Excimers Smooth the Path to Well-Lubricated Bearings

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.

Bearing dynamics

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 bearing.

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 surfaces.

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 it stops.

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.

Bearings’ lifetime

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.