problem.
B. Disconnect one end of the driveshaft, rotate the driveshaft 180 degrees and re-install it.
C. Reduce the u-joint operating angles of the driveshaft. (Remember to keep them equal as you change them).
D. Disconnect the driveshaft and remove the end yoke from the transmission or axle. Rotate the end yoke approximately 90 degrees and re-install it (torque the nut to specifications). Re-install the driveshaft.
E. Redesign the driveshaft(s) in the vehicle. Change to multiple driveshafts or change the length and/or tube diameter(s) of existing driveshafts to move the 1/2 critical speed of the assembly well out of the normal

3. An UNBALANCED driveshaft can experience a critical speed failure, in your balancer, at an RPM that is lower than the "SAFE OPERATING SPEED (RPM)" figure shown on the following screen.

4. The addition of cardboard liners or internal harmonic absorbers may need to be installed.

 

[suprahero]

I know mine isn't balanced perfectly because I have a slight to mild vibration when going down the road. I'm used to it and hardly ever think about it unless discussing it on here. I can't remember what was said, but it was something IJ brought up that got me to worrying about this. I'll have to search and see what he said. It wasn't an internet story that I heard, but I'll just have to find it.

Edit: Thanks JDub, that's what I was referring too. The very first paragraph in that article you posted.

 

[Piratetip]

jdub I came out with the same #'s as well.
http://www.supramania.com/forums/showthread.php?p=832113&posted=1#post832113

 

[jdub]

Yep...a small difference in critical speed rpm due to the extra 1" in tube length for an R154 tranny.

The one piece I have now has a vibration at 60-70 mph...not much, but is noticeable. I suspect the new shaft (due in today via UPS) will produce the same results as you got Nick

 

[Piratetip]

I would imagine so
Its a nice piece.

 

[suprahero]

jdub, what would happen if you went faster than the 120 critical speed? I do drive faster than that occasionally and would like to know if this is going to be more dangerous than the two piece. Sorry for the dumb question, but I want to know. Thanks.

 

[jdub]

Read a bit closer...that's 1/2 critical speed
It's only a problem if a harmonic vibration develops at that speed...it's more an irritation than anything else. But, over time *and* if that speed is maintained *and* the vibration is present, it can cause metal fatigue in the shaft....we're talking long periods of time here. Above and/or below the mph where 1/2 critical speed occurs is not a problem.

So you go 120 mph and stay at that speed for extended periods?


BTW - it 1/2 critical speed be a bit lower for an R154 shaft...it's longer than an A340 shaft.

 

[got_boosted]

the reduction in rotational mass is HUGE going to a 1 pc shaft.....certainly worth it.

I definately felt a difference. The car is much nicer to launch when it doesn't feel like you're trying to roll a tank up a hill. lol

Collin

 

[suprabad]

I like the idea of a stronger 2 piece.

I have driven a few cars that have been converted from 2 piece shafts to a 1piece driveshaft (including but not limited to supras) and I have not seen one that doesn't create additional vibration.

Unfortunately, it always seems to occur at highway speeds, which imho is the worst place for the vibration because that's where you are likely to spend the most time at a fixed speed feeling that vibration.

If you're building a car for the track, one piece is the way to go, but on my daily driver it's just too annoying.

 

[Piratetip]

Fortunately shaftmasters 1 piece does not do this at highway speeds. Unless of course you regularly travel at 120MPH on the freeway.

 

[drjonez]

Read a bit closer...that's 1/2 critical speed
It's only a problem if a harmonic vibration develops at that speed...it's more an irritation than anything else. But, over time *and* if that speed is maintained *and* the vibration is present, it can cause metal fatigue in the shaft....we're talking long periods of time here. Above and/or below the mph where 1/2 critical speed occurs is not a problem.

So you go 120 mph and stay at that speed for extended periods?


BTW - it 1/2 critical speed be a bit lower for an R154 shaft...it's longer than an A340 shaft.

you should also note that you would have to be at EXACTLY that RPM for a very long period of time for ANY chance of potential fatigue to START. in other words, i wouldn't worry about it.

 

[suprahero]

Thanks alot guys, I feel alot more at ease now. I don't travel at 120mph for extednded lenghts of time, but when I race my brother from time to time, I do exceede the 120mph mark, but it only took about twenty seconds I would say and then I was back down to 60mph.

For those of you that have just received your driveshaft, please keep us informed as to how well it performs on the highway and everyday driving. Thanks again for the informative answers.....................:bowdown:

 

[shaftmasters]

I like the idea of a stronger 2 piece.

I have driven a few cars that have been converted from 2 piece shafts to a 1piece driveshaft (including but not limited to supras) and I have not seen one that doesn't create additional vibration.

Unfortunately, it always seems to occur at highway speeds, which imho is the worst place for the vibration because that's where you are likely to spend the most time at a fixed speed feeling that vibration.

If you're building a car for the track, one piece is the way to go, but on my daily driver it's just too annoying.

I think that for anyone who has a worry with any modification, whether it is a driveshaft or something else, for peace of mind, don't do it! I will say though that a little education goes a long way. I have been repairing, manufacturing and balancing driveshafts for 25 years. Any two-piece shaft also has it's limitations. I have seen many failures from carrier bearings where the rubber starts to melt and you end up with a gooey mess. Whenever I balance a two piece shaft in general they tend not to run well at higher rpm's. They usually start to act-up over 2500 rpm's and are difficult to fine tune to run at what I like to refer to as "dead nuts" . The aluminum shafts easily run "dead nuts" at 2500 rpm's and well beyond.
Robert

 

[suprahero]

I'm in the construction buisiness, so I use ther phrase "dead nuts" all the time when leveling walls, or siding or whatever. Sorry,............just rambling on.