Hirth Coupling

[petertha]

I bought some plans from Modelltechnik, maybe my next project, not sure yet.
https://www.cad-modelltechnik-jung.de/construction-plans-model-engines.html

On a few of his designs he uses a built-up crankshaft method with Hirth couplings to join mid span crankshaft journal segments. These are concentrically retained within journal bearings and axially held together with a cap screw. It looked a bit intimidating, but basically looks like set the dividing head at the prescribed angle & cut across the cylinder face with a vee cutter to prescribed depth. His CAD plans shows the theoretical cut profile: 60-deg vee angle, cut inclination angle, tooth depth from point cylinder face edge & number of teeth. By theoretical I mean the peaks & valleys are defined by the sharp points. Through these links I think I'm able to replicate the cut profile. The teeth appear to mate properly in various section views & I get agreement with the axial inter-mesh distance.

General info
https://en.wikipedia.org/wiki/Hirth_joint

Solidworks + Excel. Shows underlying equations in spreadsheet. Some of the output parameters I can correlate to my 3D model, others I'm not too sure about.
https://grabcad.com/tutorials/designing-a-hirth-spline

Calculates cut path angle for standard Hirth Joint/Coupling based on given number of teeth and cutting tool profile angle. Generates downloadable OpenSCAD model but I wasnt able to open that file format.
http://thmq.mysteria.cz/hirth/

Its real life machining I'm wondering about. The cutter will have some small but defined flat or nose radius. I allowed for that distance/geometry in my CAD model. And the tooth crown should have some defined flat for trough clearance when parts inter-mesh, so factored that distance as well. BTW I'm still preserving the vee cut profile from the theoretical geometry but I guess I'm saying I'm kind of winging it here on the peak & trough details. I'm also not exactly sure how I would measure depth in-situ while machining - kind of like how you would measure a thread to know your pitch diameter is within tolerance.

- Has anyone machined this profile before? (maybe specifically on a model engine crankshaft).

- Does anyone have links to Hirth geometry/equations (ideally for simple minds LOL).

I've ordered a double angle cutter like attached pic just to experiment. Hopefully will do the job?

[petertha]

My cad tester pics

[petertha]

60-deg cutter I ordered.

[Roger B]

One of our currently not active members posted a video on this some years ago:

<a href="https://www.youtube.com/watch?v=-WacJOVzxmY" target="_blank">http://www.youtube.com/watch?v=-WacJOVzxmY</a>

It looks complicated to me 

[petertha]

Thanks, yes I saw that video.
I noticed he cuts the teeth with what looks like a boring bar style of arbor with maybe HSS cutter profile. The multi-tooth cutter I show might bite me when it comes time if I cant get proper clearance for the tool holder etc. but cross that bridge later.

[petertha]

BTW in case anyone is scratching their heads, the cad pics I show in the first post are 2 different Jung engines. The 2D is an opposed 4-cyl and the 3D is my rendition of his Vee 6-cyl. But both engines & others on his site use Hirth couplings which was my point. Lots of videos of the engines running, so feel-good confidence there. Now, how they actually made the joints is another matter LOL

[petertha]

In the MuellerNick Youtube video he defines the nose flat of his cutter. First pic is my cad equivalent to that feature that I mentioned earlier.

And also including the tooth crown clearance facet cut. Here I'm kind of winging it. Another link described this cut as being made like in a lathe with the compound set. I think this would yield an arc rather than a line but now getting into details that wont matter because its such a tiny amount, at least at these shaft dimensions.

[Vixen]

Hello Petertha

As you well know, I have a great interest in the pre-war 'Silver Arrows' Grand Prix race cars built by Auto Union and Mercedes Benz.

All the pre-war Mercedes Benz crankshafts were machine in one piece with split roller bearing for both the Main and Big End bearings

Auto Union went the opposite way. All their car cars had multi part, built up crankshafts with Hirth couplings which enabled them to use more robust, one piece roller bearings for the Mains and Big Ends.

Auto Union did not manufacture the Hirth coupled Crankshaft themselves. The crankshafts were subcontracted to Mahle.

My researches into these Hirth coupled Crankshafts suggest the individual crankshaft sections were machined with the serrated Hirth couplings but with the bearing journals left slightly oversize. The sections were then bolted up to form the crankshaft. The Main and Big End journals were then ground to size on a conventional crankshaft grinder as if it was a one piece crankshaft. The Hirth couplings gave such good alignment (and power transmission) that the crankshaft could be dismantled and reassemble with perfect alignment and accuracy. The secret of their success appears to be to do the final journal grinding of the bearing journals as if it were a one piece crankshaft.

Mercedes Benz finally went over to built up crankshafts with Hirth couplings in the 1950's and used them in the world beating W196 driven by Moss and Fangio.

Here are some photos of the W196 Hirth crankshaft, courtesy of the Mercedes F1 team at Brackly.





