Yet another Denford for the collection (Triac Retrofit thread)

[m_c]

And now to jump back to the spindle, the motor or drive has started to die. I had been considering swapping to a servo, so this has hastened that process. However, for those with experience of Triac's, space is quite limited.

After a bit umming and arring, some measuring, some head scratching, and some mock up in Fusion360 (my first real project with it - it doesn't seem to have as nice a workflow as Solidworks, but that's currently on a computer buried in a corner), a 110 frame 1.25KW servo has been ordered.
This diagram shows the required mounting position to make it fit -



As you can see, the motor has to be mounted above the head casting, so I need a spacer of about 130mm, which still puts the total height at less than the original motor. At the moment, I'm still bouncing ideas around in my head for the exact design.
My current thought until tonight, is a bit 100mm round bit steel, single bearing for support at the bottom, a rigidly mounted shaft extension on the servo, lots of the bar hollowed out, and servo mounting plate bolted on top.

I've gone through various design options.
I had thought about a welded construction, but that would introduce distortion. A flexible coupler would handle that, but then the extension shaft would have to be fully supported (i.e. lower bearing housing made about 70mm high for good support). At which point I'd be as well making the spacer from a solid bit bar and using a rigid shaft extension, so only a single bearing is needed for support.

Now tonight's thought was why not use aluminium, but then thermal expansion comes into play. Assuming a worst case temperature change of 40deg, that equates to about 0.11mm of expansion difference between steel and aluminium over the 130mm length.
Doesn't sound like much, but more than enough to put lots of pressure on bearings. Ideally I want bearings/shafts a good interference fit to avoid wear from them moving against each other.
But by using aluminium I could go back to a fully supported lower shaft with a flexible coupler. I've got 100mm diameter steel and alu available.

Weight isn't really an issue, as provided I don't go too daft, the new servo is over 6KG lighter than the DC motor, so either option would reduce weight.
Using steel, a rough guestimate weight with only moderate material removal is less than 4KG, but with a bit thought, I think I should get it under 2KG.
Aluminium should be well under 2KG, and would give better heat distribution, but the fully supported shaft adds a bit complexity to machining.

I've got a few more days to ponder this, as the servo isn't expected until next week at the earliest, but comments from others would be appreciated.