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A linchpin is a piece of metal used to keep a wheel securely on its axle. It is also referred to as a lynchpin, linch pin, or lynch pin. It's inserted crosswise directly through the axle's end where it sits securely in place until it is manually removed. Due to its shape, it should easily glide into place without a large amount of force.
The linchpin is not pointed, like a typical straight pin or pushpin. It is flat on the end and rounded on the sides. It comes in a variety of thicknesses to make it able to fit different types of wheels. The end of the pin that does not go into the axle usually has a loop of metal, similar to a key ring, so that the pin is easy to pull out. Sometimes linchpins will have chains attached to them instead of these loops.
Linchpins can be made from a variety of materials, including aluminum, zinc, brass, and stainless steel. Some are spring loaded to make removal quick and easy, while others are grooved so they can be screwed into place, allowing for an extra strong hold. There are even linchpins called "safety pins" which do not allow the user to easily come in contact with the base of the pin, which can help to protect wayward fingers.
Cars and smaller trucks do not require linchpins to keep their wheels in place, instead relying on a simple bolt system. Linchpins are primarily used on vehicles with very large wheels, such as tractors. A good place to look for a linchpin, therefore, is at a store that sells farming supplies.
The linchpin is decidedly not a new invention, since there is evidence that linchpins were used on the wheels of chariots and other wartime vehicles in ancient Rome. The word linchpin is an alteration of the Old English word "lynspin," which people began using in the mid 1300s to mean "axle pin." Exactly how the spelling changed over the years is unclear.
Since the linchpin is the device that keeps a wheel together, it is no surprise that over time a second definition of the word, meaning an abstract element that keeps a complicated process together, was born. A person might say, "The wedding planner was the linchpin that made the entire ceremony and reception a success," to indicate how good the person was at keeping track of even the most minute of details. This phrase is in common enough of use for it to appear in major newspaper headlines.