What Is An Accumulator?

What Is An Accumulator?

Whitepapers On Whiteboard







Miguel Vita, Freudenberg Hydraulic Division

Accumulator Basic Principle

What Is An Accumulator?

Click on the image to open in a New Tab.


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.

Hydraulic Bearings – Bearing the Load



Deconstructing An ASTM D2000 Line Callout

Whitepapers On Whiteboard



2 of 5




Andrew Rommann



The Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) established ASTM D2000 to help provide guidance when determining elastomer compounds. By using a method called the “line callout,” engineers have a readily available classification system.

Andrew Rommann breaks down the individual elements that compose this “line callout” and the benefits of using this method.

Click on the image to open in a New Tab.



In many elastomer products, ASTM D2000 is utilized as the standard to communicate the performance requirements of the materials based on the customer’s expectations or the demands of the application.


An ASTM D2000 Line Callout

An ASTM D2000 line callout looks like this and is the entire line that you can see between my arrows. We have a specification that applies to the line call out. We have some basic requirements information and then we have what’s known as the suffix requirements portion of the line call out.


The Basic Requirements

So, within the line call out, within the basic requirements:

  • You have usually proceeding with an M which indicates metric units, so SI units.
  • You have a number following the M that indicates the grade of the material.
  • The first letter following that number is the type. The type for the materials is actually based on resistance to heat aging.
  • After that, you have class. The class is based on resistance to oil swelling.
  • Following that, we have a single digit that is representative of the hardness of the rubber. This has a 7 that could be a 70 durometer material plus or minus five. It also could be a 65 durometer material plus or minus 5, it could be a 75 durometer material plus or minus five.

So we’ll see that the durometer if that is a specific target you’re going for, you may need to add an additional suffix requirement to explain that.

  • Then the last two digits are actually representing the tensile strength of the material given in megapascals.

What you see on the left-hand side of the line call out – this is actually the minimum requirement that you need to specify an ASTM D2000 material. With this requirement, there is a set of basic requirements automatically imposed regardless of the grade of the material and without the existence of any of the suffix requirements. Those basic requirements include tests and performance results for heat aging, oil immersion, and compression set.


The Suffix Requirements

The suffix requirements as you can see the line call out is actually the greatest portion. What I have written on the board is the longest standard line call out that you could come up with for a M2BG710 material. This is a nitrile compound. Grade 2 correlates to the performance results of each one of these tests. For a grade 2, a grade 3, grade 4, 5, and 6, each grade will have different applicable suffix requirements. It will also have different levels of minimum performance to qualify as that grade of material.

In the suffix requirements section we see that we have a preceding letter or set of letters for each suffix requirement.

  • And so B14 and B34 with the B letter – they actually are both compression set tests.
  • EA 14 with the EA preceding this is a resistance to an aqueous fluid or water resistance.
  • We have EF and this is fluid resistance. Specifically, fuel resistance.
  • We have EO 14 E0 34. These are both oil resistance tests.
  • F17 is the low-temperature requirement.
  • And our Z call-outs at the very end Z 1, 2, and 3 in this case – are what we call special requirements.


Special Requirements

These special requirements are very powerful to help clarify specific items that may be required by a manufacturing process. A typical Z could be in this case for Z1. I wanted to clarify that the seven in the durometer call out is actually applicable to a durometer of 75 plus or minus five. I wanted to make that clear so I added the Z1 call out for that.

Z2, this could be the special processing in the manufacturing that I was referring to so maybe this elastomer component goes on to an assembly that goes through a paint line and ultimately through a paint oven there could be a small degree a small amount of time short duration or we have an elevated temperature and you wanted to evaluate the effects of that Temperature of the paint booth on the elastomer itself. So in this case, I’ve included a Z2 call out to say this ASTM method D 573 and I want to check it one hour at 125 degrees Celsius.

And then Z3 in this case. I wanted to come up with something a little bit out of the ordinary and this one I wrote down is must smell like vanilla birthday cake. It’s very unlikely that you actually need your product to have a certain fragrance, but it is possible to create a Z call out to impose any special requirement of any kind on the material. Keep in mind in doing that, you can prescribe a Z call out that is impossible to meet or could have a major cost impact on the overall material price.

So with these Z callouts, you want to make sure that you’re using what is applicable to your needs and not imposing anything above and beyond your requirements on the material.


Additional Suffix Letters

Some additional suffix letters are shown here. In addition to the ones that I’ve had this particular call out did not include a C12 call out and the C suffix would indicate an ozone resistance test. You could also have a G call out which is an air resistance test and there’s a small list of additional suffix letters that correspond to different types of tests that can be applied to different types of material. The combinations of grade, type and class could have a different list of suffix letters applied.


Benefits Of Using An ASTM D2000 Line Callout

So with all of this, based around the ASTM D2000 standard, and included on your drawing the major benefits of using it –

  • provides us a standard language to communicate our performance expectations and the performance requirements demanded by the application.
  • It defines the test methods that you’re going to use so that the testing can be done at any accredited laboratory and it can be done consistently, and results can be comparable.
  • it also defines the performance requirements by the combination of grade and type/class.

So with those things defined -both the grade, type, and class – along with the ASTM D2000 suffice requirements, we know exactly what tests need to be performed on the material and what the minimum requirements of those tests need to be to qualify for this requirement. It provides very clear information to the design team, to the manufacturer, and also to the quality assurance teams for products.