Cogs with Bar attached always oriented in one axis

[David Conklin]

Assuming you have 2 nulls at either end of your bar.. the left being called “Gear” and the right being called “Hole.” This also assumes that the anchor point of your bar is at 0 on the X-axis (i.e. all the way at the left). The position of your bar should be linked to the position of the “Gear” null.

Put this on the rotation prop:

(or, check out this file):
8261_gearbarhole.aep.zip

//expression for rotation prop.

//get layers
var gear = thisComp.layer("Gear");
var hole = thisComp.layer("Hole");

//get 2 sides of triangle
var dist = hole.transform.position - gear.transform.position;

//do math
radiansToDegrees(Math.atan(dist[1]/dist[0]));


David Conklin
Motion Designer

[Mike Moon]

David thanks for your rapid response. I tried this and as soon as I put your expression into the Z rot of my bar, it immediately rotated to the Hole null however as the gear turns with the bar attached, it no longer auto orients to the hole.

[Mike Moon]

oh ok I get it. When I move the “Gear” null it does work. I guess the expression is linked to the position of the Gear Null? Right now, I have the “Gear Null” parented to the rotation of the Gear itself which is spinning on the z axis with “time*-60” Is there a way to make your expression work if the Gear Null is parented to something or do I have to animate the position by hand? Thanks.

[David Conklin]

This solution is a bit more complicated.

You essentially need to get the ‘global’ position of your gear null, not the position it’s inherited from it’s parent. You can use an expression like this to link your bar’s position to the global position of the Gear null.

p = thisComp.layer("Gear");
p.toComp(thisComp.layer("Gear").transform.anchorPoint)

Once you’ve established that link, you just need to do the same thing to your rotation expression:

var gear = thisComp.layer("Gear");
var hole = thisComp.layer("Hole");

var dist = hole.transform.position - gear.toWorld(gear.transform.anchorPoint);

radiansToDegrees(Math.atan(dist[1]/dist[0]));


Have a look at this file to help clear things up.
8262_gearbarholeworldtransform.aep.zip

Good luck!

David Conklin
Motion Designer

[Mike Moon]

Your example proj cleared things up completely! I just ran into one problem, the example didn’t work for 3D layers. It kind of shifted everything and I tried moving the anchor points but it only messed it up more. So I just pre-comp’d it and then made that comp a 3D layer. So that worked out great. Thanks so much for figuring this out for me! I’m really amazed by scripting. How did you learn to do expressions? I tried some tutorials but I’m just lost.