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.
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For optimum performance of seals, the shaft surface texture must be optimal. A rough surface texture will cause the seal to wear out quickly, while a smooth texture will cause the seal to bed incorrectly. The shaft lead, also called twist, is formed during the manufacture of shafts and has to be ideally zero.
Andrew Rommann explains the different types of shaft lead, what it does to a sealing system, and methods to measure.
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Shaft lead also known as twist. Shaft lead, if not well understood and defined on your specifications, can have a detrimental impact to the performance of a dynamic sealing system.
Typical rotary applications have an elastomeric sealing element interfacing with a rotating shaft. On the shaft surface, the characteristics are very important and critical to the proper operation of the sealing system. One of those characteristics is shaft lead.
A typical manufacturing process may be the use of a single point tool against a rotating shaft where the tools actually traversing the surface of the shaft. This operation will result in a spiraling groove pattern around the circumference of the shaft. In this type of pattern, we refer to it as macro lead – has a continuous thread-like structure.
An alternative process maybe traverse grinding.
In this case, we don’t have a single point tool rather a stone with multiple points that contact the rotating shaft. The stone is still traversing along the surface of the shaft and it does result in micro lead. The threadlike structures are not continuous, but they do have a deviation from the circumferential direction of the surface of the shaft resulting in shaft lead.
Shaft lead can have two orientations. It can be a right-hand orientation. Where it’s shown here on the moving from the bottom right to the top left corner of the image and left-hand lead where you’re actually the threadlike pattern is from the bottom left to the top right.
So what does this do in a sealing system?
Again, in a rotary sealing system, we have an elastomeric sealing element that is interfacing with a rotating shaft.
If we look closer at the surface of the shaft and in this illustration, I actually have drawn a right-hand lead type structure. We can see where the oil is in contact with the shaft surface and the sealing lip.
As the shaft rotates depending on the direction of rotation, it will actually transport oil from left to right or right to left. Because this is right-hand twist if we have a right-hand rotation – so the top of the shaft moving towards the bore – we will actually end up with movement of the oil from left to right. So in this case out of the sealed system resulting in leakage.
If we have left-hand rotation, so in this case, the top of the shaft moving out away from the bore you will result in movement from right to left. So left-hand transport. This will help retain the oil in the system.
However, if it’s aggressive enough transportation of the oil it will actually result in a lack of proper lubrication at the interface between the sealing lip and the shaft surface could lead to premature wear of the sealing lip.
So again right-hand twist right-hand rotation – the transport is to the right. A right-hand twist with a left-hand rotation – the transportation is to the left.
So how can we measure and hopefully quantify shaft lead on an actual physical component? There are two major categories out there. There are a lot of different methods developed over the years, but the oldest and most well-known method is referred to as the string method or the thread method.
In this method, you mount a shaft in a rotating device. You drape a thread with a weight attached to it over the shaft. From the side view as you slowly rotate the shaft you may observe movement to the left or to the right from its original position.
If you do observe movement, you are observing that the shaft likely has lead. You cannot easily and precisely quantify accurately the amount of lead that exists in the shaft, but you can get a qualitative sense of whether or not you have left-hand or right-hand shaft lead.
A more precise and accurate method of measurement is using an optical method. In an optical method, you are creating a 3D mapping or 3D profiling of the entire surface of the shaft.
Software can then collect the data and process it to produce a representation of the effective shaft lead of the system.
The image on the left you can see as you move in the circumferential direction from 0 to 360 Degrees. You actually have axial movement along the shaft. In the one on the right-hand side you can see as you move from 0 to 360 Degrees, you actually have zero movement along the axial position of the shaft. This one we would refer to as having right hand lead this one having zero lead.
Using the optical methods, you can get a precise and accurate quantification of the lead angle, which can be useful when you are inspecting or qualifying components for a new product.
Some of the specifications that exist and are used in the industry that you may see include ISO 6194 – 1 and DIN 3760. Both of which specify zero lead. RMA OS-1-1 actually does tolerate lead to a very small level.
There are other specifications out there. There are also specifications that exist for most major OEMs where they have expanded on the information that’s available in the industry standard specifications.
The main thing to remember is that if you are not aware of what your tolerances for shaft lead in your sealing system, or you’re not aware of what your actual lead is, you could end up with performance issues that are not easy to identify the root cause.
And in many cases we find that in a situation where you’re not able to identify the root cause of a sealing system failure in a rotating system – if it’s not a tribute attributable to the elastomeric seal or to other obvious installation design issues – in some cases it’s actually result of the presence of lead that is hard to detect and again not very well understood.
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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.