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.

Rod couplings: class T or class SM

A coupling is where a rod string normally fails, and the two classes differ in wear protection rather than in strength. Class T is the standard coupling; class SM has a spray metal finish for wells where the string rubs against the tubing. Full size and slim hole versions exist where clearance matters.

Coupling size and thread, by rod size

A coupling is identified by its thread size, not by the rod size it connects. That is the source of the “1 in × 7/8 in” confusion on requisitions: the two numbers in a quotation may be a rod size and a thread size, or two different rod sizes in a combination coupling.

Source: API 11B coupling dimension and thread tables, cross-checked against published supplier catalogues. Confirm the current edition of API 11B, and confirm availability, before the sizes are written into a contract — slim hole availability in particular varies by size.

Combination couplings — for example 5/8 in × 3/4 in or 7/8 in × 1 in — are used to change rod size within a string. On a requisition, state both ends, because a combination coupling ordered as a single size will be made wrong.

Wear, failures and what actually prevents them

A coupling is a wear part as well as a connector. It rubs against the tubing on every stroke, and what fails first tells you which fix is needed.

This is why we ask what failed first. If the tubing is worn and the couplings are intact, changing the coupling class does not fix the wear — it moves it.

Packing, marking and what to state on the enquiry

• Rod size at each end, and both ends for a combination coupling.
• Coupling thread size, full size or slim hole.
• Class: T or SM, or the sour-service qualified equivalent where wet H2S is present.
• Quantity, and whether the couplings must match an existing string’s marking.
• Certificate: EN 10204 3.1 with heat number and mechanical results; EN 10204 3.2 where third-party countersignature is required.
• Packing requirement, since couplings are handled and stored before they reach the wellsite.
• Destination port and Incoterms.

Grades supplied

Standards, inspection and delivery

Download Sucker Rod Coupling data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.

Where to go next

• API 11B sucker rod – the rod grades every accessory below is matched to
• Polished rod – the surface rod at the top of the string
• Sucker rod guide – how centralisation is chosen for the well