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Sinker Bars and Rod Guides: Do They Stop Rod Buckling?

How sinker bars move the neutral point and keep a sucker rod string in tension, why rod guides don't stop buckling, and how to choose between them.
By SRABS Engineering ·
Article / Do sinker bars prevent rod buckling?

Do sinker bars prevent rod buckling?

Sinker bars reduce rod buckling but don't remove its cause. Heavy bars placed directly above the pump add weight at the bottom of the string, which keeps more of the string in tension and pushes the compressed zone down into stiffer bars that resist bending. Sized wrong, they waste energy or just move the buckling up the string. 12

On the downstroke, forces at the pump resist the plunger's fall and push upward on the bottom of the rod string. One sinker-bar patent describes it simply: the lower part of the string "is forced upward and can be compressed," and that compression makes the string bend. 1 Rods are slender and built to carry tension, so a compressed rod bows sideways into the tubing.

Sinker bars are the most common answer, and for good reason. They're simple, they install with the rest of the string, and every rod crew knows them. But they are a way of managing buckling, and it pays to know their limits before relying on them.

Article / How sinker bars work: weight and the neutral point

How sinker bars work: weight and the neutral point

Every rod string on the downstroke has a neutral point: the depth where it is in neither tension nor compression. Above it the rods hang in tension; below it they're compressed. Sinker bars add weight near the bottom so the neutral point moves down, and the compressed section falls on heavy, large-diameter bars instead of slender rods. 123

The neutral point is a general idea for any string of tubulars: the point with neither tension nor compression, with compression building below it and tension building above it. 3 For a sucker rod string, what matters is where that point sits relative to the pump on the downstroke.

Sinker bars change two things:

  1. They add weight at the bottom. A sinker-bar patent describes them as helping "to keep the rod string in tension by providing extra weight." 1
  2. They're stiff. Sinker bars are heavy, large-diameter rods. 2 A larger diameter resists buckling far better than a slender rod under the same compressive load (bending stiffness grows steeply with diameter), so whatever compression is left falls on the section best able to take it.
schematic · sinker bars vs. rod guides
FIG. S Sinker bars and rod guides, section detailsThree sucker-rod strings in tubing, in section. A plain string buckles into a helix below the neutral point. A string with sinker bars moves the neutral point down into the heavy bars, so the slender rods above stay in tension. A string with rod guides still goes into compression; the rod bows between the guides and stress concentrates at the guide edges.FIG. SSINKER BARS VS. ROD GUIDES01 · PLAIN STRING02 · SINKER BARS03 · ROD GUIDESNEUTRAL POINTBUCKLESSINKER BARSNEUTRAL POINTTENSIONROD GUIDEEDGE STRESSNEUTRAL POINTcompression below the neutral pointDWG FIG. SSCALE NTSSRABS
FIG. S Sinker bars and rod guides, section detailsThree sucker-rod strings in tubing, in section. A plain string buckles into a helix below the neutral point. A string with sinker bars moves the neutral point down into the heavy bars, so the slender rods above stay in tension. A string with rod guides still goes into compression; the rod bows between the guides and stress concentrates at the guide edges.FIG. SSINKER BARS VS. GUIDES01 PLAIN02 SINKER BARS03 GUIDESNEUTRAL POINTCOMPRESSIONGUIDE-EDGE STRESSSINKER BARS
Article / The limits of sinker bars

The limits of sinker bars

Sinker bars trade one load for another. Too short a section and buckling simply starts above the bars; too long and the extra weight costs energy on every stroke and loads the surface unit. And they only manage the compressive force, they don't remove it. 1

  • Too short: the neutral point stays above the bars and the rods above them still buckle. The sinker-bar patent warns that a section that is too short causes rod buckling above it. 1
  • Too long: the same patent warns that an over-long section means excessive energy usage. 1 More weight also means higher peak polished-rod load, which the pumping unit and gearbox have to carry on every upstroke.
  • Design depends on conditions that change. The compressive load depends on pump friction, fluid friction through the traveling valve, viscous drag and fluid pound. As the fluid gets heavier, the well pumps off or gas rates change, the load moves and a sinker-bar design that was right at completion can be wrong a few years later.
  • Wear doesn't go away. Bars that still flex rub the tubing. Patent literature on sinker bars treats surface finish as a way to reduce abrasion and tubing wear, 1 which tells you the contact is still there.
Article / Rod guides: stopping the wear, not the buckling

Rod guides: stopping the wear, not the buckling

Rod guides and centralizers hold the rods off the tubing wall, which protects the tubing from metal-on-metal wear. They don't stop buckling: the rod can still bow between guides, and compressive load raises stress at the guide edges. 2

Molded polymer guides became the standard fix for rod-on-tubing wear. 2 But a rod-stabilizer patent points out what happens under compression: with rigid guides holding the rod to the middle of the tubing, "the sucker rod buckles between the rod guides," and compressive loads on rods with centralizers increase stress at the edges of the guides. 2 Guides change where the rod touches the tubing; they don't change the force that bends it.

