A four-bar linkage can turn a continuously rotating crank into a rocking output when its four pivot distances permit the loop to close throughout the turn. The fixed frame is one of the four links; three visible moving pieces complete the loop. Our reviewed Pump Jack uses a one-module crank, a ten-module connecting rod and a five-module rocker arm, so the crank turns fully while the walking beam nods.
Find four pivots before counting parts
Open the free Nodding-Donkey Pump Jack instructions. This existing original level 3 model has an estimated 45-minute build time. Its 24-tooth gear is driven by a worm, and the short crank connected to that gear moves the long pitman rod. The pitman attaches behind the walking beam's central pivot, making the front head rise and fall.
DIY Walkers' credited demonstration gives this article one reference job: showing that changing four-bar configuration changes which links rotate and which rock. We inspected its description and opening linkage footage. We did not copy its assembly or reproduce all of its demonstrated configurations. Our retained pump-jack model is a different educational arrangement, reviewed here for pivot closure, axial support and clear instructions.
Count joint centres rather than every plastic piece. The fixed walking-beam pivot and fixed crank pivot define the ground link. The crank pivot to its moving pin is the crank. That pin to the rear walking-beam joint is the pitman or connecting link. The rear joint back to the fixed walking-beam pivot is the rocker arm. Extra pieces can brace or decorate these links without changing the four-bar classification. The linkage explanation helps distinguish a link from a joint.
Build the frame before asking it to move
Follow the native animated instructions and free booklet. The two tall posts need their diagonal braces. Paired supports hold the walking-beam pivot, while bushes and half bushes prevent its axle creeping sideways. The crank bearing and its opposed stops keep the crank gear aligned with the worm. The pitman pivots on its pins; it must not be rigidly clamped to either rotating link.
The hanging pump rod has its own retention. Half bushes above and below its swinging block prevent it falling or being pushed out. It must clear the well head at the bottom of the nod. That clearance is a separate check from four-bar closure: a linkage may close perfectly while an attached output collides with something. Review axles and bracing, then perform one slow complete crank turn with power disconnected.
Predict which link completes a turn
Use centre-to-centre lengths in modules. The crank is 1, pitman is 10, and rocker arm is 5. The fixed pivots are six modules apart horizontally and ten vertically, so their separation is √136, approximately 11.66 modules. The shortest plus longest lengths total approximately 12.66; the other two total 15. This configuration satisfies the length condition that allows the shortest link, adjacent to the ground, to act as a full-turn crank.
That calculation is a geometry check, not a guarantee of mechanical durability. The correct assembly branch, joint freedom, support and surrounding clearance still matter. A rod fitted into a wrong beam hole changes the lengths and can change the motion completely. Count holes carefully: an eleven-hole beam spans ten modules between its first and last centres.
Try the virtual four-bar lab below. Predict whether a selected configuration gives a circle, arc or more complicated coupler path before running it. The lab is a configurable planar comparison, not an exact animated replacement for the pump jack's spatial part stack. Changing a virtual link length does not authorize changing the real build without reviewing the new closure and swept clearances.
Interactive: Four-bar linkage tracer. Change the four bar lengths of a crank linkage, watch it move and trace the path a point on the linkage draws: the start of a walking leg.
Observe a complete cycle fairly
Disconnect the motor and gently turn the crank through one full revolution. Put a fixed ruler beside the head, outside its sweep. Record highest and lowest positions using the same reference point. Then repeat twice without moving the ruler, frame or viewing direction. Do not support the moving beam with your hand: that would conceal missing physical constraints.
| Trial | Complete crank turn without forcing? | Head height range | Binding or drift |
|---|---|---|---|
| 1 | Record | Record | Record |
| 2 | Record | Record | Record |
| 3 | Record | Record | Record |
No physical trial results are supplied here. If the crank stops, identify the location before changing anything. A misplaced pivot can break loop closure; an overtight spacer can pinch a rotating beam; a loose frame can shift the fixed pivots; a displaced pump rod can hit the well. Those failures require different corrections, so a vague instruction to add more power is unhelpful.
Separate balance from motion
The rear wheel represents a counterweight and changes the mass distribution. We do not claim a measured reduction in motor effort or current. The geometry determines the permitted motion; masses, friction and loads determine the effort needed. Compare the guided straight piston in the Twin-Piston Engine: both mechanisms use a crank and rod, but a slider guide replaces the rocking output pivot. Naming that difference gives you a useful way to recognise mechanisms beyond their appearance.
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- What counts as the ground link?
- Which link is one module long?
- What should you do if the linkage binds?
- Does motor sound establish efficiency?
