"What Shape Is the Nut on a Fire Hydrant, and Why?"
In the United States, the most common shape is that of a pentagonal prism, as shown in the drawing on the right. The faces that define this polyhedron are pentagons. The 5 sided characteristic of the nut was chosen to offer a measure of tamper resistance. Hydrants can be dangerous, and only firefighters and other authorized persons should open a fire hydrant.
Sometimes the 5-sided nut has a slight taper. The shape of the nut is then called a truncated pentagonal pyramid. The pentagon face on the outside is called the minor base, and is smaller than the pentagon where the nut is is attached to the hydrant. The larger pentagon where the nut is attached to the hydrant is called the major base.
|truncated pentagonal pyramid|
In Canada and Japan, the most common shape for a fire hydrant's operating nut is a tetragonal prism, as illustrated in the photo on the left.
The tetragonal prism is also found on hydrants in some communities in the USA.
The shapes illustrated above are not the only ones found on hydrants. Other shapes for the hydrant nut include:
Can you match these names with the drawings at left?
All drawings on this page made with Cartesio 3.03e, a freeware geometric CAD program from Italy.