With its ability to dramatically impact seal life and performance, counter surface quality plays a major role in the proper function of a sealing system. Accordingly, the accurate evaluation of this attribute is critical to the overall effectiveness of a system.
Simrit has identified six additional parameters to more accurately evaluate rod surface finish in hydraulic applications. Based on the Abbott Firestone Curve, these new factors have the potential to modernize the standards for counter surface preparation, thus extending the life of sealing systems.
Traditionally, the sealing industry has used three parameters—Ra, Rmax and Rz (Figure 1)—to measure surface finish and define sealing technology requirements. While easy to measure, and based on industry standards, these traditional parameters are not enough to accurately evaluate surface quality for the following reasons.
• Ra, which measures the average roughness across a given length, does not provide the needed detail of a surface. As a result, two surfaces with entirely different profiles can have the same Ra value.
• Rmax, the maximum peak-to-valley distance across a standard length, is not able to indicate a skew toward peaks or valleys.
• Rz, which provides the average peak to valley measurement across five equal sub-segments of the standard length, masks trouble areas because of the way it is calculated.
Although these parameters are important, they are not adequate to classify surface abrasiveness and potential to hold lubricant. And as life and warranty expectations increase in the market, additional and more robust surface finish definitions are required.
Revolutionizing surface definition
Using ISO methodology, Simrit engineers set out to further define surface roughness. The Abbott Firestone Curve (Figure 2) was used, as it provides information about a surface over a defined length and the complete depth of the profile.
A straight line divides the Abbott Firestone Curve into three areas (Figures 3 and 4). Valuable parameters such as Rk (core roughness depth), Rpk (mean height of the peaks protruding from the roughness core profile) and Rvk (mean depth of the valleys protruding from the roughness core profile), help to clearly define the abrasiveness and lubricant-carrying ability of the surface. Furthermore, values like Rpkx (the full peak height) and Rvkx (the full valley depth) are indicators of abrasive peaks and excessively deep valleys.
As more sophisticated measuring machines are capable of providing these additional parameters, engineers are now able to determine the material ratio which supports the sealing function. Mr1 and Mr2 are the smallest and highest material ratios of the roughness core profile, but neither provides the material ratio of the kernel (often referred to as the bearing area) (Figure 5), which indicates the structure of the profile the seal will run against.
Even with sophisticated measuring machines, evaluation leaves room for interpretation. For example, the material ratio determination may or may not use all of the measured area (noted as Cref) and can be calculated using Rz/2 or Rz/4 as the cutoff depth. The examples below show how using different definitions can provide completely different values for the material ratio on equal surfaces (Figures 6 and 7). Simrit recommends taking into account the full peak height, as this is very important to seal wear (Cref = 0%). In addition, the cutoff depth should be Rz/2, as it more accurately relays the position of the kernel surface.
Finally, Simrit increased the number of parameters from three to nine in an effort to more accurately define surface texture and, as a result, improve the life of the sealing system. While the new Simrit system requires improved measuring equipment, no additional measurements or time are needed.
Testing the new testing system
The ultimate goal of developing these new testing parameters was to provide engineers the ability to more accurately understand—and anticipate—how seals will behave in a given application. This is important because it can be used to extend the life of a sealing system.
For common shaft materials used in reciprocating hydraulic applications, the recommended parameters are the following:
• Ra 0.05 to 0.30 μm
• Rmax 0.00 to 2.50 μm
• Rpk 0.00 to 0.50 μm
• Rpkx 0.00 to 0.50 μm
• Rvk 0.20 to 0.65 μm
• Rvkx 0.20 to 2.00 μm
• Rk 0.25 to 0.70 μm
• Mr 50% to 90% @ Cref = 0% and Rz/2 cutoff depth
In-house testing by Simrit indicates these new parameters are able to reduce part wear and extend system life. This concept can also be applied to other sealing systems, although the values will change depending upon the specific application (for example: rotating rather than reciprocating).