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Field guide

Pumping Unit Parts: From the Prime Mover to the Wellhead

Pumping unit parts run in one line from the power source to the well: prime mover, gearbox, cranks with counterweights, wrist pins, pitman arms, equalizer, walking beam, Samson post, horsehead, bridle, carrier bar, polished rod clamp and polished rod. At the wellhead the rod passes through a stuffing box, a pumping tee and a rod blowout preventer. 1613

By SRABS Engineering ·
Article / Pumping unit components at a glance

Pumping unit components at a glance

A beam pumping unit has two groups of components. The drive line, from the prime mover to the horsehead, turns rotation into a stroke. The polished-rod connection, from the bridle to the stuffing box, hangs the rod string from that stroke and seals the well around it. Many failures start where one part joins the next. 16816

This guide walks the stack in the order the load travels: what each part does and how it fails. For how the linkage moves, unit types, ratings, sizing and balancing, see the pumpjack guide. For the rods and the pump below the wellhead, see the sucker rod pump guide.

schematic · beam pumping unit, parts and geometries
FIG. J The pumpjack: parts of a conventional unit and the other geometriesSide elevation of a conventional pumpjack. A prime mover drives a gear reducer that turns two cranks with counterweights. Pitman arms connect the cranks to an equalizer at the rear of the walking beam, which rocks on a saddle bearing at the top of the Samson post. The horsehead at the front of the beam carries the bridle and carrier bar, which hold the polished rod; a stuffing box seals the rod at the wellhead. Below, three small elevations: a front-mounted unit with the beam pivoted at its rear end, an air-balanced unit with a compressed-air cylinder in place of counterweights, and a long-stroke tower unit with a drum, belt and counterweight.FIG. JTHE PUMPJACK — CONVENTIONAL UNIT, SIDE ELEVATIONCONVENTIONAL · CLASS I12345678910ITEMPARTS LIST1PRIME MOVER2GEAR REDUCER3CRANK + COUNTERWEIGHT4PITMAN ARM5EQUALIZER6WALKING BEAM7SAMSON POST8HORSEHEAD9BRIDLE + CARRIER BAR10POLISHED ROD+ stuffing boxOTHER GEOMETRIES · ▲ FULCRUMFRONT-MOUNTEDClass III leverAIR-BALANCEDair cylinderLONG-STROKEtower + beltDWG FIG. JSCALE NTSSRABS
FIG. J The pumpjack: parts of a conventional unit and the other geometriesSide elevation of a conventional pumpjack. A prime mover drives a gear reducer that turns two cranks with counterweights. Pitman arms connect the cranks to an equalizer at the rear of the walking beam, which rocks on a saddle bearing at the top of the Samson post. The horsehead at the front of the beam carries the bridle and carrier bar, which hold the polished rod; a stuffing box seals the rod at the wellhead. Below, three small elevations: a front-mounted unit with the beam pivoted at its rear end, an air-balanced unit with a compressed-air cylinder in place of counterweights, and a long-stroke tower unit with a drum, belt and counterweight.FIG. JTHE PUMPJACK12345678910ITEMPARTS LIST1PRIME MOVER2GEAR REDUCER3CRANK + COUNTERWEIGHT4PITMAN ARM5EQUALIZER6WALKING BEAM7SAMSON POST8HORSEHEAD9BRIDLE + CARRIER BAR10POLISHED ROD+ stuffing boxOTHER GEOMETRIES · ▲ FULCRUMFRONT-MOUNTEDClass III leverAIR-BALANCEDair cylinderLONG-STROKEtower + beltDWG FIG. JSCALE NTSSRABS
Article / Prime mover, gearbox and counterweights

Prime mover, gearbox and counterweights

The prime mover, an electric motor or an engine, drives the gearbox through a belt. The gearbox turns that into slow, high-torque rotation of the crankshaft. Each end of the crankshaft carries a crank arm and a counterweight, and the counterweights offset the weight of the sucker rods and the fluid column. 14

The counterweights are lifted on the downstroke and give that energy back on the upstroke, so the motor and gearbox don't lift the full load alone. One patent describes counterweights that "store energy by being lifted higher during the downward portion of the pumping cycle." 4 That answers a common fill-in-the-blank question: the counterweight is used to offset the weight of the column of sucker rods and fluids.

