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Trommel Screens 101

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mccloksey 621re trommel screenA trommel screen is named for the screened cylinder used to separate materials by size, for example, separating the biodegradable fraction of mixed municipal waste or separating different sizes of mulch or crushed stone.  Production rates can vary from 20 to 30 yards an

hour to several hundred yards of material an hour based ona number of factors described in this article.

We are going to focus on portable trommel screens in this article.  There are a number of manufacturers that provide a large range of stationary trommel systems; including several of the manufacturers of portable units that I will discuss below.

Current trommel screen manufacturers include: McCloskey, Doppstadt, Powerscreen, Wildcat (Vermeer), Komptech, Terra Select, and Screen Machine Industries.  Other brands of trommel screens that are no longer available new include: Retech, Royer, Finlay, Extec, Portec, Morbark, Diamond Z.

There are a number of differences between each brand, but some of the primary differences that you should be aware of include:

  • Drum design– There are two primary designs available on the market:
    1. powerscren 2100 trommel screenInclined screen cloth segment trommels.  These trommels use an include drum with replaceable screening segments to perform the screening process.  This is the design most commonly found in the US market.  Most trommels have a base angle built into the design which can be adjusted with hydraulic legs on the trommel and allows flow control.  The primary benefits of this design are cost, durability and simplicity of the screen segments which can be individually replaced.  The screen cloth is simply wire mesh which are commonly 4' in diameter (length is determined by the diameter of the drum) and are simply bolted inside the trommel drum.  The biggest drawback of this design is that it can take a couple hours to change out the screen cloth in order to screen different sized material, and flow control is somewhat diminished compared with the spiral drum design.  Also, this design often increases the load height of the trommel.  This design is often utilized in sand, gravel, topsoil, compost and greenwaste applications.
    2. doppstadt trommel screen drumSpiral, whole drum trommel screens.  These trommels include a spiral drum configuration that controls the flow of material through the screening drum. This design enables operators to pick the whole drum out of the trommel with a loader, and swap it with another drum to produce different sized material in about 15 minutes.  This design also enables greater flow control to maximize material sorting capabilities per load.  Load height in this style trommel is often much lower than the inclined design.  The primary drawback is the cost of the drums and inability to replace portions of the drum if damage occurs.  This design was first introduced to the US market around 2001 and is mostly commonly utilized for topsoil, compost or greenwaste applications.
  • Sorting Ability – Most of the machine on the market are 2-sort, although a few of the largest machines on the market offer 3-sort capabilities.
  • mccloskey 621re trommel screen drumDrum Size – With any trommel, the amount of the screen cloth will be the most important factor in determining production capabilities.  Please note, all manufacturers quote drum size rather than the amount of screen cloth.  The drums often have several feet of flange at the beginning and end of the trommel, such that the actual amount of screen cloth is less than the length of the drum.  Drum diameter is just as important as length.  The diameter of trommels on the market range between 5' and 8', with most machines having a 6' and 7' drum diameter.
  • Durability – As with all equipment, the various brands offer substantial differences in durability of the machines.  Many of the lighter-duty brands are not well-suited for tougher applications such as sand and gravel.
  • Conveyor System – Most trommels today come equipped with two attached stacking conveyors; one for the "fines" that fall through the drum screen, and the second conveyor for the "overs" that go all of the way through the trommel and out the back.  Some larger or older trommel screens do not have conveyors attached and require separate stacking conveyors to stack the material.  Separate conveyors are handy for creating larger piles of material without moving the trommel or pile, but make moving the machine around the yard or to different locations much more difficult and expensive.  Also, some trommels come equipped with a radial stacking conveyor for the "fines" which allows the operator to create a larger pile before having to move the material or the trommel.
  • Infeed Hopper Size – The hopper size will determine how much material can be feed into the trommel at a time.  Larger hoppers will permit the use of larger loader buckets, make better targets for the loader and can often help prevent bridging in your trommel screen.  Hopper sizes range from a couple yards to as much as 9 yards in some of the larger machines.

