#Relations

A relation ties two degrees of freedom together with a constant factor. It holds nothing and places nothing: the parts move exactly as before, but now they move together, in a fixed proportion.

That is how drives are assembled: turn the pinion and the rack travels; turn the screw and the nut runs along it; drive one axis and the second one covers half the distance.

#How to add one

Press Relation in the assembly bar, click the joints in the mate list and press Enter.

You pick finished joints, not geometry: a relation works on top of what is already there. If the joint you picked has no suitable degree, the program says so at once — “this joint has no free rotation” or “no free travel”.

#Kinds

The screw relation stands apart: it lives inside a single cylindrical joint and turns it into a thread — rotation and travel stop being independent.

For rack and pinion the order of the sides is fixed: rotation first, travel second. That is why the value is called “travel per turn, mm” — how many millimetres the rack covers per full turn of the pinion.

#Value and direction

• Ratio — the factor for gear and linear: 2 means “the second one runs twice as fast”.
• Travel per turn, mm — for rack and pinion and for screw: the thread pitch or the gear module.
• Reverse — flips the direction of the second degree: gears in external mesh turn towards each other, in internal mesh the same way.

#When a relation does not act

The relation row in the mate list shows its state:

• “Joint lost — the relation does not act” — one of the tied joints was deleted. The relation has nothing to hold on to; add the joint back or delete the relation.
• “Degree of the wrong kind: this relation expects another” — a gear was handed a slider that has no rotation at all, for instance. Change the kind of the relation or pick another joint.

#See also

• Joints — what relations work on top of.
• Joint limits and drives — how to drive motion and keep it in bounds.