Me Taylor White MEng

[TaylortrysEngineering]

Hello, 

I was wondering if I can get some input from Machinist and Engineers. 
 

to give a bit of background, I’m a Mechanical Design Engineer who works for a Centrifugal Pump Manufacturer and Distributer. We have had countless debates on incorporating standards on our drawing and for most it’s black and white but a topic that keeps occurring is surface roughness, in particular O ring grooves. I’m an Engineer from the UK and use BS ISO and ASME for pipes. We have a lot of concession from suppliers saying they can’t achieve a certain surface roughness, for example. For a static application on an O ring groove (BS 862) the surface roughness is 1.6um, our supplier can only achieve 3.2um for example and the O ring in question is like 300mm diameter roughly, which were told it’s impossible to achieve and the cost will sky rocket reducing profits massively, also we have had no problems of failure either. Do we change the drawing or keep dealing with concession So our drawing conforms to standard? 
I’m a fairly Young Engineer with no machining experience but am desperate to know the capabilities what suppliers can achieve (I know all suppliers are different but seems to be a common occurandr across the board) Has anyone experienced problems like this in the past. Be really good to hear your input/advice. Thank you. 

[G B Reid MIMechE]

Hi!

Normally, you would not reduce the standards requested on the design, but this is an interesting case.

You imply htis is a long-standing product and have stated that you have had zero failures...you have also stated the manufacturers cannot match the requirements. This implies that you have granted the concessions on many occasions without detriment.

This results in - effectively - an empirical proof of design.  In this instance, as the change has been proven empirically, it would be safe to do.

There are a multitude of additional information that would otherwise need to be investigated: o-ring compression, dimensions of the groove, material of the O-ring, fluid type (liquid/gas) etc, but this all becomes moot as the design has been proven in real life.

Basically, keep a complete log of all items and any failures that do occur. These must be investigated to determine the root-cause of the failure. Records are key here to your protection. Should there ever be a catastrophic incident, your records will prove the empirical safety...

good luck