Surface Finish Confusion 1

[Brammer]

Hello all,

I'm looking to get a steel rod to work with a seal I'm getting. The manufacturer of the seal tells me I need a rod with a surface finish of 16uin Ra or better. A manufacturer of steel rod tells me that he has a precision ground rod that has a tolerance of +0 -0.0005 in. How can I tell if this is good enough?

Thanks!

 

[redpicker]

How can I tell if this is good enough?

You'll have to ask your supplier if they can meet both OD dimensional tolerance and surface finish requirements. They are two independent characteristics.

rp

 

[remetaper]

Redpicker is right ! In my opinion, for sealing performances , it's better to talking about Rmax or Rt ( max peak-to-valley height ) than Ra ( Average roughness height ) . You have to evaluate that a isolated pit, scratches or dent may cause leaking in zones where these ones could happen. Sometime,the centerless grinding process of manufactorer of steel could cause some grinding scratches ( defined as "comets" or "comma schleifen " in German Langauge) in case of poor care of grinding wheel during the process or not suitable grinding wheel for the specific steel grade.

 

[EdStainless]

And for most seals there is such a thing as the surface being too smooth. This results in too much surface contact and friction.

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Plymouth Tube

 

[mfgenggear]

Brammer

all what is said above & also Edstainless is correct, there is a tolerance for seal surfaces. generally like 15-20 micro.
have an independent lab or your inspection department to verify the surface finish & diameter. never rely on your vendor for attributes. It must verified to ensure it's been correctly being mfg.

Mfgenggear

 

[Brammer]

Hi Guys,

Thanks alot for all of the help. It makes sense that the roughness and diameter tolerance are two separate things, just seemed odd that the vendor didn't know much about it. I'll see what else I can find out. Thanks everyone. And good call mfgenggear, I'll be getting a third party to look at the rod after I get it.

Thanks,
-Brammer

 

[Metalhead97]

Brammer,

I've worked with high-vacuum equipment for years in graduate school. The only rods I've ever used with o-ring seals were smooth with no visible signs of machining marks. Make sure you use plenty of vacuum grease.

metalhead