What Is Torque Retention, and Why Do I Care?

This is the sixth 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 about. Click the links below to read the first five installments:

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

 What Is Bolt Load, and Why Do I Care?

 What Is Creep Relaxation, and Why Do I Care?

 What Is Ignition Loss, and Why Do I Care?

 What Is Air Aging, and Why Do I Care?

This week, we’re going to talk about torque retention (or torque loss, depending on your particular viewpoint) and how it affects your gasket performance. Torque retention is related to many of the topics previously discussed, but we thought we’d take a closer look at it specifically.

 

Torque Retention

Torque retention isn’t necessarily something that can be measured and published as a gasket material property, but compressibility, recovery, and creep relaxation values have a direct effect on a good torque retention value (along with a good seal). Below, we have listed a few general questions to help those of you that are less familiar with this concept start to gain an understanding.

1. What exactly is torque retention? It is a measurement of how well a bolted joint retains the force applied to each bolt. How much of the initial torque is retained by the fastener after a certain time or exposure? Zero percent torque retention means that the bolts have lost all of their load and are no longer compressing the gasket and thusly, the bolts are falling out and the joint has failed.

2. What causes torque retention to change? Changes in the gasket due to time or temperature are the main reasons you tend to see changes in torque retention. Other reasons would have to do with the design of the joint, the assembly of the joint, the bolting procedure, the fasteners themselves, and other factors.

3. What do I do about torque loss? After the initial heat cycle, some reduction in torque retention (torque loss) is expected and compensated for with the initial bolt load. In an ideal situation, bolts are just re-tightened and the retention should be sufficient. While this isn’t always possible, the installation load is typically high enough to achieve the initial seal, withstand creep and load loss, and still maintain sufficient load for a long term seal.

4. What should I look for in materials to maximize retention (or minimize loss)? The answer to this really depends on the requirements of your application and the conditions of your joint, so you’ll never find one perfect material. However, when selecting a material, you want to pay attention to creep relaxation values. The lower the better, meaning that a material with low creep will provide more “push-back” on the bolts and thusly, retain more torque.

5. What else should I know? All compressible gasket materials have some level of creep or load loss which can be expected. The challenge as a designer is to provide sufficient load for a long term seal.

 

Now That You Know

Knowing about and understanding torque retention can help you to better select a material for your joint. It can also help you to understand what you are seeing after the application begins to log runtime hours. You can always expect some reduction in torque retention, the key is to understand why it is happening and how much is too much.

Is the cup half full or half empty? Torque retention or torque loss?

What Is Torque Retention, and Why Do I Care?

Are Surface Finishes Important When Working With Composites?

In many of today’s newer applications, the surface finish of the flange is almost always something that must be addressed due to the popularity of selecting SLS (single layer steel) and MLS (multi-layer steel) gaskets for applications. The cost associated with the machining required to create and maintain these mirror-smooth surfaces is something that needs to be considered during the design phase.

For some of you newer engineers, you probably see this as a way of life when it comes to designing a joint. However, it is important to know that there are other solutions that do not require the expensive machining for the flanges, and they work well in the aftermarket (where the stainless steel shims typically do not). Have we caught your interest? Keep on reading for more.

 

Does Surface Finish Matter?

Composite (a.k.a. perforated core, metal reinforced) gasket materials are known for their performance and ability to seal at elevated temperatures. (See our blog post, “What Qualifies as ‘High Temperature’ When Talking About Gaskets?”) Not only are they one of the best technologies out there for high temperature applications, they are also one of the best at compensating for imperfect flanges.

Imperfect flanges…this leads us to think of two questions immediately.

1. What causes imperfect flanges? – They can be caused by many things, all of which cause detriment to the joint if there isn’t a proper gasket material holding the seal. Imperfect flanges can be a result of cast surfaces, improper machining, temperature distortion, or damage from gasket removal, to name a few.

2. Why are composites good at compensating for imperfect flanges? – This is due to the construction of the materials. The laminate is comprised of a compressible facing material that will conform to flange irregularities and configurations, while a metal core provides rigidity and structure to the gasket while maintaining the facing against the flange.

 

One Less Worry

There is no perfect gasket material. However, it is important to do your homework on the various technologies and decide what is going to reduce the risk (and the cost) the most for you in your design. While it is comfortable to keep using a technology because you’ve used it in the past, it is good to at least look around once in a while to be sure you have the best fit for a particular application. If a more forgiving gasket material is going to increase your chances of maintaining a seal, then talk to your trusted gasket material supplier to see what your options are.

Until next time!

Are Surface Finishes Important When Working With Composites?

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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