The performance of an O-ring seal depends largely on two factors— the right material and the right size. However, determining the right size O-ring is usually the hardest part when it comes to choosing the right O-ring for an application.
Unfortunately, selecting the right O-ring size is paramount to avoid leakage and other costly damages that may occur in the long run. Additionally, If you’re unsure what O-ring size you need, you’ll have a difficult time purchasing an O-ring that will work for your application.
So, here’s a guide that will help you size an O-ring properly.
Measuring O-ring Cross Section and Inside Diameter
To identify the right O ring sizes for your application, you need to know the required cross-sectional (CS) diameter and the inside diameter (ID). The CS can be best measured using a calliper, but you can also get a close estimate using any other sizing tool.
When it comes to measuring the ID, it is advisable to use a calliper for very small IDs and a Pi-Tape or O-Sizer for larger IDs. Once you’ve determined the CS and ID, you can use an O-ring size chart to determine if the dimensions correspond with the recommended O-ring size for your application.
Understanding O-ring Sizes and Tolerances
O-rings are normally described by either standard (inch) or metric (mm) dimensions. There normally exists some variance in the O-ring dimensions as a result of manufacturing limitations. These variances are referred to as tolerances.
Aerospace Standard AS568B defines standard O-ring sizes. The CS of these O-rings range from 0.040 to 0.275 inches, and their ID range from 0.029 to 25.940 inches.
Metric O-ring sizes, on the other hand, are defined by International Standard ISO 3601-1, which classifies metric sizes into two categories, G and A. The G series O-rings are available in a wide range of ID and are ideal for general purpose applications. The A series O-rings, on the other hand, are ideal for aerospace applications, which require tighter tolerances.
Choosing the Right Cross-sectional Diameter
Generally, O-rings with a larger CS offer better resistance to compression set. Moreover, they have less volume swell in fluids and won’t leak easily if their surface is scratched.
O–rings with smaller CS boast superior physical properties. They also can resist explosive decompression better than their larger CS counterparts. Moreover, these O-rings are more space-efficient.
When it comes to dynamic applications, you would want to go for O-rings with large CS as they are more resistant to rolling in a groove. Smaller CS O-rings will normally slide and roll in a groove, resulting in leakages due to the ring’s spiral failure.
Now that you have some knowledge of O-ring size selection, you can go shopping for your O-rings at Specialist Sealing Products, one of the leading O-ring suppliers in the UK. The company offers a wide range of high-quality O-rings. For more information on O-rings and other sealing products offered by Special Sealing Products, click here.