What fails here is usually the setting, not the steel. Peak crankshaft torque depends on how well the unit is balanced, and that peak decides the smallest acceptable gearbox and motor. 5 Each well has a counterweight setting that evens out the largest upstroke and downstroke torques, and the gap between the ideal and the actual setting has to be managed for the life of the well. 5 Fully loaded motors use less electricity, so an oversized motor wastes power. 5 See counterbalance and gearbox in the glossary, and the pumpjack guide for balance checks.

Article / The wrist pin: where the pitman meets the crank

The wrist pin: where the pitman meets the crank

The wrist pin is the pin that joins each pitman arm to its crank arm. It passes through a bore in the crank and is held by a lock nut. Its known failures are the nut backing off under vibration, which can drop the pin, and wear that enlarges the crank bore until the crank has to be replaced. 23

Each pitman attaches to the walking beam at the tail bearing and to a crank arm at a wrist pin. 2 Some patents call the same joint the crank pin bearing. 1 The usual design is a pin through a tapered bore in the crank, secured by a lock nut on its outer end. 3 Vibration loosens the nut, which "frequently results in an inadvertent loss of the wrist pin," and wear enlarges the crank bore until the whole crank has to be replaced. 3 As general field practice, wrist pin nuts are checked on routine unit inspections.

Article / Pitman arms, equalizer, walking beam and Samson post

Pitman arms, equalizer, walking beam and Samson post

The pitman arms carry the crank motion to the equalizer, a bar that pivots on the tail bearing at the rear of the walking beam. The beam rocks on the saddle bearing on top of the Samson post, which acts as the fulcrum. Wear in the saddle or tail bearing lets the rod drift out of line over the well. 18

Their wear often shows up at the wellhead, not at the beam: saddle bearing and tail bearing wear, or a damaged walking beam, can keep the polished rod from traveling straight up and down. 8 A unit set correctly can also move as the ground under its base settles over the following months, so check alignment again after a unit is set or swapped. 8

Article / Horsehead and bridle

Horsehead and bridle

The horsehead is the curved head on the front of the walking beam. The bridle, made of wire rope, hangs from its arc-shaped face and carries the carrier bar. The curve keeps the bridle hanging straight over the well as the beam rocks. A badly reinstalled head or an uneven bridle bends the polished rod. 18

The horsehead comes off for well service and is sometimes put back wrong. 8 A bridle installed longer on one side, or fraying on one side, lets the carrier bar lean and puts a bending moment in the polished rod. 8 The bridle leaves wear marks on the face of the horsehead; marks that have shifted mean something in the unit has moved. 8

Article / Carrier bar vs polished rod clamp: what's the difference?

Carrier bar vs polished rod clamp: what's the difference?

The carrier bar is the horizontal bar that hangs from the bridle. The polished rod passes through it without being gripped. The polished rod clamp is the bolted clamp that grips the polished rod and sits on top of the carrier bar, so the bar carries the rod string and fluid load through the clamp. 67

In short: the carrier bar is part of the pumping unit, and the clamp ties the rod string to it.

  • Carrier bar. Hung by the bridle cables from ears on each end. Its function is "supporting a polish rod clamp." 6 Some texts call it the hanger bar. 17
  • Polished rod clamp. Fixed to the polished rod so the rod cannot slide back down through the carrier bar, which makes the rod string move with the unit. 7

Because the clamp sets where the rod hangs, it also sets pump spacing, so the plunger moves freely and doesn't hit bottom on the downstroke. Spacing is changed by moving the clamp along the rod or by putting a spacer between the clamp and the carrier bar. 7 A second clamp can act as a backstop: one patent sets a secondary clamp just above a lower bar, so it takes the load if the primary clamp or the rod fails. 6

What goes wrong here. Carrier bars must be level: under load, the bottom of the clamp is forced flat against an unlevel bar and bends the rod. 9 A worn bar develops a dished spot under the clamp with the same result. 8 Clamp bolts should be torqued to the maker's figure, with clamp, rod and bar clean where they touch. 8 A pump-off controller load cell sitting down into an oversized hole in the bar also bends the rod; a leveling plate fixes it. 8 (For what that load cell is used for, see the pump-off controller guide.)