It can be difficult for first time buyers to recognize the difference in trommel screen performance.  It is easy to get caught up watching how fast material goes through the machine rather than how well the machine actually sorts the material.  True trommel performance is based on its ability to sort the greatest percentage of "fines" out of a given material over a specified amount of time (such as per hour).  Performance will vary greatly based on the size of the screen cloth as well as across different materials types.  It is sufficient to say, however, that great amount of screen cloth, and the longer the material stays in the drum, the higher the percentage of fines will be sorted out per load.  Each operator will have to experiment some with their material to find the best balance of sorting ability and production for their operation.  At most, you want to ensure that the feed rate for the trommel is no faster than your ability to feed the machine.  This will maximize the sorting percentage that the trommel is capable of without slowing down your loader.  Your ability to load the trommel will depend primary on: the size of your loader and bucket, type of material, and distance of the trommel from the raw material.  Keep in mind that your loader will have to spend time moving the output piles if you do not have separate long stacking conveyors.  With smaller trommel screens, your loader may be able to easily keep up with the machines processing capabilities in which case you will have to balance the sorting percentage with the cost of a idle loader and operator.

Other features of trommel screens:

  • mccloskey 621re trommel screen brushesBrushes – Nearly all trommels utilize brushes mounted along the screen cloth which helps to keep the screen cloth from becoming clogged with material.  As the brushes wear, they can be adjusted to utilize the full length of the brushes before they must be replaced.  The brushes are generally about 4' long on most trommels, similar to the screen cloth segments.
  • Remote Controls – Some trommels are equipped with remote controls which allow the operator to activate the infeed and drum rotation as well as control their operating speed.  The remote can also often turn on the conveyors and include emergency stop.  Remote controls are not generally necessary for trommel operation, but can improve production rates on machines that offer variable infeed rate and drum rotation speed.
  • Grizzly – A grizzly is simply a set of large bars across the infeed which perform an initial sort on material dumped into the infeed of the trommel.  They are primarily utilized in rock and gravel operations and enable the trommel to sort out large rocks before they enter and damage the trommel screen.  Many of the grizzlies are angled so that material simply falls off the side of the machine.  Many are also equipped with hydraulic cylinders to lift and dump any material that has piled on top. 
  • doppstadt sm720k trommel screenTracks vs. Rubber Tires – Most trommel screens on the market have rubber tires as opposed to tracks.  Tracks add considerable cost and weight to trommel screens and require a separate trailer to be transported.  Tracks primarily offer the benefit of moving the trommel screen rather than the output piles as material accumulates.  This helps prevent any break in trommel operation as operators work to move output material away from the screen.
  • Reversible Infeed – this is a fairly common option, especially on later model trommels.  The primary purpose is to help alleviate bridging that may occur between the hopper and screen drum.  This relevance of this feature will vary based on application.

Tips and Alternative Applications

  • Trommel screens work best on dry material.  Material that is too wet can stick together and clog the screen cloth, reducing production rates.  Keeping your brushes correctly adjusted can help keep screen cloth clean and working properly.
  • Color Mulch With Your Trommel Screen – Trommel screens also work very well for coloring mulch.  Simply seal the drum with plastic, insert a spray bar attached to a color pump and begin making perfectly colored mulch with minimal colorant.  This dual ability of trommel screens to both sort mulch, and then produce perfectly colored mulch makes trommel screens very popular in mulch yards.

If you have any questions, or anything you would like to add to this introduction to Trommel Screens, please let us know.  Also, if you need any help finding the right machine for your application don't hesitate to contact us via phone, or through our online contact form.  If you are looking for a used trommel screen, please see our selection of used trommel screens for sale.  If you need any help selecting a new trommel screen, we can provide an unbiased opinion of the various brands currently available.