| The most common type of bearings
used in CMM’s today are air bearings. The benefits of air bearings stem
from the fact that there are no moving parts, which eliminates noise and
vibrations. Air bearings are non-contact, minimizing wear and are also
"self-purging" which keeps the machine guideways clean and free from particulate
matter. This eliminates problems that are found with mechanical bearing
systems.
The two common types of air bearings used are orifice type and porous
graphite. The benefit of the porous graphite bearing is that the air flows
through the bearings’ microscopic honeycomb structure. The result is a
larger bearing support surface and a significantly stiffer bearing with
a small air gap between the air bearing and the granite or ceramic guide
member. This gap can be as small as 0.0002" while the orifice type bearing
system gap is usually 0.001" or greater. This small gap can only be accomplished
if the guide surface is manufactured STRAIGHT and FLAT.
Note the
porous graphite type bearing allows air to diffuse through the whole surface,
preventing any high or low pressure variances in the gap between the bearing
and the surface.
In this superior porous graphite air bearing system, rigidity is further
enhanced by using a "wrap around" preloaded system. This design, incorporated
in all Starrett machines, is far superior to the more typical free floating
air bearing systems.
Since an air bearing is similar to a spring, it will attempt to move
away from the guide member or compress toward the guide member whenever
the machine is in motion and especially when there is a weight transfer
as the carriage assembly moves across the bridge structure.
The wrap around preloaded system has bearing sets. One bearing
is a guide bearing and opposite this guide bearing is a preload bearing
which acts as a constraint to minimize the compression-decompression effect.
The bearings are adjusted as pairs to a gap of 0.0002".
Adjusting the bearings together increases the bearing lift capacity
and stiffness. This principle is carried all the way through to include
the outboard leg support where the granite plate has a lapped, parallel
ledge opposite the top bearing surface, allowing even these air bearings
to be preloaded.
The benefit to this design is that acceleration and deceleration, along
with the repositioning of the Z-axis carriage assembly, will not cause
bearing collapse, which would result in changes of the machine geometry
and ultimately, loss of accuracy and repeatability.
Bearing Aspect Ratio
To maximize the benefits of a superior guide way and air bearing design,
the bearings must also be positioned correctly. The distance that the bearings
are spaced relative to each other in relation to the measuring range is
referred to as bearing aspect ratio.
The greater the aspect ratio, the smaller the motion and measuring deviation
will be from the guiding member.
Additionally, bearing aspect ratio can only be effectively utilized
if the primary guide surfaces are manufactured straight and flat, and if
the mass of the moving object is well positioned at the center of gravity. |
