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In most applications, to create an effective seal an o-ring must be slightly smaller than the groove it sits on – stretched during installation. But how much can it stretch and still be effective?
In the second part of Not All O-Rings Are Created Equal, Don Grawe covers elongation – how it’s defined and how it’s impacted by the base polymer, hardness, and curing process.
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Welcome to our next installment of how all o-rings are not created equal. Today we are going to talk about elongation and to start us off we’re going to use the trusty Parker handbook to give their definition and then go from there.
Parker defines elongation, as it pertains to o-rings: as an increase in the length expressed numerically as a percent of initial length. It is generally reported as ultimate elongation and further, like tensile strength, elongation is used throughout the industry as a quality assurance measure on production batches of elastomer materials.*
Okay, what’s all that really saying?
What we need to know when looking at how far an o-ring can stretch is:
The primary reason that’s important is especially the smaller the o-ring gets, there are applications where you have to be able to stretch it over a fitting, over some type of a ledge to get to the groove that you ultimately need to be able to get to.
Always consider the elongation when it comes to installation. With that, elongation gets measured by ASTM requirements in our physical properties. We talked about compression set, when we talked about tensile strength, we talked about abrasion resistance – they all fall within the ASTM D2000 requirements.
How that affects the different materials and the ratings for elongation. Let’s look at three of the most common materials that you work within the industrial marketplace. That being nitrile or NBR, FKM or fluorocarbon, and EPDM.
A 70 durometer NBR o-ring, typically by ASTM D2000 requirements, will stretch 250%. In practicality, most quality compounds will exceed that significantly. In the case of a 70 durometer EPDM, you’re looking at a 200% stretch. Again, most compounds, quality compounds, will exceed that by a good measure.
The closer you get down to 100%, the tighter those variances will be. With an FKM or a fluorocarbon material – 75 durometer very common – now you’re looking at only 150% stretch. So from a pure base polymer standpoint standard compounds, these are your norms.
Let’s stay with the two common NBRs and FKMs. If the only thing I change is hardness and go from a 70 durometer nitrile to a 90 durometer nitrile, look what happens to my elongation. Now, my elongation is only 100%. Common applications for fittings and couplings etc. 90 durometer is a common durometer because of the properties that that hardness will give but also understand you can’t stretch it near as much as you can the 70 durometer nitrile. In the case of fkm also a significant hit from 150% for 75 durometer to only 100% for the 90 durometer.
Let’s throw one more variable in there. If compression set is a significant factor in your application, a peroxide cure o-ring is a common go to when it comes to compounds and materials.
Well, a sulfur-cured 70 durometer nitrile is really the baseline that we were comparing over here at 250%. But by going to a peroxide cure material, higher compression set material, one with a little higher tensile strength – look what happens to your elongation at the same 70 durometer hardness. Drops from 250% down to 125%
Base polymer, hardness, curing process all have an impact on elongation.
There’s some give and take as there is with most compounds and most elastomers when you’re trying to reach a particular physical property within that. Elongation just like any of the others.
To get performance in one property sometimes means giving up performance in another physical property.
You want elongation, you got to stretch this part significantly without breaking, but in order to get that based on the other criteria:
Know the basics – elongation- how far can I stretch but what else do I need my material to do?
Again, not all o-rings are created equal!
SOURCES: *Parker Hannifin Corporation, “Parker O-Ring Handbook – ORD 5700”
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“Shelf life” is the maximum time (beginning with manufacture date) that an o-ring or elastomeric seal – with proper packaging and storage, becomes unable to meet its original specifications.
Aerospace Recommended Practice (AP 5316) is the most comprehensive basis for establishing shelf life, however, it is not a binding specification.
Let’s look at a few Methods at which shelf life is being calculated.
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Some materials are listed as unlimited, however unlimited is not equal to forever. For some companies, they use 25 to 30 years as unlimited shelf life. According to the study EPRI NP-6608, which is one of the references in ARP 5316, which replaced by Aerospace Standard 5316, in the study it talks about five different methods to obtain elastomer shelf-life.
It is acceptable to use field experiences data or lab test data, however, using military standardization handbook or rubber products or warranty, then other methods are recommended.
Data is collected by products being stored for a number of years in an average room temperature. This method is time-consuming and yet it is a case-by-case basis.
Shelf life can be obtained by using this extrapolated graph or EST equation, which requires materials in qualification data or using EDT equation and only materials data is needed.
By using a test chamber at a given temperature, data can be obtained by measuring critical properties periodically until data drop below acceptable values. Then the EST equation can be used to estimate the shelf life.
Appendix B is where roughly 70 generic materials were summarized by using Method A, B, and c and is listed in years.
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Rotary shaft seals, also known as lip seals, are used to seal rotary elements. Deciding on the type of radial shaft seal is a challenging process that requires selecting specific seal design characteristics to match the system parameters.
Today let’s dive into the basics; the components and materials that make these complex seals.
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Going to talk today about features of radial shaft seals. A radial shaft seal is a component typically employed to seal in fluids and seal out contaminants in a dynamic application. Typical features include:
A radial shaft seal is typically installed in this orientation with the main sealing lip facing inward towards the oil or the fluid being sealed in.
It’s installed in the stationary bore as a press-fit and it has the main lip and the dust lip engaging the rotating shaft.
