Typically defined as resistance to indentation under specific conditions, the hardness of an elastomer is more accurately thought of as two related properties: inherent hardness and processed hardness. As a result of chemical structure, each elastomer has its own inherent hardness. This inherent hardness can be modified (and is typically supplemented) via compounding and vulcanization. Hardness in molded rubber articles (processed hardness) is a factor of cross-link density (and the amount of fillers). The more cross-linking a given material undergoes during vulcanization, the harder the final molded part will be. When judging the potential effectiveness of a molded seal, (processed) hardness is one of the most common criteria in the rubber industry.

Unfortunately, hardness is also one of the least consistent concepts in that the most-used measurement scales have only limited comparability. There is no single “universal hardness” unit, so it is often impossible to draw a clear and easy correlation between readings on two different scales, even when the samples being measured are absolutely identical. There are currently two hardness tests that predominate in the rubber industry: Shore durometer and International Rubber Hardness Degrees (IRHD) (see Figure 16).

Because Shore Instruments led the way in the marketing of durometer gauges, the words “Shore” and “durometer” have become virtually synonymous within the rubber industry. Now a division of Instron Corporation, Shore Instruments offers a wide range of durometer scales conforming to the ASTM D 2240 standard.

The Shore A durometer is a portable and adaptable device that uses a frustum (truncated) cone indentor point and a calibrated steel spring to gauge the resistance of rubber to indentation. When the durometer is pressed against a flat rubber sample, the indentor point is forced back toward the durometer body. This force is resisted by the spring. Once firm contact between the durometer and the sample has been made, a reading is taken within one second unless a longer time interval is desired. Five readings are typically taken, then an average value calculated. The amount of force the rubber exerts on the indentor point is reflected on a gauge with an arbitrary scale of 0 to 100. Harder substances generate higher durometer numbers. A reading of 0 would be indicative of a liquid, whereas 100 would indicate a hard plane surface (e.g. steel or glass).

Though the elastomeric lips of most standard shaft seals fall in the 70 to 90 Shore A range, the application in question will always govern the necessary hardness. Softer compounds offering less resistance may be perfectly fine for low-pressure seals, but high-pressure seals will likely require a harder, tougher lip material. Making decisions about a property such as hardness often demands compromise, however, in order to ensure the long-term usefulness of the seal. For example, a relatively hard compound may resist high pressure, but its use can also lead to increased frictional buildup. Increased friction leads to increased heat, which can, in turn, degrade the sealing lip and decrease the seal’s life span.

It is also important to realize that measuring the hardness of a rubber sample is an imprecise art (see Figure 17). Depending on both the specific gauge in use and the expertise of its operator, it is possible (even probable) that the same sample will yield two or more different readings. The rate at which the durometer is applied to the sample, the force used, the amount of time that elapses before taking the reading, and the temperature of the specimen at the time of testing can all impact a test result. For this reason, all durometer readings normally include a tolerance of ± 5 points, but sometimes even this may not be enough to fully anticipate all of the variances to be seen in testing. Technological advances have reduced many of the discrepancies, but sometimes at the expense of the simplicity and portability that initially made durometers popular. It is generally a good idea to test a given specimen several times and average the results to ensure accuracy.

Despite the long-standing close association between “Shore” and “durometer,” there are a number of other companies that manufacture high-quality durometers, including PTC Instruments and Rex Gauge Company. And while durometers are fine for measuring the hardness of material samples, measuring the hardness of actual shaft seal lips is a different matter. The cross-sections of sealing lips tend to be too small, and the lips themselves too thin, for hardness to be accurately measured using a Shore device. Gauging the rubber hardness of a sample taken from an actual sealing lip generally requires use of a microhardness tester such as those manufactured by Wallace (see Figure 18).

The Wallace tester is similar in design and function to a Shore durometer, with the main difference being that the indentor point on the microhardness tester is smaller. This smaller point can more accurately reach into small places (such as the spring groove of the lip) to take measurements.

Microhardness readings are generally expressed in terms of International Rubber Hardness Degrees (IRHD) and come with the same tolerance (± 5 points) as Shore readings. Though not as common in the United States as abroad, all IRHD testers are designed to conform to the ASTM D 1415 standard.

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