What grade of steel would be best for a transmission input shaft? 7

[LJDCRF]

Currently we are using a material that I am told is a modified 4340 called 300M. The shafts that fail show helical cracks down the length of the shaft. The failures have all shown up in trucks that are subjected to large amounts of towing. The currently the shafts are machined from 300M billet and are hardened. I do not know how hard they are making them.

Because of the configuration I want to look into forging this design. Machining from billet creates a large amount of wasted material. The billet must be 5.25” in diameter and 9.380” long. Approximately 90% of this material is wasted.

The shaft is 9.380" long. The first 7.5 inches of the shaft has an OD of .970 the remaining length is a splined drum that measures 5.25 OD.

Reading the machinery’s handbook I found 4150 as a suggested material for hard gears that require strength and toughness, but its tensile strength is not much higher than 4340.

If the design continues to be manufactured out of billet, what materials are being used currently in high torque, cyclic loading configurations? What would be the optimal hardness?

If a forged construction is feasible what material should be used? What treatment should be used to finish the part?

 

[NickE]

If you're using 300M you are probably using the strongest production material you can.

4150 is only going to be harder than 300M on the outside. The core is going to be soft at 1" dia, and worse at 5.25".

I think you are going to have to go back to the mechanics of the design not the materials. unless you go to some sort of exotic material like Elgiloy (at ~$185/lb last I checked.) ... even then you will still see failures.

What amounts (magnitude) of stress is the shaft seeing? (FEA it, or just do a hand calculation to get you in the ball park.)

More than 600MPa and I dont think materials are going to help.

Does the cracking initiate at the transistion btw the .970" and 5.25"?

What is the surface finish of the shaft after hardening?

What is the hardness of the case?

What is the hardness of the core?

What is the torque output of the motor?

What is the max load of the vehicle?

What is the lowest gearing usable?


Nick
I love materials science!

 

[TVP]

LJDCRF,

You really need to understand the forces and stresses applied to this shaft in order to make a meaningful improvement. As NickE said, 4150 is not a better choice. Can you provide a more thorough description of the failure mode, or provide a link to an image? Also, you need to determine how the current part is processed including heat treating. If it is a ductile failure due to torsional overstressing, then this will likely require a different approach than if the part is failing due to fatigue, etc.

 

[LJDCRF]

Currently I have a design to increase the .970 portion of the shaft to 1.275.

At the same time we will be offering the stock size shaft in a stronger billet / forged construction. If we are already using the best material then I will focus on the design and eliminate as many stress concentrations as possible.

Before I started, much of the design work was handled by our suppliers. Many of the details are known only to them. I am trying to get heat treating details and surface finish specs from them, so that I can at least establish a baseline.

I just found a broken shaft and the failure is different than I described. It looks like the shaft failed because there is no relief at the end of the splines.

What would I specify if I wanted this to be forged? How are forgings finished? Are they heat treated the same as billet?

Picture to follow.

 

[LJDCRF]

Where can I post the pictures? I assumed I could post them in this forum, but I was unable to.

 

[LJDCRF]

Cracking does not occur btw the diameter change.

The lowest gearing usable would be 1st gear 2.54, 4 low 2.72, and rear diff ratio of 3.73. Total reduction: 25.68

The stresses calculate to be 24000psi ~165MPa. This number was determined using the torque measured on a chassis dyno and does not consider any loses. Dyno measured 1300ft-lbs output through 3:73 gears, transmission in third gear (1:1), with the torque coverter fully locked.

The max load on the vehicle can be as much as 100% of the vehicles capacity in sled pulling. Granted the tires are always slipping a little, but when the sled starts bearing down on the truck the tires grip pretty hard.

 

[kenvlach]

LJDCRF ,
Post your pictures on a web server where they can be directly accessed as jpg or gif files, e.g., at

http://i3.tinypic.com/.

Then, to show them on Eng-Tips: enclose the URL, preceded by img & a space, within [].
E.g.,
[ignore]

[/ignore]
will show

 

[TVP]

What would I specify if I wanted this to be forged? How are forgings finished? Are they heat treated the same as billet?

Are you asking how to specify that the splines are forged instead of machined (broached, etc.)? If yes, then I recommend creating a "forging" drawing that is separate from the "finished" drawing. The forging drawing includes all the features that are to be forged with appropriate dimensions, tolerances, etc. The finished drawing then shows the final shape with expected machined dimensions, requirements for shot peening, coating, inspection, etc.

