why are trays of cans more stable to stack tall than boxes?

11 views
0

I work stacking grocery pallets for a living. The rule is that cardboard cases/trays of cans make the best base for stacks and should always go on the bottom of pallets, and cardboard boxes, even dense and heavy ones, should go on top of the can layers.

What I’m after is the basic physics of why trays of cans are so much more stable than cardboard boxes. Thanks to anyone who responds!

In: 6

Beverage cans are pressurized, so the pressure acts opposite of outside crushing forces in the can, making them more stable. Cylindrical shape is also optimal for evenly distributing the load along the can so there isn’t a single weakpoint on the can that will buckle under load.

As for food cans, they’re not pressurized, but they’re made of a decently thick metal (often times corrugated to give extra rigidity), and again, they’re cylindrical.

Cardboard is relatively weak and non-rigid, therefore and easily buckles under load.

Furthermore, the whole can act as one single structural unit. As you apply load on top of the can, the entire can participates in resisting that load, as opposed to a cardboard box that’s basically a couple of flat pieces of cardboard loosely connected together. Load doesn’t transfer well from one side of the box to the other.

A cardboard box only holds the roof up from the walls. Whereas the stacked cans hold up the roof from the walls of every can. Many more walls of can and metal strength.

The cans hold up from all the area including it’s center. The cardboard boxes have a lifting weak point in it’s center

When stacking anything “heavy stuff goes on the bottom” is the universal rule of thumb. This is because of a thing called the [Centre of Gravity](https://en.wikipedia.org/wiki/Center_of_mass#Center_of_gravity). (Since that page redirects to Centre of Mass, I’ll also give you [this link](https://www.geeksforgeeks.org/difference-between-center-of-mass-and-center-of-gravity/) to help explain the difference and similarity between CoM and CoG.) Basically, more weight on the bottom = lower centre of gravity = more stability. (Good ol’ [Bill Nye does a great half hour](https://youtu.be/GAj-BM8gJNQ?t=916) ELI5 bit on this!)

But why not put the heavy boxes of not-canned stuff on the bottom sometimes?

Partly this is just an efficiency thing. Even if there are *some* cardboard boxes that are about the same weight/density of the cans, the majority of cardboard boxes are going to be lighter than trays of cans. So it’s just easier to say always put the cans on the bottom, boxes on top. (Ideally it’s going to be cans on the bottom, then heavy boxes in the middle, then light boxes on top.) Saves time not having to test the weight of each box.

There’s another factor contributing to stability here, and that’s the uniformity/consistent pattern that trays of cans have, which most other products do not.

A regular predictable and simple pattern is more stable than an irregular one.

Consider the shapes of the products in those boxes which are equally heavy/dense to the trays of cans. Likely these are things like containers of liquid, which tend to have awkward shapes, or bags of stuff like grain, whose shape shifts and changes regularly.

Rarely are those containers going to have the simplicity and uniformity of the equal-size regularly shaped cylindrical cans placed in rows. Even if the box itself is a simple and fairly uniform shape, inside it probably isn’t, so the parts of the box that aren’t occupied with heavy dense dish soap or whatever are likely to crush inwards, warp, or deform, ruining the shape and consistency of the pattern. Even if it’s a box that’s very tightly packed with bags of sugar, it’s still more likely to warp than a tray of cans. (The strength of the metal cans is also a factor that retains the strength of the pattern better than cardboard boxes or plastic bags/jugs/etc.)

So the TLDR is: putting the trays of cans on the bottom creates a lower centre of gravity and a more regular pattern, which increases the stability of the stack, and having “cans always go on bottom” as the default rule means less choices have to be made in the moment, increasing efficiency.

Hope that helps!