Sucker Rod Coupling

Class T and Class SM Couplings

API 11B defines two coupling classes distinguished by surface treatment and intended wear performance. Class T is the standard coupling, made of the same material as the rod and phosphate or otherwise treated for corrosion protection. Class SM is a spray metal coupling: a hard, wear resistant metal coating, typically a nickel based or tungsten carbide alloy, is applied to the outside diameter. The purpose is to resist the abrasive wear that occurs where the coupling rubs against the tubing wall, particularly in deviated or horizontal wells, and to reduce the risk of galling when the coupling is repeatedly made up and broken out. Class SM couplings are longer than class T for the same rod size, and they wear the tubing less than a bare coupling in most circumstances, but they cost more. In a horizontal well the choice between them is often driven by a wear prediction based on dogleg severity and the number of pumping cycles.

Full Size and Slim Hole

Sucker rod couplings are supplied in full size and slim hole, also called slim line. A full size coupling has its outside diameter at or slightly above the API specified value and gives the greatest thread engagement and strength. A slim hole coupling has a reduced outside diameter, which increases the annular clearance between the coupling and the tubing wall. This matters in two situations: in wells with small clearance between the rod string and the tubing, where the larger coupling would rub and cause wear, and in wells where the annulus carries the produced fluid and a larger coupling increases the flow restriction. The trade-off is that a slim hole coupling has less wall thickness and therefore lower torsional and tensile capacity, so it should not be used where the rod string experiences high torque or heavy load. Confirm the required type when ordering, since the two are not interchangeable.