'Air tight' tolerance?

[Peter.]

Another couple of questions if I may:

What tolerances must I hold to prevent air bleeding past the spool on a basic valve, typical of the type I see used on small engines on this site. I want to achieve as close to zero as possible.

I see many valves from brass running in cast iron. Is there any reason why I may not use delrin in cast iron for a valve?

 

[Tin Falcon]

Peter A very off the cuff no scientific answer here but I would thing a .0003 to .0005 inch difference in the mating parts would be ideal .
Sometimes in model building you make the bore to size and lap if needed then make the internal pat to fit. also when fitting make sure there are no burrs to interfere with the fitting test.
Give the delrin a try if you are just running on air. One of the great things about this hobby is unless you sell your machines you always have a source of original parts and you can make a few spares if you want and you can experiment with materials etc.

Tin

 

[Blogwitch]

Peter,

I see many valves from brass running in cast iron. Is there any reason why I may not use delrin in cast iron for a valve?

The only problem I could envisage is the actual hardness, or lack of it, if you use delrin.

You would require the bore to be polished, as if it wasn't, the grain structure of the metal would act like a file on the material.

I actually usually use delrin for making my small bore laps out of, and can attest to the fact that it does wear, and the surface gets very rough and gnarled rather quickly when faced with a rough bore.


Bogs

 

[Peter.]

Thanks guys.
I want to use Delrin to keep down the weight of the spool. I could just as easily use cast iron or brass I guess, but I kinda think I'm going to be making a few prototypes before I get an actual working valve so I'd rather use the much more available plastic.

I guess I could do my experimenting with Delrin then make the final offering from metal.

 

[Tin Falcon]

peter my car has plastic what i believe to be delrin sleeves on the door catches They do wear out in time I have replaced some with delrin and have held up a couple years so far.
tin

 

[joeby]

It might be a challenge to keep air leaks to a minimum without using seals of some type. Spool valves normally have o-rings or similar to seal, I believe.

Delrin would work as a spool, but depending on the valves end use, another material may be more suitable. I don't remember if Delrin is hygroscopic; if it is, exposure to water from the air source may cause problems.

Something you might try if you are making a spool without some type of seal; cut narrow sharp-edged grooves some distance apart on the o.d., similar to ring grooves.
I don't think they would need to be very deep. It does seem to slow the leaking somewhat.

Kevin