Talk to us
Article / hero
Field guide

How a Pumpjack Works: Parts, Unit Types and How They're Sized

A pumpjack is the machine at the top of a rod-pumped well that turns a motor's rotation into the slow up-and-down stroke of the rod string. A motor drives a gearbox, the gearbox turns two cranks, and the cranks rock a walking beam whose horsehead lifts and lowers the polished rod. Counterweights balance the load so the gearbox doesn't carry it all on the upstroke. Beam pumping units like this serve about two-thirds of artificially lifted oil wells. 19

By SRABS Engineering ·
Article / What is a pumpjack?

What is a pumpjack?

A pumpjack, or beam pumping unit, is the surface half of a sucker rod pumping system. It sits over the wellhead and moves the polished rod up and down, one stroke for every turn of the cranks. The rods carry that motion down to a pump near the bottom of the well. 12

The machine on the surface and the pump downhole are two halves of one system. This guide covers the surface half: the parts, how the linkage works, the main unit types, how units are rated and sized, and why counterbalance matters. For the rod string, the downhole pump and the pumping cycle, see the sucker rod pump guide. To see how rod lift compares with other methods, see artificial lift methods.

Article / Pumpjack parts, from the motor to the wellhead

Pumpjack parts, from the motor to the wellhead

A conventional pumpjack has ten main parts in its drive line: the prime mover, gear reducer, cranks with counterweights, pitman arms, equalizer, walking beam, Samson post, horsehead, bridle with carrier bar, and the polished rod sealed by a stuffing box at the wellhead. 1210

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
  1. Prime mover. Typically an electric motor or an internal combustion engine. 1
  2. Gear reducer (gearbox). Turns the motor's fast rotation into the low-speed, high-torque rotation of the crankshaft. 1
  3. Cranks and counterweights. A crank arm on each end of the crankshaft carries a counterbalance weight. 1 The weights offset the load hanging from the horsehead (see counterbalance below).
  4. Pitman arms. Each crank arm is pinned to a pitman arm, which pulls the rear of the walking beam up and down. 1
  5. Equalizer. Joins the two pitman arms at the rear of the beam, on a bearing usually called the tail bearing. 1
  6. Walking beam. The long beam that rocks on top of the Samson post.
  7. Samson post. The structure that holds up the beam. Its top acts as the fulcrum, and the beam pivots there on the saddle bearing. 1
  8. Horsehead. Mounted on the front end of the beam. Its curved (arcuate) front face is what the bridle hangs from. 1
  9. Bridle and carrier bar. The bridle cables hang from the horsehead face and hold the carrier bar, which clamps the polished rod.
  10. Polished rod and stuffing box. The polished rod is the uppermost joint of the sucker rod string. Its smooth surface lets a hydraulic seal be made around the moving rods. 2 That seal is the stuffing box, secured to the wellhead around the polished rod, with packing rings inside it. Because the rod reciprocates through the packing, the packing wears, and leaks at the stuffing box are a common maintenance item. 10
Article / How a pumpjack works: rotation into a stroke

How a pumpjack works: rotation into a stroke

The motor turns the cranks in a full circle. The pitman arms turn that circle into an up-and-down push and pull at the rear of the walking beam, so the beam rocks on the Samson post. The horsehead at the front swings the opposite way and lifts and lowers the polished rod: one stroke for every turn of the cranks. 1

On a conventional unit the Samson post is the fulcrum between the two ends of the beam. 1 When the pitman arms pull the rear of the beam down, the horsehead rises and lifts the rod string and the fluid above the pump. When the pitman arms push the rear up, the horsehead drops and the rods go back down.

That is what the horsehead's curved face is for: as the beam swings, the bridle keeps hanging straight over the well, so the polished rod moves in line with the wellbore instead of being pulled sideways. The load on the polished rod over one stroke, plotted against position, is the dynamometer card; see the dynamometer card guide for how to read it.

Article / Pumpjack types: conventional, front-mounted, air-balanced and long-stroke

Pumpjack types: conventional, front-mounted, air-balanced and long-stroke

Pumpjacks differ mainly in where the beam pivots and how the load is counterbalanced. A conventional unit pivots mid-beam with crank counterweights; a front-mounted unit pivots at the rear of the beam; an air-balanced unit uses a compressed-air cylinder in place of weights; and long-stroke units drop the walking beam for a tower and belt. 1345611

TypeWhere the beam pivotsHow it's counterbalancedSource
Conventional (Class I lever)Samson post between the horsehead and the pitman armsCounterweights on the cranks1
Front-mounted (Class III lever)At the rear end of the beam, with the horsehead at the frontCounterweights on the cranks4
Air-balancedAt one end of the beam, on the post, with the horsehead at the otherA compressed-air cylinder under the beam511
Beam-balanced—Counterweights on the walking beam itself3
Long-stroke (tower)No walking beamA counterweight on the other end of a belt or wire rope6

