Positive seal coupling system

A hollow coupling for joining together two pipe ends and having a longitudinal axis and an inner profile, the inner profile comprising:

• a plurality of serrations extending over at least a portion of the inner profile and bounded by a first smooth end surface and a second smooth central surface of the coupling;
• a first end serration bounding the plurality of serrations at a first end, and providing a transition from the first smooth end surface of the inner profile to the serrations;
• and
• a second end serration bounding the plurality of serrations at a second end, and providing a transition from the serrations to the second smooth central surface of the inner profile;
• the first end serration having a first radius and a second radius, the first radius originating at the first smooth end surface and curving radially inward relative to the longitudinal axis of the coupling from the first smooth end surface toward the plurality of serrations, and the second radius originating from the first radius, and curving radially outward relative to the longitudinal axis of the coupling from the first radius toward the plurality of serrations;
• and
• the second end serration having a third radius and a fourth radius, the third radius originating at the plurality of serrations and curving radially outward relative to the longitudinal axis of the coupling from the plurality of serrations toward the second smooth central surface, and the fourth radius originating from the third radius and curving radially inward relative to the longitudinal axis of the coupling from the third radius toward the second smooth central surface of the profile;
• the first and second radii of the first end serration, and third and fourth radii of the second end serration having rounded profiles that reduce stress concentrations in the coupling.

A coupling for joining together pipe ends and having a first side with a first end and a second side with a second end, the coupling having an inner profile comprising:

• first and second serrated sections proximate the center section on the first and second sides of the coupling respectively, the first and second serrated sections having a plurality of threads extending radially around the inner diameter of the coupling;
• first and second curved sections proximate the first and second serrated sections respectively, the first and second curved sections each having a surface that curves radially outward relative the longitudinal axis from the first and second serrated sections, respectively, toward the first and second ends of the coupling;
• first and second smooth sections adjacent first and second ends of the coupling;
• and
• first and second transition sections between the first and second curved sections and the first and second smooth section respectively, each of the first and second transition section having a surface that curves radially inward relative to the longitudinal axis from the first and second curved sections, respectively, to the first and second sections, the first and second transition sections having a smooth, rounded profile that minimizes stress concentrations.