The main reason to use a spring would be to overcome the limitations of the primary seal element.
Generally, seals made from elastomer and most polymers can energize themselves at least for a short time under certain conditions. However, as application parameters broaden, spring energizers are needed to provide consistent loading.
There are four types of springs commonly used in the industry:
1. Garter Springs
2. Cantilever or V Springs
3. Helical-Wound Springs
4. Canted-Coil Springs
The garter spring is constructed of a thin wire that is coiled and back-wound on itself, which resists stretching or being pulled apart. They are most commonly used on a radial shaft seal and helps the lip engage the shaft. There are many different combinations of wire and coil diameter, which allows us to select predictable loading across a wide range of seal diameters.
They are a good option when we have high runout in our shaft allowing the shaft seal to maintain contact with the shaft and unlike the rest of the seals we’ll talk about today. Garter springs do not require an opposing surface to push against.
The next type of spring is the cantilever or more commonly known as the V spring. It is made up of a metal strip that is punched and formed into a “V” shape. This spring geometry allows for a wide deflection range in a predictable linear load.
The shape of the spring also concentrates the load at the front of the seal – making it a good choice for excluders or scrapers in reciprocating applications. It can also be a good choice for static applications where we have wide tolerances or misaligned glands.
These are commonly selected for static applications. But sometimes we can use them in slow or infrequent Dynamic conditions.
The small deflection range of this type of spring prevents us from using them when we have wide intolerances or misalignment.
These types of springs have a very flat load versus deflection curve – making them a great option when we do run into a large gland tolerance or a misaligned condition.
They are also good options when we run into a friction-sensitive application because as the engagement of the lip changes the amount of loading generated by the spring does not change.
Welcome to our next installment of ESP’s “Not All Seals Are Created Equal.” Today we are going to talk about a seal that’s been around for a long time, but in some cases may be one of the best-kept secrets in the utilization of rotary shaft seal applications. And that is the Parker JM Clipper® seal.
What makes the JM Clipper® seal a little unique is that when we think of radial shaft seals, oil seals, we think of a metal can on the OD with a rubber element that has been molded to it that acts as the sealing element. Sometimes it has a spring to help provide the necessary pressure on the sealing lip – the necessary load – to allow it to function properly.
With the Clipper® seal you have a composite OD that is integral with and molded with, the elastomeric sealant component – can also have a spring to it for added load. Many of the same characteristics as what the traditional metal seal would be. So very robust seal and it provides a lot of rather unique benefits and features from the normal oil seal.
Let’s take a look at a few of them.
1. As mentioned, the composite OD already provides a gasket type seal. So, it exactly does perform as a sealing element as opposed to just metal-on-metal and it does not require any type of plates or secondary components to keep it in place.
2. It’s composite so it’s not going to rust or corrode – creating contamination issues.
3. It is a one-piece molded construction for the entire size range, which makes it a little bit more robust than the normal mechanical crimping or in some cases adhesive gluing of the rubber to the outer can.
4. It eliminates a lot of the problems of thermal expansion when you have extreme heat, extreme cold, and metal-on-metal – sometimes losing some of that friction fit that is necessary for the oil seal to stay in place and perform.
5. It is much easier to install and is very user-friendly.
6. It is also able to be designed for split models as well.
If you have any questions or need more information regarding the JM Clipper seal, please contact ESP International, check out our website and learn more.
Hello and welcome to another installment of “Not All O-Rings Are Created Equal.” I’m going to take a few minutes today to talk about one of the common failure modes of an o-ring and some of the factors that should be considered or be aware of to alleviate any such failures.
For example, did you know that in dynamic reciprocating applications an o-ring is not recommended for strokes of 12 inches or longer? A lot of applications out there, especially reciprocating that tend to stroke longer than the 12 inches. There’s a lot of factors that must be considered and we’ll talk about some of those as we go along.
As a rule, anything over 12 inches is not recommended.
In addition, in dynamic reciprocating applications, the direction of pressure and the seal friction should oppose each other.
Failure will likely occur if pressure and friction both are of the same direction.
