Subject: Re: o-rings metric vs english
Author: Robert Hawkes
Date: Jul 20, 2001, 7:12 PM
Post ID: 1707584795
Another e-mail to keep in my Guzzi Arcana file!! Bob
----- Original Message -----
From: stephen brenton <sbre-@williamstech.com>
To: <Loopfram-@topica.com>
Sent: Friday, July 20, 2001 10:46 AM
Subject: o-rings metric vs english
| ============================================================ Hot. Cool. And FREE. Get outrageously awesome offers on stuff you want emailed direct. http://click.topica.com/caaacgGb1dfltb1FlG8a/TopOffers ============================================================ As one would expect there are industry standards for o-rings and as it turns out they seem to recognize the need to satisfy metric needs as well as english. When it comes to the size of an o-ring it is sufficient to completely describe an o-ring with ID (inside diameter) of the ring and CS (cross section diameter of the cord). When we use these sizes we are actually using nominal dimensions, similar to saying a framing stud in building construction is a 2 x 4. O-ring cord is only manufactured in a limited number of standard cross section diameters. I'm sure there are special cases which require a size outside of the standards, but most designers and engineers try to use what is available off-the-shelf. The standard metric cross sections are 2, 2-1/2, 3, 3-1/2, 4, 6 and 8mm. The actual cross sectional diameter for a 2mm o-ring and a 1/16" o-ring is .070 +/- .007 inch. The list below shows sizes that we will commonly encounter in working with motorcycles: Nominal CS Actual CS Nominal metric 1/16" .070 +/-.007" 2mm 3/32" .093 +/-.007" 2-1/2mm .103 +/-.007" 3mm 1/8" .125 +/-.007" 3-1/2mm .139 +/-.007 4mm What does all this mean? Well if I have an o-ring that measures about .070" it could be a 1/16" or a 2mm (nominal) o-ring. From CS alone I can't distinguish it as a metric or english. Inside diameter follows the same pattern. O-rings have a nominal ID which is "close" for description purposes but different from the "actual" ID. For example a 3/8"(nominal) o-ring has an actual ID of 364" or 9.25mm. There are tolerances applicable to this actual ID which adds even more complexity because there are different "classes", similar to threads on a bolt. A class I o-ring of this size (3/8) has a tolerance of +/-.005"(.13mm); a class II has tolerance of +/-.007"(.18mm). A 3/8 nominal ID o-ring would be a reasonable choice to seal a 10mm cylinder stud. Maybe there is a 10mm nominal ID metric o-ring available, but it's not listed in the supplier catalogs I have. So if I choose to use a 3/8 o-ring then the only decision is the CS size. Good choices are 1/16 or 3/32, which coincide with 2 or 2-1/2mm. I'm going with 3/8ID x 1/16CS. I'll let you know how it works out. stephen in sc. |