Motion Control Engineering & Manufacturing Resources

Hinge Design: How to Find Center of Gravity of an Irregular Object

Posted by Weber Knapp on Jun 27, 2019 5:03:40 PM

There are many reasons a design engineer would want to calculate center of gravity. One of the big ones? Keeping your lid, door, or cover from slamming into something -- or someone.

Designing a no-slam hinge won’t just keep your customers safer, it’ll also keep you safer … from lawsuits. On a lighter note (pun intended), a steady open/close also improves ease of use for customers.

Keep reading to learn how to find the center of gravy of an irregular object. There are actually three ways:

  • The Easy Way
  • The Easier Way
  • The Easiest Way!

How to Find Center of Gravity of an Irregular ObjectDefining Center of Gravity and Balance

Center of gravity is a geometric trait. Simply put, it’s the average location of the weight of an object. Center of mass and center of gravity are the same thing.

We can ID the motion of any object by the shift of its center of gravity from one place to another, and the rotation of a hinged object about its center of gravity.

You need to find your center of gravity in order to find your balance. This is crucial for heavy: 

  • Doors
  • Lids
  • Covers

The Importance of Center of Gravity

Why is balance important to a design? It's the point that the whole design hinges on (pun #2). Whether you figure out the center of gravity yourself, or use an app, or let your counterbalance manufacturer help ... it's a must.

Once your hinge manufacturer has your lid’s weight and its center of gravity, it can develop the geometry of your custom counterbalance hinge design. You also need to know these figures to use the Vectis design-your-own hinge app.

How to Find Center of Gravity of an Irregular Object

The Easy Way

We refer to the easy method as the intersecting lines method.

Normally, a sheet of paper’s center of gravity doesn’t change, no matter which edge or corner you pick it up by. But if you fold one of the paper’s corners over, the center of gravity shifts because the weight distribution is different.

When figuring out a lid or door’s center, you can hang it by its pivots and draw a straight line down. Then you can hang it by its handle and draw another line. Where those two lines intersect is your center of gravity. 

Your hinge manufacturer will need to know the point, both horizontal and vertical, from the main pivot in the closed position.

The Catch

  • If your lid is very heavy, you might not be able to use this method.

  • Another potential issue lies in the shape of your cover -- think of a tanning bed door. Since it’s a C shape, the center of gravity will be in the middle of the C, where there is no actual door to put your line on. That’s where using CAD software would be easier.

Speaking of which ...

The Easier Way

If you’d rather skip the old-fashioned method, you can consult your CAD software model to calculate center of gravity in 3D. If you can change your coordinate positioning to the main pivot in your model, you can get those same dimensions right in SolidWorks or pretty much any other software you use. 

In the case of SolidWorks, you can find center of gravity by clicking Center of Mass in the Reference Geometry toolbar, or by heading to Insert > Reference Geometry > Center of Mass.

In the graphics area, you’ll see an icon resembling crosshairs appear at the center of mass of your model. In the FeatureManager tree, center of mass appears below Origin.

The position of your center-of-gravity point updates as you add, move, and delete features in a component.

The Catch

  • Simulating a cover is not the same as having the real-life cover on hand. You’ll need to be very careful when specifying material properties (density, grade, etc.).

  • If you don’t properly put in the specs above, you probably won't realize it until it’s too late. When your heavy lid or door is manufactured, the perceived quality will be poor. Basically, any sloppiness on your part will set your project back a long, long way.

The Easiest Way

The easiest way? Simple -- just send your lid design to a complex-hinge manufacturer.

For example, at Weber Knapp we have a proven method in which we take force readings at different angles. We use that information combined with proprietary software to determine your lid or door design’s center of gravity.

The Catch

  • There isn’t really any downfall here unless you’re insistent on DIYing your counterbalance design. We find customers get far better results when we take the measurements ourselves, but we’ve also shown customers how to do it.

How to Find Center of Gravity of an Irregular Object 2About the Balance Point

The balance angle is the number of degrees at which the center of gravity is directly above your pivot.

The balance point is the location where the center of gravity should line up directly over the top of the pivot. This is the point at which your lid or door stays put and will not swing violently in either direction.

If the user pushes the lid forward, though, it’ll crash -- unless you have a counterbalance.

The balance point is important because it will help you verify whether you have the correct center of gravity.

Better Left to Another Engineering Department?

Even though they can go on the internet and find plenty of information about center of gravity, a lot of engineers don’t think it through or truly understand what center of gravity is or why it's important.

This leads into the other major downfall of bad door or lid design. If you mix up your x and y coordinates, for example, you’ll end up with a severely underbalanced design that’ll crash instead of slowly closing. This is a huge injury and liability issue that isn’t worth the risk.

If you want to get started the right way with a true no-slam hinge, click the Vectis DYO button or the Contact Us button below.

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Topics: Counterbalances, Motion Control, Design