Sucker Rod Grades and Rod String Design

Sucker rod grades: C, K and D

The API steel grades are defined by tensile strength. Grade C is a carbon steel and Grade K an alloy steel, both rated at 90 to 115 kpsi ultimate tensile strength. Grade D, made in carbon and alloy versions, is rated at 115,000 to 140,000 psi. Higher-strength rods beyond Grade D are made by manufacturers to their own specifications. ¹²

The grade sets the ceiling, not the life. A rod string is subjected to cyclic loads as it moves up and down, and such loads often lead to fatigue failure; corrosive attack by well fluids, or abrasion of the rods against the tubing, may hasten it. ¹ So a stronger grade is only an improvement if the well's corrosion and contact problems are under control too. The why sucker rods part guide covers those failure modes.

The patent literature puts it simply: other things being equal, a stronger sucker rod is preferred over a weaker one. ¹ The catch is "other things being equal". The rest of this guide is about the other things.

Steel, fiberglass and continuous rod

Most strings are steel sucker rods joined by couplings. Fiberglass rods trade steel's toughness for corrosion resistance and lighter weight, but don't tolerate compression. Continuous rod removes the couplings along the string, which spreads side load in deviated wells. ⁸⁹

Fiberglass. Fiberglass/polyester composites have much greater resistance to corrosion than unprotected steel, and the patent describing them notes that all-steel strings in corrosive wells have been known to last less than three months. ⁸ Lighter rods allow a smaller pumpjack and reduce gearbox loading. ⁸ The weakness is compression: compression impact can cause fiber buckling, after which a pultruded rod is usually rendered useless. ⁸ The steel couplings at the rod ends are exposed to sour fluid and subject to corrosion, and the rod is bonded to its end fitting on only one surface. ⁸

Continuous rod. A continuous string has no couplings between the polished rod and the pump. In deviated wells that matters, because the side load is distributed over an increased area of contact, which results in longer run times; one study converted five high-failure deviated wells from conventional rod to continuous rod for that reason. ⁹

Tapered rod strings: why the rods get thinner with depth

The top of a rod string carries the weight of every rod below it plus the fluid load, so it needs the largest rods. A tapered string uses sections of decreasing diameter from the surface down, sized so that the stress at the top of each section is about the same. ⁶⁷

The upper end of a sucker rod string must support more weight than the lower end, so the string is made larger near the surface and smaller near the pump. ⁷ A single-size string would either be oversized at the bottom or overstressed at the top.

The design rule for a tapered string is equal stress: each section is sized so that the equivalent stress at the top of the section matches the other sections. ⁶ Below the smallest rods, sinker bars are often added to keep the bottom of the string out of compression, which is the buckling problem.

Taper percentages: where they come from

A taper percentage is the share of the string length given to each rod size. The published values in API RP 11L come from empirical formulas made for hand calculation. Deeper, higher-volume wells need tapers designed for the well, because the right split changes with stroke length and pump size. ⁵⁶

The taper tables in API RP 11L were a practical tool: current RP 11L taper percentages are based on empirical formulas that simplified the calculations so they could be made by hand. ⁵ As wells got deeper and pumped more volume, rod loads and stresses rose, and more accurate taper percentages became necessary; with modern computing, a taper can be designed for each well. ⁵

The right split isn't fixed even for a given rod combination. Research on taper design found that a shorter stroke length gives a greater rod taper percentage, and that the share of the top section grows with plunger diameter. ⁶ Use RP 11L as a starting point and design software for the final string; this guide doesn't reproduce the table.

The modified Goodman diagram, in plain terms

Rods fail in fatigue from the stress range, not just the peak. The modified Goodman diagram turns that into a limit: the allowable maximum stress is a quarter of the rod's minimum tensile strength plus 0.5625 times the minimum stress, all multiplied by a service factor. ³

The formula, as manufacturers publish it for API grades: ³

Sa = (T/4 + 0.5625 × Smin) × SF

• Sa: the maximum allowable stress.
• T: the rod's minimum ultimate tensile strength.
• Smin: the minimum stress in the stroke, calculated or measured.
• SF: the service factor (next section).

In plain terms: if the rod's stress fell all the way to zero on every stroke, you could load it to a quarter of its tensile strength. Every psi of minimum stress you keep in the rod lets the maximum go 0.5625 psi higher. That is the logic of the diagram: the stress range, not just the peak, drives fatigue. A string is acceptable when the maximum stress at the top of each section stays below Sa for that section's minimum stress.

The diagram has been used since the 1960s to set allowable loads for API grade rods, and it is considered very conservative. One short-course paper combining field review and fatigue testing proposes dividing tensile strength by 2.8 instead of 4. ⁴ That is a proposal in the literature, not the published rule; design to the formula above unless your rod supplier says otherwise.

