Mad about Science: Working trucks

By Brenden Bobby
Reader Columnist

Living and working around here, we’ve seen our fair share of unique work trucks on the road.

Logging rigs, cement trucks, garbage trucks — all of them perform specific and special roles in our community. But what happens inside these vehicles? Have you ever thought of the engineering that goes into a garbage truck? Why does the cylinder on a cement truck spin?

Let’s start with what everyone calls a cement truck. In actuality, these are concrete trucks and the distinction is important if you work in construction. Cement is an ingredient in concrete, acting as a binding agent with water to lock sand, gravel and cement together. Keeping it still allows a chemical reaction to occur that binds it all together to create concrete. In a nutshell, this is why the drum of a concrete truck spins — but there’s a bit more to it than that.

The inside of a concrete truck’s drum is lined with steel formed into a helical pattern that acts like an Archimedes’ screw lever when the drum is rotating. This agitates the mixture and keeps it from fully curing, giving the mixture more time to be transported before it’s delivered to a job site. 

During travel, the drum rotates to pull the concrete mixture upward. Upon reaching the site, it rotates the opposite direction to feed the concrete out of a spout and into its final destination. The spout can be guided by a worker to place the concrete with relative precision.

The speed at which the drum rotates is important. Higher speeds of rotation allow for more thorough mixing, but can add excess air to the mixture or create friction. Increased friction means increased heat, which can alter the flow rate of the wet concrete, its workability once poured and damage the mixing equipment. Concrete trucks will rotate their drum between four and 12 times per minute. The speed may vary based on the ambient temperature or humidity, as well.

There are different types of concrete trucks. The standard drum mixer is the one you’re most likely to see around here — large trucks with rotating drums. These trucks collect mixed concrete and transport them to other sites. Another version is a volumetric mixer truck, which is a much larger vehicle with segmented compartments that allows for mixing onsite.

What about garbage trucks? These engineering marvels are a common sight around town. They use a set of mechanical arms to grab bins and lift them to dump their contents inside a chamber before driving off and repeating the process.

Side-loader trucks are most commonly seen around Sandpoint. These are efficient for tight neighborhood streets and require fewer staff members to operate than a rear-loader truck you’ve likely seen in movies and on TV. A side-loader truck grapples the wheelie bins and dumps them into its trash compartment, where a hydraulic press compacts the trash to force out air and create more room for garbage.

Front-loader trucks are also seen in Bonner County. These utilize a sort of forklift in the front that grabs metal dumpsters and lifts them over the cab to pour their contents into the hopper before compaction. These deal with a larger volume per bin than side-loaders and are usually quite large.

There is a huge variance in how garbage trucks handle compaction. Some trucks use a hydraulic press like the trash compactor walls in Star Wars: A New Hope. Others may use a screw press. Whatever system any particular truck uses, it’s capable of exerting up to 30,000 pounds of force. That’s equivalent to two fully grown African elephants standing on the same spot. In short, you don’t want to take a trip into the hopper of a garbage truck.

The storage compartment of a garbage truck is more than a metal box. These spaces are specially designed to carry all manner of waste as efficiently and safely as possible. A hydraulic wall presses the garbage into layers, forcing out as much air as possible to leave room for more trash. Sensors alert the driver when the compartment is reaching capacity or if dangerous materials like flammable batteries or aerosol cans have compromised the pressure or integrity of the load. The surfaces of the truck itself are also coated in a very tough paint and sealant designed to protect the metal from corrosion and the immense forces being exerted within.

Trucks may also be equipped with a reservoir for collecting leachate, which is effectively garbage juice. This stuff isn’t just gross, it can be dangerous depending on what people decide to throw away, and it cannot simply be drained out onto the street.

Believe it or not, these trucks are also designed to minimize escaping odors, which you’ll be thankful for if you’re ever stuck behind one on its route for more than a few minutes.

Garbage trucks transfer their contents to a transfer station, where material is collected, sorted, weighed and then transferred to its final destination, which could be a recycling facility, landfill or incinerator.

In California, a number of natural park areas are actually built on top of old landfills. In many places around the world, trash is incinerated to generate energy from waste. This makes sense when plastic is derived from petroleum. While it’s not great for the atmosphere or our climate, the alternative is likely to be drowning in juicy plastic waste as far as the eye can see.

Alternatively, we could just stop individually wrapping every single item in so much plastic, especially if it’s not compromised in any way by washing in a kitchen sink.

Stay curious, 7B.