First, they take the broom and swish it left, then they swish it right -- okay, street sweepers haven't been men with brooms for more than a century. These days, they're drivers steering boxlike vehicles that look like a cross between a Zamboni machine and a dump truck.
There are a couple of types of street sweepers. There are those that clean the streets with brushes and those that clean with air. The basic function of any street sweeper is the same, though: The dirt has to be scrubbed from the road; it needs to be put in a little pile, and that pile needs to go into a bin until it can be disposed of properly.
Conventional street sweepers have jets under their trucks that shoot water onto the street's surface to keep flying dust particles to a minimum, while spinning brushes scrub the dirt free from the streets and gutters. A cylindrical brush under the truck sweeps the debris onto a conveyor belt that leads to a storage container. Sometimes, instead of the cylindrical brush, a vacuum-like mechanism will suck the debris into the container.
Regenerative air sweepers, on the other hand, have hydraulic systems that use air jets to churn up the dirt on the road, then swirl it toward the center of the truck. Negative pressure under the truck creates a vacuum that carries the dirt up into the receptacle at the truck's rear. Filters clean the air in the receptacle and reuse that air to loosen debris on the street, while water is sprayed to keep down the dust.
Some sweepers use spinning brushes instead of regenerative air or water to get the debris underneath the truck, where a vacuum moves it into the storage hopper. These models are usually smaller than the big street or highway sweepers, and they're used at industrial sites where cleaning up particulates is paramount.
All of this activity gives us a clue to why these trucks, small or large, are so darn noisy. First, they're usually powered by a heavy equipment-grade diesel motor. Second, there are either jets of water squirting away or a hydraulic system pumping air onto the street, not to mention the brushes spinning like crazy at about 4,000 revolutions per minute (rpm). There's a lot going on in a small package -- with very little sound dampening. Plus, they run when neighborhoods are quiet, either late at night or early in the morning, so we're more likely to notice the racket.
Let's move on to how these machines are used in our everyday lives, and what innovations we're seeing in the street sweeper's third century.