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Engineering360: "Translating Imperial Units to Metric Sizes in O-Rings"

08/30/2017 9:54 AM

Read Engineering360 article: Translating Imperial Units to Metric Sizes in O-Rings.

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Power-User

Join Date: Jun 2007
Location: Center of the Known Industrial Universe - TUGGERAH 2259 - Australia
Posts: 258
Good Answers: 52
#1

Re: Translating Imperial Units to Metric Sizes in O-Rings

01/03/2019 12:41 AM

Dear Seals-Shop GmbH:

1) You say "The inside and outside diameters are the
distances from the center to the O-ring’s inside and outside surfaces, respectively." I regret you are mistaken. You have just described the inside radius and outside radius, NOT the diameters.

2) You say "The cross-section is the difference between the outer and inner diameters". For an o-ring, the cross-section is HALF of that difference.

3) Then you proceed to state regarding cross-section, "the first step is to convert the fraction to a decimal, then perform the unit conversion". Regrettably, imperial sized o-rings with fractional nominal cross-sections do not have that measurement as their actual cross-section. You chose a non-existent imperial nominal cross section (1/3 inch) and ended up with 8.5mm as the wanted metric size. Sorry, imperial standard o-rings allow for a 10% compression, so 1/8 inch o-rings have a measured 0.139 inch cross- section, 1/4 inch o-rings have a measured 0.275 inch cross-section. See this typical reference chart:
https://www.marcorubber.com/o-ring-size-chart-british.htm

4) Metric o-rings however are normally described by their actual dimensions, therefore do not have that 10% assumption of installed compression. Hence, in the case of your mythical 1/3 inch imperial o-ring's nominal cross-section, its actual free cross-section would be 0.366 inches or ~9.3mm, not ~8.5mm.

5) Lower down, you imply that Fahrenheit degrees are imperial units and that Celsius degrees are metric units. No, neither of those measurement systems has any relationship with the inch or the meter. But your conversion formulae are spot-on.

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Join Date: Dec 2017
Posts: 22
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#2
In reply to #1

Re: Translating Imperial Units to Metric Sizes in O-Rings

02/07/2019 7:50 AM

Relativity PL,

Good catch. I would like to know who the 'Sponsored Content' author is. They don't appear to have a review process before publication and must have assigned this task to an intern. Anyone familiar with O-rings would know the difficulty of finding its center with any accuracy. That is why O-rings are sized & measured by diameter. Although this author is fair with basic algebra, they have very little understanding of definitions (e.g., diameter vs radius) and its applications to the real-world.

Good luck in keeping the Center of the Known Industrial Universe focused.

Regards,

__________________
"The lazy mind mistakes words for realities."
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