Lesson 1 gave the rocker's motion. This lesson reads the other links, follows a point on the coupler, and checks where the mechanism works well.
Start from the fourbar of lesson 1 (40, 120, 80, 100 mm with a point in the middle of the coupler), or download fourbar.linklab.json and open it with Open in the top bar.
1. Any link as the output
Set θ₂ to 60 in the box at the bottom. In the Kinematics panel, change Output to Angle of link 3. The panel now reads the coupler.
| At θ₂ = 60°, ω₂ = 10 rad/s | LinkLab |
|---|---|
| θ₃ | 18.376° |
| ω₃ | −0.3955 rad/s |
| α₃ | 26.68 rad/s² |
The coupler's angle is measured from A to B, the order the link was drawn in. Its angular velocity is negative here: the coupler is turning clockwise while the crank turns counter-clockwise.
2. A point on the coupler
Click Plots and choose the preset Point path. The three plots give the x and y of the traced point and its speed |V| over the cycle. Point at a plot to read a value; click or drag on it to move the mechanism there.
Check your numbers
At θ₂ = 60° the point P in the middle of the coupler moves at 382.6 mm/s. By hand: V_P = V_A + ω₃ × r_PA, with V_A = aω₂ = 400 mm/s at 150° and r_PA = 60 mm at 18.376°. Add the two vectors and compare.
3. Transmission angle
Choose the preset Transmission. The third plot is the transmission angle μ, the angle between the coupler and the rocker. Force passes from the coupler to the rocker best when μ is 90°, and poorly below about 40°.
Here μ is smallest at θ₂ = 0°: 26.4°. At that position the crank and the coupler point almost the same way, and the rocker is pushed along its own length rather than across it. This is the position to improve if the mechanism has to carry a load.
4. Time and data
Plot x-axis switches the plots from the input angle to time over one cycle, at the speed set in the panel. Export data (CSV) writes every plotted quantity at every step, for a spreadsheet.
Try this
Move the coupler point to a corner of the coupler (drag it with the Select and move tool) and watch the coupler curve change. Which point on the coupler has the smallest speed over the cycle?
Answer
There is no single answer; the point closest to the instant centre of the coupler moves slowest at each moment, and that centre moves as the crank turns. The point path plots let you compare candidates: the one with the lowest peak in |V| is the slowest over the cycle.