- Excimers Smooth the Path to Well-Lubricated Bearings
Machining microgrooves into spherical surfaces is a task made for excimer lasers.
Todd E. Lizotte
Applications in the defense and aerospace industries have increased
demands for spherical plain bearings that require less maintenance and that yield
higher performance and longer life. These demands, in turn, have initiated the development
of laser micromachining systems to improve bearing process technologies.
In most cases, bearing performance and lifetime
directly relate to the manner in which lubricants recirculate within the bearing
assembly. Laser-machined blind-depth lubricating grooves can help improve transport
of lubricants over the entire surface of the bearing and inner rings. Laser processes
also can adjust critical groove parameters, enabling bearing designers to produce
optimized lubrication grooves that meet performance specifications.
Excimer lasers can machine materials,
even a human hair, without incurring thermal effects. The hair shown is approximately
100 μm (0.004 in.) in diameter and has one square 50-μm-wide (0.002 in.)
slot machined into its side.
State-of-the-art processing systems
use seven-axis computer numerically controlled laser-etching techniques to produce
the grooves on the surface of the bearing. A high-energy UV laser beam illuminates
a mask that defines the groove geometry. The mask, similar to a stencil, is several
sizes larger than the actual groove, but it is scanned in two axes, while the bearing
is simultaneously scanned in four axes.
The mask defines and shapes the beam,
and directs it to the spherical surface of the bearing with a multielement imaging
lens, which compresses the image on the mask to the proper size and shape. The mask
and bearing are then scanned. The reduction or compression ratio of the mask to
target size determines the optimal energy density needed to etch the bearing material.
Using a multiaxis controller, the system
can simultaneously synchronize all axes of motion and interpolate the laser pulses
to precisely etch the grooves over the spherical surface. The process produces grooves
up to 2 mm wide and 0.25 mm deep, with tolerances of ±0.004 mm, and can etch
hardened steel, ceramics, crystalline structures and polymers.
Because of the unique way in which
the ultraviolet excimer laser etches, the bearing material does not suffer from
adverse thermal effects, such as heat-affected zones caused by infrared laser beams.
The precision of laser processing is especially
effective for specialized spherical bearing applications in which high-speed performance
is critical. Bearings typically have surface finishes of 3 μin. that, under
a microscope, appear as well-defined hills and valleys. When the surfaces of processed
bearings rub together under high pressure, irregularities are thought to weld together
and break off. This event, called adhesive wear, ultimately results in a failed
The function and operational lifetime
of high-speed bearing assemblies depend on lubricants, which act as a separating
layer between two sliding surfaces. Lubricants inhibit surface wear and maintain
the lowest possible friction torque level.
One way to inhibit adhesive wear is
to develop an elastohydrodynamic film, which is created dynamically in a rotating
bearing, depending on the lubricating fluid’s property of increasing viscosity
with increasing pressure. High viscosity generated under high pressure can effectively
separate this inner-ring-to-outer-ring contact. Once a distance greater than the
typical surface finish roughness separates the surfaces, they will not contact,
eliminating adhesive wear. Elastohydrodynamic films are dependent on lubricant viscosity
at operating temperatures and maximum bearing speed.
High-viscosity elastohydrodynamic films generated under high pressure
can effectively separate inner and outer bearing rings. Lasers enable etching of
grooves into curved bearing surfaces. The curvature of these grooves creates a self-priming
and -pumping action of lubricants that ensures a continuous flow over the bearing
Designers purposely develop micro-grooves
of varying geometry to create a self-priming and -pumping action. This ensures that
a continuous flow of lubricant flushes over the bearing surfaces. Tailoring the
design of microgrooves can allow a bearing to operate over a larger dynamic range.
Laser processing techniques can produce
a variety of complex microgrooves on bearing surfaces. Developed to tackle tough
spherical surfaces, the technology applies also to simpler geometries, such as spindle,
radial, angular contact and thrust-bearing configurations. In fact, laser techniques
developed for high-end bearing assemblies can be used in a vast array of industries,
including automotive, aerospace, medical, microelectronic, semiconductor and commercial
manufacturing (see table).
Understanding the application requirements
is key to implementing — or not implementing — the technology. Whether
adapting microgrooves to slow- or fast-moving bearing assemblies, the goal should
be to deliver lubricants efficiently and continuously over their surfaces.
In slow-moving bearing assemblies,
the lubricants are often thicker and more viscose, requiring larger microgrooves
not only to keep lubricants moving, but also to retain them in the bearing once
Tolerances for the mating distance
between inner and outer bearing surfaces ultimately determine the type of lubricant
used. If tolerances are very tight, specialized channels are required to force the
lubricants onto the bearing surface. These channels either are microgrooves, such
as in turbine applications where lubricants take the form of aerosolized oils in
the pneumatic lines that feed the turbine, or are drilled through porous media used
to retain lubricants and to release them during turbine operation.
The operational lifetime of the bearing is another
factor that helps in the decision of whether to use microgroove technology. In sealed
systems and in most aerospace applications, access to bearings is limited, and their
lifetimes may need to match the operational lifetime of the total assembly. These
situations may dictate the use of continuously self-lubricating pumping schemes
where the lubricants are sealed within the bearing assembly, and the microgrooves
provide the pumping action.
Application of microgroove technology
is becoming easier with the development of new process technologies. Micromachining
and micro-technology are revolutionizing the way we live. However, their benefits
do not always find their way to more general use in conventional technology. Bearings
are in some cases termed a traditional market, and the potential of applying micromachining
to existing macrosize products offers manufacturers new possibilities and payoffs
in extending product lifetimes. The technology may even provide added safety benefits
to some key products already in use in the aerospace industry.
Meet the author
Todd Lizotte is chief development officer and
vice president of research and development at NanoVia LP in Londonderry, N.H.
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