Engineering task: How prevent metal balls from getting stuck when flowing down and out of a cylindrical container?

[abrek]

TL;DR Summary

Problem with getting stuck

Hello, I have a cylindrical container with a conical bottom and a hole from which a tube with a diameter of 1 cm departs. This whole system is filled with metall balls with a diameter of 1 cm, which displaces the piston. The problem is that the balls get stuck because the hole is not big enough. Is it possible to somehow eliminate the jamming of the balls without changing the diameter of either the hole or the balls? Thank you in advance for your answers!

 

[Baluncore]

Is it possible to somehow eliminate the jamming of the balls without changing the diameter of either the hole or the balls?

Where the items in the container have a fixed size, you can use a slotted disc that rotates above the hole. Only one ball may drop into each slot as the disc rotates, that ball can then fall into the hole if there is space in the tube. Other balls near the hole are disturbed by the rotation, which prevents static blocking.

 

[Chestermiller]

Even if the balls were smaller than the exit hole, the system could experience jamming due to static friction between the balls, and between the walls of the unit and the balls. This is because the piston puts the contents into compression, which increases the contact forces between the balls. This type of feeding problem is often encountered in plasticizing extruders.

 

[Lnewqban]

Could you introduce vibration within the mass of balls?

 

[jrmichler]

Flow of granular material from bins is a challenging problem. Try searching granular material bin flow. Some good hits are from companies selling solutions. Here is an example from one supplier that I randomly found from an internet search: https://polimak.com/en/how-to-improve-bulk-material-flow-in-silo-bin-hopper/, with a figure from their web site.

Another search criteria is granular solids bin flow. That search found this site: https://bulkinside.com/bulk-solids-...rtation/ten-steps-to-an-effective-bin-design/, with this graph:

This last hit has a lot of good information on the flow of coarse (larger than 3 mm) solids.

Standard techniques for getting granular solids to flow from bins include vibration, mechanically cycling bins with flexible walls, air cannons, rotating screws, and more. My old copy of the Fifth Edition of Chemical Engineers' Handbook by Perry and Chilton has a few pages on bin flow of solids, but you are probably better off using internet searching. Martin Engineering Big Blaster Air Cannons are overkill for your application, but have interesting stuff to read on their web site.

I have never seen anything related to your application, where solids size is almost equal to discharge hole size. I can only suggest that you do a lot of reading starting with the search terms I suggested.

 

[Baluncore]

I have never seen anything related to your application, where solids size is almost equal to discharge hole size.

That transforms into a problem of selecting individual items from a bulk pack. One obvious example is a lottery machine, where numbered balls are randomly selected from a mixing drum.

Bulk handling machines are used in the pharmaceutical industry, where tablets or capsules are counted from bulk and placed in blister packs.

Another example is a bottle capping machine. Bottle caps are delivered in bulk, then caps are selected and oriented correctly, before being applied to individual bottles on the production line. That requires a "cap feed" machine, just as you require a "ball feed" machine.

I have an old beer bottle capping machine. It takes bulk crimp caps in a hopper at the top, then feeds caps from bulk, into a slot where they are fed into the crimper at the top of the bottle. It is based on one face of the hopper having a rotating disc that accepts correctly oriented caps from bulk, then allows those caps to fall individually into the slotted line.

In agriculture, some seed drills are supplied with perforated discs that rotate at the ground speed of the machine. The soil is first cut and opened to the correct depth by a pair of cutting discs, (coulters), then seed are dropped down a tube into the cut, before a press roller closes the ground behind. A perforated disc meters the flow of seed from bulk, and regulates the spacing of plants in the row. Maybe it is time to investigate pea and bean sowing machines.

The problem with this hopper feed into a tube, will be designing a slotted rotating disc for the hopper, that will drop the balls individually into the tube, and cannot jam. Ball feed machines will be used in the manufacture of ball bearings, but they will probably be of a proprietary design.

An alternative design is a small chain elevator, one that selects balls in small elevator buckets. The buckets are lifted through the bulk hopper, collecting and holding only one ball each. At the top of the flight, balls are then tipped into/onto the tube opening, with any excess falling back into the bulk bin.

 

[tylerkelley1980]

I’ve dealt with this in bulk handling, and the key was adjusting the outlet design. Using a conical hopper with a steeper angle reduced bridging, and adding a small vibration motor kept balls moving. Sometimes a rotating agitator near the outlet helps too. The goal is to maintain constant flow and prevent points where balls can interlock or jam.

 

[Baluncore]

Two balls, with surface friction, can build a wedge across the exit tube. That is because the balls counter-rotate and can lock like a wedge. Three balls in a line cannot do that, as the middle ball can spin. So replacing one wall or side of the hopper with a roller or a rotating disc, (to emulate the end ball of three), will break the two ball bridge whenever it forms.