Conventional and front-mounted. The difference is the lever class. On a conventional unit the fulcrum sits between the load and the drive. 1 A front-mounted unit is built on what the patent literature calls "API front-mounted geometry Class III": the beam is pivoted at its rear on the Samson post and the horsehead is at the forward end. 4

Air-balanced. Instead of steel weights, a cylinder of compressed air does the counterbalancing. The downstroke compresses air in the cylinder, and on the upstroke the air expands and pushes the beam up, the same job counterweights do by being lifted on the downstroke. 5 In one described layout the beam is pivoted at one end on the post and the other end, with the horsehead, is supported by the air-balanced cylinder. 11

Long-stroke units. These replace the walking beam with an upright tower and a drum on top. A belt or wire rope runs over the drum, with the polished rod on one end and a counterweight on the other, and a motor drives the drum back and forth. 6 The patent that describes them lists the walking beam unit's drawbacks as its cumbersome structure, heavier weight and larger base. 6

Article / What the numbers on a pumpjack mean

What the numbers on a pumpjack mean

A pumpjack's size is given by three numbers: the gear reducer's peak torque rating in thousands of inch-pounds, the structure's polished-rod load rating in hundreds of pounds, and the maximum stroke length in inches. A unit rated 80-109-42 has an 80,000 in-lb gearbox, carries 10,900 lb on the polished rod and strokes up to 42 in. 3

The American Petroleum Institute rates pumping units with a number made of three groups: 3

  1. Torque rating. The gear reducer's torque rating, divided by 1,000 and given in inch-pounds (80 means 80,000 in-lb). 3
  2. Structure rating. How much polished-rod load the unit can support, divided by 100 (109 means 10,900 lb). 3
  3. Stroke length. The maximum stroke in inches (42 means 42 in). 3

Designations on real units usually carry letters as well. This guide doesn't decode them; check the maker's own designation key rather than guess.

Article / How a pumpjack is sized

How a pumpjack is sized

A pumpjack is sized so that each of its three ratings covers the well: the gearbox must carry the well's peak torque, the structure must carry the peak polished-rod load, and the unit must reach the stroke the pump needs. Peak torque is usually the expensive one, because it sets the smallest gearbox and motor that will do. 37

  • Gearbox torque. The peak net torque on the crankshaft decides the smallest acceptable gearbox and motor sizes, and with them much of the capital cost. 7 That peak depends heavily on counterbalance, which is why sizing and balance can't be separated.
  • Structure load. The beam, Samson post and bearings must carry the highest load on the polished rod over the stroke: the rod string plus the fluid it lifts, plus the dynamic loads of the stroke. The sucker rod pump guide covers where those loads come from.
  • Stroke length. Set by how far the pump has to stroke to move the well's fluid at a sensible speed. It must fit inside the unit's maximum stroke. 3

A unit that's too small runs overloaded; one that's too big costs more to buy, and its motor runs lightly loaded. Fully loaded motors use less electricity, so an oversized motor wastes power. 7

Article / Counterbalance: why a pumpjack has counterweights

Counterbalance: why a pumpjack has counterweights

Counterweights balance the weight of the rods and fluid so the gearbox doesn't lift it all on the upstroke and then brake it on the downstroke. The weights store energy on the downstroke and give it back on the upstroke. When the counterweight doesn't match the well, peak torque rises and the motor uses more power. 579

The load on the polished rod is lopsided: heavy on the upstroke, when the rods lift the fluid, and light on the downstroke. Without counterbalance the motor and gearbox would carry the full upstroke load and then hold the rods back on the way down. Counterweights even that out. They store energy by being lifted higher during the downstroke and release it on the upstroke, the same way an air-balance cylinder compresses and expands. 5

How well that works matters in three ways:

  • Peak torque. Peak gearbox torque depends on how well the unit is balanced, and each combination of well conditions has a counterweight setting that minimizes the peak net crankshaft torque. 7
  • Energy. A well-balanced unit puts the motor's excess energy into lifting the counterweight on the downstroke and gets it back on the upstroke. If the counterweight is not properly matched, the motor consumes more power and turns part of it into vibration and noise. 9
  • Equipment. Lower peak torque means less load on the gearbox, and lets a smaller gearbox and motor do the job. 7

Balance doesn't stay put. The gap between the ideal and the actual counterweight setting has to be managed over the life of the well, because the well load changes. 7

Article / How to check whether a pumpjack is balanced

How to check whether a pumpjack is balanced

Compare the downstroke with the upstroke. A balanced unit shows similar peak motor current, power and gearbox torque in both halves of the stroke. For a direct number, measure net gearbox torque from motor power, from the dynamometer card with the counterbalance moment, or with a static counterbalance-effect test. 89

Practical balance checks compare the two halves of the stroke by time, peak current, total power consumption or peak torque. 9 Peak current is the quickest field check. As general practice: much higher peaks on the upstroke mean the counterweights are too light for the load, and much higher peaks on the downstroke mean they're too heavy.