With regards to heat treating, the only difference would be a potential normalizing treatment prior to machining. Normalizing creates a better microstructure for machining, with the part subsequently quenched and tempered, induction hardened, etc. It is important to understand whether or not the shaft has multiple heat treatments, e.g., initially quenched and tempered to create the bulk properties and then induction hardened or carburized to produce a locally hardened surface either over the entire shaft or just in the splined region.

 

[LJDCRF]

More I am asking how to specify that I want to start with a forging and then machine the part to the finished dimensions. The details from forging to finish part is what I am hoping to gain from this part of the discussion. The normalizing before machining is an excellent example. Thank you TVP. If forging the splines would be stronger than machining, then I would definitely be interested in pursuing that route.

Thank you Kenvlach for the pic posting advise.

 

[Tmoose]

Poor geometery choice can create stress concentrations over 3X.

http://www.mechengcalculations.com/jmm/beam68.html

 

[CoryPad]

The description of the material and the failures is incomplete.

Properly produced shafts made of 300M can withstand 165 MPa for almost unlimited cycles.

Your stress estimate is incorrect by at least a factor of five, or your material quality is very poor.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[LJDCRF]

I over simplified my stress calculation description. The stress I calculated did not consider stress consentrations or loses in the transmission. I merely calculated the max torsional shear stress for a .970 OD X .312 ID tube subjected to 350 ft-lbs of torque (686hp high performance truck). Upon further discussion with our transmission shop, I found out that these shafts are being broke by trucks with completely stock engines (285hp) when they are towing huge amounts of weight. The shafts are failing more in high cycle situations.

 

[TVP]

LJDCRF,

Obviously the failure is due to the geometry of the splines and how these transition into the shaft. Increasing the diameter of the splines while maintaining the same shaft diameter while blending the two sections with a large radius would be the first step to improve the design. Having said that, I will address the follow-on items that you requested regarding specifying this shaft as a forging.

First, as you already know there is a quite large difference between the shaft diameter and the drum diameter, which complicates the forging process. One option would be to manufacture the drum and shaft separately and then join them by friction welding. The following link shows a part manufactured in this manner:

http://www.hirschvogel.de/de/_pdf/Brosch_HKG.pdf

Now the next thing I should clear up is the thermal processing that should take place after forging and prior to machining. Normalizing is frequently used for lower alloyed and lower carbon steels. For 4340Mod/300M normalizing to a ferrite + pearlite microstucture is not a huge improvement in machinability-- I think the hardness is still around 350 HB in this condition. Annealing or spheroidize annealing to a hardness in the range of 187-241 HB will result in optimum machinability. Now, keep in mind that this type of processing is based on conventional machining, and does not take into account hard turning/milling concepts that have become more popular recently, primarily for precision tools and dies.

If this were my component to develop, I would either make it in two pieces as I mentioned above, or I would look at making it on hot extrusion equipment like the Hatebur HM75XL. The maximum billet diameter for this machine is 90 mm, and this can be upset to a maximum diameter of 145-165 mm and a maximum length of 180 mm. This would create a preform in four die stations that would be roughly the proper drum diameter, but the shaft would be larger and shorter than required. The next step would be to cold extrude this part to the final near-net shape, only requiring final grinding to the finished diameter. Turning would be limited to the drum end and for the grooves. The splines could be extruded as well. Obviously this entails a lot of forging tools to be created, so if the number of parts to be manufactured is not very high, then this will be overly expensive.

Using this process, the part would be annealed after hot forging and prior to cold extrusion. The splines could be finish extruded, and the annealed + cold worked microstructure would be adequate for machining. The part would the quenched and tempered to the desired bulk hardness, probably in the range of 42-47 HRC. Finish grinding would create the final contours, accounting for distortion. Induction hardening of the splines to a hardness of ~ 53-60 HRC and a depth of 0.5-0.8 mm would probably be sufficient. Tempering for 1 hour at ~ 180 C would improve the toughness without deteriorating the hardness much.

The last item I would investigate is shot peening. You may want to do this as the very last process, making off areas that you don't want a change in surface roughness. Alternatively, this could be done prior to induction hardening, and tailoring the process to achieve the desired residual hardness, surface roughness, etc. Metal Improvement has a nice website and a lot of technical information that can be downloaded. The Forging Industry Association also has a nice summary of the questions and details that should addressed regarding specifying forgings (

http://www.forging.org/engineer/pdf/HowtoBuyForgings.PDF).

Good luck and feel free to ask any further questions.

 

[LJDCRF]

Thank you TVP.

I also want to say, thank you for your patience, to everyone else that participated in this discussion.