Now, these are just two examples of a common failure that is given the term spiral failure. I want to talk a little bit more about spiral failure and an understanding of what that is. According to the Parker o-ring handbook, which is his kind of the Bible to the o-ring industry, this type of failure was given the name because when it occurs, the seal looks like it’s been cut halfway through the o-ring.
The cross-section of the o-ring actually has a corkscrew or spiral look to it. An o-ring usually seals in that condition until you get a complete break, but it does cause fatigue and it doesn’t lend itself to susceptibility to breaking once it has spiraled.
Having said that, a properly used o-ring slides during the reciprocating stroke. There’s actually movement of the o-ring. The hydraulic pressure produces a greater holding force within the groove that produces the sliding effect. If it doesn’t slide, it is going to roll. The smoother finish of the sliding surface in relation to the groove surface finish produces less friction. Stands to reason. Running friction is lower than breakout friction. The torsional resistance of the o-ring also resists twisting. The compound, the material, as we discussed in other sessions, what are the differences in o-ring materials themselves?
I want to talk a little more about the torsional, or spiral failure, and the fact that it is not limited to just an o-ring cross section.
There are many applications where you’ll use what they call a quad ring, a square section or a lathe cut, an x-ring – these types of seals are also susceptible to spiral failure given those same conditions. I’m not going to go into a lot of detail in all of those but there’s actually quite a list of things that need to be factored into the design when applying o-rings in reciprocating applications to reduce or eliminate spiral failure:
– The speed of the stroke itself
– The media that is being used, or the lack of lubrication
– We talked about the pressure differential
– Squeeze – if you have too much
– What’s the shape of the grooves or are the grooves split?
– The length of stroke
– Surface finish
– The type of metal that is being used
– Is there a side load? Is it absolutely concentric all the way around?
– And there are several others
These are all factors that can a tribute to spiral failure which can be a catastrophic failure in o-rings. To learn more about it, go to the ESP International website to find the Parker O-Ring handbook. It’ll give you much greater detail to how to avoid spiral failure in application.
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Today we are going to talk about the differences between piston and rod wear rings.
What I’ve got up on the board – you can kind of imagine –on this side [left] we’ve got your piston groove. Where you have this piece of metal running inside of your cylinder.
And on this side [right], you’ve got your cylinder head where the rod is going to be sliding in here. If you know what a wear ring is, it supports your moving piece of hardware, here in the case of the piston, and stops it from banging the inside of your cylinder. How are you going to get this super rigid, tough material wear ring to fit into its groove when you’ve got this extra piece of metal, this lip, keeping it from falling off? You can’t just bump it in and slide it over because you have a larger diameter.
What’s the answer? You cut it.
There are three types of cuts. The butt, the step, and the angle cut. They are all essentially the same for the purposes of this discussion.
The same question over here.
How are you going to get this rigid rod wear ring inside when you’ve got the larger diameter for the wear ring OD and the smaller diameter where you’re going to have to allow the rod to slide through?
The answer again is to cut it.
The answer is no.
PISTON WEAR RING: The fundamental difference is that the piston wear ring you’re going to want to stretch it and allow it to collapse into its groove to eliminate its gap as much as possible.
ROD WEAR RING: The opposite is true for the rod wear ring because you are going to want to crumple it into a smaller diameter and allow it to spring and expand to that larger diameter.
We are out here at the mobile hydraulics training center where we train our salesmen and engineers on how to rebuild hydraulic cylinders.
What I’ve got on the bench are two cylinder head glands like you’d see on the back or front of a piston cylinder, as well as two pistons.
What I’m going to show you is how to install the rod vs the piston wear ring on each of these.
Like we saw on the whiteboard, how do you get this larger diameter rigid wear ring through the lip into where it seats on the inside?
The answer is:
– You crumple it up
– You put it through
– And allow it to expand
And that’s going to stay right where we want it because it’s forcing itself outwardly.
– We pull it apart
– And allow it to collapse into its gland
This is on pretty tight, and it’s not going to fall out during assembly.
Miguel Vita, Freudenberg Hydraulic Division
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Hi everybody. Welcome to white papers on whiteboard. My name is Miguel Vita. I work for Freudenberg in the hydraulic accumulator division. I was invited today by our partners ESP International to talk about hydraulic accumulators.
