Choosing Screens for Your Grinder
“Wood waste processors face a variety of considerations when selecting the screen configuration to best accomplish a desired end product. Screen selection and grinding strategy will vary based on a variety of factors including the type of grinder being used – horizontal versus vertical – as well as the type of wood being processed, and will also differ based on tree species.”
A general rule of thumb is round holes for tub grinders and square holes for horizontal grinders.
Screen selection is usually based on two main factors – the type of material being processed and the desired end-product.
Each tree species will produce a different end product. Various tree species will react differently to grinding and can make a widely varying product that can have a big impact on the type of screen used. Moisture content will also play a big part in determining screen and end product size.
All shapes and sizes
The most commonly used screens feature round and rectangular holes as these two geometric configurations tend to produce a more uniform particle size and end-product across a wide range of raw materials. There are other options, however, and each serves a specific function depending on the application.
Diamond Hole – Ideal for processing wet and difficult-to-grind material such as compost, palm, wet grass and leaves. These materials have the potential to build up on the horizontal plane of a square-hole screen or between the holes of a round-hole screen, causing the screen to blind and the material to re-circulate, thus reducing overall productivity.
A diamond hole screen is designed to channel the material to the point of the diamond, which allows the cutter to swipe past the screen, helping to remove material that may otherwise accumulate.
Grid – Bars are welded horizontally across the screen face (as opposed to a rolled punch-plate screen) and function similar to a secondary anvil. A grid screen is often used in processing of construction debris or in land clearing applications where there is less concern about the specification of the end product.
Rectangular – Allows more material to pass through the screen due to the increased geometric size of a rectangular-hole opening compared to a square-hole opening configuration. One potential drawback, however, is that the overall consistency of the end product may be compromised. It is possible for pieces substantially longer than the width of the hole to pass through the screen. That being said rectangular holes generally produce a fairly uniform product. The height of the hole rather than the width is the determining factor in product size.
Hex – Provides a more geometrically consistent hole and uniform opening, as the distance between the corners (diagonally) is greater on a square hole than in a hex hole which is straight. In most cases more material can be processed with a hex screen — versus a round-hole configuration — and similar production values can still be achieved, compared with a square-hole screen. It is important to note, however, that actual production rates will always vary based on the type of material being processed.
The cutting dynamics of a tub and horizontal grinder differ dramatically. As a result, horizontal grinders may require the use of special screen setup configurations to attain a specific desired end product.
When using a horizontal grinder it is recommended to use a square-hole screen with the addition of baffles to help reduce the likelihood of producing oversized material. The baffle is a piece of steel that is welded on the back side of the screen – a design configuration that will help prevent long shards of wood from passing through the hole before it is properly sized. A good rule of thumb for adding baffles is that the length of the steel extension should be half the diameter of the hole. In other words, if a 10.2cm (four-inch) screen is being used, the steel baffle should be
5.1 cm (two inches) long.
Proactive screen maintenance
Manufacturers recommend that grinder engines be serviced every 200 to 250 hours, during which screens and anvils should also be inspected for wear.
Maintaining consistent spacing between the cutter and anvil is critical to producing a quality, consistent end product. Over time increased wear to the anvil will result in increasing the space between the anvil and cutter, resulting in the potential for material to pass through unprocessed. This may impact cost of operation, so it’s important to maintain the grinder wear surfaces.
The space between the cutter and screen is another area that should also be inspected on a regular basis. The gap is likely to increase over time due to wear, and so productivity may be affected. As the distance increases it will result in recirculation of processed material, which can also affect end product quality, productivity and increased fuel consumption.