Take Inventor Assembly Constraints to the Max — and Min

13 Jan, 2011 By: John Hackney

IMAGINiT Tricks Tutorial: Add limits to assembly constraints in Autodesk Inventor 2011.

Editor's note: This tutorial courtesy of IMAGINiT Technologies.

When working in assemblies, the designer would often like to flex some of the assembly constraints within a predetermined range of motion. You could accomplish this with a drive constraint, but that may be too much for just a quick motion check. This is a great opportunity to use Inventor assembly constraint limits and physically drag your constrained components through the range of motion.

The constraint limits can be found by selecting the double chevron in the lower right corner of the Place Constraint dialog box.

Limits are available for the following constraint types:

  • Mate
  • Flush
  • Angle (Undirected Angle and Explicit Reference Vector only)
  • Tangent
  • Insert.

Most of these will be very easy to apply, but I will caution you: The angle constraint limits may have you scratching your head.

The Maximum check box allows you to enter the maximum range of the constraint. For example, if you are mating two parts and you want to drag them apart a maximum of 2 inches, you would check the box labeled Maximum and enter 2 inches. By the same token, if you wish to limit your motion on the minimum side of the constraint, say to .5 inch, simply check the box marked Minimum and enter that distance.

If the check box labeled Use Offset as Resting Position is selected, the program will return the constraint value back to the offset value you entered when you created the constraint. For example, you have a mate constraint in your assembly with the minimum value set to .5 inch, the maximum value is set to 2 inches, and your offset value is .5 inch. Left-click and drag the moveable part, then release it, and it will return to a .5 inch offset.  A word of caution: The offset value must be equal to one of the other limit range values or somewhere between them. In my example, the offset is equal to the minimum limit's value.


A very useful trick is to set only one limit; for example, I could set the offset to .5 inch, leave the box beside Use Offset As Resting Position checked, and set the minimum limit value to zero. Without a maximum value set, I can drag the mated part an unlimited maximum distance away for the mated part and back to zero. When I release the part I am dragging, it will return to the value of the offset value.

If you apply limit values to a constraint, Inventor will place a +/- symbol beside the constraint listing in the browser. This will allow you to quickly identify which constrains have limits as your model grows in complexity.

If you apply limits to your constraints during design, the limits may interfere with other constraints. I would suggest restricting the number of constraints containing limits, and applying required constraint limits after you finish constraining all assembly parts.

Autodesk Inventor 2011 has another little surprise waiting for you that bears mentioning. Let's imagine you have a part that is mated to another fixed part. You have also constrained other parts to the one you intend to drag between limits. You add your maximum and minimum limit values, then check the Use Offset as Resting Position option. When you drag one of the moveable parts, you may experience a sudden return to the offset value when you approach the maximum or minimum limit values — just one of those quirks in the software. To overcome this, left-click and drag on another moveable part.

After learning about constraint limits, I immediately thought of one application where I wanted to use it. We know we can use contact sets to have one part push another into a new position, but it's hard to have the part return to the original position. It is pretty easy if both the part pushing and the part being pushed are in contact at all times, but if they are separated at some point during the movement, constraint limits can come to the rescue.

In the simple assembly layout above, the roller cam rod does not touch the pin in the block during part of the cycle. As the cam rod moves down, a contact set between the pin and the cam rod causes the pin to move.  As the cam rod reaches its maximum travel and begins to recede, the constraint limit between the end of the cam rod and the pin (A) pulls the pin back to the original position. There is a constraint limit on the pin (B) to keep it a maximum distance from the block. 


About the Author: John Hackney

John Hackney

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