playground material and crumb rubber

Upgrading the Shredder: Cleaner Cuts and lower knife costs

by Tom Schroeder, Columbus McKinnon  October 6, 201

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Figure 1 – Hook knives.

If you own and operate a shredder, you know that the capital cost of the equipment is just a small part of the story. Shredders, by nature of their operation, require frequent maintenance. Most maintenance time with shredders is spent replacing hooks and knives.

Shredders operate in two ways: shearing (cutting like a scissor) and tearing (ripping material apart). Many types of materials can be ripped or cut apart. Sometimes having the two methods together can be an upgrade to a shredding operation. Beyond that, though, the ability to replace shredder knives without removing the shafts can save tons of time and money for recyclers and other shredder users.

Take for example figure 1 above. This is the typical hook knife arrangement found in almost all shredders. The principle is clear - grab the product to be shredded, force it against an offset hook and tear it apart. Brute force is applied in this case to accomplish the goal of shredding. The typical product produced by tearing material is not uniform or cubical. In contrast, if you shear, or scissor-cut the same tire, you end up with product as seen in figure 3 (below). In this case, the shreds were "scissor cut" using CM-patented replaceable knives, as shown in figure 4 (below).

Shredder owners might wonder why they would care how the shredding action occurs, as long as you get shredded materials or destruction of the product. The answer is really a matter of economics, and in some cases what the final product will be used for.

In many cases, shredding is for simple reduction in size of a product, so that it can be disposed of and take up less space in a landfill. In other cases, it could be for assurance that a product is destroyed. Industries such as food, alcohol, pharmaceuticals, cosmetics and consumer goods want products destroyed when they are expired, mislabelled or recalled. This ensures that the product never reaches the black market or, worse, injures or kills someone.

With recycling, materials ranging from tires to electronics can be shredded to make other products, or to recover valuable metals and plastics.
Shredded, recycled tire material, processed to a specific size and shape as is achieved by scissor cutting, can be very efficiently used as fuel (tire-derived fuel), mulch, playground material and crumb rubber, as well as in asphalt or turned back into raw materials using pyrolysis, as some examples.

Breaking down the Cost savingsFirst off, it should be remembered that it is the knives that are doing the cutting. The sandwiched hook blade is there for one purpose only - to grab the product to be shredded and introduce it to the knives. The sharp edges of the knives act similarly to hand scissors, and tolerances are approximately 0.001 inches from the opposing knives. The slow rotating shafts are very "beefy" and the gearbox is extremely powerful. 

So where do the cost savings come from in using replaceable knives in shredders? 

CM replaceable knives can be sharpened and reused. In some cases, when using the patented CM Double Stack knives arrangement, you can get up to six uses from one knife. What you can see in figure 4 is that bolt-in CM knives can be swapped to the other side for the first reuse. Then, for the second reuse, you grind the sides of the knives down from 2.2 inches to 2 inches. When they become dull, you then swap to the other side for the third reuse. Then for the next reuse, you grind the knives down to 1.8 inches - and then the last reuse is to again swap them to the other side of the cutter. This gives you a total of six uses from the same knife with no welding and no shaft disassembly. This is a major cost savings.

Other savings in time and money come from the clear fact that these knives can be replaced while in the shredder. You do not have to disassemble the shafts at all. This can save up to two days of maintenance time. The CM system also incorporates replaceable side wear plates. 

With replaceable, reusable knives, the savings add up. CM replaceable knives provide much lower spare parts cost, the ability to reuse knives multiple times, less maintenance time and more in-service running of the shredder during maintenance. 

This article was originally published in Recycling Product News, September 2017, Volume 25, Number 6.