One Time Opportunity: Upgraded (“bulletproof”) transmission coupler order

[Nicholas R]

No, I don't think anything was e-mailed out yet. Ed told me on July 6 that he had placed the order for 20 couplers and that the leadtime is about a month so it may be until early August until you hear from him.

Thats correct. You will hear from Eddie for order information.

Eddie and I are kind of teaming up on this. I made a drawing for a coupler a while back when I pulled mine out of my transmission to swap for my one piece shaft. Always thought I may have them made but didn't know if there was interest, and I've been very invested in producing my 1 piece shaft (which is still happening). My company is also sending me out of the country for about 5 months so I figured it would be some time after that. However, since the need is here, I'm having the couplers made and shipped to Eddie since I'll be out of the country, and Eddie is handling all of the orders, and installing them in the cars at his shop that require them.

I spoke to the manufacturer yesterday and they said that they had acquired the material and were in the process of machining them now. They have a transmission shaft that I sent them so that they can test fit the splines before they are sent. They said that they are still shooting for early August for shipping (to Eddie) barring any major issues.

I have always hated the idea of taking money ahead/pre-orders so I will not accept money until the parts are complete and ready to ship. Sometimes there are problems, delays, etc. in the process, and though often unavoidable, they are a pain; but to me that pain is amplified exponentially when customers have already put money on the line.

For those curious, I performed some Finite Element Analysis (FEA) on the part for steady torsional loading on the fully splined coupler with the 4340 material at the heat treatment level I have chosen, and the principle and shear stresses are below the yield strength of the material up to 500ft/lbs of torque. [PLEASE NOTE I AM NOT RECOMMENDING PUTTING 500FTLBS THROUGH YOUR TRANSMISSION] That being said, steady torsional loading isn't usually what breaks things, it is typically shock/impact loading (aka: your burnouts, potholes, quick changes in traction, railroad tracks, etc.) and fatigue. Still I am confident that in this specific part should no longer be the weak link in the input shaft line.

Coupler FEA Simplified.jpg

The weak link will much more likely move to one of the low cross section areas of the shaft where the shaft was necked down so that splines could be rolled, the worst of which being where it is down to ~17mm-17.5mm diameter coming out of the coupler. The other known issue is at the overhung 5th gear as under heavy load the 2 shafts can begin to spread apart causing the teeth to improperly engage, or an excessive alternating stress in the shaft causing fractures.

Original Shaft Cross Section Dimensions.jpg

Here is a stripped 5th gear from a transmission of mine.
Failed 5th.jpg

I have performed the similar FEA on the original shaft and it doesn't fair anywhere near as well at that level of torque. Not knowing the exact material for the original shafts makes it a little difficult (it's clear that its nothing special, likely just general alloy steel), but I can make some assumptions about it based on the case hardening and the tested hardness. The stresses calculated are far beyond the tensile strength of the material for steady load (not even considering fatigue). I would still recommend that people drive responsibly as they approach the 300hp/300ftlb area.

Original Shaft FEA Simplified.jpg

Also bear in mind that this part is useful for not just higher powered cars. I have talked to multiple owners and vendors about transmissions that have failed on stock cars at the coupler. A transmission in my possession failed at the coupler due to fretting corrosion (which occurs at non-lubricated connections where vibration/movement is present) and broke the main shaft at the roll pin. This happens in the coupler because the main shaft is case hardened, where the stock coupler is significantly softer (it is not a hardened part), and the connection is completely dry as its outside the lip seal.

Here is a photo one of the transmissions I have where the coupler splines corroded away due to fretting, eventually putting all of the load on the roll pin, shearing the end of the main shaft (the end of the shaft is still inside the coupler). This is a shame because the result was that the main gear shaft was sheared, which is the most expensive shaft to replace. Some say that the coupler is a "mechaical fuse" but to me, if it's failure leads to the most critical and expensive shaft becoming damaged and unusable, it's not serving that purpose. That roll pin is only there to control axial positioning and not meant to carry torsional load; but when the splines fail, thats where the load goes.

Failed Coupler.jpg

Broken Shaft.jpg

Anyway, here's hoping the machining process continues smoothly!