Article / The polished rod

The polished rod

The polished rod is the top joint of the sucker rod string. Its smooth surface lets the stuffing box seal around a moving rod. It carries the whole rod string, the fluid load and the dynamic loads, and when it breaks, it almost always breaks by fatigue at or just below the polished rod clamp. 8911

The polished rod is usually the strongest member of the rod string, yet polished rods can fail regularly while the sucker rods below them don't. 9 Two short-course papers explain why:

  • Bending, not tension. Almost all polished rod breaks are fatigue failures at the bottom of the clamp, 9 and nearly all trace back to misalignment. With the rod off-center, the bend concentrates just below the clamp as the carrier bar comes down toward the stuffing box. 8
  • Where the bending comes from. An unstable foundation, bent structural members, a badly adjusted horsehead, a wellhead stack out of plumb or an unlevel carrier bar. 9
  • Clamping on spray metal. A clamp on the hard coating cracks it and can start a failure. These rods commonly have 4 ft of bare rod for the clamp; the paper prefers 6 ft. 8
  • Liners and couplings. Over-tightened liner set screws create a stress riser, and corrosion can hide under a liner. A regular sucker rod coupling in place of a polished rod or combination coupling also causes failures. 8 That connection is one of the highest loaded points in the system. 10

A polished rod that parts near the top of the stroke drops the whole string the length of the stroke onto the pump, and the shock carries into the seating nipple and the tubing. 8 The rod can also fall through the stuffing box and leave an opening for well fluid to spill. 15 For the rest of the string, see why sucker rods part.

Article / The rod pump wellhead: stuffing box, pumping tee and rod BOP

The rod pump wellhead: stuffing box, pumping tee and rod BOP

A rod-pumped wellhead stacks, from the bottom up, the tubing head adapter, a production blowout preventer, the flow tee (pumping tee) and the stuffing box, all on one bore. The rod BOP can shut in around the rod; the tee sends produced fluid to the flow line; the stuffing box seals the moving polished rod. 1314

  • Rod BOP. Shuts in the well while a rod is in the hole. Two opposed elastomer ram blocks close around the polished rod and seal the space between the rod and the tubing. 14
  • Pumping tee (flow tee). Connects the production flow line to the tubing. Usually a separate piece, bolted flange to flange to the BOP. 14
  • Stuffing box. Secured to the wellhead around the polished rod, with packing rings that bear on the rod. 12

What goes wrong here. The rod reciprocates through the packing, so the packing wears; sand in the crude and poor alignment make it worse. 12 An out-of-line rod presses on one side of the packing, and "wear and leakage soon occur." 13 Repeated repacking, or packing worn on one side, is a good sign of an alignment problem on the unit. 8 The stack itself must be straight: API Recommended Practice 11B suggests the wellhead assembly be vertical to within 1 1/2 in in 20 ft. 9

Article / A walk-around check from the ground

A walk-around check from the ground

Most polished-rod and stuffing-box trouble can be seen without climbing. Watch the rod from the side and the front, check the carrier bar with a spirit level, look at the bridle and its marks on the horsehead, and stop the unit at the top and bottom of the stroke to check the rod for plumb. 8

  1. From the side, check that the polished rod tracks straight up and down.
  2. From the front, check that the rod is centered on the horsehead and the bridle marks haven't moved.
  3. Check the bridle for even length and damage, and the carrier bar with a spirit level.
  4. Stop the unit at the top and bottom of the stroke and check the rod for plumb from two sides.

If any of these look wrong, have the unit inspected for the cause. 8 The surface card from the polished rod load cell is the other view of the same parts; see the dynamometer card guide.

Article / Where SRABS fits

Where SRABS fits

Nothing in this stack changes with the SRABS Pump. It works downhole and it's a drop-in to API spec, so the pumping unit, carrier bar, clamp and wellhead described here stay as they are. The surface equipment you run today stays in place.

The SRABS Pump is "a drop-in anti-buckling rod pump that keeps the string in tension where it would otherwise buckle." See how it works, the system and the rod failures it targets on solutions. 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.

Article / The bottom line

The bottom line

A pumping unit is a chain of parts, and the polished rod sits where the chain meets the well. The drive line wears at its pins and bearings. The polished-rod connection fails mostly through misalignment: an uneven bridle, a worn or unlevel carrier bar, an over-tightened clamp or a crooked wellhead stack. Check them from the ground, and fix the alignment before replacing the rod again.

