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Technotes - 1st Quarter 2001

Preferred Surface Finish and General Information for Steel Mating Plates Used with Paper Friction Materials

The preferred mating plate surface for paper-faced friction plates will have a microfinish of 0.2-0.6 micro meters [8-25 microinches] arithmetic average (or center line average). Since paper materials are designed to ride on a thin boundary layer of lubricant, variations from a perfectly flat surface must be controlled. However, protruding deviations from perfect smoothness are much less desirable than intruding deviations.

In the ideal case, this finish will be achieved by barrel tumbling or vibratory finishing. If the mating surface is a cast iron, the best finish is obtained by roller burnishing; an acceptable finish may be obtained by grinding in a circular pattern. For paper friction materials, the best finish will exhibit a profile tracing with the following characteristics, measured tangentially to the circumference:

Trace length: 20 mm [0.75 in]
No peaks in profile exceeding 2 micro meters [80 microinches]
No more than 2 peaks greater than 1.5 micro meters [60 microinches]
No more than 20 peaks greater than 1.0 micro meters [40 microinches]

Material type and hardness are not critical if the surface finish meets these criteria. However, nodular cast iron may cause increased wear rates. Typically, hardness of 20 Rockwell "C" is specified for spline wear resistance. However, hardness of less than 80 Rockwell "B" is acceptable for the mating surface of the disc.

Mating plates should be flat within reasonable values. Acceptable limits are typically established using parallel plates to check the combination of flatness and parallelism. The spacing for these plates should be calculated as follows:

Take 0,1% of the maximum diameter of the disc.
Add this value to the maximum thickness of the disc.

For example, a disc that is 300 mm in diameter and is 2,10 mm thick would be checked with plates set to a gap of 2,4 mm {(0,001 * 300) + 2,1}.

Flatness should be assured by a process that will give stable results at elevated temperatures. Typically, this can be accomplished by stress relieving at 500 degrees C. However, a balanced state of residual stress can also be achieved by roller leveling. There are elaborate x-ray diffraction tests that can be performed to assess the state of residual stress in a plate. However, a simple test to determine stability consists of exposing a plate to a temperature of about 200 degrees C for 20 minutes, and then checking the part with parallel plates after cooling to room temperature. A stable plate will remain flat after this test.

Since the majority of mating plates are made from steel, the usual practice to manufacture these discs using cold rolled steel. Since hardness is not a critical issue, the carbon content is not a critical component. SAE 1020 through 1080 materials have all been used successfully, with SAE 1035 by far the most common material. Although heat treatment of these steels may be used for strength or wear resistance of the spline teeth, it is not essential for the rubbing surface. As a rule, heat treatment often creates difficulty in maintaining proper microfinish.

The easiest method of obtaining the desired characteristics in micro finish is to use AISI "#2 Bright" finish steels. "#2 Bright" material is routinely produced from rimmed steel ingots. Rimmed steels usually have a thin layer of pure iron, or ferrite, on the outer surface of the ingot. As the ingot is successively rolled to strip thickness, this layer remains. It is very soft, easily polished, and provides an excellent surface when treated by tumbling or vibratory finishing.

Grinding or sanding of "#2 Bright" material almost always causes deterioration of surface finish. This is a result of the solidification dynamics of casting rimmed ingots. The rimming process causes a higher level of non-metallic inclusions to be formed in steel, as compared with killed or capped steels. These inclusions increase as a percentage of volume as the center of the ingot is approached. Since rolled strip is just a thinned plane of the original ingot, the internal structure can be quite "dirty". As material is removed from the outer skin of the strip, increasing amounts of inclusions are exposed. In practice, the more grinding or sanding that is performed, the worse the finish will become.

As more steel producers turn from ingot methods to continuous casting, obtaining "#2 Bright" material becomes more difficult. With strip produced from continuous cast material, or from killed steel, obtaining the desired finish may be more difficult. If the non-metallic inclusion level is low, it may be possible to produce the proper finish with light-duty abrasive methods, such as wet-linishing or circular-pattern grinding. These methods may also be used with hot rolled steels to obtain a high quality micro finish.

Selecting the optimum mating plate material, manufacturing process, and microfinish specifications is a problem that requires attention to detail. It is essential to know what facing materials will run against the reaction surface, what material availability might be in different circumstances, and what cost constraints may be important.

All of the important variables--including flatness, parallelism, and precise details about the surface configuration--may be important. Determining a set of parameters for a particular application often involves trade-offs that should be based on a clear understanding of the issues and options.

Livonia Technical Services can assist you in specifying reaction plate materials, processes, and finishes, as well as many other design questions for brake and clutch systems. LTS can also provide in-depth, on-site training in the areas of brake and clutch technology.