Heat-treated 4130 for transmission input shaft coupler

[deloreanz]

I'm looking for some feedback on a project I'm working on to replace a weak input shaft coupler in a Renault 369 transmission. Rather than make the input shaft one piece, Renault used a relatively weak metal for the coupler and only partially splined its ID.


Someone had them remade in heat treated 4130 and it seemed to work out pretty well. I'm about to do a batch of my own and was curious if anyone had a better material recommendation of similar cost? I'm planning on running them through a cryo process as well after they're done.

Thanks

 

[redpicker]

Do you know the hardness of the factory part, or you just going with the "heat treated 4130 has to be better than the crappy material the factory uses" theory. For the cost of making custom parts, it would probably be worthwhile to spend $100-$200 on a metallurgical analysis (Chemical analysis, hardness, microstructure) of a factory part so you know what you are trying to duplicate.

Are you planning on having them heat treated before or after machining? This is an important processing step and the sequence of operations is important; if you don't get it right, it could create a lot of scrap.

Have you considered that perhaps the factory part is designed purposely to be weak so, in the event of an overload, only a relatively easy to replace, inexpensive part needs replacing instead of a lengthy and expensive overhaul? I am not saying that you are making a mistake, just asking if you have thought about it.

Finally, in my opinion, the risk in cracking the parts by subjecting them to a cryogenic treatment is far greater than any improvement you will see. I will admit that the risk of cracking is rather low, but it is still far greater than any improvement.

rp

 

[deloreanz]

I'm looking into getting the stock coupler tested to get more info on what it is made of. I do know there was a qualitative difference before and after people installed this upgraded coupler.

The theory Renault had was that the coupler would fail before the rest of the transmission. Unfortunately when the coupler fails it ruins the input shaft splines and throws metal bits everywhere else. For my batch, the plan was to heat treat and then broach it, which is how the first batch was made which ensured the fit would be exact.

As I understand it, as long as the cryogenic treatment is applied properly cracking shouldn't occur. I've heard only good things from people who either do or use the technique.

 

[tbuelna]

deloreanz-

If you want to use a CrMo steel, 4140 is probably a better choice than 4130 for this application. A cost effective commercial alternative to heat-treated 4140 is Flexor M. It comes pre-hardened to Rc38-42, which is about the max hardness level you would want for turning/broaching operations. For a limited number of parts it would probably be more economical to use heat-treated stock material, and eliminate all of the post machining heat treat operations.

Hope that helps.
Terry

 

[deloreanz]

Why do you recommend 4140?

One limitation I'm not sure how to get around is that the broaching said they can't cut splines in anything higher than Rc32-34. Broaching before hand would potentially make the spline fit too tight, but I don't know if it would be negligible.

The shop was going to handle procuring the material, basically the entire process but perhaps I can ask them about 4140. I know how some shops can get if you ask them any engineering recommendations.

Thanks again

 

[tbuelna]

4140 would respond to heat treatment much better than 4130 with the section thicknesses shown in your example part, and the cost difference would be negligible. 4130 is preferable mostly in applications where welding is required.

Internal splines can definitely be broached or shaped in material harder than Rc34, but harder material does reduce tool life. If your spline vendor prefers a material hardness below Rc34, take a look at the Flexor alloy instead, which comes pre-hardened Rc28-32. As I noted above, for your small batch of parts I would highly recommend avoiding any post-machining heat treat operations. It is quite easy to scrap parts during heat treat operations from cracks or distortions. It is very costly to scrap a batch of parts during heat treat operations that are already 95% complete.

Good luck with your project.
Terry

 

[NickE]

properly heat treated 4130 or 4140 will not show significant property differences due to a post-heat treat cryogenic treatment.


Nick

 

[Frederick]

NickE
Your statement that "properly heat treated 4130 or 4140 will not show significant property differences due to a post-heat treat cryogenic treatment" flies in the face of the research. Yes, hardness will change only a few HRC points and tensile strength will not change a whole lot, but the part life will greatly increase.

As to the part cracking as mentioned in a previous post, if you dip the part in liquid nitrogen you probably will crack it. Dipping a component in liquid nitrogen is not cryogenic treatment. A proper cryogenic treatment is a slow reduction in temperature, followed by a hold, followed by a slow increase in temperature to room temperature. This should be followed by at least one tempering cycle for hardened 4130 or 4140. I don't know where people get this cracking stuff. It is kind of like saying don't heat treat a part because it loses tensile strength in the austenitic zone.

Nick

 

[deloreanz]

At this point based on the info here and discussions I've had IRL, I'll probably stick with the heat-treated 4130 and do a cryo treatment thereafter. It remains to be seen if I'll need to heat treat before or after the broaching.

