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# 4340 transmission output shaft

## 4340 transmission output shaft

(OP)
Hello All,

I am new here and this is my first time posting a thread. I would like some feedback from either expert advise or real world experiences. I have Machined a few aftermarket performance transmission main shafts for a transmission that did not see much attention in aftermarket upgrade industry(would rather not disclose what Transmission they are designed for). So far the two I have made are holding up quite well with zero issues. BUT I feel I can make them easier and or better on the heat treating end of the spectrum. The design portion is limited as it is a direct replacement. The two prototypes I have now running are machined from 4340 annealed steel. All machining and cutting done prior to heat treatment. The shaft is Gas nitrided on the surface with a hardness of 52-56 HRC and a depth of .025" case. Overall after heat treating dimensionally the shafts did not change with this process enough to have to remachine anything. I have two more shafts completed also from 4340 annealed and possibly looking to go another route on heat treatment to better the life of the shaft. I am just not sure if the case depth of the Nitride will stand the test of time. Any opinions or recommendations? I am also looking into Carbo-Austempering, anyone had any experience with this? The less amount of post machining I have to do after heat treatment the better. Below are some numbers I have wrote down so you all know what type of power I am dealing with. Ultimately I am after the maximum torque output combined with the best heat treatment process. Aren't we all?

Engine= 600 ft lbs, 7,000 rpm
Trans= 3:1 ratio, Manual clutch
Speed @ shaft = 2333 rpm
shaft length at this given radius = 11" (estimate)
Torque @ shaft = 1800 ft lbs (did not figure in any clutch loss/ slippage)

With these numbers I am getting a shear psi of 42,317. Does this seem accurate?

Thank you everyone!

### RE: 4340 transmission output shaft

Assuming that your shear stress was determined for the surface of the shaft, it won't be all that much less just under your case hardening (shear stress in torsion is directly proportional to the radial distance from the center).

Chances are the tires won't be able to hold several thousand ft*lbs of axle torque (1800 x some axle ratio), so you're probably off the hook for looking at 1800 ft*lbs taken as a constant load case. But suddenly engaging the clutch will involve an impact factor of some sort, and the wheels and tires represent rotational inertia (resistance to being accelerated rotationally), and such details as splines and shaft thickness transitions will introduce stress intensification effects. That gets into a fatigue evaluation of some projected usage.

Norm

### RE: 4340 transmission output shaft

(OP)
The shear stress was taken from the rear most part of the shaft which in this case is the longest/ smallest cross section at 1.375" diameter with the spline major diameter being the same size (estimate). In this world racing it is not uncommon to see 10k ft lbs of force on the drive axles and with almost instant power through the drive train and a clutch intended for minimal slippage I just want to be sure I am getting the most out of my shaft in terms of the heat treating process goes. Like I stated before the size, geometry and overall design of the shaft is pretty well fixed with being a replacement part. So I want to be sure I can gain the most I can from the material and treating process and I am not sure I am doing all that I can from that stand point. If that makes sense.

### RE: 4340 transmission output shaft

If you can deal with some finish grinding or turning I would first furnace heat, hot oil quench & temper the shaft. Then follow the furnace heat with induction hardening and tempering to the same surface hardness but with up to 0.300 effective case depth (to 50 HRC).

### RE: 4340 transmission output shaft

(OP)
dbooker630, Thank you for your response. I have actually contacted a lot of companies in U.S about induction hardening this shaft. Once I send them the drawing of the shaft they tell me that the shaft has too many steps in it and it would cost a fortune for me for a company to have to supply a coil pattern/ multiple size coils to effectively induction harden this shaft. The shaft ranges from 2.25" to 1.375" in diameter with multiple steps and diameters in-between. This is why I went the Nitride route. BUT looking into other options. Unfortunately the induction cant be one of them. Has anyone on here familiar with the Carbo-Austempering of 4340 and can shed some light on this? I have done a lot of research and reading but want someone with more knowledge on this subject to be on the same page.

thank you all!

### RE: 4340 transmission output shaft

Steps are always an issue with induction but they can be worked around through a series of dwells and power adjustments throughout the scan. But development time would be required. In your case a single-shot coil would be preferred. Depending upon the size and length of your part a single shot can range anywhere from $3K up to$20K. For a single part I agree this is not the most cost effective option. I am not familiar with anyone carburizing a 0.40% carbon steel.

