relationship between tolerance and roughness

[MarcoUNIPI]

Good evening to all are new to the forum.
I am a first-arm mechanical engineering student and today I have a doubt.
realizing a particular of a total representative a volumetric compressor, for precision the cylinder. I have come to quote the cylinder diamter in which the piston runs and to assign its tolerance (it seems to me to remember an h6/h5). Here I wanted to know if I should for completeness indicate a roughness on this surface (e.g. a rectification at 0.4 or 0,8) or if roughness can be considered already included or secondary to dimensional tolerance.
Thank you all in advance and still good evening.
:finger:

 

[paulpaul]

roughness should always be put, general and/or local, regardless of dimensional tolerances.
its value depends on the application: contact and lubrication conditions, materials, loads, relative speed, etc.

ps: was the h/h evaluated well? for a piston/canna coupling seems quite risky to me! :

 

[Focus]

ps: was the h/h evaluated well? for a piston/canna coupling seems quite risky to me! :

more correct "wrongly." :smile:

Hi.

 

[Kakisan]

Hi.
roughness should always be indicated (especially on such critical surfaces).
However, I think it makes no sense to make the outer surface of the piston work as a sealing surface.
I think you should do the piston with a greater game and insert 2 or 3 elastic bands.
In this way you can also eliminate a tight tolerance between piston and cylinder and reduce the demands on the surface of the piston..which is no longer functional.

 

[paulpaul]

Hi.
roughness should always be indicated (especially on such critical surfaces).
However, I think it makes no sense to make the outer surface of the piston work as a sealing surface.
I think you should do the piston with a greater game and insert 2 or 3 elastic bands.
In this way you can also eliminate a tight tolerance between piston and cylinder and reduce the demands on the surface of the piston..which is no longer functional.

It is usually so! but different solutions are possible according to the type of compressor

 

[MarcoUNIPI]

more correct "wrongly." :smile:

Hi.

I found that tolerance between those recommended in the textbook I use.
quote: "free running. for internally lubricated couplings." I thought it was suitable for that job.
Unfortunately the course of drawing leaves much running on these things by focusing more on the representative part, which is why I am not very careful about the topic.
What kind of tolerance would you recommend for such a job?
:smile:

 

[Kakisan]

Hi.
what you propose is a very precise coupling.
If you could produce the 2 perfect details (so to 0) you wouldn't even play to pass an oil film to lubricate.
instead in your case you will have a frequent flow, constant and I think fast... so widens those tolerances. and since you're there, add the elastic bands.