If the surface stack is in order and you're still pulling rods, look downhole next. Test that on one well.

Loan us a well →
Article / FAQ

FAQ

What are the main pumping unit components?

The prime mover, gearbox, cranks and counterweights, wrist pins, pitman arms, equalizer, walking beam, Samson post, horsehead, bridle, carrier bar, polished rod clamp and polished rod. At the wellhead the rod runs through the stuffing box, the pumping tee and the rod blowout preventer.

What is the difference between a carrier bar and a polished rod clamp?

The carrier bar hangs from the bridle and has a hole the polished rod passes through. The polished rod clamp grips the rod and sits on top of the carrier bar. The bar carries the load; the clamp transfers the rod string's weight onto it and sets where the rod hangs.

What is a wrist pin on a sucker rod pumping unit?

The pin that connects each pitman arm to its crank arm. It sits in a bore in the crank and is held by a lock nut. If the nut loosens the pin can be lost, and wear can enlarge the crank bore until the crank has to be replaced.

What is used to offset the weight of the column of sucker rods and fluids?

The counterweights on the cranks, or a compressed-air cylinder on an air-balanced unit. They are lifted on the downstroke and help lift the load on the upstroke, which lowers peak gearbox torque.

What are all the components on a wellhead for a rod pump?

From the bottom up: the casing head and tubing head, a production rod blowout preventer, the flow tee (pumping tee) that sends fluid to the flow line, and the stuffing box that seals around the polished rod.

Why do polished rods break at the clamp?

Bending fatigue. When the rod isn't centered over the well, or the carrier bar isn't level, the bend concentrates just below the clamp as the carrier bar nears the stuffing box. Over-tightened clamps and clamping on spray metal add stress at the same spot.

Article / Sources

Sources

  1. US Patent 10,815,984, "Beam pumping unit with geometry optimized for bearing stress reduction" (2020). https://patents.google.com/patent/US10815984
  2. US Patent 4,703,665, "Well pumping unit" (1987). https://patents.google.com/patent/US4703665
  3. US Patent 4,445,800, "Wrist pin" (1984). https://patents.google.com/patent/US4445800
  4. US Patent 3,971,213, "Pneumatic beam pumping unit" (1976). https://patents.google.com/patent/US3971213
  5. Laine, R.E., Keating, J.F. and Jennings, J.W., "Counterweight Torque (and Counterbalance Equivalent) Versus Gearbox and Motor Loads," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/counterweight-torque-and-counterbalance-equivalent-versus-gearbox-and-motor-loads
  6. US Patent 4,286,656, "Composite carrier bar device" (1981). https://patents.google.com/patent/US4286656
  7. US Patent 9,963,945, "Hydraulic pump spacing device and system" (2018). https://patents.google.com/patent/US9963945
  8. Sands, R., "Polished Rod Failure Prevention," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/polished-rod-failure-prevention
  9. Angelo, L., "How to Minimize Polished Rod Breaks," Southwestern Petroleum Short Course, 1994. https://www.swpshortcourse.org/paper/how-minimize-polished-rod-breaks
  10. Link, B. and Williams, B., "Modified Polished Rod with Sucker Rod End: Ensuring a Stronger Connection," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/index%2Ephp/paper/2024030-modified-polished-rod-sucker-rod-end-ensuring-stronger-connection
  11. SLB Energy Glossary, "polished rod." https://glossary.slb.com/en/terms/p/polished_rod
  12. US Patent 7,861,771, "Rod pump stuffing box" (2011). https://patents.google.com/patent/US7861771
  13. US Patent 6,637,509, "Wellhead stuffing box support assembly" (2003). https://patents.google.com/patent/US6637509
  14. US Patent 5,617,917, "Integral blowout preventer and flow tee" (1997). https://patents.google.com/patent/US5617917
  15. US Patent 4,377,206, "Well blowout prevention device" (1983). https://patents.google.com/patent/US4377206
  16. SLB Energy Glossary, "beam pump." https://glossary.slb.com/en/terms/b/beam_pump
  17. US Patent 6,135,670, "Polished rod clamp" (2000). https://patents.google.com/patent/US6135670