DIYbanter - View Single Post - Clausing spindle bearings-help!

In article , says...
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.

An overhung load applied at the spindle nose creates a moment that needs
to be balanced by the spindle assembly. In the two bearing spindle that
moment is resisted by the widely spaced bearings, causing the spindle to
bend between the bearings.

With the three bearing spindle, the angular contact pair at the nose not
only provides radial and axial support, but also resists the moment that
results from the overhung load. In addition, with 2 bearings at the
nose, the total radial stiffness of the bearings themselves is greater
close to the load. Clearly, since there is some angular deflection of
the spindle as it passes thru the bearing pair, there's also a bending
moment in the spindle between the front and rear bearings, but it's
smaller than with two bearings, and adds to the resisting moment
provided by the pair at the nose.

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.

No doubt that's one reason that bearing arrangement is so widely used.

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.

I'd expect to find there's both an inner and outer spacer, match ground,
though the outer spacer may be fixed in the spindle cartridge.

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.

I hear those guys work cheap, too. I wonder if the 3000RPM top speed of
the HLVH is a consequence of the the bearing spacing. I often wish my
Feeler would turn faster. Likely the permanent grease lube is a limiting
factor as well.

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.

As long as the angular contact pair is carrying the thrust loads, the
fact that the radial bearing is floating axially has no effect on its
ability to carry a radial load. A bigger issue is the fact that a single
bearing will inevitably have some small amount of radial clearance, so
it's likely that light loads at the nose may not load the rear bearing
until its clearance is taken up. This is likely one reason it's common
to see a preloaded axially floating pair at the back end of a spindle in
place of the single bearing.

Ned Simmons