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The Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) established ASTM D2000 to help provide guidance when determining elastomer compounds. By using a method called the “line callout,” engineers have a readily available classification system.
Andrew Rommann breaks down the individual elements that compose this “line callout” and the benefits of using this method.
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In many elastomer products, ASTM D2000 is utilized as the standard to communicate the performance requirements of the materials based on the customer’s expectations or the demands of the application.
An ASTM D2000 line callout looks like this and is the entire line that you can see between my arrows. We have a specification that applies to the line call out. We have some basic requirements information and then we have what’s known as the suffix requirements portion of the line call out.
So, within the line call out, within the basic requirements:
So we’ll see that the durometer if that is a specific target you’re going for, you may need to add an additional suffix requirement to explain that.
What you see on the left-hand side of the line call out – this is actually the minimum requirement that you need to specify an ASTM D2000 material. With this requirement, there is a set of basic requirements automatically imposed regardless of the grade of the material and without the existence of any of the suffix requirements. Those basic requirements include tests and performance results for heat aging, oil immersion, and compression set.
The suffix requirements as you can see the line call out is actually the greatest portion. What I have written on the board is the longest standard line call out that you could come up with for a M2BG710 material. This is a nitrile compound. Grade 2 correlates to the performance results of each one of these tests. For a grade 2, a grade 3, grade 4, 5, and 6, each grade will have different applicable suffix requirements. It will also have different levels of minimum performance to qualify as that grade of material.
In the suffix requirements section we see that we have a preceding letter or set of letters for each suffix requirement.
These special requirements are very powerful to help clarify specific items that may be required by a manufacturing process. A typical Z could be in this case for Z1. I wanted to clarify that the seven in the durometer call out is actually applicable to a durometer of 75 plus or minus five. I wanted to make that clear so I added the Z1 call out for that.
Z2, this could be the special processing in the manufacturing that I was referring to so maybe this elastomer component goes on to an assembly that goes through a paint line and ultimately through a paint oven there could be a small degree a small amount of time short duration or we have an elevated temperature and you wanted to evaluate the effects of that Temperature of the paint booth on the elastomer itself. So in this case, I’ve included a Z2 call out to say this ASTM method D 573 and I want to check it one hour at 125 degrees Celsius.
And then Z3 in this case. I wanted to come up with something a little bit out of the ordinary and this one I wrote down is must smell like vanilla birthday cake. It’s very unlikely that you actually need your product to have a certain fragrance, but it is possible to create a Z call out to impose any special requirement of any kind on the material. Keep in mind in doing that, you can prescribe a Z call out that is impossible to meet or could have a major cost impact on the overall material price.
So with these Z callouts, you want to make sure that you’re using what is applicable to your needs and not imposing anything above and beyond your requirements on the material.
Some additional suffix letters are shown here. In addition to the ones that I’ve had this particular call out did not include a C12 call out and the C suffix would indicate an ozone resistance test. You could also have a G call out which is an air resistance test and there’s a small list of additional suffix letters that correspond to different types of tests that can be applied to different types of material. The combinations of grade, type and class could have a different list of suffix letters applied.
So with all of this, based around the ASTM D2000 standard, and included on your drawing the major benefits of using it –
So with those things defined -both the grade, type, and class – along with the ASTM D2000 suffice requirements, we know exactly what tests need to be performed on the material and what the minimum requirements of those tests need to be to qualify for this requirement. It provides very clear information to the design team, to the manufacturer, and also to the quality assurance teams for products.
Pressure velocity, or PV value, is the combination of the pressure of the application and the speed of either the rotating or reciprocating shaft. The PV limit is the maximum value of that combination where the seal will function and wear normally. If we exceed that value, we’re going to see excessive wear which will lead to sealing failure.
There are several factors to consider when selecting a seal. Each factor has a direct impact on the performance and lifespan of your application. One of the most significant, but often overlooked, is the pressure-velocity, or PV, of your seal.
Jason Huff spends some time defining pressure-velocity, the calculations, and walking through examples to show its significance.
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When selecting a seal, there are several factors that we need to consider. Including pressure, speed, temperature, the media you’re trying to seal, the hardness, and surface finish of the mating hardware.
And arguably one of the most important things that we need to take into consideration is the PV value or pressure velocity.
This is the combination of the pressure of the application and the speed of either the rotating or reciprocating shaft. The PV limit is the maximum value of that combination where the seal will function and wear normally. If we exceed that value, we’re going to see excessive wear which will lead to sealing failure.
For a reciprocating application, to calculate the PV value:
If we had an application that had a stroke length of 3-inches and a cycle rate of 80 cycles per minute and a pressure of 600 PSI:
– 600 PSI should be no problem for a quad ring
– A u-cup will handle 600 PSI – no problem
– And then obviously these two versions of a cap seal can handle 600 PSI
The issue becomes when we combine that with the speed of 80 cycles per minute, which is fast for a reciprocating application.
We’re going to take our:
– three-inch stroke length divide that by 12 to get it in feet
– multiply that by 2 to capture the entire distance traveled
– multiply the 80 cycles per minute
– multiply 600 PSI
That puts our PV value at 24,000.
When we reference our seal selection chart you can see both the quad ring and u-cup are no longer viable options and we’re going to have to stick to one of these cap seal options.
Similarly, if we want to calculate the PV value for a rotary application, we’re going to take:
– the circumference of our shaft in feet
– multiply that by the speed in RPM
– multiply that by the pressure in PSI
If we had a 2-inch diameter shaft, and it was rotating at 1500 RPM and a pressure of 30 PSI:
– 1500 RPM for a traditional rotary lip seal – no problem
– A Flexi-lip or PTFE lip seal – no problem
– The same with these spring energized PTFE seals
Now that we have to consider 30 PSI that automatically puts are rotary lip seal out because that’s exceeding its max range – 30 PSI for the PTFE lip seal is no problem. Not a problem for the spring energized PTFE seals either.
But, when we combine the two:
– our 2-inch shaft divided by 12 so that we’re in units of feet
– multiply that by pi to get the circumference
– multiply 1500 RPM
– multiply 30 PSI
That puts our PV value at 23,562.
Again, now it eliminates those first two options as being acceptable seals.
It’s very important to not only consider the pressure and velocity independently – we need to combine the two so that we get a true understanding of what the seal is going to see in application.