Service factors: derating for corrosion

The service factor scales the allowable stress down for the environment the rods work in. A value of 0.9 is recommended for corrosive environments, so a corrosive well's rods are allowed only 90% of the stress the same rods would carry in clean fluid. ³

Because SF multiplies the whole allowable line, it lowers every point on it. For the same loads, a string in corrosive service runs closer to its limit, which is why corrosion control and rod design go together. Check your rod supplier's service factor guidance for the specific fluid.

Couplings: the weakest link in the string

A coupling has to carry the full rod load through a thinner section than the rod body. A slimhole coupling with a Grade D rod withstands only about 89% of the stress the rod could sustain alone, and a full-size coupling with a high-strength rod about 85%, so couplings can be the weakest link. ¹⁰

• Full-size vs. slimhole. Slimhole couplings fit tighter tubing clearances, but the reduced wall thickness reduces the effective stress area. A 1-in slimhole coupling with a Grade D rod can typically withstand only about 89% of the stress the rod could sustain alone. ¹⁰
• High-strength rods. Stronger rods raise the stakes for couplings: a 7/8-in full-size coupling can withstand only about 85% of the stress a high-strength rod could withstand alone. ¹⁰ Match coupling strength to rod grade.
• Coatings. Spray-metal coatings are applied to reduce wear, but such coatings are themselves susceptible to stress cracking, and the coating tends to become brittle during later heat treatment. ¹⁰
• Make-up. Most connection failures come from loose couplings and improper make-up. That's covered in why sucker rods part.

Putting it together: a rod string design checklist

Design the string from the load and the environment, then check it: pick a grade for the load and corrosion, choose the material, size the taper so the top of each section sees similar stress, check every section against the modified Goodman line with the right service factor, and match the couplings to the rods. ³⁵⁶¹⁰

1. Know the loads. Peak and minimum polished-rod load from the dynamometer card or a predictive design (see the dynamometer card guide).
2. Pick the grade for the stress level and the well's corrosion. ¹²
3. Pick the material: steel, fiberglass with its compression limits, or continuous rod for deviated wells. ⁸⁹
4. Size the taper for equal stress at the top of each section; start from RP 11L and refine for the well. ⁵⁶
5. Check Goodman at the top of every section, with the service factor for the fluid. ³
6. Match the couplings to the rod grade, and watch slimhole limits. ¹⁰
7. Check the bottom of the string for compression; see sinker bars and buckling.
8. Make sure the surface unit can carry it. Peak polished-rod load must stay inside the pumpjack's structure rating; see the pumpjack guide.
9. Re-check when the well changes. Fluid level, water cut and pumping speed change the loads the string was designed for.

Where SRABS fits: "The SRABS Pump is a drop-in anti-buckling rod pump: it solves buckling at the source instead of adding weight to fight it." See how it works, the system, and the modelled scenarios on cases. To estimate what fewer rod failures would mean on your fleet, run the ROI calculator. Its default 100-well scenario is modelled, not a field result.

The bottom line

A rod string is a fatigue design, not just a strength design. Grade sets the ceiling, the taper shares the load, the Goodman check with a service factor tells you whether each section will last, and couplings decide whether the joints keep up with the rods. Design for the well's loads and fluid, and re-check when the well changes.

If your rod failures cluster in the same section or the same depth, start with the string design and the bottom of the string, then test closed-loop control on one well.

FAQ

Sources

1. US Patent 4,486,249, "Method of making class D sucker rods" (1984). https://patents.google.com/patent/US4486249
2. Tenaris, "API Sucker & Pony Rod" product data sheet (SRAPI R01, 2025). https://www.tenaris.com/media/x2wlmird/srapi_r01.pdf
3. Tenaris, "API Grade Stabilizer Bars" product data sheet (SRSTABAPI R18, 2025). https://www.tenaris.com/media/o1boyhpr/srstabapi_r18.pdf
4. Hein, N., "Modifying the Modified Goodman Diagram," Southwestern Petroleum Short Course. https://swpshortcourse.org/paper/2014010-modifying-modified-goodman-diagram
5. Gault, R.H., "Rod Stresses From RP11L Calculations," Southwestern Petroleum Short Course. https://swpshortcourse.org/paper/rod-stresses-rp11l-calculations
6. Liu, C.H., "Sucker rod string design of the pumping systems," Ingeniería e Investigación 35(2), 2015. https://doi.org/10.15446/ing.investig.v35n2.48667
7. US Patent 8,113,277, "Sucker rod end fittings and method of using same" (2012). https://patents.google.com/patent/US8113277
8. US Patent 4,602,892, "Sucker rod" (1986). https://patents.google.com/patent/US4602892
9. Pons, V., Weaver, A.M., Guillotte, L.J. and Wlazlo, A., "Continuous Rod: Improving Run Times in Unconventional Wells," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/25-continuous-rod-improving-run-times-unconventional-wells
10. US Patent 5,405,457, "High strength sucker rod coupling" (1995). https://patents.google.com/patent/US5405457