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!