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Michigan Metrology Articles

The following article excerpts provide helpful information for engineers and quality control professionals. Please click on the links after each excerpt to download the entire article.

If you need less sliding friction, should you make the surface rougher or
smoother?

How does the surface texture affect the friction between two dry parallel sliding surfaces? The key understanding of the relationship of dry sliding friction to surface texture is linked to the notion of the "Real Area of Contact." Depending on the surface roughness and the material properties, the real area of contact
may not be too dependent on the surface roughness—in other words, if you make the surface
"rougher" or "smoother" you may not affect the sliding friction. On the other hand, for various types of texture and materials the nature of the surface roughness may affect the dry sliding friction. In some case, making the surface smoother will reduce the friction, whereas in other cases, making the surface rougher will reduce the friction.

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How Many Measurements?

The measurement of a component's surface texture may be described by many texture parameters. The question often arises: how many measurements do I need to make for a given parameter in order to obtain a particular level of confidence in the result? This article will give some basic guidelines for determining that number of measurements.

The many texture parameters are typically organized into categories including: average height parameters such as roughness average, Ra; extreme height parameters such as the maximum peak-to-valley, Rz; spatial parameters such as the mean profile spacing, Rsm; functional parameters such as the reduced peak height, Rpk; and hybrid parameters such as rms surface slope, Dq. A description of all the various texture parameters may be found in the ASME B46.1 2009 standard.

Multiple measurements made in different locations over a surface will generally demonstrate some variation in the specific texture parameter being assessed. The ratio of the sample standard deviation of the measurement (S) to the sample mean value of the parameter (M) is termed the intra-surface variation...

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Surface Texture Measurement - Did You Set The Cutoff Lengths Properly?

...Along with specifying the surface roughness with Ra one must establish the cutoff lengths used to measure the surface. A specification of Ra alone is inadequate since measurement with different cutoff lengths may result in differences in the measured Ra values.

A surface profile is composed of a spectrum of different spatial wavelength components of various amplitudes. The measured profile will consist of a “filtered” version of the true profile since the instrument will be physically limited in sensing the finest spaced features...

As per the ASME B46.1-1995 standard (Page 36) on surface texture specification, the long wave cutoff length, λc must be specified on all drawings. Per the ISO 1302-2002 (Page 9) standard on surface texture, the long wave cutoff length λc and the short wave cutoff length, λs must be specified on all drawings...

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Surface Texture Measurement — Rz...Which Rz?

When measuring surface texture, typically the average roughness, Ra is specified. Ra is a measure of the average of the absolute value of the variation of the surface texture profile about a best fit mean line.

The next popular parameter is Rz, which is more sensitive to extreme peak/valley structures than Ra. However Rz has taken on many different definitions over the years and unfortunately has beenimplemented differently among the various surface roughness measurement machines available.

Since 1995, the American Standard, ASME B46.1, has defined Rz as the average over the evaluation length of the successive values of the  peak to valley heights found per sampling length. Thus one may use any number of sampling lengths to determine Rz. Typically if the number of sampling lengths is not specified, it is assumed that 5 sampling lengths are used....

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Patent Attorneys and Surface Texture

...Consider the case of a novel surface finish for a medical device, such as a heart implant, stent, or prosthesis. Assume that without the novel surface developed by the inventor that after implantation the medical device fails prematurely or causes complications. Often, an attorney familiar with medical device technology can be found, but finding a patent attorney knowledgeable in tribology/surface metrology may be difficult...

Assisting the attorney in understanding the invention requires the inventor to take the initiative in clearly identifying the novel features of the invention. For example, assume a novel feature is the "surface roughness" of an implant, stent, or prosthesis. In this case, the inventor must ensure that the attorney understands the concept of "surface roughness." In some cases, it will be appropriate to completely define "surface roughness" in a claim of the patent. In others, the term "surface roughness" is included in a claim and more explicitly defined in the section of the patent application often titled "Detailed Description of the Invention". This decision is generally left to the patent attorney. However, it should not be assumed that the patent attorney understands the concept of "surface roughness" as used by a tribologist/surface metrologist.

Ensuring that the patent attorney understands "surface roughness" is the responsibility of the inventor.  For example, the inventor should make clear to the attorney that "surface roughness" may be defined as the average roughness, Ra or a number of other different texture parameters, measured with a particular spatial wavelength bandwidth.  Failing to emphasize the importance of specifying the spatial wavelength bandwidth to the attorney may result in a less effective patent than desired...

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