What Is Creep Relaxation, and Why Do I Care?

Here is the third installment of our “Why Do I Care” series. Every so often, we’ll blog about some of the questions people new to the gasket industry may be looking for more information on. Click the links to read the first two installments:

 What Is Compressibility & Recovery, and Why Do I Care?

 What is Bolt Load, and Why Do I Care?

This week we’re going to talk about creep relaxation in gaskets and gasket material. This is something that will happen to some extent in most gasket materials. How much or how little of this you see depends on your gasket material selection and the application itself. It is important to have an understanding of creep relaxation and how it relates to a particular material.

 

Creep Relaxation

In basic terms, creep relaxation is the measurement of how much a particular gasket material spreads (thins) out when force is applied. As the gasket loses thickness, the bolts can relax, which leads to a loss of load. When the level of creep relaxation is too high for the application, that’s when you’re going to have problems maintaining the seal.

Thickness of your gasket material plays a big part in the amount of creep relaxation you see in the material. If you’ve read our previous blog post How Thick Does Your Gasket Need to Be? Here Are 4 Ways to Know For Sure., you’ll see that thicker is not always better. Generally, the thinnest gasket you can use is going to be the best option. The amount of creep relaxation seen in a material is going to be directly proportional to the thickness of the gasket. With that said, sometimes a thick gasket is necessary. If that is true for your application, then you should expect a certain level of creep to take place. Ideally, the bolts and joint will be loaded sufficiently to overcome the relaxation that is present and maintain the seal.

The majority of the creep relaxation seen in a material occurs early in use. Some suggest that material will continue to creep indefinitely, but the loss is negligible. If your application can withstand the creep that occurs within the first duty cycle, chances are it will maintain its seal.

Materials have different amounts of creep, depending on the temperatures and pressures used in the joint. Generally, denser materials will have less creep than softer materials. So, in stiff flanged joints with plenty of load, generally a denser material is a better choice.

Softer materials are more compressible and provide better conformance to flange surfaces.  However, some types of softer materials due to their make-up, exhibit high creep/relaxation under load and temperature. This can be addressed by providing sufficient bolt load, to overcome any expected creep effect and maintain a long term seal. Minimizing gasket thickness will also help in this area.

High temperature and high pressure also have an effect on the amount of creep you will see in the material. If you work in these types of applications, you will need to pay special attention to the specifications of the material you are selecting to work within the suggested temperature conditions. Taking material higher than its capability can lead to the breakdown of various components, additional creep and ultimately load loss on the fasteners.

 

Material Selection Matters

Being aware of the creep relaxation characteristics of a specific material is paramount to choosing the right material for your application. Don’t hesitate to talk to your trusted gasket material supplier for help in identifying the type of material that you may need for your application.

Until next time!

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What Is Bolt Load, and Why Do I Care?

Here is the second installment of our “Why Do I Care” series. Every so often, we’ll blog about some of the questions people new to the gasket industry may be looking for more information on. The last installment was “What Is Compressibility & Recovery, and Why Do I Care?”.

This week, we’re going to talk about bolt load. You may also have heard the terms: torque load, flange load, or compressive force, which all refer to the same thing. Bolt load is something to keep in mind when you are designing your joint and selecting your gasket material. Without knowing or paying attention to it, you may be setting yourself up for an insufficient load situation…and a problem.

Bolt Load

Putting it simply, bolt load is the key to the joint. Sufficient load must be available to achieve the initial seal, and then maintain that seal long-term over the life of the joint. The load must be comfortably more than needed, to overcome obstacles so a sufficient safety factor is present.

Achieving the proper load isn’t something that simply happens by grabbing a wrench and tightening the bolts until they feel snug. There is a lot that goes into the calculation and subsequently, the assembly, to ensure that the joint will stay properly sealed.

Bolt loading is not only the sum of the force from all of the fasteners, but also how it is applied and the geometry of how it is applied. Generally, the bolts should be placed such that they distribute the load evenly throughout the sealing surface and in line with it as best as possible (bolt circle).

Load is divided by sealing area to determine average flange load and thus target loading. This can be correlated to material studies so the engineer can make a determination of how much load is needed, and thus, how many bolts, what size bolts, and the torque to apply. Manufacturers will tell you that fasteners should be sized and loaded just before, at, or just after yield strength to apply the most load properly. This is also the reason they should not be reused, such as in a head gasket application.

Loading issues such as these are a key component in FEA (finite element analysis) and should be as accurate as possible to make a good model. Engineering evaluations such as this are often very helpful in establishing a successful design.

Gasket Material Makes A Difference

Bolt load is only as good as the compressibility and recovery your particular gasket material can achieve. One can figure out the perfect design and exactly what bolt load is needed in your application. But, if you don’t have the right gasket material (either because a poor selection was made, or there isn’t really a “right” material for your application because of how it was designed…oops), you aren’t going to achieve the seal that you need. Selecting the proper gasket material to meet your requirements is probably one of the more important pieces of the design, and the sooner in the design process you do this, the better off you’ll be.

Now That You Know

As you can see, bolt load is something that generally falls within the scope of a design engineer, but a good gasket material supplier can be a great resource. If you are unable to achieve the proper load, or you want to ensure that there a material to meet your loading requirements, talk with your supplier. They should be able to give you some insight, and maybe even a different material to consider.

What are some of the main reasons for your insufficient load problems?Request a Call

What Is Compressibility & Recovery, and Why Do I Care?

Here is the first installment of our “Why Do I Care” series. Every so often, we’ll blog about some of the questions people new to the gasket industry may be looking for more information on.

This week, we’re going to talk compressibility and recovery. These are two of the most important properties in selecting a gasket material, and ones you definitely need to pay attention to. Having a material with the right compressibility and recovery for your application makes all the difference in the performance of the application.

Compressibility

How hard or soft is the material? How much will the material compress when pressure is applied to it? This is a key factor involved in the joint. What are the flanges like, and how much load is available? Are they cast or stamped, rigid or not? “Soft” sealing materials such as cork, rubber, sponge, foam, etc., compress large amounts and can be used when low flange load is present. Denser materials such as fibers, hard rubbers, composites, metals, and others require more load to achieve a seal and only compress a small amount. Gasket materials chosen should possess sufficient compressibility to overcome flange imperfections, internal pressure from fluid being sealed, and deflection.

Recovery

How springy is the material? How close to “normal” will the material be when pressure is removed? Recovery is what it’s all about in flat gasket applications. Materials must have sufficient recovery to maintain a seal after initial cycles and relaxation. Stiffer, denser products, while having less compressiblity, often have the best recovery characteristics.  The magnitude of the deflection and recovery is less, but better.

Look At The Data

This is an example of compressibility charted against load. As you can see by this simple chart, the higher load creates more displacement. For many materials, they are nearly linear within their usable ranges, for “non-soft” materials such as fibers and composites, this would be up to 5000 psi. Engineers designing joints can benefit by this information to compare to the load available in the joint by the bolts (both #, type, and pattern) to assure adequate compression.

Compressibility test

This chart is actually a load/deflection curve, which shows the full scope of both compressibility and recovery in a graphic form.

Now That You Know

You may be wondering why someone just can’t design a material that is “perfect” for compressibility and recovery? Well, that could be done, but what properties are you willing to give up? With any material, you need to find the balance you need between all properties, and that sometimes means giving a little of this for that. Doing your research and fully understanding what each gasket material has to offer is going to help ensure your application runs as smoothly as possible.

What are some other properties that you pay particular attention to?

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