"The hardness of small components—microhardness—is difficult to measure but necessary to ensure that a component meets performance requirements," said Greg Vassmer, ARPM technical coordinator for the Dynamic Seal Committee and president of Fluid Sealing Science L.L.C., his entrepreneurial consulting company based in Rockford, Ill. "Microhardness measuring devices developed in recent years have improved the accuracy of the equipment, but operator skill and technique are still critical."
Researching, validating and editing the revamped standards on the committee are representatives from across the oil and gasket manufacturing industry, from the larger Freudenberg-NOK Sealing Technologies and Trelleborg A.B. to smaller seal manufacturers.
The OS-3 standard—now in its third iteration—saw its first guidelines drawn up in 1965, Vassmer said. As an organization (since 2010) dedicated to non-tire rubber component producers, ARPM manages an entire series of non-tire standards—for conveyor belts, power transmission belts, shaft seals, hoses, protective linings and others.
As such, the OS-3 guideline is one of 23 publications put out by ARPM, with 20 of those dedicated to oil seal standards.
The process for updating all the guidelines has been ongoing since 2015.
"We are probably about three-quarters of the way through the process," Vassmer said. "Sometimes it's a research process, sometimes it's not."
According to Vassmer, microhardness is the measurement of the stiffness of a piece of material when a small indentation is applied, not unlike durometer, but on a very small scale, for small cross sections of rubber components.
One of the primary analytics that microhardness can offer an operator or customer is the cure state of the product.
"Once you create a complex part out of rubber, with curves and odd shapes, you need to know if it is cured properly—this (measured stiffness) is what gives the part its correct performance characteristics," Vassmer said.
There is a high expectation in the automotive industry for repeatability and performance, especially as those traits relate to wheel, engine or transmission seals—"dynamic seals" that interface between a moving part and a stationary part.
Such seals, which also are crucial in the industrial segment as shaft, pressure and hydraulic seals, often are subjected to thousands of rotations per minute, Vassmer said.
"These seals can be reciprocating or otherwise, but some relative motion must exist in this interface," he said. "Static seals are opposite—they are put in place and are stationary as a space filler."
Measuring devices to validate the cure state and stiffness of a part have been around for more than 30 years, Vassmer noted.
Wallace Instruments makes a common microhardness tester, which features an indenter—a ball at the end of a small probe—that pushes in to the rubber part.
Whereas previously an operator needed to know a lot about the settings of the microhardness measuring device itself, today that same operator requires more knowledge about the proper mounting techniques for a small, asymmetrical sample.
"The current versions are getting away from trying to tell the operator how to use the tester—they have control elements built into them, and an operator just needs to follow instructions," Vassmer said.
The OS-3 guideline focuses on the mounting of the seal and the variations that can be obtained if the mounting technique is not controlled properly.
"How big of a piece do you take for the sample? What are you measuring? This talks about the handling of the part and what variations you might expect to see—the differences with customers—so that manufacturers and customers can come to a common understanding," he said.
The OS-3 guideline discusses the operating principles, specimen preparation, test settings and the sources of variation for the resulting measurements.
The OS-3 standard is available for $225 to non-ARPM members at arpminc.com/publications. ARPM members can access this and other publications for free.
"This is the only place a supplier or user of rubber seals can go to get an industry-approved understanding of how to apply these microhardness devices to the small and many times odd-shaped components molded from rubber," Vassmer said.