What is the “compression set” or “permanent deformation at compression”, relaxation under stress and viscous sliding or “Creep” in relation to the elastomeric gasket or an antivibration system?
All these concepts are related but different stress/strain and compression measurements are all influenced by temperature and compound formulation.
Here is a brief overview of the compression set, stress relaxation and viscous sliding or “creep” and how each relates to commercially available gasket materials and custom-made vibration dampers and mountings.
What is “compression set”?
The compression set of rubber materials expresses how much a material does NOT regain in thickness after being compressed for a period at a specific temperature.
The sample is first compressed to a certain percentage of its thickness, possibly also subjected to heating, for a certain period of standard time.
Then freed by compression and measured and the test results (e.g., ISO 815 / ASTM D395 / UNI 4913) are expressed as a percentage of lack of initial thickness.
The temperature of the test should be similar or in relation to the conditions of use. The most common are 23°/25°C (ambient), 70°C and 100°C.
The compression set, relaxation under stress and viscous “Creep” are all affected by both temperatures and formulations. A material that has a high compression set at room temperature may have a compression set greater than 70°C.
Having a low compression set (lower percentage of crushing memory) means having greater tightness over time, for example for an O-ring or a gasket. But depending on the applications the needs change.
For static applications where the gasket will not be exposed to significant pressures, such as the gasket of a glass, a light rain compared to pressure washing is a not so critical situation.
However, once the gasket is crushed permanently, it is susceptible to leakage. Door seals for an electrical or control casing are subject to periodic compression and recompression; therefore, the rebound or compressive strength set is important.
What is relaxation under stress?
Stresses in a compressed gasket are generated in response to the clamping load. An easy way to imagine this effect is the return pressure of the gasket. Over time the polymers will relax or push less.
The extent of relaxation depends very much on the polymer and temperature. Imagine crushing a gasket in a vice. Initially it may take for example 100 kg to obtain this 50% deflection, but over time, a possible load cell can indicate 75 kg.
The amount of return pressure will decrease after a certain period. If you expose the rubber to higher temperatures, the value may be lower.
This return pressure is important during sealing because the gasket must push harder than the stresses or pressures exerted by the external environment or from the inside, for example in an electrical box or a hydraulic valve.
What is “Creep”
Creep refers to the long-term deformation in gaskets or vibration dampers from being under a constant load.
So like imposing a weight resting on a rubber plate, and measure the thickness at constant intervals, after being loaded, for a predetermined time and temperature.
In operation, sliding or “creep” in compressed gaskets or vibration dampers is often controlled with compression limit switches or compression heights set.
In motor vibration dampers, for example, if the motor is coupled with rotation transfer systems (gearboxes, propeller axes or drive shafts), the Creep value is very important to control over time the offset of the motor with respect to external components.
The compression set is the most commonly tested property for elastomeric gaskets/rubber materials. Test data is available in most commercial product data sheets.
It is correct to deduce that the material with a good resistance to the compression set will also have good relaxation properties under stress, however, verification still requires testing.
Understanding how materials deform and bounce is critical to long-term sealing performance. Applying an incorrect gasket can lead to costly damage to equipment in operation.
Even using an anti-vibration device with an incorrectly formulated compound can be detrimental to the engine/transmission coupling system.
In gasket applications, a material with excellent characteristics is generally silicone, thanks to its temperature stability, as well as for fluoroelastomers as long as you formulate and produce the articles with the correct precautions to obtain maximum performance.
At high temperatures, an incorrectly processed fluoroelastomer would be similar to a common oil-resistant rubber. Similarly, silicone would be more sensitive to the compression set.
Even common elastomers can have good characteristics compression sets or creep, it is necessary to formulate the compounds correctly to obtain the best results.
Per ulteriori informazioni sugli elastomeri ad alte prestazioni per applicazioni di guarnizione e ammortizzazione, rivolgetevi a Cavallero Gomma o compilate il form presente sul sito.