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What is the difference between a no-hub coupling (corrugated metal sleeve): enter image description here

Versus a shielded no-hub (smooth metal sleeve):

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Then there is a flexible coupling, which oddly is not called any kind of "no-hub" even though the connection style is exactly the same, but without metal sleeving, hence flexible:

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There is a significant price difference between the no-hub and shielded no-hub, with the latter more than double in price. Odd, since I would have thought corrugated metal should be stronger and also cost more due to the required processing.

Most of all, how are no-hubs and shielded no-hubs different?

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The simple answer is that there are many types of couplings for many different pipe types and appliction. Shielded couplings are stronger, and are load tested to be used in the ways that code permits.

You can see all the types of couplings and ways that they're used in the Fernco product catalog: https://www.fernco.com/literature-downloads/product-line-catalog

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    This answer describes the general function, not the physical differences between the no-hubs in question. Adding to the confusion, Fernco categorizes all their no-hubs as flexible couplings, whether bare, sleeved, corrugated, etc.
    – adatum
    Jul 15, 2023 at 17:45
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I've never heard the term "no-hub coupling" used except in regard to the couplings used with no-hub cast iron pipe. They are cheaper than "shielded couplings" because they contain less rubber and metal than is typical of other sorts, and they are produced and used in much greater quantities than the other sorts, as no-hub DWV installations are all you will find in any newer non-residential building in most areas.

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