Measuring an O-Ring

How to Calculate Maximum O-Ring Cross-Sectional Thickness

The physical properties of O-rings determine how well your industrial equipment can maintain seals through every motion. If your O-rings don’t have the right materials or don’t fit, there is no way they will perform the way you need them to.

One often overlooked aspect of your O-ring selection is cross-sectional thickness. Learn more with our guide.

Why Does the Cross-Sectional Thickness of O-Rings Matter?

It comes down to diametrical clearance. O-rings need to have a slight squeeze to create the seal. Every seal has a slight gap between the two mating surfaces, called the diametrical clearance. The O-ring’s job is to fill the gap at the right moment to make sure nothing leaks when the two surfaces come in contact with each other.

Another thing to consider is that the O-ring’s cross-sectional thickness determines all of the other dimensions of the O-ring. If your O-ring is thicker, the inner diameter is less while the outer diameter is greater.

Third, the higher the cross-sectional thickness, the more material and surface area you have on your O-ring. It will weigh more and will need more force to form a better seal.

O-Ring Cross-Sectional Thickness Goes Beyond Groove Depth

When you have a moving part, the O-ring’s cross-sectional thickness must measure the gland depth and not just the groove depth. The groove depth is measured using the machined depth into one part. The gland depth is the groove depth plus the diametrical clearance. So, a bit of the O-ring will either stick out of the groove or receive another part to fill in that groove. Rarely does groove depth equal the cross-sectional thickness of the O-ring.

Dynamic Vs. Static Seals

The amount of squeeze you need on O-rings is another physical aspect that can change the cross-sectional thickness of your O-ring. Squeeze causes the part to seal correctly and might change based on a dynamic versus static seal. Dynamic seals, where the seal is in near-constant motion, usually need a low squeeze to prevent friction. And friction is the bane of an O-ring’s existence, which is why cross-sectional thickness must be calculated properly to prevent too much friction.

How to Calculate the Maximum O-Ring Cross-Sectional Thickness

  1. Start with the bore diameter or the diameter of the hole.
  2. Subtract the bore tolerance, or how much it can deviate from its physical properties, from the bore diameter. Keep this figure in the calculator memory or write it down.
  3. Enter the groove diameter.
  4. Add the groove tolerance to the groove diameter.
  5. Subtract line 4 from line 2.
  6. Divide line 5 by 2. Write this number down or keep it in the calculator’s memory.
  7. Enter the maximum % compression that the O-ring can tolerate while maintaining the right seal. Depending on your process, the compression ranges from 3 to 35 percent.
  8. Divide line 7 by 100.
  9. Subtract line 8 from the number 1.
  10. Divide line 6 by line 9.
  11. Enter the O-ring cross-sectional tolerance, or how much the O-ring’s cross-sectional diameter can deviate from its norm.
  12. Subtract line 11 from line 10 for the answer.

Consider an O-Ring Surface Treatment by EZ Coating

Want an effective way to reduce the coefficient of friction to make your industrial equipment last longer? Our O-ring coatings outperform PTFE in several important factors, such as the coefficient of friction and abrasion resistance. It can make your O-rings more effective and prevent breakdowns.

Contact us for more information or if you need a test sample. We’ll respond within 24 hours and then discuss your requirements.