Monday, October 29, 2012

Hey Grinder Guy, does it make a difference what type of hole I use on my Grinder Screens?

WHEN Magazine Article- Nov 2012

This is one of the most overused benefits made by all kinds of Salespeople, Operators and Manufacturers.
My screens are round and they give me a better product!
My screens are hexagons and they give me a better product!
My screens are a square and they give me a better product!

I’ve heard it all. And all may be correct. They all may be wrong also.
So lets figure out what they are really saying.

Here are some facts:
  • The same size hole does not give the same size product in all materials. Green wood is different than dry wood, logs are different than yard waste, etc. The more moisture in materials, you will need a larger screen to be able to grind it. Dry wood requires a smaller screen because you need much more cutting action on all the pieces for consistency.
  • The thicker the screen, the more baffling effect you will get, which means more fines
  • Screens cut on an angle will give you less fines and more production but also larger material
  • A great piece of advice from Lori Rheinberger of, an Aftermarket Manufacturer: As a rule of thumb, 70% of your material will be ½ the dimension of your screen size or smaller. So, for example, with 4 inch screens, 70% of your material should be 2” minus with the balance being larger than 2”. Good rule to start from when trying to size a product initially.

Any screen you buy, from the OEM or Aftermarket Manufacturer, are all design engineered before they are cut, unless you and your torch are making them in your shop.
On every engineered drawing the actual open hole space should be shown. That and only that can actually compare 2 screens and how they compare to the actual product and production versus each other. Because if a screen doesn’t have the same open area, they are not equal.
What does that mean? With 4” screens, maybe 50% of the actual screen is open space for material to fall through and the other 50% is the steel framework making the 4” holes. If you increase to 6”, that open space may increase to 60%, thus more production and larger product.

But what about using the same dimension?
For example:
  • A round 1 ½” hole measures an area of 1.77 sq. inches
  • A square 1 ½” hole measures an area of 2.25 sq inches
  • That is over 25% more open space in the square hole
  • This means you will get larger pieces in the square hole and more production.
  • Multiply the 25% times every hole in the screen and you see how much more open space is in the screen and how your production will increase because of it.

In addition, when the square hole pattern is layed out on the steel for cutting compared to a round hole, the pattern lays out much more efficiently creating more holes, which means much more production because of the extra space created by the extra holes.
So the operator that says, “My square hole gives me a better product” might be comparing his production to a round hole screen.
But because the Round hole screen is smaller and then will have less holes per screen, it will make a finer product than the square screen, which the other operator may say, “My round hole gives me a better product” because he has smaller material.
Lastly, the hexagon, which seems to be the latest greatest in the industry. Layed out for cutting on steel plate it is probably the most efficient. But be careful, because a 2” hexagon does not have the same dimension area as a 2” square hole, but just by having more holes, it will give you more open space and more production. So the operator that says, “My hexagon hole gives me a better product”, may be comparing his production to a square hole he used previously. You never know what the comparisons truly are?

In conclusion, what do we do?
·         Try different screens
·         Ask your supplier for engineered drawings so you can compare open area of the screens
·         Ask your supplier what customers are buying now compared to last year or the year before
·         Thinner is better for production quantity and fines reduction
·         Try to get screens that are angle cut to increase production and reduce fines

I have some photos posted at for you to compare and review.
Questions, comments? Dave Whitelaw,

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