Hirth Coupling

[Elam Works]

One side has a course pitch right hand thread while the other has a fine pitch left and thread

Actually, don't they need to be the same hand? The one pitch just needs to be coarser than the other so that it gains a slight amount over the trailing fine pitch. The differential being the difference in lead. I used M16x1.5 and M16x1.25 because taps were readily available for an effective lead of 0.25mm*rev. Some experimentation was required to figure out how many thread 'head start" the fine pitch was given before engaging the coarse so that when it nipped up tight the ends of the differential bolt (or stud, actually) ended up flush with the adjacent surface as desired.

If the threads were opposite hands, wouldn't the pitches be additive (in the previous example, 2.75mm*rev)?

Re- The PH Horn carbide insert interface. Another disadvantage is the lack of surface area carrying load. just six flank surfaces. The rest of the face does not contribute and indeed cannot touch the opposing surface least it prevent the joint from clamping up tight. Not unless you were able to maintain very precise tolerances like the combined short taper/face joint of a D1/A1/A2 lathe spindle nose. As said, vee-serrations are probably easier.

-Doug

[Vixen]

One side has a course pitch thread while the other has a fine pitch thread

Actually, don't they need to be the same hand? The one pitch just needs to be coarser than the other so that it gains a slight amount over the trailing fine pitch. The differential being the difference in lead. I used M16x1.5 and M16x1.25 because taps were readily available for an effective lead of 0.25mm*rev. Some experimentation was required to figure out how many thread 'head start" the fine pitch was given before engaging the coarse so that when it nipped up tight the ends of the differential bolt (or stud, actually) ended up flush with the adjacent surface as desired.

If the threads were opposite hands, wouldn't the pitches be additive (in the previous example, 2.75mm*rev)?

-Doug

Hello Doug,

You are correct about both threads being the same hand. I have corrected the original post.

Mike

[Niels Abildgaard]

Otto Pulch was known in  Germany for some very advanced aero engine concepts.He showed me once how he made a poor mans Hirth coupling.
The divided shaft to be was made as one piece on final dimensions.
From memory around 60mm diameter with 40 mm hole.
He then drilled and reamed 5mm holes in a ringzone and had some colleaque at work in Jullich divide the shaft with wire EDM .
9mm long 5mm rollers from INA was then used as locking keys.
After assembly there was no dimensional changes.
It was part of crankshaft for a 6 cylinder 2 row radial motor based on BMW motorcycle cylinders.
I will try to make a drawing of the shaft

[Roger B]

In Schilling's book (page 25) there appears to be a crankshaft design using 3 square keys like the PH Horn grooving tools. It also seems to suggest using a high strength aluminium alloy for the crankshaft with INA bearing inner races as the journal surfaces.