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.
FDA, 3A, and NSF are common standards for the production of o-rings within the food grade and potable water industries. Each standard is derived from a different agency who contains unique sets of rules, requirements, and regulations.
They are very different, and very specific in their place in the market.
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Welcome to our next installment of not all o-rings are created equal. Today, I thought we would take a little bit of time to talk about the differences in FDA, 3A, and NSF. These are common standards in the North American marketplace that seemed to have a lot of confusion about certifications, validations, approvals etc. We’re going to try and simplify it a little bit and then in future editions will get into greater detail to each. Again, specific to North America – the China GB4806 the European WRAS – those are standards to be addressed another time as well. But in general, as we talk about food grade and potable water, there are three standards that in the production of o-rings you need to be able to consider and be aware of; and those specifically being FDA, 3A, and NSF.
FDA is the standards that are derived and managed by the Food and Drug Administration. What this is, is simply a list of ingredients that are neither toxic nor carcinogenic that can be included in the formulations of compounds for o-rings.
They go through a series of extraction testing so that as the o-rings are in use in application, what is leaching of those o-rings that may or may not create a problem. Once that is done, the o-rings and the o-ring material is deemed to meet FDA requirements. Not to be confused with being certified.
The FDA does not certify o-ring materials or compounds but rather creates a list of acceptable ingredients.
The next one we want to talk about is 3A. It is not, despite what some people might believe, part of FDA. It is derived from the International Association of Milk, Food, and Environmental Standards. A secondary group is the Milk and Dairy Supplier Association. What 3A does is it determines the rubber that’s capable of contact with antibacterial treatments and the maintaining of the physical properties of the o-ring after those treatments.
As you can imagine in in the milk production world, there’s a lot of chemicals. There’s a lot that goes on in order to continue to service that equipment. What is the o-ring able to survive in that environment? There’s also an E3A Standard – basically similar to the 3A standard, but specific to egg production.
Finally, we’re going to talk about NSF. NSF not to be confused with the Food and Drug Administration or the Dairy Group, but it is specific to the National Sanitation Foundation. NSF has two primary standards or certifications that we need to be aware of:
With the NSF and the various testing that is required in order to meet those approvals, the NSF does the testing and does, in fact, certify to the compounds, to the materials for their standards.
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.
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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.
The AS568 o-ring size chart, published by the Society of Automotive Engineers (S.A.E), sets a standard for universal o-ring sizing. The chart specifies the inside diameters, cross-sections, tolerances, and size identification codes for 349 o-rings used in sealing applications.
DJ Rodman spends some time covering these attributes at a high-level.
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Today we’re going to talk o-rings. We get frequent calls, request for quotes, emails, regarding o-ring and o-ring sizing. Today we are going to run through and explain how o-rings are sized and talk a little bit about the language of o-ring sizing. O-rings are sized using AS568; which is an aerospace standard that was published back in the seventies. The original AS568 was published in 1971, since then there have been several versions A through D – you can see the years that they were published – 1995, 2001, 2008, and 2014 is the latest.
When we talk o-rings, we are of course are talking about circular seals, but more importantly, when we say o-ring, we mean the o-shaped cross-section – or round cross-section. Because a number of seals are round but the cross-sections are not circular. So today we are talking about the circular cross-section seals.
O-rings are sized using three different dimensions. There’s the ID which is the inner diameter. There is the OD which is the outer or outside diameter, and then there’s the cross-section which is also known as the width. So those are the three dimensions that are most important.
As we look at the standards themselves. The first thing that I do want to clarify is that sizing is important when it comes to o-rings. You can get close to dimensions, but you want to be careful because those o-rings are sized using a squeeze and fill percentage, which is critical in sealing and it does affect the overall sealing of the application. So if you have questions about the groove or groove size console an application engineer.
The AS568 dash numbers refer to a specific ID and cross-section. The OD is listed when you look at tables, or in particular when you look at the tables for a manufacturer, but it’s not necessarily needed.
The AS568 numbers are uniform across manufacturers. It does not matter who. You are going to see the same dimensions and you’re also going to see the same tolerances.
So in our example, we’re looking at a -001 o-ring. That o-ring has a 1/32 inch ID and it has a 1/32 inch cross-section. We can look at it from a nominal standpoint and we can also look at it from an actual standpoint. That actual standpoint- it would be an ID of .029 inches with a tolerance of + or – .004. The cross section would be .040 with a tolerance of .003 inches. Again, this is universal across manufacturers. As long as it’s AS568 referenced, all of these dimensions should be the same.
As you look at the dash numbers – as they go up higher you get into thicker cross-sections. So for instance, a -120 o-ring is 1 in by 3/32 of an inch – actually translates to .987 + or – .010 with a cross section of .103 + or – .003.
Again, use the table, use any manufacturer – we have Parker in particular – use the table provided by the manufacturer and then use tools. We don’t necessarily recommend a caliper, although you can get close to those dimensions. Also, you can use an o-ring cone which would get you the actual AS568 reference for that part.
All in total, there are 349 AS568 dash numbers that are available – these are tooled-up. Any o-ring manufacturer should have 349 sizes available.
Now in addition to your standard AS568 dash numbers, when it comes to o-rings, I do also want to reference that there are 3-9XX sizes available – few limited. These are considered boss or the tube-fitting o-rings – sized specifically for threads and the end tube fittings.
There are also nonstandard sizes that are available, and those sizes depend on the manufacturer. They’ll be a list of available sizes that have been tooled up outside of the AS568 references.
One last note as you’re talking about AS568 sizes, do keep in mind that tooling for these sizes at manufacturers is unique to the material or the compound that the o-ring is made up of and this is due to shrinkage rates and shrinkage rates on material varying. So the tooling itself will have to vary with that.
I appreciate the time. If you have any questions, feel free to contact our website at espint.com or industrial seal.com.