Interesting article to know how trailer balls actually work.  Its not rocket science, but interesting.

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How Trailer Balls and Switch Balls Work

Pick-up truck tows boat on the road.
Ivan Cholakov/
I know, it's such a small thing, but you need a trailer ball so your tiny pick-up can safely bear the weight of hauling that huge boat.
You've got your boat mounted on your trailer, and you're getting ready to hook it up to the receiver hitch on your pickup truck. Suddenly, a problem arises that could derail your trip before you leave the house: Do you have the right trailer ball?
Trailer balls and switch balls are hitch accessories mounted on your receiver hitch's ball mount. It's the piece of towing equipment that couples the trailer to the towing vehicle and bears the load of thetrailer's weight.
Trailer balls, also called hitch balls, are rounded to allow for lots of vertical and horizontal movement. After all, the hitch connection will be bouncing around a lot due to the motion on the road [source: Sierra Nevada Airstreams]. However, depending on what you'll be towing, it's important to make sure you have the right trailer balls equipped.

Choosing Trailer Balls and Switch Balls

So you're ready to buy a trailer ball. But how do you go about choosing the right one for your vehicle? There are four factors to consider here: trailer weight, the diameter of the hole on the ball mount, the thickness of the ball mount platform on the receiver, and the size of the trailer coupler [].
Trailer being towed by car on highway.
Richard Hartt/Time & Life Pictures/Getty Images
Make sure you know your cargo's gross trailer weight and tongue weight before you start.
First, let's get to know a little more about weight - specifically, gross trailer weight and tongue weight.Gross trailer weight is the fully loaded weight of the trailer you're towing, including any and all gear placed on it. Tongue weight is the downward pressure that the trailer exerts on the ball [source: Truck Stuff USA].
The most important factor in choosing a trailer ball is its gross trailer weight rating. Each ball lists its own weight capacity. Be careful here: Your trailer receiver may be rated to carry up to 10,000 pounds, but if your ball is rated at 5,000 pounds, that's the maximum you can tow. In addition, always make sure the hitch ball's weight rating is greater than the gross trailer weight so you don't overload the towing equipment.
Trailer balls are classified by the dimensions on the different parts of the unit. There's the diameter of the ball itself (the distance around the center of the ball), the shank's diameter, the shank's length, and the size of the circular piece of metal between the ball and shank. The most common ball size is two inches in diameter, but heavy-duty industrial trailer balls can go up to 2 1/4". The diameter of your ball mount's hole must be no more than 1/16" greater than the ball shank diameter in order to fit. The necessary shank length is determined by the thickness of your ball mount platform - you don't want to get a ball whose shank is so small you can't lock the nut in place. You'll also need to buy a ball to fit into your coupler.
In short, you need to buy a ball that's 1/16" smaller than your ball mount's hole and that can support an amount of weight greater than your trailer, has a long enough shank to fit into your ball mount platform, and whose diameter also fits into your coupler.