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:
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.
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.