That makes guides a wear-protection tool. They work best alongside a design that keeps the compressed section short, and they're no substitute for one.

Article / Sinker bars vs. rod guides: how to choose

Sinker bars vs. rod guides: how to choose

Use sinker bars to manage how much of the string is in compression, and rod guides to protect the tubing where the string still touches it. Most buckling wells end up with both, plus operating changes to cut the downstroke load itself.

Sinker barsRod guides / centralizers
What it changesMoves the neutral point down; puts compression on stiff bars 12Holds rods off the tubing wall 2
Stops buckling?Reduces it, if sized right 1No: rods can buckle between guides 2
Main costWeight: energy and polished-rod load 1Edge stress under compression; guide wear and replacement 2
Main riskToo short: buckling above the bars. Too long: wasted energy 1False sense of security if compression stays high
NeedsA design based on actual downstroke loadsSpacing matched to where the string contacts the tubing

Cut the load, too. Whatever you install, the compressive force comes from the downstroke. Slower strokes reduce fluid friction, and stopping fluid pound removes its impact load. Check the dynamometer cards: a calculated pump card that drops below zero load on the downstroke means the bottom of the string is in compression.

Article / Designing buckling out instead

Designing buckling out instead

The alternative to fighting compression with weight is a pump that keeps the string in tension in the first place. With no compressed section, there's nothing for sinker bars to manage and nothing for guides to rub against.

That is the approach behind the SRABS Pump: a mechanically re-engineered sucker rod pump that keeps the rod string in tension where it would otherwise buckle. In the site's words, "the buckling problem, solved at the source." It's a drop-in to API spec. For the full mechanism walk-through see how it works; for the problem in depth, the sucker rod buckling guide.

To estimate what fewer rod failures would mean for your own fleet, run the ROI calculator. Its default 100-well scenario is modelled, not a field result.

Article / The bottom line

The bottom line

Sinker bars and guides manage buckling; they don't remove it. Bars move the compressed zone onto stiffer steel, guides protect the tubing where rods still touch it, and both need re-checking as the well changes. Keeping the string in tension at the pump removes the compressed zone altogether.

If you're adding weight and guides and still pulling rods, test that on one well.

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Article / FAQ

FAQ

What are sinker bars used for in a sucker rod string?

They're heavy, large-diameter bars placed directly above the pump. Their weight keeps more of the rod string in tension on the downstroke, and their stiffness means the bottom section resists buckling better than ordinary rods.

Do sinker bars prevent rod buckling?

They reduce it when sized correctly, by moving the neutral point down. A section that's too short leaves rods buckling above it; one that's too long wastes energy and adds load at the surface.

What is the neutral point in a rod string?

The depth where the string is in neither tension nor compression. Rods above it are in tension; rods below it are compressed, and that's where buckling happens.

Do rod guides stop sucker rod buckling?

No. Guides keep rods off the tubing wall, which reduces wear, but the rod can still buckle between guides and compression raises stress at the guide edges.

What's the downside of sinker bars?

Weight. More weight means more energy per stroke and higher polished-rod load on the pumping unit. Sinker bars also need a design based on downstroke loads that change as the well changes.

Is there an alternative to sinker bars?

Yes: keep the rod string in tension at the pump so it never goes into compression. That's the idea behind anti-buckling rod pumps.

Article / Sources

Sources

  1. US Patent 10,024,118, "Sinker bar" (American Sinker Bars LLC, 2018). https://patents.google.com/patent/US10024118
  2. US Patent 11,261,673, "Stabilizer for inhibiting sucker rod buckling during compression moments in artificial lift wells" (Black Mamba Rod Lift LLC, 2022). https://patents.google.com/patent/US11261673
  3. SLB Energy Glossary, "neutral point." https://glossary.slb.com/en/terms/n/neutral_point