O-Ring Spiral Failure

“Not All O-Rings Are Considered Equal”

Spiral Failure

Whitepapers On Whiteboard







Don Grawe


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.

O-Ring Spiral Failure Examples

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.

What Is O-Ring Spiral Failure?

o-ring spiral failure

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.

The Sliding Effect

O-Ring Sliding Effect

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?

Not Just O-Rings

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.

Are Piston & Rod Wear Rings Interchangeable?

Are Piston & Rod Wear Rings Interchangeable?

Whitepapers On Whiteboard







Jesse Thomas

Accumulator Basic Principle

What Is An Accumulator?

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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.

What are the differences between these two? Are they interchangeable?

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.

Let’s go out to the shop and I’ll show you.

Accumulator Hydraulic System

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.

Installing the Rod Wear Ring

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.

Installing the Piston Wear Ring

Piston Wear Ring Install

The opposite is true on the piston. This time we don’t want to force itself out. That will just cause it to flop out without any warning. Instead, we want it to collapse inwardly. That’s why on the piston wear ring we have very little space initially.

– 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.

What Is An Accumulator?

What Is An Accumulator?

Whitepapers On Whiteboard







Miguel Vita, Freudenberg Hydraulic Division

Accumulator Basic Principle

What Is An Accumulator?

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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?

What Is An Accumulator? The Basic Principle.

Accumulator Basic Principle

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.

What Happens On The Hydraulic System?

Accumulator Hydraulic System

We have here a schematic of a hydraulic system.

  • hydraulic cylinder
  • valve pumps
  • a tank

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.

For example:
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.

Basic Functions Of An Accumulator

Accumulator Functions

  1. Starting with the pulsation dampening. On the hydraulic system, you have pulsation. This pulsation is coming basically from the hydraulic pumps. So the accumulator will make a dampening on this pulsation and will stabilize your system. You’ll reduce the noise, you’ll reduce the vibration of the system and you’ll have the system working this move.
  2. Also, the accumulator can keep constant pressure in your system. If you have a leakage, for example, you’re going to lose the pressure of the system. The accumulator will stabilize the pressure and you keep the pressure at a certain level until you can stop your system for maintenance.
  3. Another function of the accumulator is really to be an emergency source of power in your system. For example, when you have your system being applied on hydraulic brakes and you need a sudden release of pressure in your system, the accumulator will help you release this pressure whenever it is needed.

Three Types of Accumulators

3 Types Of Accumulators

And here we come to the three different types of accumulators. We have the bladder, diaphragm, and the piston type of accumulators.


The Bladder is the bread-and-butter. You can use bladder accumulators everywhere. Most of the hydraulic systems use bladder accumulators.

  • You have a bladder bag.
  • You have the pre-charge of nitrogen.
  • Connected to the hydraulic system.

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.


Then we can go to the diaphragm type accumulator.

  • Basically, the same where you have a carbon steel shell but instead of a bladder, you have a diaphragm.
  • Also, the pre-charge of nitrogen and this portion is connected to the system.

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.


  • Instead of a bladder or a diaphragm, we are using an aluminum piston to make a barrier with the nitrogen.
  • You keep the nitrogen pre-charged.
  • The system is connected to your hydraulic system.

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.

Thank you.

Why PTFE Rotary Seals vs normal rubber?


Why PTFE Rotary Seals vs Normal Rubber?

Whitepapers On Whiteboard







Jason Huff



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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.

High Speed, Low Pressure Profile

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.

High Pressure, Low Speed Profile

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:

We can do a traditional scraper lip, which is good at scraping fluid and keeping contaminants out.

We could change the lips style to a taper and that’s going to be better for sealing and lower friction.

Several different spring options:

– I’ve got the cantilever or V Springs shown in there right now.

– 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:

We could eliminate this o-ring and do a flanged design. This flange would get clamped in the hardware and ensures that the seal cannot rotate in the bore and allows you to increase your speed rating just a little bit.

