Ropes, cables, springs and joints
The Connect tab joins bodies: ropes and cables, joints, bars, and the drives (gear meshes, belts, thrusters). A label beside the cursor always says what the next click will do (Start · Free end, Wrap over Pulley 1, End · Tie to Box 2 …).
Rope or Cable?
| Rope R | Cable ⇧R | |
|---|---|---|
| What it is | a chain of small links with mass: it sags, swings and drapes | an ideal string: massless, inextensible, exact tension |
| Ends | free, tied to a body, or pinned (⇧ on empty space) | tied to a body or a fixed point |
| ⌥+click on the way | ties the rope there | an eyelet the cable slides through |
| Use it for | hanging chains, whips, things that look like rope | textbook pulley problems, tensions you want to read |
| A motor on a pulley | a Rope over a wheel is turned by its friction | a Cable winds only where it grips the wheel: turning a wheel's Axle Motor on makes the cables over it grip it (the route list switches it back to an ideal sliding pulley) |
| Out of the box | a ~12 mm braided rope: 0.1 kg/m, links 12 cm apart, a little bending stiffness (0.002 N·m²); a new scene gives ropes 100 kN of stretch stiffness and internal friction 10/s (World → Ropes) |
Both: click the first end, then any circle to wrap over it (the side is the side you come from; ⇧+click ties to it instead), then the last end. Enter finishes at the cursor, Backspace removes the last point, Esc cancels. A Rope can also be drawn freehand by dragging.
A cable can carry mass per length (kg/m): its weight and inertia then act along it (a heavy chain over a pulley), without sagging. Properties shows a cable's length, slack and tension (at each end and per span when it has mass), and an Active switch that cuts it without deleting it. Cables over a drum with friction follow the capstan equation.
A rope's internal friction (scene Properties → Ropes) damps wiggles along it while the rope still swings as a whole; 0 gives the undamped rope.
Tying a rope to a rope. With the Rope tool, start or end on another rope (or ⌥+click on it on the way) to tie to it there: the label reads Tie to Rope 1. The knot moves with that rope, so a rope can hang from a rope, and a net or a cradle can be tied together.
Stiff ropes. A rope's Properties → Stiffness and Breaking → Bending stiffness (EI, N·m²) makes it a wire, a rod or a fishing line that holds a curve: 0 is a limp rope. Rest shape is the shape it springs back to: as drawn or straight; Hold the current shape takes the shape it has now. A rope whose start is pinned and whose next link has a tie point is clamped: it bends under its weight as a cantilever does, w·l⁴/(8·EI) at the tip.
Ropes that snap. Break tension (N) is the most a rope can carry: past it the rope snaps at its most loaded link (a hanging rope at its top), and the status bar says so. Going back to the start of the run brings it back whole.
Stretch. A rope stretches a little, as real ones do: scene Properties → Ropes → Stretch stiffness EA (100 kN in a new scene, 1 to 30 000 kN). That is why a rope drawn taut between two pins sags a few centimetres under its own weight instead of staying a perfect line; a larger EA makes it tauter (30 000 kN is a steel cable of about 15 mm).
Springs, rods and joints
| Tool | Key | Joins | Properties |
|---|---|---|---|
| Spring | K | two points at a distance | stiffness k, damping c, rest length; drawn as a coil |
| Rod | L | two points at a fixed distance | a massless rigid link |
| Pin Joint | J | two bodies at one point | free turning; a torsional spring and damper, friction, angle limits, a motor and a break force |
| Slider | two bodies along an axis | limits, a motor | |
| Weld | two bodies rigidly | — |
Click the first body (or empty space for the world), then the second. Points snap to centres, corners and side middles, to any point along a side when you are close to it, and to the rims of circles and rings; a ring shows where the click lands. A ring is taken from a click in its hole too: its inner rim when you click near it, its centre otherwise. Where bodies overlap, a menu asks which one you mean; challenge goal zones and liquids are never joined. A Pin Joint goes where you clicked: where the two points meet, or at a point clicked on one body that lies on the other too; when the two points are apart, a loose second body (nothing joined to it yet, not fixed) is moved so its point lands on the first, so clicking a Pin Support and then the end of a bar hangs the bar from the pin.
Joints are solved together, exactly, when they form a mechanism (a four-bar linkage, a truss, a wheel of welded parts), so heavy loads on light parts do not stretch them. Scene Properties → Simulation → Exact joints switches this off.
The Bar tool: trusses and frames
Bar draws a solid bar between two points: click the two ends (each snaps to what is near the cursor: another bar's end, a body's centre, corner or the middle of a side, a pin or a ground; a ring and a name show what it will join), move the cursor to set the thickness (or type it in the options) and click or press Enter. Where an end sits on a body the bar is joined to it: welded by default, or pinned with Ends join → Pin. Bars pinned end to end make a frame or truss that moves and carries loads like the real thing (bars that meet at a joint pass each other there, as truss members side by side on a pin do); a free-body diagram shows the forces on one bar. (Analyze → Member Forces works on trusses made of rods.) Select a bar to change its ends later (Properties → Bar ends).