For a measured value of gearbox loading there are three methods: 8

  1. From motor power. Measured motor power, motor and drive efficiencies, and pumping speed give the gearbox torque through the stroke.
  2. From the dynamometer card. The surface card combined with the unit's torque factors and the counterbalance moment from the cranks and weights.
  3. Counterbalance-effect (CBE) test. A static test: the cranks and counterweights are held level until there is no upward or downward movement when the brake is released, which gives the counterbalance moment at that crank position.

Moving the weights along the cranks, or adding or removing weights, then brings the two halves back into line.

This is where continuous data helps. SRABS Brain "reads every dynamometer card, motor current and Echo fluid level, and checks them against the physics of your rod string." Cards and motor current are the same measurements the balance checks above rely on. See the system, how it works, and the modelled scenarios on cases. To estimate what fewer 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 pumpjack is a lever, a gearbox and a set of counterweights working one well. Its parts turn rotation into a stroke, its three ratings say how much torque, load and stroke it can handle, and its counterbalance decides how hard the gearbox and motor work on every stroke. Size the unit to the well's peak torque and load, and keep it balanced as the well changes.

If you run a fleet of pumpjacks and balance gets checked only when something breaks, start with the cards and motor current you already have, then test closed-loop control on one well.

Loan us a well →
Article / FAQ

FAQ

How does a pumpjack work?

A motor drives a gearbox that turns two cranks. Pitman arms connect the cranks to the rear of a walking beam, which rocks on the Samson post. The horsehead at the front of the beam lifts and lowers the polished rod, one stroke per turn of the cranks.

What are the main parts of a pumpjack?

The prime mover, gear reducer, cranks and counterweights, pitman arms, equalizer, walking beam, Samson post, horsehead, bridle and carrier bar, and the polished rod with its stuffing box at the wellhead.

What is the difference between a conventional and a front-mounted pumpjack?

Where the beam pivots. On a conventional unit the Samson post sits between the horsehead and the pitman arms (a Class I lever). On a front-mounted unit the beam pivots at its rear end, with the horsehead at the front (a Class III lever).

What do the numbers in a pumping unit designation mean?

Three groups: the gearbox torque rating in thousands of inch-pounds, the structure's polished-rod load rating in hundreds of pounds, and the maximum stroke in inches. A unit rated 80-109-42 has an 80,000 in-lb gearbox, a 10,900 lb structure rating and a 42 in stroke.

Why does a pumpjack have counterweights?

To balance the rods and fluid. The weights are lifted on the downstroke and help lift the load on the upstroke, which lowers the peak torque on the gearbox and the power the motor draws.

How do you know if a pumpjack is balanced?

Compare the upstroke and downstroke: peak motor current, power or gearbox torque should be similar. A counterbalance-effect test, motor power or the dynamometer card gives a measured value.

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. SLB Energy Glossary, "polished rod." https://glossary.slb.com/en/terms/p/polished_rod
  3. US Patent 8,083,622, "Systems and methods for automatic belt tensioning in low speed, low volume fluid recovery operations" (2011). https://patents.google.com/patent/US8083622
  4. US Patent 4,520,683, "Pump jack slant wells" (1985). https://patents.google.com/patent/US4520683
  5. US Patent 3,971,213, "Pneumatic beam pumping unit" (1976). https://patents.google.com/patent/US3971213
  6. US Patent 4,665,761, "Long stroke pumping unit" (1987). https://patents.google.com/patent/US4665761
  7. 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
  8. Rowlan, O.L., McCoy, J.N. and Podio, A.L., "Best Method to Balance Torque of a Pumping Unit Gearbox," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/best-method-balance-torque-pumping-unit-gearbox
  9. Lv, H., Liu, J., Han, J. and Jiang, A., "An Energy Saving System for a Beam Pumping Unit," Sensors 16(5):685, 2016. https://doi.org/10.3390/s16050685
  10. US Patent 7,861,771, "Rod pump stuffing box" (2011). https://patents.google.com/patent/US7861771
  11. US Patent 4,462,759, "Oil well pump shutdown system" (1984). https://patents.google.com/patent/US4462759