If we needed to go higher than the 3,000 PSI for pressure, we could extend this heal a little bit. It makes it a lot more rigid, a lot more stable profile. So, in something like this, there would be an o-ring in there as well. This could bump our pressure rating up to close to 10,000 PSI.

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:

  • A lot of times you’ll see them in gearboxes or motors
  • Pressure washers for when we’ve got high pressure
  • Rotary unions
  • Swivels
  • Compressors
  • The Virgin PTFE is FDA compliant. So it’s a good option for those types of applications.
  • Cryogenics due to the wide temperature range
  • Robotics

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.



What is FDA, 3A, & NSF?


What is FDA, 3A, & NSF? Food Grade & Potable Water.

Whitepapers On Whiteboard







Don Grawe



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.

What is FDA?

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.

What is 3A Sanitary?

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.

What is NSF?

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:

  1. NSF 51 – specific to food and beverage applications.
  2. NSF 61 – specific to potable water.

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.

FDA, 3A, NSF. They’re not all equal. They’re very different and very specific in their place in the market.



What Is Tensile Strength?


Not All O-Rings Are Created Equal! (Tensile Strength)

Whitepapers On Whiteboard







Don Grawe



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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What Is Tensile Strength & Why Is It A Factor?

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.

What Am I Giving Up When I Improve My Modulus?


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.

The Importance of the PV Value


The Importance of the PV Value When Selecting A Seal

Whitepapers On Whiteboard







Jason Huff



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.


Reciprocating PV Calculation

For a reciprocating application, to calculate the PV value:
– take the stroke length in feet
– multiply that by the cycle rate in cycles per minute
– multiply that by the pressure in PSI


Reciprocating Example


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.


Rotary PV Calculation


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


Rotary Example


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.

Shaft Lead A.K.A “Twist”


Shaft Lead A.K.A “Twist”

Whitepapers On Whiteboard



3 of 5




Andrew Rommann



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.


Macro 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.


Micro Shaft Lead

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.


Two Orientations of 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.


Results In A Sealing System

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.


Twist, Rotation, Transport

 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.


String Method & Optical Method

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.


Industry Specifications

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.

Durometer Scales – The Basics


Durometer Scales – The Basics

Whitepapers On Whiteboard



1 of 5




Jason Huff



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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Shore Hardness Scales

Shore OO Durometer Scale:


If we look at some of the most common scales used across the industry, we can start with the Shore OO The indenture for the Shore OO is the conical bottom there in this scale is used for extra soft rubbers. Some examples of this would be like sponge rubber or Gummy Bears or even the gel insoles for your shoes.

Shore A Durometer Scale:

Another very common durometer scale used in the industry is the shore A durometer scale. You can see that the indenture for this one is a truncated cone. This durometer scale is going to cover your soft medium and hard rubbers. Some soft rubbers could be like rubber bands or a pencil eraser. Some more medium hardness rubbers would be car tires or a little bit harder than that, maybe the sole of your shoe.

Shore D Durometer Scale:


And then after that, for much harder materials, we’re going to jump up to the shore D. You can see this is a 30-degree cone. Some examples of products you would use the shore D hardness tester on would be like a shopping cart wheel, which is a very hard rubber or even some plastics when you get to the higher range of the shore D –  like a hard hat or something like that.

IRHD Durometer Scale:

Another very common durometer scale used in the industry is the IRHD – the international rubber hardness degree. This scale is very closely aligned with the shore A but it is not exactly equivalent. We can’t compare an IRHD to the shore A and think that their equivalent.

Important Note For Testing:

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.

Shore M Durometer Scale:

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.

Understanding AS568 Aerospace Standard


Understanding AS568 Aerospace Standard

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DJ Rodman



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.

The Three O-Ring Sizing Dimensions

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.

AS568 Universal Table

 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.

Non-Standard O-ring Sizes

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.