June 28, 2010

Fire Hydrant Pentagons

Fire hydrants are an inescapable part of the urban landscape. They are so commonplace that it's easy to take them for granted or to overlook them—except perhaps in the case of an emergency.

These devices for delivering water, however, are worth a second look. If you examine the operating nuts that control the flow of water, you'll see that their shape is a regular pentagon. Most nuts that you might encounter in home building projects or car repairs are hexagonal or sometimes even square, but rarely pentagonal.

You can't use a standard wrench to tighten or loosen one of these pentagonal nuts. Indeed, firefighters have special wrenches to do the job.

One example of a tool for turning a fire hydrant's pentagonal nut.

But the novel geometry presumably deters others from tampering with the hydrants.

Notice that although the operating nut at the top of the hydrant (along with others on the side) to control water flow is pentagonal, the nuts holding the hydrant together are hexagonal.

Fire hydrants with pentagonal operating nuts are common in the United States. I have heard, however, that this is not the case in Canada or in other countries. I'll have to check that out the next time I travel outside the United States.

Photos by I. Peterson


Adriana said...

your mathematical eyes, and then I smile. The world is still fun.

Tom Leathrum said...

There are some interesting features of the adjustable wrenches firefighters use on these pentagonal nuts, too. They tighten securely onto three sides of the pentagon with a very simple mechanism, where a standard crescent wrench used for hexagonal or square nuts only tightens onto two opposite sides with a more complicated mechanism. Here is a link to an image (from Wikipedia) showing the adjustable part of the wrench:


Pacha Nambi said...

Here is a related YouTube video:


Anonymous said...

I suspect the reason for pentangle nuts goes further than tampering, since it's very easy to get around with a pipe wrench. I think the pentagonal nuts are easier to deal with when they're rounded off and messed up by tampering too.

Anonymous said...

Hex nuts are susceptible to rounding off with use with a loose-fitting wrench. Penta nuts less so, square nuts less still and triangular nuts "never". Triangular nuts don't have a lot of clamp area for the bolt to tighten down on if used to put tension on the bolt but if use simply to turn it, or if used with a flange, they would work well and sockets are not common. I suspect that the penta won over the triangle because a triangle (and square) would be more susceptible to being rounded at the apex by use of a pipe wrench and then may be come unusable with the fireman's wrench. The firemen's wrenches are intentionally loose-fitting and adjustable to deal with this issue.

Anonymous said...

Pentagonal nuts are used in British Columbia, Canada