Some is Good, More is Better? (part 2)

The question is, “How do I hang a heavy gate”? The answer starts long before that in the design phase of the job. Simply put, the farther apart you can place your two hinges, the stronger the overall system will be. This means designing a gate with hinge points as close to the top and bottom of the gate as possible.

  Whether you are building the gate from scratch, or you already have the gate, once you know the distance between the hinges, the next factor in the equation is the width of the gate. How big is the opening? With these two numbers, and the overall weight of the gate, it is possible to compute how much rotational force, or “moment” the gate produces. This is the force that destroys hinge systems. Choosing a hinge designed to carry a given “moment” is crucial to making a strong gate system.

  The wider the opening, the longer the gate, and the greater the leverage exerted by the gate on the hinge, and the gate post. Spreading that leverage over a larger hinge distance lessens it's overall impact on the system.

I only know of one manufacturer that rates hinges using these calculations. You can find their “Proportional Load Charts” by registering at their website, www.mftfence.com, and downloading their 2013 Catalog.

  I started this installment with the statement that the farther apart your hinges are, the stronger the gate system will be. There are several ways to optimize this distance. For ornamental gates, gusseted corners can help support that weight. Never place a hinge in an unsupported area of the upright, as it will flex under load. For wood gates, consider placing hinges where top and bottom rails of the gate meet the upright. Chain link can offer some interesting variations. In industrial and commercial applications, one way to gain strength for gates on very wide openings is to build an “extension”. Extend your gate post above the fence line to the distance you want to gain between hinges, then extend the gate upright to the same height, bracing it to the top rail with a brace at no more than 45 degrees, for rigidity). 

If you look closely at this gate, you can see that it is almost as described.  The gate builder has added an extension to the top of the gate to gain strength.  Unfortunately, he is not taking advantage of this height.  He has kept the top hinge just below the top horizontal rail of his gate.  It would be a much stronger system if the top hinge were at the top of the extension.  The other flaw in this design is that the angle brace comes down at an angle greater than 45 degrees, which prevents taking advantage of it's strength to keep the gate rigid.  Notice the addition of a wheel at the bottom of the gate.  It could be eliminated, and the gate would be much stronger, if the top angle brace were attached to the top horizontal rail of the gate at the point where the lower angle braces meet in the center of the gate.  But that's a topic for the next post!

Some is Good, More is Better? or, Why Three Hinges on a Gate Isn't a Good Idea

In the past month I've talked to several fence contractors who shared the same misconception. In each case they thought that adding a third hinge to a gate would allow a heavier gate to be hung safely. There are several reasons why this is not so.

The first is that, no matter how hard you try, you will never be able to get all three hinges aligned in a straight line. There will always be a straight line between any two of the hinges, but to get all three in perfect alignment would be a tough job with a laser in perfect lab conditions, much less in the field. Why does that matter? Because the “odd man out” hinge will tend to push or pull on the gate frame, twisting it and leading to damage to both the gate and the hinge affected. To make matters worse, the hinge that is odd man out can change, depending on how far open or closed the gate is in its arc of travel.

Several of the contractors I talked to pointed out that the doors on their house have three hinges. That's often the case, and as someone who worked as a carpenter for nearly twenty years, I can say that the chief reason for that is wood doors are not stable without a third hinge. The center hinge on a door is there to make the door more rigid, not to support the weight of the door. See the following article for deeper understanding of this. http://bit.ly/1hgisiZ

A second reason is that the weight of a gate is primarily borne by one hinge, not two, or even three. Unless you spend a lot of time perfectly balancing the hang weight of the gate from the hinges, one hinge will likely carry the bulk of the weight, most of the time. Think of an old wrought iron gate installation. The ring and pin hinges used for hundreds of years work well as an example. The bottom pin carries the weight, the top pin serves as a pivot to guide the gate through its arc and keeps it from tipping over.

“So how do I hang this heavy gate”, you ask? That's the subject of the next installment.