Replacing Media

Question:

We have a long, continuous vibratory machine, about 30 inches in diameter by 16-ft long. We add media every day to keep the level up. The time cycle is about 30 min. The trouble is that worn-down media gets lodged, resulting in high inspection and dislodging costs. We are considering starting with all new media—eliminating regular additions—and running until lodging occurs. Then, we will completely replace all the media. Will this work, and how long will we run before the media has to be thrown away? And, what problems will we encounter with this method? Thanks, in advance, for any light you can shed on this problem. J.M.

Answer:

You ask a tricky question. The size of your machine does not matter. The final answer will be determined by actual experience—my estimate is 150 hours. Here is how I got there:

The load level necessary to give you the 30 minute time cycle must be maintained. Your present method is to replace worn media on a regular basis. If you do not do this, the ratio of parts to media will increase, leaving more and more parts in the machine to maintain the process time. This happens automatically if the input rate of parts is constant. You will have to empty the machine and start with a new load of media when one of two events occur: 1) parts start damaging each other or 2) media starts getting lodged.

Which event happens first depends on how small the media gets before it starts lodging. It is likely that part damage will occur before the media begins lodging.

My guess is that when the media volume is one-half of the original volume, you will start to see part damage. To determine if this will occur before media starts lodging, you need to know the relative mass of lodged media to new media. Take a number of pieces of new media (20 pieces is a good number) and weigh them on a lab scale. Then, take the same number of the largest pieces of lodged media and weigh them. Subtract the weight of used pieces from the weight of new pieces. Divide that result by the weight of the new media and multiply by 100, and you have the percent of the media life you can expect before lodging occurs. This number might be 50–75%. If the number is higher than 50%, and if I’m right that 50% media load is when part damage occurs, then you will be removing the media before lodging occurs. In this case, your media cost will increase with this method.

Most popular ceramic media wear out at about 1% in three hours. If you can use it until it is 50% worn out, that will take about 3 x 50, or 150 hours running time. And, if lodging occurs before there is part-on-part damage, the hours will be three times the percent useful life as calculated in the weight test. For example, if you calculated that 65% of the media life could be used before lodging, and the parts were still not damaging each other, you would go 65 x 3, or 195 hours before changing the media.

In addition to possible damage to parts, you will have trouble getting all the parts out at the end of a production run as media levels shrink. This may require manual unloading—a costly process with your style of machine. On the positive side, you may have some other mass-finishing applications in your shop that can use the smaller media discarded from this operation—or, another company may buy the media.

It is unfortunate that continuous machine manufacturers have not perfected undersized media sorting. There are attempts to do so with second screen decks, but these methods aren’t very successful in the straight through designs. Bowl style machines do, however, have more effective sorting screens under the main separation screen. Some companies have even installed separate media sorting systems. This allows regular removal of undersize media—though at a rather high cost.