Strange Half shaft problem

[Rakesh Murali Team Ojas]

http://www.formula-hybrid.org/forums/download/file.php?id=10

Hi, we are using a Westgarage Sierra LSD. But due to size limitations, we cannot include the coupler part- the part which has a splined rod going into the differential and has a flange sort of structure for bolt-on type CV joint. I have attached a picture of the same. To eliminate this, we are using a Maruti 800 half shaft with internal splines on the inner tripod (picture attached) and hence takes a rod as the differential output. We are planning to machine a stepped rod which has matching splines of the differential on one side and that of the maruti half shaft on the other and thus couple the differential and the half-shaft. I have the following doubts and it would be great if you could reply asap:

1) The half-shaft is 22mm dia, solid. The maximum torque output of the maruti car is 960 N.m
What material will you suggest to make the coupling stepped rod of? We were planning for Chromoly as AISI 4340 is not available in India.
2) The MAX. Torque output of our car is around 940 N.m. Do I need to heat-treat the material of the coupler rod? If yes, how do I decide the hardness to achieve?

I know the question is pretty confusing, kindly refer the images, it will help you in understanding the problem. Please help ASAP!!!

[Rakesh Murali Team Ojas]

http://www.formula-hybrid.org/forums/download/file.php?id=10

Hi, we are using a Westgarage Sierra LSD. But due to size limitations, we cannot include the coupler part- the part which has a splined rod going into the differential and has a flange sort of structure for bolt-on type CV joint. I have attached a picture of the same. To eliminate this, we are using a Maruti 800 half shaft with internal splines on the inner tripod (picture attached) and hence takes a rod as the differential output. We are planning to machine a stepped rod which has matching splines of the differential on one side and that of the maruti half shaft on the other and thus couple the differential and the half-shaft. I have the following doubts and it would be great if you could reply asap:

1) The half-shaft is 22mm dia, solid. The maximum torque output of the maruti car is 960 N.m
What material will you suggest to make the coupling stepped rod of? We were planning for Chromoly as AISI 4340 is not available in India.
2) The MAX. Torque output of our car is around 940 N.m. Do I need to heat-treat the material of the coupler rod? If yes, how do I decide the hardness to achieve?

I know the question is pretty confusing, kindly refer the images, it will help you in understanding the problem. Please help ASAP!!!

[kcapitano]

Hi Ojas

I'm afraid you're going about this business a little backwards.

The geometry of your half shaft and differential will determine the geometry of your "stepped rod". Once you have a rough idea of the size of your "stepped rod" you can calculate the stress in the part based on your vehicle's torque output. With the stress found, you can then determine what material properties you need and therefore what material/treatment you need.

[Rakesh Murali Team Ojas]

Hi, http://fsae.com/groupee_common/emoticons/icon_smile.gif
Thanks for replying.
I know the approximate dimensions of the stepped rod. I have the differential and the inner CV joint housing. I can find out the spline geometry on both the differential and the casing.
I have been using the basic torsion formula E=16/Pi*d^4*Torque. The stress value which I get for 22mm dia is approx. 570 MPa. Now there are several materials which have their ultimate stresses in this range. I thought Chromoly will be better because I read in some forum that they use Chromoly inserts to increase half-shaft length. What is your opinion about Chromoly? Do you have any other material suggestion?

Coming to the heat-treatment, I tried referring to the Hardness:Tensile strength charts for steels. But a tensile sterngth of around 570 Mpa has a HRC of around 15-16. Again in some forums I read that while manufacturing half-shafts, the y heat-treat it to a hardness of HRC 40-50.

Kindly help me out here. By the way, our team is participating in FS UK 2012 with a hybrid car.

[Paul Achard]

First of all, make sure to take into account the stress concentration cased by the considerable step on your coupling shaft.
Second, when you look at torque in a metal shaft, the shear stress will typically be your limiting factor, not your normal stresses.
Third, safety factors are typically based on yield stress, not ultimate stress. In this case yielding is not a huge issue as long as you retain concentricity in your shaft, but I personally wouldnt feel safe knowing my parts were yielding.
I suggest you read up some more on shaft design, because you are apparently overlooking important factors.

[buggaero]

Give taper in the rod at diameter step to reduce local stress concentration at the step.

[kcapitano]

My opinion? Pick up a copy of Engineer to Win by Carrol Smith and any textbook on mechanical design. Carrol Smith will get you in the ballpark, the textbook will take care of the fine details.

oh, and don't believe everything you read in forums http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Rakesh Murali Team Ojas]

Hi guys, thanks for the replies again.

@Plooba: The formula I stated gives me 'shear stress'. I approximated shear stress to be 75% of Ultimate tensile strength and hence got the UTS. As far as yield strength goes, I didnt consider it as the length of the rod is not very high and hence i.m.o yielding wont be a huge concern. Anyways, as you have pointed out, I will consider it too. For stress conc. reduction, we are giving a fillet at the step. The diagram I drew was drawn in MS Paint http://fsae.com/groupee_common/emoticons/icon_biggrin.gif.

@Buggaero: Thanks, I was thinking of making it a fillet but even taper will reduce stress conc.

@kcapitano: I understand what you are trying to say. This is the 'first' time we are making an FSAE car, that too a hybrid one and so am an amateur. I did read shaft design from my course books but the fundamental problem I faced was in selecting the material!

[shark.ashwa]

@Team Ojas,
You do get 4340 in India, you just have to look hard! http://fsae.com/groupee_common/emoticons/icon_wink.gif
I wouldn't design stub axles based on simple yield strength calculations, I would put a considerable amount of safety factor into it.
Our stub axles (your coupler! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif) were made from EN 24, yield strength in the range of 850-900 MPa, hardened to 35 HRC.

Regards,
Sharath

[Rakesh Murali Team Ojas]

Thanks for your help Shark.ashwa! Now that was useful. Once am done with all the calculations and the machining, I will post the details. It will be useful in future!

[RCV Performance]

Ojas,

Just to toss another option into the mix for you, we (RCV Performance) do make custom tripod housings for our FSAE axle kits. This way you wouldn't need to have a stepped rod, as the tripod housing would plug right into the diff. Plus the entire assembly would be very light.

If you are interested in this, let me know!
RCVFSAE@gmail.com