O-ring surface finish is more often the cause of leakage than the ring itself. The groove is correct, the O-ring is the right material, the compression has been calculated: and yet it still leaks. In most cases, the cause lies in the surface finish. A single turning groove, scratch or machining mark on the sealing surface is enough to create a continuous leak path along the O-ring. This article covers the normative roughness values for O-ring surface finish and what they mean in practice for static, dynamic and pulsating applications.
An O-ring seals by elastic contact with two surfaces: the groove bottom and the counterface. The seal works as long as the contact pressure of the ring is higher than the operating pressure of the medium. That contact pressure is determined by the squeeze, the hardness of the ring and the quality of the surface on which the ring rests.
At high operating pressures, the pressure helps: it presses the ring harder against the surface and bridges small surface defects. At low operating pressures, the contact pressure of the ring is limited and the ring is not able to span small roughness peaks. Especially at low pressures, under vacuum or when sealing gases, surface quality is therefore the most critical variable in the design. A properly ground sealing surface can compensate for a small geometric error in the groove. A rough or damaged sealing surface can nullify a perfectly dimensioned groove.
Four times tighter: the sealing surface in dynamic applications (Ra 0.4 µm) is four times tighter than in static applications (Ra 1.6 µm). Every stroke over a rough surface wears the ring.
In O-ring seals, there are three separate surfaces, each with its own roughness requirement: the sealing surface (the counterface on which the ring seals), the groove bottom (the surface on which the ring rests in the groove) and the groove sidewalls (the sidewalls of the groove). The requirements differ by surface and by sealing type.
The sealing surface has the strictest roughness requirement. This is the surface that is in direct contact with the outside of the ring and along which the medium is sealed. In static applications, Ra max. 1.6 µm is required. In dynamic applications, where the ring slides over this surface with every stroke, the requirement is four times stricter: Ra max. 0.4 µm. Under pulsating pressure, where pressure surges cyclically press the ring against the surface, Ra max. 0.8 µm applies. Damage such as turning grooves, scratches or shrinkage cavities on the sealing surface is directly critical. Even a fine turning groove in the radial direction forms a continuous leakage path along the circumference of the ring.
The groove bottom has a less strict requirement than the sealing surface, but it should certainly not be treated as a non-critical surface. In static applications, Ra max. 3.2 µm is sufficient. In dynamic applications, that is Ra max. 1.6 µm. The groove bottom is the surface on which the ring rests when it is not under pressure. A groove bottom that is too rough does not directly damage the underside of the ring, but at higher pressures it causes uneven support, which leads to locally excessive squeeze at the roughness peaks.
The groove sidewalls are the least critical of the three surfaces for the sealing function itself. In static applications, Ra max. 6.3 µm is the standard. In dynamic applications, Ra max. 3.2 µm. But the groove sidewalls are critical for installation: sharp edges or burrs on the transition from the sidewall to the sealing surface damage the ring during insertion. Always check the sidewalls for burr formation after machining.
The table below shows all normative roughness values from the Anyseals specification for the three surfaces and three sealing types. Ra is the average roughness, Rz the average peak height, Rmax the maximum peak height.
|
Surface |
Dynamic Ra |
Dynamic Rz |
Dynamic Rmax |
Static Ra |
Static Rz |
Static Rmax |
Pulsating pressure Ra |
|
Sealing surface |
≤ 0.4 µm |
1.2 µm |
1.6 µm |
1.6 µm |
6.3 µm |
10 µm |
0.8 µm |
|
Groove bottom |
≤ 1.6 µm |
3.2 µm |
6.3 µm |
3.2 µm |
10 µm |
12.5 µm |
1.6 µm |
|
Groove sidewalls |
≤ 3.2 µm |
6.3 µm |
10 µm |
6.3 µm |
12.5 µm |
16 µm |
3.2 µm |
For daily use, the compact comparison at Ra level is the most practical:
|
Surface |
Static Ra max. |
Dynamic Ra max. |
Pulsating pressure Ra max. |
|
Sealing surface |
1.6 µm |
0.4 µm |
0.8 µm |
|
Groove bottom |
3.2 µm |
1.6 µm |
1.6 µm |
|
Groove sidewalls |
6.3 µm |
3.2 µm |
3.2 µm |
In addition to roughness, there are specific forms of damage that always lead to problems, regardless of whether the Ra value is within the standard. The standard explicitly states that the sealing surfaces must be free of the following defects.
Scratches: axial or radial scratches on the sealing surface form leakage paths along the ring. Even a hairline scratch is critical for gases or at low pressures. Scratches are the most common cause of leakage in newly assembled systems.
Shrinkage cavities: small cavities in the surface caused by casting or welding create local interruptions in the contact between the ring and the surface. Visible as small pits or blisters, sometimes only under magnification.
Deep machining marks: too high a feed rate during turning or milling leaves spiral or linear marks that greatly exceed Rmax, even if the average Ra remains within the standard. Therefore, always measure Rz or Rmax in addition to Ra.
Ra alone is insufficient as an inspection criterion. A surface may have a low Ra value and still contain deep, isolated scratches that greatly exceed Rmax. For sealing surfaces, it is standard practice to measure both Ra and Rz, and for critical applications also Rmax.
The measurement must be carried out in the critical direction: for cylindrical sealing surfaces, the axial direction (parallel to the axis) is the most critical, because scratches in that direction run parallel to the ring and can form a continuous leakage path. A measurement in the radial direction gives a different picture for cylindrical surfaces than the actual sealing situation.
With every stroke, the O-ring slides over the sealing surface. A rough surface mechanically wears the ring, a little more with every movement. In static applications, this does not occur: the ring remains stationary on the surface and can bridge small roughness peaks through elastic deformation.
Ra is the average deviation of the roughness profile from the mean line: an average value over the entire measuring length. Rz is the average height of the five highest peaks minus the five deepest valleys: it is more sensitive to outliers. Rmax is the largest measured peak in the profile. For seals, Rmax is the most critical, because one deep scratch is enough to create a leakage path.
The most common cause is that Rmax or Rz exceeds the standard, even if Ra is within the specified values. One or a few deep scratches or machining marks can create a leakage path without noticeably affecting the average roughness. Also measure Rz and Rmax, and visually inspect the surface for scratches, pits or spiral machining marks.
In static applications, light scratches can sometimes be ground out or polished, provided the dimensional accuracy still meets the requirements afterwards. In dynamic applications, reworking is almost always necessary because the requirement is stricter. After reworking, always check the dimensions and the roughness again. In the case of severe damage, replacing the component is the safest option.
No. In dynamic use, the groove bottom may have a maximum Ra of 1.6 µm, which is four times less strict than the sealing surface. The groove bottom is not in contact with the moving counterface and does not directly contribute to wear of the ring during the stroke.
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