Lower Transfer Case Gears

Not wanting to clutter up jpets thread, I have been documenting this gear adventure on my thread - Barrys Builds in Jeep Photos and Stories.  I wanted to share this idea here though for others working on this project.

Hi all,

A quick update. I've discovered a few problems that may have contributed to the failure of my input gear. I'm confident I can fix it but I have to remake those gears.

Also ... I got the seven gear sets back today and the heat treating company screwed them up ..... gotta start over ... Thanks for your patience.

Barry S wrote: Sorry hear that after all your time and expense.  Hope the heat treaters aren't planning to charge you again.  Wish I could lend a hand somehow.  No pressure on this end though - I appreciate all your hard work. -Barry

I feel like the design is good and that I will get to a good finished product, it's just some of the processes that I don't seem to have control of that are wearing at me.

Hi all,

I am still on this project. Here is and update. I took these parts to another heat treat company that was highly recommended to me. This company uses a thermal vacuum hardening processes that is supposed to be much more uniform and with less distortion to the part. (and more expensive).

First, a recap on the gears I have already made. On my prototype gears, I had the input and intermediate gears carburized and induction hardened (teeth only). The output gear is carburized, heated and quenched in its entirety to harden all outer surfaces, since all of the outer surfaces are doing duty. The sides are thrust bearings, the ID is a bearing, the clutch teeth have to be hard and of course the drive teeth. I did not have the input gear or intermediate gear "thru" hardened like the output gear because I wasn't sure how much the parts would change after heat treat, and I wanted to be able to file or machine some out of the internal splines in case they shrunk enough that they would no longer fit on the tranny output shaft. In hind site, I should have had all the parts thru hardened. The process is actually cheaper, and appears to be a more thorough process as I will show.

I had the gears RC tested by Midwest Thermal Vac:



... turns out that these induction hardened gears have a variance of 60-43 RC:





The spec is 59-60 RC

The potential good news is that Fred, at Midwest Thermal Vac says he can anneal these parts and Vacuum harden them correctly since they have already been carburized on all the outer surfaces. I am sending him some parts.

Even though I am frustrated, I am very encouraged by these discoveries. In my TC, I broke the input gear even though the intermediate and output gear were in good shape. Well the output gear was "thru" hardened, not induction hardened like the input gear.

Here is a pic of the output gear:




True, the output gear is not working in low range, but in high range, it is under more load than the input gear so to see it in such good shape compared to its sister gear is encouraging to me. I feel very confident that this design is going to work, but it may take a little more time to debug. I can't put this case in and head to the Rubicon untested but I hope to get the parts back and put in right after. Thanks for your patience.

Edited by jpet - 16 June 2021 at 11:34am

(x2)  I'm loving this tech.  I have a neighbor who works at a heat treat facility down the road.  They offered a tour and they frequently carborize.  I have been dreaming of giving this a shot myself as I have some older machining equipment, mills and a lathe.  This would definately be stepping outside my normal comfort zone.  Maybe one day.  

I got the gears back from heat treating:



These are vacuum hardened. It’s looking more and more like my previous problem was the induction hardening process. Fred did some analysis that were way over my head:



... but in a nutshell, because the induction hardening is done out in the open air, contaminates in the air can get into the metal surface. You can see this in the sheet on the right.

I am working with another specialist now who is analyzing my design and looking to improve on it but the good news is, he likes it and thinks it’s adequate for a 4.3 Chevy with 6.55 first gear. He is more concerned with the spur gears after these gears. This guy knows his stuff. Might be a little expensive though.

Teraflex gears:

Here is a little info, you may or may not find interesting:



This is an original 26 tooth input gear sitting on top of a teraflex input bowl gear. Both are 26 tooth but you can see that the original gear has a superior helix angle of 30 degrees compared to the 20 degree helix angle of the teraflex gear. It’s not surprising to me why the teraflex gears have a reputation for being loud. The teraflex tooth is approximately 8 DP.

Edited by jpet - 15 July 2021 at 8:45am

The loud versus the strong.  The Muncie M20/21 versus the M22.  In short, there is no free lunch.  You obviously get a more quite gear set with the higher helix angle but you sacrifice some potential strength in the process and develop more longitudinal thrust..  Ear plugs are cheap-----

duffer wrote: The loud versus the strong.  The Muncie M20/21 versus the M22.  In short, there is no free lunch.  You obviously get a more quite gear set with the higher helix angle but you sacrifice some potential strength in the process and develop more longitudinal thrust..  Ear plugs are cheap----- The spur gears have always struck me as wimpy but they do not seem any more prone to failure than the rest of the D18.  The 10 spline output shafts also leave a lot to be desired.  Just making those with the common 26 spline used on the later pinion yokes would no doubt increase the strength 20 to 30%.

I don’t disagree with this. I’m just saying that is why they have the potential to be loud. When I talked to the teraflex salesman, he said that was there biggest complaint is the noise. I’m not certain but I think the other gear is 42 teeth. If so, a 20 degree helix angle is about the limit for a 26/42 gear set that is 8 DP 20 degree pressure angle between 4.2857 centers.

Compromises ... right Bob?, roc? ...

When Greg, over at first gear questioned the strength of the spur gears, I told him that I have never heard of the spur gears failing other than rusting out because water got in the case.

Do 26 spline shafts exist for D18?   I think the u-joint would fail before the shaft but I’d switch to them if they exist.

oldtime wrote: For comparison…Standard Tera Low 4 gear set for D18 ….

Those spur gears appear to be 8 DP 20° pressure angle 23/45

Sometimes, increasing the helix angle increases strength and or durability. Sometimes it helps one gear but hurts the other. Yes, there is more thrust which is harder on the case. Sometimes as the strength increases, the wear characteristics go down. Example:



The first set of numbers (black), is the original 21/39 7 DP 22.5° pressure angle, .625 wide spur gears. The left is strength, and the right is wear

The second set of numbers is the same gear set with a 10 degree helix angle added. Notice that the design has similar strength numbers but better wear characteristics than the spur gears.

The third set of numbers is a 20° helix. The pinion is way stronger, the gear is less strong and the wear numbers start to decrease.

For fun, I ran the numbers for the teraflex spur gears (red) 23/45. I assume they are 8 DP and 20° pressure angle since 7 DP on 4.2857 centers would be impossible.

Also, for fun, I am posting the numbers for my gear set (blue). Mine are .933 wide.

Edited by jpet - 19 July 2021 at 1:41pm