25th November 2001, 2:29
Screw Trough Design
We are possibly looking at replacing our troughs that we use to convey bark and wood waste into our boiler.
The reason is that they have worn through in several places.
As I stated a while back we have tried to put in uhmw liners but they keep peeling out. We have been also looking at either a ceramic lining or putting in an industrial version of a "Rhino Lining" in the new troughs.
Our maintenance group is thinking about converting the screw troughs to a flat bottomed or box like design instead of the typical rounded bottom design.
I am leery of the flat boxlike design primarily because every screw system that I have ever seen where there is one screw per trough always has a round bottom that is just slightly larger than the screw diameter.
Could everyone please comment on all three of the issues
1) Plastic spray-on linings
2) ceramic linings
3) Square, flat bottomed screw troughs
Thank you all,
1st December 2001, 1:39
I offer the following comment based on Discrete Element Method (DEM) modeling of vertical grinding mills (they are a screw conveyor with the axis turned vertical, and must move steel balls to grind hard rock ore). Your idea to make a square box may have merit. Depending on the size of the clearances and the product size and physical properties. Wear is due to shear work. Wear is proportional to shear work. Mapping the wear pattern in crossection and down the axis give the clues as to what you need to do to improve wear life. What about increasing the diameter of the trough? This does the job of the square box, just better (my opinion). Added clearance reduces the direct high slippage of screw to wall. This creates an autogenous zone (ie. conveyed material shears more on it self than on the wall due to the lower slip rate with the wall. Their may be other complications such as compression binding that defeats the benefits. This would also be true of the box section.
All this can be modeled mathematically using DEM. We have published the concept at the Vancouver BC Canada SAG2001 conference this year reference www.SAG2001.org Tues. evening session paper 107 "Prediction of Wear on Mill Lifters Using Discrete Element Method by Dr. X. Qiu
Conveyor Dynamics, Inc.
I have been working with granular material. The gap between the screw and the casing changes the effectiveness of the screw. This is because the screw will not fill properly, and may lose some of the product through recirculation. But it is not a dominant variable. Also it depends on the angle of the screw.
If you go for a square box you will lose some of the capacity, but it may be only 20 %. You can always fill in the corners to make it more hexagonal.
Ceramics is fine. It needs a good bonding material, and a backing which does not more. Otherwise you may get cracking and peeling.
With plastic linings you need a test of some kind. It works well with some products, and doesn't with others. It's the same with rubbers. Can you do a test of some kind, or even a bench test in a laboratory? I imagine your product comes in chips or small strips of say 12 mm by 50 and binds up into balls.
Plastics also need a very good bond, and a very good backing layer. Will a local tear cause a large chunk of lining to pull off? Ask your Vendor for evidence of similar applications.
In addition to the right material you need very good and experienced tradesmen to do the job. The job can easily be messed up in the installation.
We had a case with polyurethane where spray-on was fine but the curing was not to specification. We delivered the pipes to site and within a few weeks they developped bubbles and sections turned to a cheese like brittle consistency.
5th August 2002, 18:10
I believe that in your case the UHMW liners was an excellent choice. However those liners are not very good foe adhesive applications. I have good sucess with the liners when mechanical attachement was used.
They have a very low coeficient of friction, are durable with wood products and they are relatively inexpensive.
4th December 2002, 16:02
screw trough desing
you should make your trough whit hard material as "CHT 100" or "CHT 360" CHT means canadian-heat-treated. I produced a lot of trough with hard material for boiler feeding and it last many years before change. May be the UHMW liner is not realy the right place to put it. that may cause problems with temperature variation as humidity and the friction of bark with sand may affect the life expectancy of your liners. I do not know about ceramic. You may have bridging problems with square trough and that may runs out as fast as the other ones. A polygon shape works because when we use hard plate we can't roll them. It have to be bend so it always give a polygon shape. Be sure that your new bottom part of trough is easily removeable so your next shot down will be shorter. It is possible for us to supply those or even make shop dwg for you to built them. regards.
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