Additional information and details on mechanical couplings are provided in Chapter 9 of the Plastic Pipe Institute (PPI) Handbook of Polyethylene Pipe.

Internal Stiffeners

The mechanical couplings generally utilize an internal stiffener within the relatively flexible PE pipe to support the pipe under the area of the seal ring and any gripping devices. Stiffeners are included in the compression and stab type coupling packages, but may need to be provided for other type couplings, such as the bolt type, as well as other applications. When connecting PE pipe to the bell end of a ductile iron or PVC pipe, it is recommended that a stiffener be added to the ID of the pipe to insure a good connection between the seal in the bell and the pipe. There are two types of stiffeners available on the market: a fixed diameter stiffener that matches the ID of the pipe being repaired (mainly used with smaller diameter service lines), and a split ring stiffener. Stiffeners are normally made of stainless steel and provide a thin yet strong pipe wall reinforcement without disturbing the flow characteristic of the pipe.