Four-bar linkages

A four-bar is the simplest closed linkage: an input crank a, a coupler b, an output rocker c and the ground d between the two fixed pivots. cadagent.formulas solves where it goes from the four lengths alone. It gives the Grashof class, the coupler and output angles on either assembly, the transmission angle, and the input angles where the linkage locks. Agents call it from Python. The animator below runs the browser port of the same functions, tested against the Python on shared vectors, so it never shows a number an agent would not get.

Try it: the four-bar animator

Set the four links. The crank turns, or rocks and stops where the linkage locks, and the coupler and rocker follow on the assembly you pick. The other assembly of the same links is dashed. The dotted curve is the path of the dot on the coupler plate (the coupler curve). The chart plots the transmission angle μ over a turn of the input, with the band below 40° marked. The default is Norton's Example 4-1, a crank-rocker. Its crank turns fully, but μ falls to 26.4° at 0°, below the guideline.

Try these:

• Make the crank the longest link (crank 100, coupler 80, rocker 70, ground 60). Now no link turns fully: it is a triple rocker. The input rocks and stops at ±137.9°, and the crank's circle turns rust where the links cannot close.
• Make the coupler the shortest (crank 80, coupler 40, rocker 120, ground 100). This is a double rocker. Its input locks at 51.3° and at 125.1°, and the same links fit a mirror-image range below the ground link. The coupler turns fully only by passing into the dashed assembly at each lock; the animator keeps the assembly you chose, so here the coupler swings back.
• Switch the assembly. Open and crossed are the two ways the same links fit together at one input angle. The transmission angle is the same for both.

How to read it

The input locks where the coupler and rocker fall into one line. There the diagonal from the crank pin to the output pivot equals b + c (stretched) or |b - c| (folded), and μ is 0. The input cannot drive past that point, and both assemblies meet there. A real linkage can leave that point on either assembly; the animator stays on the one you chose. A change-point linkage (S + L = P + Q) can lay all four links in one line at 0° or 180°, and its branches meet there too.

flowchart TD
    L["four lengths<br/>S shortest · L longest<br/>P, Q the other two"] --> T{"S + L<br/>against P + Q"}
    T -->|"less: Grashof"| G{"which link<br/>is S?"}
    T -->|"equal or more"| N["equal: a change point,<br/>the links can fall into line<br/>more: a triple rocker,<br/>no link turns fully"]
    G -->|"a or d"| CR["the input turns fully<br/>crank-rocker (S = a)<br/>double crank (S = d)"]
    G -->|"b or c"| DR["the input rocks<br/>double rocker (S = b)<br/>rocker-crank (S = c)"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class L,T,G here
    class N near
    class CR,DR out

The transmission angle μ is the angle between the coupler and the rocker, folded to 0–90°. Near 90° the coupler pushes the rocker round; near 0° it pushes along the rocker, and the joints bind. Norton gives 40° as the usual lowest value for a linkage that must run smoothly. That is a guideline for a drive, not a force calculation.

From Python

Any one length unit works; angles are radians, counter-clockwise from the ground link.

import math
from cadagent import formulas as fx
fx.grashof(0.040, 0.120, 0.080, 0.100)                 # kind 'grashof', inversion 'crank-rocker', crank_rotates True
fx.fourbar_angles(0.040, 0.120, 0.080, 0.100, math.radians(40))         # (theta3, theta4): 20.3° and 57.3°
fx.transmission_angle(0.040, 0.120, 0.080, 0.100, 0.0)                 # 0.4605 rad, 26.4°: the lowest
fx.fourbar_input_limits(0.100, 0.080, 0.070, 0.060)    # stretched 2.406 rad: the input stops at ±137.9°

Each function states its units, limits and source in fx.catalogue(). They are listed with the other formulas on Engineering formulas.

Limits

• Rigid links on ideal pins, planar, position only. No velocity, acceleration, force, dynamics or joint clearance. A printed pin joint has play that the ideal linkage does not, so a printed linkage reaches its limits early or late and rattles near them.
• The 40° guideline is not a load check. For the joint forces, do a force analysis or a test. CAD and closed forms do not prove fit or strength.
• Which assembly the links take at a limit is not modelled. The animator keeps the one you chose.
• Synthesis is out of scope. Finding the lengths for a wanted motion is not done here; pylinkage does that.

Sources: R. L. Norton, Design of Machinery, 5th ed., McGraw-Hill, 2012, Chapters 2 (the Grashof condition and its inversions) and 4 (position analysis, the transmission angle and the toggle positions); F. Freudenstein, Approximate synthesis of four-bar linkages, Transactions of the ASME 77 (1955) 853–861.