### RE: 4340 transmission output shaft

Take a look at ASTM A646 (N135) and think about nitriding as an option. This is Nitralloy 135 mod as used in aerospace applications.
We used to use this alloy, we would quench and then temper at 1050F, then nitride at 1000F for 48 hours.
The results were very reliable and the properties were great.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

### RE: 4340 transmission output shaft

(OP)
dbooker630, Carbo-Austempering is a quite different process then the normal carburizing you see done today. I have done quite a bit of reading on this process myself and also read some literature using this process with 4340. I just wanted insight from someone else whom may of had some real world experience with this and some Do's and Don'ts maybe.

EdStainless, I will look into that material for future shafts as a possibility. The process you did on the N135 is indeed very similar to the process I did on the last shafts. Shaft was quench and tempered back down to mid 30s RC and then gas carburized @ 975F for 48 hours with a surface depth of .025" and hardness of mid 50s RC.

### RE: 4340 transmission output shaft

The 135M with the 1050 temper will be 37-41 HRC and still have great toughness.
With nitriding you get a two step effect, the near surface (about 0.010") will be 80HRC, and then deeper (at least 0.060") will be about 60HRC. After that it will taper off to the core properties.
I should add that 4130 nitrides fairly well also, just not as high of surface harness as the 135M.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

### RE: 4340 transmission output shaft

(OP)
Right, After doing research I noticed the max achievable surface hardness was much higher on the 135 material but the max achievable case depth was more on the 4340. With the 4340 material I have the shafts made from right now that I am looking to get heat treated. What kind of strength percentage gain could I expect with 4340 core hardness 28-32 HRC and gas nitrided surface of 55HRC, compared to the same shaft 4340 in the annealed state I guess is the question? Which is about 17 HRC.

Thank you,

### RE: 4340 transmission output shaft

We are working with a similar situation on a sand buggy. Same material splined shaft. We have broken two of them spinning wheels then hit pavement do big shock load. One sheared right off just outside if the bearing. The other tore the welded part and a section of the hub out. This was carefully TIG welded. But no heat treatment other than slow cooling. Maybe a 1045 material might be better. I think grade 8 bolts are made from this but the heat treating is not my specialty. These are used as floating axels. They really need a redesign to abetter floating situation. The welded one was a repair for being sheared. It was re machined to remove the torn metal. I suspect the weld created a weakened area in the hub. Snap rings hold the axels in place but the groove is outside off the load. The redesign would probably have the bearing riding on a splined sleeve arrangement. I’ll look at it later the shel is pretty expensive as it is so tricky heat treating might put them in the unreachable expense. Many race car axels are made from 300 m. They may even be room formed. I did the spine of a missile like this but there was no limit on expense. I know the bill was astounding.

### RE: 4340 transmission output shaft

You do not want to weld a part that has been carburized or nitrided, unless you leave a good section masked off and not surface hardened.
If you have any steel over 0.25%C that has been through Q&T you can weld on it with pre-heat and then afterward re-temper it at 25F below you original temp. This won't weaken the parts and it will reduce the stresses. Of course you also have to make sure the parts are clean, use good shield gas (or low hydrogen electrodes), and watch your heat imputs.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

### RE: 4340 transmission output shaft

(OP)
Yes, Exactly what EdStainless said. Without seeing the axle or break point I would assume it broke because it was a heat treated part that became brittle. I would only recommend welding on anything in its annealed state. Then Heat treating afterwards. If you do have to weld on it use the sequence EdStainless mentioned. Going to a 1045 material will not help the issue if it is done the same way the other axles were made.

### RE: 4340 transmission output shaft

Sorry I misspelled in my previous post. I have a double vision situation that makes typing extremely difficult. Anyway I meant roll forming similar to spinning sheet metal. This process was required in the project.it was very expensive as it required additional tooling. However it did survive a very hard impact in a rocky area. But getting and machining a relatively small and one off item is out of the question for our little hot rod. The new parts will have some heat treating following what I learn here. Thanks everyone for commenting on these parts.and materials. The welded part is out of the question due to my new medical and vision issues. Even my service dog can’t help me here. She is good but I’ve not heard of a welding dog be safe everyone there is an invisible killer about.

### RE: 4340 transmission output shaft

Pictures and drawings of the shaft would be helpful.

Nice closeup Pictures of the fracture surface are essential.

### RE: 4340 transmission output shaft

The out put haft on the sand buggy is still up in the air. The draft goes part way through a flanged piece of a cv joint. There is enough left of it that it could be cut down then bore the flange to fit. In the old days of drag racing we narrowed rear ends by doing this with axels then drilling a blind hole on the bore axel centerline and pressing a dowel in it then welding around the center line this held up to some pretty high powered cars in the day. Axels are one piece today or splined tubes but this may be our solution untill a custom axel is made. If we could spline the hub it would be nice but they are too hard unless we spring for an Edmonton cut.

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