Some of the major considerations to make for selection of material include the temperature, the speed, the fluid, and the pressure of the application you are being sealed in.
The most important thing to remember is you need to understand all of the parameters of your application when requesting or working with an engineer to provide a recommendation for the proper radial shaft seal for the application. There are a wide variety of materials, profiles, and other parameters that can be adjusted to customize a radial shaft seal for your application.
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A premature o-ring failure is often times due to a combination of root causes. Let’s dive into one of those causes – Compression Set – what is it, how to diagnose and how to keep it from happening.
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I will start by saying not all o-rings are created equal; different materials, different hardness’s, different curing processes, they’re not just round, they’re not just squishy and just to fill in a hole. Having said that many of you also know a lot of the basics when it comes to o-rings, but one of the things that I want to focus on today is what Parker Hannifin states is the number one cause for o-ring failure and that is compression set.
Compression set is a term that’s used for one of those physical properties that gets measured with every batch of material that gets produced by reputable o-ring manufacturers. Which is maybe clue number one. If you’re buying o-rings from a manufacturer that does not test every batch and every lot and has full traceability- that’s a red flag.
Let’s go on the premise that you’re working with people that do have full batch and lot control of every material that they produce. Tensile strength, elongation, specific gravity, TR10 values; these are all different physical properties that get measured. Compression set is one that, in my experience, [I will validate what Parker’s telling us] is probably the most important of all of them. If you don’t know what compression set is – in simplest form – it’s taking o-ring material squishing it between platens, applying pressure to squeeze at 25%, then baking it in an oven for a prescribed period. Taking it out. Undoing the platen, pull out the material and measure how much did it come back.
Now in a perfect world, it comes back all the way.
If it comes back all the way, it is taking no compression set. Also, factor in that when you design o-rings for seals you are utilizing a squeeze rate that will vary from 10%, 15%, to 20%. So, in the scenario where you have an o-ring that takes a compression set and it doesn’t come back 10%, 20%, 30% – what does that do to that design squeeze that you’ve put in there? So now similar to other variables in your design now, in essence, you have another form of tolerance stack-up.
So, let’s dive a little bit more into compression set. Compression set values vary widely by material. You use different materials for different applications. From NBRs for a lot of hydraulic applications [oil/petroleum uses in the like]. You use HNBR for lot of the same things, but when you get involved in higher temperatures or different types of fluids some compatibility issues. You use EPM, again for different applications often for ozone resistance external uses for brakes in some of those applications. FKM is used often for chemical compatibility and for high-temperature applications. Probably the four most common o-ring materials in the industrial market. Yes, there’s silicone, AFLAS, Neoprene, and others, but these are the four common.
Within these four common materials is a variance in the amount of compression set they just inherently take. Start with NBR. One thing to note about NBR is how it’s processed. There is a difference between a standard sulfur cure – which the world has known for decades – and a peroxide cure material. A peroxide cure material is a step up from the sulfur cure and one of the reasons for that, [you can see in the illustration here] is the compression set. Under standard hundred degrees C testing for 70 hours, you can expect a good sulfur cure to still have 15% to 25% compression set. Peroxide cure material you’re going to reduce that to 10% to 15%. Significant Improvement for a lot of face seals, for both static as well as dynamic applications.
You go to an HNBR which is considered an upgrade in the NBR family, and you’ll see very similar compression set characteristics of the peroxide cure. If compression set is all that we’re dealing with peroxide cure might be an alternative because there’s also a premium on price to go to the HNBR.
Another reason to go with the HNBR would be for higher temps and for other application concerns. Look at EPDM. Again, EPDM, something very similar to the sulfur cured – 15% to 20% compression set on EPDM material. FKM which gets tested at 200 degrees, so you’re not exactly comparing the same thing, but because of the higher temperature material is often tested at 200° C – you’re looking at 15% to 20% compression set. Now I want to point out that these compression set figures are also going to vary on what and how the methods are being used to test it. Slabs, buttons, dog bones – often when you’re just testing material, it’s not done with the actual o-ring. Which brings us then to the next set. When you’re looking at compression set values by o-ring, you’re going to get some wide variables as well.
Taking an o-ring material that has been molded into a shape, gone through all its curing processes and everything like that, you’re going to get variance. If nothing else do to sheer size alone. A very thin small cross-sectioned o-ring to a big fat o-ring. What are those values going to differ? You are going to see significant differences from the o-ring to just the material. At a hundred degrees for 70 hours a standard NBR 1/16th cross-section can take a 100% set. The fatter or thicker cross-sections take a 60% set.
You go to a peroxide cure, the thicker cross-sections not quite as much difference, but when you get to those thin cross sections, there’s a lot of applications out there that use the small cross-section, 90%. That 10 points can make a difference of sealing and not sealing. HNBR again a little bit better improvement. 85 to 60. 85 being the thin cross-section.
So, wide variances when you’re looking at physical properties and when you’re looking at compression set from your suppliers, understand what are you are looking at? Am I looking at raw materials, slabs, buttons, dog bones, or am I looking at actual o-rings? What are those figures and those values that you want to be able to measure to – size and cross-section do matter. Where I’m going with this – beware of low-cost materials. Not all o-rings are created equal. There are differences and low-cost materials often times will impact your compression set values. Know what you’re getting.