We decided to start with the basics:
What is an accumulator?
How can I use an accumulator?
What are the different technologies that we have on an accumulator?
Let’s start with – “What is an Accumulator?” Think of an air balloon inserted into a bucket and apply a force to the balloon. You increase the pressure on the airside of the balloon. This is the basic principle of an accumulator.
You have an accumulator with a hard shell. Normally carbon steel – very similar to the bucket that I showed you before, and you have an elastomeric diaphragm. This elastomeric diaphragm will make a barrier to a pre-charged nitrogen section. You can compare the pre-charged nitrogen with the air that you have in your balloon.
The port is connected to the hydraulic system. To the hydraulic system, we will apply pressure in this portion and will be translated on the same action that you have with this Force. So basically, when you have the hydraulic system, you increase the pressure in the nitrogen area.
We have here a schematic of a hydraulic system.
And we have added an accumulator in the system.
When the hydraulic system has no pressure, you have the pre-charge of the nitrogen using the whole cavity of the accumulator.
You have a shovel on your tractor and the shovel hits a stone. You have a huge force being applied here that will increase the pressure in the whole system. This pressurized oil will move to the accumulator and will increase the nitrogen pressure. So, this nitrogen inside the accumulator will work as a cushion. You have dampened the system using an accumulator.
And here we come to the three different types of accumulators. We have the bladder, diaphragm, and the piston type of accumulators.
1. BLADDER ACCUMULATOR
The Bladder is the bread-and-butter. You can use bladder accumulators everywhere. Most of the hydraulic systems use bladder accumulators.
Those accumulators are used in pulsation dampening where you have high frequency, especially in a small amplitude. A lot of applications, right? But this type of accumulator has a restriction. The bladder has a vulcanized seam, and this is the weak point of the bladder system. If you have high frequency and high cycle demand, you can have a rupture in this seam. This is the restriction of this type of accumulator.
2. DIAPHRAGM ACCUMULATOR
Then we can go to the diaphragm type accumulator.
Very similar applications as the bladder type accumulator. However, the diaphragm accumulator has an advantage.
Since you don’t have a seam in the diaphragm, you don’t have the restrictions that you have with the bladder type accumulator.
So applications pretty much the same, but this one is really a reliable accumulator, especially when you have high cycle demands. Applications with 1 million, 2 million, 3 million cycle demands – this is where to use a diaphragm accumulator.
3. PISTON ACCUMULATOR
But really you don’t have limits for this type of accumulator.
Since you machine the accumulator, you can make it in any size. You can make accumulators with a quarter gallon. You can make accumulators with 300 gallons. You can make accumulators going to 40,000 PSI.
Custom ports, custom design, and materials so the piston accumulator is really for limited applications where you can make custom design accumulators.
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Today we’re going to talk about PTFE rotary seals.
PTFE is more commonly known in the industry as Teflon, but today we’re going to refer to it as PTFE because Teflon is the DuPont trade name. Reasons we would choose to use PTFE over your normal rubber elastomer style seal would be that we’ve exceeded the capabilities. Whether it be speed, pressure, temperature, and maybe the chemicals that it meets. PTFE is a very low friction material – so it’s able to operate at very high speeds.
It’s got a very broad temperature range. Virgin PTFE can handle ranges from -425 Fahrenheit up to 450 Fahrenheit and we can even shift that range a little higher depending on the fillers that we add to it. So, it can handle a wide range of temperatures and basically any fluid or chemical that you throw at it.
For this one, we’ve got very lightly loaded and very flexible lips that are machined. They’re very lightly engaged, so we need a shaft that runs very true – no runout because PTFE is not a very resilient material. It needs some form of energizer in order to make sure that it remains in contact with the shaft.
So with this profile here, we’re going to be limited to about 50 psi, but we can run up to about 5,000 surface feet per minute for speeds. We have an excluder and then the main lip to retain the fluid.
We can change that a little bit – we could do away with that lip and possibly add a redundant lip for fluid retention.
If we had additional pressure, we could reinforce this a little bit with a metal band – increasing the rigidity of it. We could increase the pressure rating of up to 150 psi.
