Correct hole size for a given O-ring 1

[rcr22b]

The O-ring is AS568a -008.

This is a nominal size 3/16" ID x 5/16 OD" x 1/16" wide.

For a shaft .311" I decided on a gland grove of .0625" depth x .070" wide.

Tolerance on the shaft is +.000 / -.002.

The shaft is mild steel. And the application is a compression screw running in 6061 aluminum head on a model diesel engine.

What is the correct size for the bore?

I guessing +.001"

Meaning that if the shaft is in fact .311" then the bore would be .312"

Or, shaft .310" and bore .311"

Am I correct or not?

Should I be tighter than .001? Or less?

 

[swertel]

Parker's catalog and free software tells you what size to make the mating components of an o-ring depending on the type of joint and material being seals. I'd check into those.



--Scott

http://wertel.eng.pro

 

[Timelord]

rcr22b,

I suggest you download a copy of the parker o-ring handbook

http://www.parker.com/o-ring/Literature/00-5700.pdf

The optimal dimensions depend on whether it is a dynamic or static seal and whether the shaft spins or is stationary etc. See page V-25 for the various dynamic seals or section IV for static seals. I have carried a copy of this handbook for 35 years (first published 1957). I consider it the bible for o-ring applications. If you are really lucky, the local Parker dealer can get you a hard copy.

Timelord

 

[KENAT]

Yeah, look at an Oring manufacturers catalogue as they usually give formulas/guidance for this.

In the UK I seem to recal a British Standard that had this information for standard sizes, not sure I've seen the same in the US.

KENAT, probably the least qualified checker you'll ever meet...

 

[rcr22b]

Timelord,

I studied the Parker catalog but while it helped me decide on the proper grove for the ID I could not find anything to tell me the correct size for the bore.

This is a static seal, but it does have limited rotation if the compression screw is adjusted to raise or lower the compression slightly.

Anyway, thanks to everyone for the replies.

 

[Timelord]

rcr22b,

Page IV-5 of 20 shows the gland you want. The dimensions are tabulated on the following several pages for each size o-ring including the bore size (columns A or D). A Parker 2-008 is the nominal size you gave. Another thing, you may have trouble streching this small of an ID over the OD of the rod.

Timelord

 

[CorBlimeyLimey]

From the Apple Rubber Products catalogue;

Static Radial Seal (for -008 O-ring)
Bore Dia = .295" +.002/-0
Piston (Shaft) Dia = .294" +0/-.001
Piston Groove Dia = .186" +0/-.001
Gland Width = .105" +.010/-0

Pressure limited to 1500 psi.


Obviously the above only applies for suitable surface finishes.

 

[rcr22b]

Timelord,

I see it now.

Thanks.

 

[hydtools]

The bore to shaft clearance depends on the o-ring material and pressure to be sealed. The clearance determines whether or not the o-ring will extrude into the clearance. If your pressure is low you can allow more clearance.

The gland dimensions determine the correct squeeze for the o-ring and allowance for the o-ring to displace when squeezed in the gland without rolling. I believe the actual cross-section of the 1/16 o-ring is .070 inches.

The Parker handbook describes the pressure/material/extrusion relationship as well as other seal and gland data. The Parker handbook was my friend for years, too.

Ted

 

[Pud]

Note that the ether used in model diesel engine fuels is very aggressive with any standard O ring material (eg Viton, nbr, etc, etc.)

Hence the reason contra pistons are usually a transition fit.


Cheers


Harry

 

[rcr22b]

Pud,

"Note that the ether used in model diesel engine fuels is very aggressive with any standard O ring material (eg Viton, nbr, etc, etc.)"

This is not a contra piston. It uses a compression plug and Viton o-ring and I don't know how ether affects it.

But it's worth looking into.

Thanks.

 

[Cockroach]

You can mathematically determine the compression and stretch of any elastomer, given its cross sectional diameter and gland geometry.

For a male gland of seal diameter A, gland diameter of B1 and elastomer cross section CS then the percentage squeeze is:

%squeeze=[1-(A-B1)/(2 CS)) X 100.

Likewise, the percentage stretch is:

%stretch=[(B1-d)/d] X 100, d=inner diameter of o-ring.

The female gland uses exactly the same equations, except the seal diameter is now B and gland diameter A1.

%squeeze=[1-(A1-B)/(2 CS)) X 100
%stretch=[(B-d)/d] X 100, d=inner diameter of o-ring.

Typically the percentage stretch regardless of o-ring size or application, static/dynamic, is 2% to 3%. However, the percentage squeeze varies according to static application and cross sectional diameter, or dynamic application and cross sectional diameter. I follow the Parker-Hannifin suggested squeezes to the letter in this regard.

A lot of engineers will simply interpolate between line entries of their o-ring size and application. This is alright, but I've found that in the case of bastardized gland geometries or cross sectional diameters of o-rings made from primer cord of differing thicknesses, that the pure mathematical approach is much simpler.

Analytically, the equations regarding squeeze can be mathematically determined noting the o-ring shape deforms from a circle to an ellipse. This deformation is responsible for "gland fill", which is why the gland width needs to be larger than the cross sectional diameter of the o-ring. You can determine this width by usage of Poisson's Ratio, noting the mechanical deformation of the rubber.

But I do agree with the previous entry, just use the Parker-Hannifin Handbook. It is absolutely the best source on the market and should be sitting right beside the Machinist Handbook on your book shelf.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[rcr22b]

Thanks for all the replies.

I have downloaded the Parker handbook and think I've got it figured out.

The bore size should be .312 with a gland depth of .050.

 

[ewh]

For the full Parker handbook, use

http://www.parker.com/o-ring/Literature/ORD5700.pdf

The other link only provided me with the first 3 pages

 

[Cockroach]

Ewh, the rest of the pages are ear marked to the left of the main text. They are arranged like chapters in a book, just click on it and that section will open.

It's the way Adobe organizes the virtual book these days! The whole book is there.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada