Constraint Headaches

By Nelson Wagner,
MCAD Solutions Engineer
Hagerman & Company
Mishawaka, IN

Have you ever had a constraint issue that you spent way too much time trying to figure out? Most of us have had problems constraining assemblies to make them behave in the manner that we want them to at one time or another. One of these problem constraints is transferring a constraint from one item to another at the point of collision. This situation does not have a direct constraint solution. Sometimes on these types of assemblies, you have to think outside the box a little bit.

Look at the assembly below. There is a tangent constraint between the lever and the pin on the right. There is also an alignment constraint so that the lever rotates about the pivot pin on the far right. See how the lever interferes with the pin on the left as we move the block with the two pins down?

What we want to see happen in this example is when the lever hits the pin on the left; we want the tangent constraint to be applied to that pin. There is not a constraint rule that allows us to make this happen, so we have to get a little bit creative.

The best way we have found to accomplish this task is to create a ghost part for the lever to follow. In the example below we created a guild for the lever to follow. The sketch for this part is based off of the two pins so that if we change the pins in the assembly, the ghost guide would follow.

Now we apply a transitional constraint between the guide and the lever. We also want to right click on the guide part and turn visibility off, as we do not want to see this part in the assembly; we simply want the lever to follow it. These steps are shown below.

Now you can see that as we move the center plate with the two pins down the transitional constraint gives the illusion that the lever is following the two pins when in reality it is following the invisible guide. It mimics the behavior that we want.

The last thing to consider is the bill of materials. We would definitely want to make sure that we exclude this ghost part that we have created as we would not want to see it in the final assembly bill of material.

So, you can see that sometimes you will have to be a little creative to get the end result you want. Just remember, there are usually several ways to get the end result you are looking for. That’s what makes Inventor so exciting and fun to use-it accommodates creativity.