If we did have a little bit of runout that we needed to handle we could modify this lip a little bit and include what we call an “elf toe”. And then we could add a small spring to help the lip maintain contact with the shaft. But still, the runout must be minimal. We’re talking about maybe 20 thousandths depending on the speed.
If we shift gears and go over to this other style, we’re looking at high pressure but relatively low speed.
The pressure rating on this profile would be about 3,000 PSI and your surface speed is going to be limited to about 1,000 surface feet per minute.
We’ve got quite a few different options as far as lip styles:
Several different spring options:
– Another option would be a canted coil if we wanted to reduce the lip loading a little bit.
And we’ve got several different options for the bore:
So, we’ve got a wide range of possibilities with PTFE and we can tailor the fillers depending on the application conditions and the performance criteria needed. There’s a wide range of additives that we can put in the PTFE to tailor to the needs.
Wide range of applications that these seals can be used in:
Another benefit of PTFE versus a rubber elastomer seal is that there’s no tooling required for these. These get machined out of a sleeve or billet of material. So prototyping and initial samples are very fast and inexpensive.
And that’s it for PTFE seals.
Welcome to our latest edition of “Not All O-rings Are Created Equal.” In previous editions, we’ve talked about some of the physical properties of the o-rings and things that you need to be aware of and account for when selecting an o-ring when selecting material for your application.
Some of the more common physical properties that we’ve talked about so far are your durometer and hardness, your compression set, elongation and the important factors that they are in determining the compound of materials to use. Today we are going to talk a little bit about tensile strength.
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Short version according to the Parker Hannifin O-ring Guide:
Tensile strength is simply measured as the PSI or megapascals required to rupture a specimen of a given elastomer when stressed.
Put it under stress and at what point does it burst – at what point does it break?
One thing I want to point out, as I’ve done this in other sections regarding compression set and elongation, it varies by material, but it also varies within the material. Which is why I emphasized that they’re not all created equal. A nitrile isn’t just a nitrile, isn’t just a nitrile.
Specific to tensile strength, it can range anywhere from 6.9 to 27.6 megapascals. Just within the nitrile family alone. FKM can very 3.4 to 20.7 – a pretty good size range. Even EP which tends to be a fairly simple, stable material in most applications – 2.1 to 24.1 megapascals.
Why is that important? In the grand scheme of things, as you increase tensile strength, you improve the modulus of the material. As stated in other sessions, when you make a change to a compound or material, to get something you’ve got to give up something.
What am I giving up when I improve my modulus?
I’m sacrificing elongation. In most applications that’s a non-factor. As we’ve talked to the elongation section, elongation is predominantly about installation. How far can it stretch? If you’re not having to overstretch your o-ring, it’s less of a factor but rather gives you some important properties.
With the modulus – which is the stress at a predetermined elongation usually measured at 100% – the higher modulus is apt to recover from peak overload. That’s important. It helps support and aid to the strength and wear-ability of the o-ring and it usually increases with hardness.
Tensile strength while not talked about a lot can be an important factor and one of those key physical properties that you need to be aware of as you evaluate why and how all o-rings are not created equal.
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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Today we’re going to talk about durometer of rubber products. Durometer is a measurement of hardness and like other hardness test measures the depth of indentation in the material created by a given Force using a standardized pressure foot. The ASTM D 2240 standard recognizes 12 different durometer scales.
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One important aspect to note is the test methods used when testing durometer.
When you’re using any of these Shore OO, Shore A , or D, or the IRHD scales, you need to be using a large thick flat piece of rubber. The piece of rubber needs to be a minimum of six mm thick and large enough that all of your measurements can be taken at least 12 mm from the edge of the material. This can create some obstacles when you’re trying to measure small rubber products such as o-rings. Most O-rings don’t have a 6 mm thickness. So if you’re trying to use a Shore A durometer tester on the typical o-ring that’s going to be an incorrect measurement method. It’s not valid.
In the case that you do need to measure physical parts with small cross sections, you’re going to have to use the shore M durometer scale. Again, you can see that this is a 30 degree cone. It is a little bit smaller diameter than the short D and also would have a different spring force applied to it. But with this scale you can measure samples as little as a 1.25 mm in diameter.