Popular Topics

How Big of a Strut? Motion Has Us Covered.

One of the benefits of being the engineer in a group of friends is that you get to help with all your friends’ projects. The same friend who I helped design a Trebuchet found in this 4 part blog series asked me to help create a concealment flag. A concealment flag is basically a lock box that looks like wall art. These are used to hide several things including candy from kids.

The box will have an RFID lock on the underside for easy access, and one of his requirements is that the box open on its own.

I modeled a quick representation of the box in SOLIDWORKS to gauge size, form, and fitment. Assigning the wood material to the box gives an accurate depiction of weight.

Using a gas strut seems to be the best way to have the box “open on its own”. One question that came up was how do we know what strut to use to lift the lid? Here is where SOLIDWORKS Motion Analysis part of SOLIDWORKS Simulation Standard, shines. Motion Analysis utilizes mates in the assembly and the material properties of the model to provide kinematic outputs accurately and efficiently to the designer.

I started with a gas strut position pictured below. I took a best guess as to where we may want to place the strut. To utilize Motion Analysis inside of SOLIDWORKS you simply turn on the add-in.

SOLIDWORKS Motion Analysis lives at the bottom of the SOLIDWORKS window, and can be opened by selecting the motion study tab.

Once open you can assign many different motors, forces, contacts etc. In this initial study I used a linear motor that moves 5 inches over 7 seconds. This will open the lid against gravity which was applied as an external load. With gravity turned on the strut must overcome the weight of the lid, and any mechanical disadvantage in the system.

Mechanical Advantage / Mechanical Disadvantage noun: The ratio of the force produced by a machine to the force applied to it, used in assessing the performance of a machine.

After calculating the study, we can request a force output graph. What we see is that the strut is trying to push up and not out on the box lid. This causes a high force requirement to initially move the lid. I did not do a great job of placing the strut mounting locations, but we can iterate and find a location that suits our needs.

Moving the strut to a new position 1 inch further towards the back of the box allows for an outward stroke that is more horizontal. This new location adds mechanical advantage into the system, causing a drastic reduction in force required to lift the lid. Adding an extra inch to the stroke of the strut allowed the box to open to the same height.

The above graph shows that the force required to initiate movement on the lid is now low and as the lid opens the force increases. The maximum force to lift the lid is 19 lbs. This can easily be accommodated by splitting the force over two struts having each strut carry 9.5 lbs.

Of course, I will continue to iterate to find a position and strut that matches our overall requirements, but this initial calculation is good enough for “government work”.