I am going to pursue a billet machined two-piece design using 300M for the shaft and 4340 for the drum. A stress relief will be added behind the splines.

I have a few more questions.

Do the hardness targets also apply to a billet construction?

If not...

Using 300M for the shaft, what hardness would produce high toughness and strength characteristics? i.e. 52-55Rc for the bulk and 58-60Rc for the splines. Would shot peening be beneficial?

Is a friction weld different from a press fit?

If so...

Using a friction weld, what should the interference between the mating surfaces be? What surface finish should the mating surfaces have? How hard should these surfaces be prior to friction welding?

 

[GMIracing]

"The stresses calculate to be 24000psi ~165MPa. This number was determined using the torque measured on a chassis dyno and does not consider any loses. Dyno measured 1300ft-lbs output through 3:73 gears, transmission in third gear (1:1), with the torque coverter fully locked."

"I merely calculated the max torsional shear stress for a .970 OD X .312 ID tube subjected to 350 ft-lbs of torque (686hp high performance truck)."

There is a very large problem with your analysis posted above.

"The max load on the vehicle can be as much as 100% of the vehicles capacity in sled pulling. Granted the tires are always slipping a little, but when the sled starts bearing down on the truck the tires grip pretty hard."

Based on your descriptions of "High Performance and Sled pulling", I am going to assume that you are running this shaft in a pulling truck.

I don't know what TCM you are using, or even what transmission for that matter, but at 100% engine load, I would assume that your TC is no longer fully locked. If this is the case, depending on your torque ratio of the TC, you are looking at 600-700 ft-lbs of Input shaft torque, much more than your posted 350.

Two simple suggestions:

Given that you currently don't forge this part, i am guessing that you do not have a very high production volume. Forging may prove to be an expensive solution to a entirely unrelated problem. To save material cost, i would still suggest a two piece design. If space allows, riveting the Input shaft to the drum is an easy, cheap, and low tech solution. This also allows for different metals for the two pieces, with additional savings when machined from solid stock.

part II. If possible, I would suggest rolling the spline form in with an induction hardening process afterwards in the spline area. Rolling usually forms a very nice tooth root radius, and also removes the stress risers in the spline transition area.

 

[LJDCRF]

Thanks GMI

We produce the converters we use.

I incorrectly assumed the converters to be fully locked at the end of the sled pull. Our converters have a torque multiplication of 2.5-2.8. This does help explain the failures. I will revise the calculation and assume 25% loss in the transmission.

Revising my calculations:


1300/3.73 =~ 350 (Dyno torque measured / diff ratio)

350*1.25 = 437.5 (25% loss)

437.5*2.8 = 1225 ft-lbs (Torque converter Multiplication)

1225 * 12 = 14700 in-lbs (converting to in-lbs)

J = .086 (polar moment)

? = (.970/2) = .485

?max = T? / J

?max = 82901 psi =~571.6 MPA

 

[LJDCRF]

The forum did not accept the greek characters I imported.

Please note

?=(.970/2) = .485 should read: rho = (.970/2) = .485

?max = T?/J should read: Tao max = T * rho / J

 

[TVP]

L,

1. The hardness targets that I provided will produce a tougher part than the range that you suggested (52-55 HRC).

2. Visit MTI website (

http://www.mtiwelding.com/)

for more information on friction welding. This is not at all similar to press-fitting.

3. Surface roughness really depends on stresses, etc. I would start with something like Rz 16 micrometers maximum for the turned/ground surface. Shot peening will increase this. I definitely recommend shot peening, and you should work with a company like Metal Improvement to develop the requisite process.

4. Process sequence should be friction welding --> finish machining --> heat treating --> shot peening. The exact sequence for shot peening and spline making will depend, meaning that masking of the splines would be required if this occurs first.

 

[Tmoose]

"shot peening. You may want to do this as the very last process, making off areas that you don't want a change in surface roughness. Alternatively, this could be done prior to induction hardening........."

Shot peened steel parts must be kept below approximately 400F after peening in order to prevent inadvertently relieving the beneficial compressive stresses. Peening very hard steel, over RC 50, reduced tooth failure completely on a record holding Harley ProStock dragster's gears in the 70s. Then again, the magnafluxing, deburring and polishing of critical areas may have helped as much. It's tough to say.

 

[TVP]

Tmoose is correct about the temperature limits and shot peening. However I did not mean to imply that the shaft surface should be induction heated, only the splines on the drum. The shaft surface could be induction heated to an even higher hardness than the bulk, but then this would be another variable.