Brake Master Cylinder surface finish 1

[quadracer12]

So I found some roughness inside the cylinder of a brake master cylinder that we are producing in China. I know I cannot accept the roughness, but I am trying to find information on an acceptable surface finish inside of the cylinder. I am having very little luck.

I believe somewhere around Ra 24-32 or 0.6-0.8 micrometers would be acceptable, but I'm hoping to get some opinions from the fine engineers in this forum.

This is for vintage cars so no ABS or other electronic factors, simply the hydraulic cylinder. Thanks for the help!

 

[swall]

Are the cylinders cast iron, bare aluminum or anodized aluminum?

 

[GregLocock]

The supplier of the seals will have a relevant opinion.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[TVP]

My opinion: definitely no higher than Ra 0.6-0.8 micrometers. This range is not really that smooth, and most seal manufacturers recommend lower surface roughness.

 

[quadracer12]

The cylinders are cast iron, but we also have bare aluminum cylinders that we are playing with. Would it be different for each material?

Thanks TVP, I was thinking that 0.6-0.8 was the upper limit, I will probably hold them to a higher standard (lower roughness).

Any additional info would be great!

 

[IRstuff]

Seems to be plenty of info on the web:

http://pdf.directindustry.com/pdf/james-walker/o-ring-guide/28272-15628-_24.html

http://www.allorings.com/oring_groove_design.htm

http://www.usseals.com/hydraulic_seals.htm

Note the caveats against too fine a finish.

http://www.allsealsinc.com/parker/dynamic-oring-sealing.pdf

TTFN

FAQ731-376

 

[swall]

What I was getting at was that if you are working with aluminum master cylinders, you have to take into account the roughening that will occur if you anodize the bores and start with a smoother finish than would be the case if running in bare aluminum bores.

 

[dgallup]

In general, 0,8 Ra (in microns) is considered the upper limit for sealing applications. That being said, there are a lot of different production processes that will produce a 0,8 Ra with very different roughness profiles and sealing properties. We regularly allow 1,2 Ra on turned parts with static o-ring seals because the direction of lay matches the seal. If the direction of lay is across the seal, a better finish is required. Dynamic seals, with their lower compression, need a somewhat better finish. However, it can't be too smooth or it won't hold any lubricant.

In any case, for a safety critical item like a brake cylinder, I would hope you are doing some serious leak testing at extremes of fit and finish and at high and low temperatures. There are too many variables involved to just take a stab in the dark.