In article , Ned Simmons
says...

It would be interesting to talk to the engineers who decided how
and when to make those changes.


The change from 3 bearings to two seems consistent with
Hardinge KISS philosophy. By removing redundant constraints
the 2 bearing design makes it easier to build a very true
spindle at the expense of a bit of stiffness.

It's not clear that the two bearing spindle is less stiff.

In the three bearing one, the last bearing runs true radial, and
its outer races *must* be free to float inside the bore of the
headstock. All the thrust loads are taken by the front duplex
pair.

I suspect the main issue that lead them to three bearings was the
differential thermal contraction rates between the cast iron
headstock and the steel spindle. In principle the three bearing
one does not suffer from any change in preload if the casting
expands more than the spindle.

The other issue is there's no real precision item inside a
three bearing headstock. The preload on the duplex pair is
set in the bearing factory, and once installed will be developed
correctly as long as all the races are up tight against each other.

In the HLVH though (for example) the preload depends critically
on the separation between the bottoms of the bearing recesses in
the casting, and the length of the 'preload cylinder' which is
nothing more than a precision spacer that separates the two
inner races. Maybe the engineers realized that the differential
thermal expansion wasn't that much of a killer, and they could
hire guys who liked cats and give 'em lots of Mt. Dew so they
could do the tricky job of assembling the bearing stack through
the headstock on the machines.

Because the spindle is constrained axially and radially by two
bearings almost a foot apart that probably does make for a fair
degree of rigidity. As opposed to having the backside only
constrained by a radial bearing floating axially in a bore.

Jim


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