Understanding Durometer

 

You’ve heard of odometers, speedometers, altometers (lots of –ometers). But what, pray tell, is durometer? Hint:  It’s not an instrument you slap on the inside of your cockpit.

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Figure 1 – Where to find durometer or “Hardness Rating” on an O-ring product page on SkyGeek.com (CLICK TO ENLARGE)

Basically, durometer refers to both the measuring device and the measurement of a material’s hardness. In fact, “duro” is Latin for “hard/tough.”  The “-ometer” refers to the fact that it is based on a scale; it is a measurement of some kind. Durometers indicate a material’s resistance to permanent indentation. In the aviation industry it often corresponds to products with an elastomer, polymer, or rubber composition. This is especially true when dealing with O-rings.

You will find specifications on the product pages of many O-rings we sell. Durometer is synonymous with “Hardness Rating.” (See Figure 1)

So why is this important? When applying an O-ring to a part of your aircraft that requires one, its material composition will give you an indication of its function and durability. Some assemblies call for harder elastomers, others softer. It’s the durometer that will indicate if the O-ring is the right fit.

For a brief overview of durometer, you can watch this informative Youtube clip. The content is great at giving a casual viewer a basic understanding of the topic. However, be warned: the speaker in the video is weird and his mannerisms may or may not be amusing.

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Shore Durometer Chart courtesy of the incredibly useful blog site from Alan Garratt. (CLICK TO ENLARGE)

Another aspect of durometer you may be wondering about is the use of the word Shore. Not only does Shore refer to the creator of the durometer, Albert Shore (1876-1936), it also is used in classifying the types of durometer scales.

There are many types but the two most common are Type A and Type D. According to Alan Garratt, “There is a simple logic laid behind the sequence of the early Shore scale letters and how they evolved to measure harder and harder materials. Later the need to measure materials softer than the range of Shore ‘A’ accuracy proved more difficult to fit into this progressive sequence of lettering.”

Garratt’s blog, Shore Durometer History,  is an excellent resource on the topic and is shore sure to provide those technically-inclined geeks out there with information on the development of this system. In particular I recommend reading the section “Evolution of the Shore Scales.” The bulleted points briefly explain why the classification is the way it is. Even better – there is a convenient chart of the various scales to compare and contrast (as seen above).

As mentioned, Type A is considered perhaps the most common of the Shore durometer scales. But within that type is a range. Numbers on this range go from softer to harder as they increase. Still, that may be too vague to understand so we retrieved this comparative list from Mykin Inc. The list provides real-world examples associated next to the numbers to give you a better idea.

SHORE

REAL-WORLD EXAMPLE

20A

Rubber Band

40A

Pencil Eraser

60A

Car Tire Tread

70A

Running Shoe Sole

80A

Leather Belt

100A

Shopping Cart Wheel

 

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About Skylar
Skylar

Woah! This is Skylar. I am THEE Sky Geek. I like planes, trains, and automobiles, but really just planes. I love aviation and anything related to it. If you shop on our site you will see my mug plastered throughout since I am the official mascot. I enjoy talking to Sandy, Otto, and Mach since they share my passion for flight. Researching for news and events and other information that affects our customers is what I can't wait to nose dive into. The sky's the limitless!