Perhaps the Auto Union/ Mahle way of finish grinding the journals would also help you and me achieve the required accuracy in our model crankshafts.

Stay safe

Mike

[Vixen]

Hello again,

Hubert Schillings also uses built-up crankshafts in many of his multi cylinder engines. See his book "Boxer-, Reihen- und V-Motoren als Modell"

Schillings uses a much simpler way to couple the crankshaft pieces than the Hirth coupling. Shillings uses four round dowel pegs, in reamed holes, for alignment and power transmission. The round dowel pegs may be an easier prospect for model engineers rather than the complex Hirth coupling. Both would appear to give similar alignment accuracy, the Hirth however would transmit much greater torque. The post assembly final journal grinding process, I described earlier, could be applied, with the obvious advantages, to either coupling method.

I am still debating which method to use on my Mercedes Benz W165 engines.

Stay safe

Mike

[petertha]

Thanks Mike. Very nice detailed pics. When I did a 'Hirth' keyword search on the forum I didn't get many hits, but yes I saw your references (possibly a few others). I wasn't quite sure if that eventually resulted in making the couplings on a specific project, hence my fishing expedition post.

I'm going to make some testers from blanks of stock. That won't incur major expense, just some time & effort. If results turn out OK, I'll have confidence in making the CS itself which is kind of key to the overall project. The Hirth geometry is kind of interesting to me unto itself, so part of the fun. I guess on one hand its small & fiddly (at 12mm OD dimension in this case). But the potential payoff is making a rather complicated multi-throw crankshaft via more manageable smaller components versus a one-piece CS and all that that entails (large chunk of metal + hogging + interrupted cuts + finishing all the journals + potential stress relief distortion + clamshell bushings vs bearing race.... etc). Choose your poison I suppose.

There are other elements to the built-up crank that I need clarification on, but I'll reach out to Mr. Jung specifically & report back any useful information. Thanks for your interest. Lets keep comparing notes.

[petertha]

Hubert Schillings also uses built-up crankshafts in many of his multi cylinder engines. See his book "Boxer-, Reihen- und V-Motoren als Modell"
Schillings uses a much simpler way to couple the crankshaft pieces than the Hirth coupling. Shillings uses four round dowel pegs, in reamed holes, for alignment and power transmission.

I have been searching for a copy of that book & basically given up. But thanks for that description, I understand the layout. Hmm.. interesting option, something to include in the mock-up test.

[petertha]

Magnification of your pic kind of looks like what I was guessing at. The flat that runs down the tooth crown looks to be of constant width. Another reference I saw inferred it was cut like you would turn it in a lathe. But I think that would result in a tapered flat & more importantly, inconsistent with the meshing fit up because the tooth cut itself is extruded straight through the part. I didn't validate this though. My searching for Hirth geometry basics was hit & miss. Either kind of simple stuff or research papers above my pay grade, but not much in between. Strange we are trying to figure this out when the designer had pencil & paper many years ago LOL.

Hmm.. the dowel pinned joint is looking rather appealing LOL

[Vixen]

Hi petertha,

It was by pure coincidence that today I was reviewing my Mercedes W165 crankshaft drawings when you started this post. I am torn between the Hirth and the Schilling coupling and still cannot decide which, if any, will give me a straight crankshaft at the end to the day. Accuracy of this order is totally dependent on the machinery available in your workshop. No option of subcontracting the work to Mahle.

The only reason to use a built up crankshaft is to allow the use of one piece roller bearings throughout and one piece conrods. The cost is high as you need to make so many intricate pieces to high accuracy. But that accuracy is not enough by itself. The assembled crankshaft still needs the bearing journals accurately finished off on a conventional one piece crank journal grinder.

I think the flat bottom to the Hirth V grove has a lot to do with preventing crack propagation. The serrations only need to contact over a small part of their flanks, likewise the radial contact area is also very small.

Mike

[Vixen]

Dont get too concerned about finding formulii defining the flats at either side of the serrations. They only need to prevent contact with the bottom of the Vee, so just cut a little off.

I find it helps to think of the Hirth coupling as two identical bevel gears. Once you have defined the cut angle, you have almost everything you need.

Mike

[Vixen]

Hubert Schillings also uses built-up crankshafts in many of his multi cylinder engines. See his book "Boxer-, Reihen- und V-Motoren als Modell"
Schillings uses a much simpler way to couple the crankshaft pieces than the Hirth coupling. Shillings uses four round dowel pegs, in reamed holes, for alignment and power transmission.

I have been searching for a copy of that book & basically given up. But thanks for that description, I understand the layout. Hmm.. interesting option, something to include in the mock-up test.

Hi petertha

JasonB sent me a link to download the Shillings book a while back. Have a look at     https://www.dropbox.com/sh/q8ijzp3h7kqr3fs/AAA_-bbREnB6pBcNP9EZXT1za?dl=0     it may still be there. The text is in German and does not give away much detail. The drawings are where the detail is hidden.

Mike