# Surface Finish Correlation between Ra, Rz and Tp (bearing surface ratio)

#### Ron Rompen

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This is a shot in the dark, but it doesn't hurt to ask. Does anyone know of any accepted studies showing a correlation between Ra, Rz and Tp (bearing surface ratio)? I am manufacturing parts which have an Ra callout of 1.0, OR Rz of 6.0, but with a Tp of 50% or better.

Anyone have any history/information to share?

#### dwperron

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#### Ron Rompen

Trusted Information Resource
Yeah, I asked him first, DW, but he had no information that I was prepared to acknowledge publicly

#### Miner

##### Forum Moderator
Staff member
Disclaimer: I only have a small amount of experience in surface finish, so I am going from my engineering background and some theory.

Ra and Rz are measuring in the same axis, but are measuring different characteristics (Ra ~ average roughness, Rz ~ max roughness). If Ra increases, it makes sense that Rz must increase. However, Rz might increase without Ra increasing. Therefore, there should be a rough correlation between the two, but is will not be perfect. There is an article that discusses the correlation here.

Now we get to Tp. Tp is measured on an axis that is 90 degrees to the axis in which Ra and Rz are measured. This means that Tp may vary significantly with little or no impact on Ra/Rz until you get to extreme values. You could get a weak correlation, but not one that is particularly useful, or that you would want to rely upon.

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