×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Correct hole size for a given O-ring

Correct hole size for a given O-ring

Correct hole size for a given O-ring

(OP)
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?


 

RE: Correct hole size for a given O-ring

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

RE: Correct hole size for a given O-ring

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

RE: Correct hole size for a given O-ring

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...

RE: Correct hole size for a given O-ring

(OP)
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.

RE: Correct hole size for a given O-ring

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

RE: Correct hole size for a given O-ring

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.

cheers

RE: Correct hole size for a given O-ring

(OP)
Timelord,

I see it now.   

Thanks.

RE: Correct hole size for a given O-ring

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

RE: Correct hole size for a given O-ring


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

RE: Correct hole size for a given O-ring

(OP)
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.

RE: Correct hole size for a given O-ring

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

RE: Correct hole size for a given O-ring

(OP)
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.

RE: Correct hole size for a given O-ring

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources