An odd aerodynamic phenomenon

In My Humble Opinion
1

The machine pictured is very similar to one I worked around in a corregated box plant. Where I worked, a die cutter would cookie-cut small boxes out of larger sheets and form a little stack which would be advanced to the guy who strips away the trim and stacks the finished boxed. The trim would be kicked into this thing, which shreds the scrap which then gets sucked up by the vertical tube behind it. On this machine there is a lip on the bottom front edge. Our chopper didn’t have that lip, and small pieces of scrap would get under the box, and actually pack in tight enough to raise up the box. And it was heavy. Many’s the time the worker would be trying to pry it up with a crowbar and blow out the scrap. I never understood it. Similar suction tubes came down from the ceiling elsewhere in the building which didn’t have a chopper. They ended with a short slanted section which rested on the floor and had the same packing problem. I finally sealed the lips with grout, which should have been done decades earlier. I’m probably not doing a great job describing this machinery.

https://live.staticflickr.com/65535/51285326626_b11521416a_m.jpg

2

I can think of a couple of things that may be going on. First, the air velocity entering the intake of the machine is predominantly horizontal, so there is nothing to lift a piece of scrap headed towards the intake over the gap. So the scrap goes into the gap. There may also be a slight attractive force between the scrap and the floor (especially if the scraps are flat) such as static electricity, though I think that may not play much of a role here. Second, once a piece of scrap is in the gap, it will be hit by other pieces of scrap, further driving it in. And while the intake duct may be heavy, it is moving during operation due to the machine vibration, and this movement allows the scraps to be driven further under the intake duct. The lip on the machine pictured prevents scrap from getting under the intake, but a wedge or something forming a ramp into the intake would be better, but the vertical lip is good enough to prevent problems.