Surface Roughness - Guidelines 1

[KevinZ7]

Greetings,

I deal with sealing against low pressure (<500psi) water using elastomeric seals, turned, milled and molded parts.

I have found lots of literature detailing different ways of specifying surface finishes (Ra, Rp, Rz, Rv, Rt etc etc).

However, I have found very little information about recommended values for these parameters, the only information I have found is the Parker O-ring Design Guide which recommends an Ra of 0.8um for sealing applications and a Ra 0.4um for vacuum applications.

It seems to me that their must be information and test results out their for minimum requirements for various sealing applications using all of these other parameters, I just don't know where to look.

The reason I ask is I feel I may be over-specifying the surface finish in some instances, for instance it doesn't make sense to me that a surface produced by the tip of an end mill with machining grooves crossing the sealing surface would have the same Ra spec as a turned surface with circumferential marks.

Can anyone point me in the right direction of where to look?

Much appreciated!

 

[dgallup]

I can't point you toward a document but I can say that when we have turned parts with the lay parallel to the seal we allow Ra 1.2 with very good results. For molded parts we require Ra of 0.8 and for machined parts where the lay crosses the o-ring we require Ra of 0.6 or 0.4. Ra is almost universally used as it is the most repeatable parameter. Rmax may be more important in some sealing cases and we use it in come critical cases. However, the repeatability of the Rmax measurement is not good. Rmax will typically be 8 to 10 times Ra. Rz falls somewhere in between but I hate to use it as there are multiple definitions of Rz and it's hard to be sure what a particular profilometer it indicating. Most of the other parameters are for very specialized applications.

One other thing that has a big impact on reliable sealing is the size of the o-ring cross section. It becomes increasingly difficult to get good sealing as the o-ring cross section decreases. I have seen some companies that had a lower limit on allowable o-ring cross section. Since you are dealing with water I envision large seals so it's probably not an issue.

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