# Gears and Joints with SketchUp & Sketchy Physics Part 2

Tutorial by Rich Decibels.

Following on from last week's introductory tutorial, here's a bit more information on how to make mechanical models in SketchUp with SketchyPhysics (SP). Here's a great resource if you want to learn more about mechanical linkages, gears, and all that good stuff (hat tip to Edgar Castelo for the link).

So here's were we left off: two gears meshing happily.

Before we go any further, I'm going to hide those ugly purple hinge symbols - they're still there, just invisible. Next I'm going to make some bars (I'm making something like a four-bar linkage). My method is: draw rectangle (R), then push/pull it into 3D (P), then select the whole thing (triple-click) and make it a group (right-click -> Make Group).

It's a good idea at this point to hit Play and start your simulation: try picking up the bar and moving it round to be sure the physics are working correctly. If it's not working right, check the Gotchas at the end of this tutorial.

I want one end of the bar attached to one of the gears by means of a rotating hinge. To do this I start by dragging it into position:

Then double-click the bar to edit the group, and add a hinge:

Now click outside of the group to exit group-edit mode. Next we need to use the Joint Connector tool to connect the hinge to the gear. This part of the process can be frustrating because SP has some weird behaviour. Select the Joint Connector tool, then click on the hinge. You'll probably find that the first time you click, it selects the whole bar+hinge group. We don't want that, so keep clicking on the hinge until the selection changes from the whole group to just the hinge. Don't ask me to explain this behaviour: sometimes it takes 2 clicks, sometimes 10. When you have just the hinge selected, hold down Ctrl and select the gear. The bar should now be correctly connected to the gear.

At this point you can hit Play again to check that the hinges and connections are working as intended:

Next I'm going to add another bar and connect it to the other gear, using the same process.

Clearly, I don't want the two bars flopping all over the place like that, they should be connected by another joint. Remember the process: edit the second bar group, add a hinge, close the group, then use the Joint Connector to connect this new hinge to the first bar.

Repeating this process with two new bars gives me the final mechanism (note all hinges are hidden in this view):

And if you can't guess a practical use for such a weird looking mechanism, check out this machine (and stay tuned for my laser cut acrylic version).

### Sketchy Physics Gotchas

• Save all the time! My installation at least crashes very frequently.
• Sketchy Physics only 'knows about' groups and components
• Groups of less than a certain volume are ignored, so scale everything up 10x if you're seeing weird behaviour.
• Turn gravity off unless you need it.
• Always attach a hinge to a grouped object (see dummy square hack).
• Put damping on all your hinges if you're seeing weird behaviour