Thanks for the feedback everyone, but please chime in if you have any additional thoughts.

 

[gearcutter]

You are after a part which needs to have a greater resistance to wear than the original.

I would make the part out of 4140, rough turn, through harden & temper to 32-35 HRC, finish turn and then gearcut/broach.
Then I would have the part GAS or PLASMA nitrided to a depth of 0.50mm. If this is done correctly, you will end up with surface hardness of around 48 - 52 HRC.
In this case; 4340 would not be suitable.
I feel that this would be a far better outcome than what is being proposed.
It is highly unlikely that there will be any significant distortion as a result of the nitriding process as the process temperature is quite low and there is no quenching required.
The quenching process is generally what creates most of the distortion in a part.

One thing to keep in mind though; the splined sleeve is part of a system that is effectively a flexible coupling. Therefore, due to misalignment, something has to absorb the misalignment. The purpose of the sleeve (if replaced regularly) is to be the softer sacrificial member so that the shafts do not wear.
Note how I said that the sleeve should be regularly replaced.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[deloreanz]

I'm not sure how valid it is to say the coupler is meant to provide play or that it should be replaced. Many instances of this transmission show the coupler is on quite snugly, while in others it is looser and can be removed by hand. As for replacing it, I know of no renault recommendation to do this. I am only aware of the design intent for this coupler to shear before input shaft damage, which wasn't an idea that worked since the coupler ruins the input shaft splines when it shears.

I take it that 4140 can handle the nitride process whereas 4130 is not suitable (you mentioned 4340)?

 

[gearcutter]

I would suggest then that your information is incorrect as this is not a system designed to, as you say, "shear".



Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[deloreanz]

I don't get your implied meaning with the quotations. I've always understood that torsion on a shaft can cause it to shear, or in this case forces applied to the spline faces.

I find it hard to believe that they would have under-engineered the part using a soft metal and removed ID splines if it was purely meant for flexible torque transmission.

All that is beside the fact that they do fail, and very prematurely.

 

[gearcutter]

[highlight #EF2929]"I don't get your implied meaning with the quotations."[/highlight]
I don't understand what you're meaning is. I only used one quotation and it was yours.

[highlight #EF2929]"All that is beside the fact that they do fail, and very prematurely."[/highlight]
Have you conducted a professional investigation on any of the failed components so as to determine the true cause of failure? If not, then nothing of any benefit can be taken from your statements as they are not based on facts...........and I don't mean to offend by saying this.

[highlight #EF2929]"I find it hard to believe that they would have under-engineered the part using a soft metal and removed ID splines if it was purely meant for flexible torque transmission."[/highlight]
This is based on your level of experience and, once again, is not based on fact.
In the industries that I work in; this is a very common form of splined coupling. Used to both act as a flexible coupling and aid in assembly/disassembly.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[deloreanz]

"All that is beside the fact that they do fail, and very prematurely."
Have you conducted a professional investigation on any of the failed components so as to determine the true cause of failure? If not, then nothing of any benefit can be taken from your statements as they are not based on facts...........and I don't mean to offend by saying this.

I agree much can be gained from a professional investigation but in this case, it seems pretty clear if there are coupler splines sheared off past a certain torque while stronger couplers don't have this issue. If I can get my hands on a failed unit I can go about doing more research, but really at this point we know stronger couplers solve the problem at higher torques (aside from a very expensive 1-piece input shaft solution).

"I find it hard to believe that they would have under-engineered the part using a soft metal and removed ID splines if it was purely meant for flexible torque transmission."
This is based on your level of experience and, once again, is not based on fact.
In the industries that I work in; this is a very common form of splined coupling. Used to both act as a flexible coupling and aid in assembly/disassembly.

I can see how using a softer material might allow some flexibility but I don't see how missing splines would be a good idea. And it seems pretty clear to me that from a pure torque perspective, that the expected failure point is the coupler.

 

[gearcutter]

To some extent I might agree with you that the removal of spline teeth may act as a crude form of a torque limiting device.
I would advise against removing the coupling and making the connection out of one piece.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mrfailure]

I would go about examining both the spline coupling and the mating shaft for material properties to understand the nature of the failure and proper corrective action. Is there really such a high hardness mismatch between the two components that shearing off of spline teeth is a possibility? Also, I would try to determine whether failure mode really was overload or was it from fatigue (not to presume, but these are the two most likely based on provided information). Obtaining this information will then help you to answer the questions about material, heat treatment, and surface treatment required for the coupling and its teeth. Good luck!