This blog is the first in a three-part series about plastics in drag conveyors.
Many single-chain and twin-chain conveyors use plastics for their paddles. We, too, make our paddles from plastic. But we don’t limit our use of plastic to these components. We also use plastics to reduce wear and friction throughout the conveyor.
It’s a worthwhile study to consider where plastics can and should be used in drag conveyors. How a manufacturer designs their conveyor and what materials they use impact how well the machine performs.
There are three reasons why it’s a good idea to use plastics in drag conveyors beyond the paddles. 1) Plastics reduce friction. 2) They reduce wear. 3) They control the wear the machine inevitably incurs.
Friction and Wear in Drag Conveyors
First, a word about why reducing friction and wear is important. The more friction a system has to overcome, the less efficient that system will be. In other words, the more friction in your conveyor, the more electricity you’ll need to power the conveyor beyond simply moving material. And electricity, of course, costs money.
As for wear, more of it means additional maintenance and possibly a shorter system lifespan. The faster components wear, the faster they’ll need to be replaced, which means the conveyor will require more time, attention, and cost. And it means there’s an increased risk that something might break unexpectantly, thereby requiring you to shut the conveyor down and halt operations.
Enough said. Reducing friction and wear is important. Now, as to how plastics accomplish this:
Reducing Friction in Conveyors
Plastics reduce friction because plastic is more slippery (has a lower coefficient of friction) than steel. Specifically, ultra-high-molecular-weight polyethylene (UHMW), which we use for our wear strips, has a kinetic coefficient of friction between 0.10 and 0.20 with steel, depending on the UHMW used. Steel, on the other hand, has a kinetic coefficient of friction with itself of about 0.6.
Plastics also reduce friction with the material being conveyed for the same reason. Wood, for example, will slide over plastic easier than steel. This is especially so when the conveyor is horizontal, and gravity pulls the material straight down into the plastic.
Reducing Wear in Conveyors
Plastics also reduce wear—specifically in the chain pin and bushing—by supporting the chain over the full length of the conveyor.* The plastic wear strips in our conveyor provide the maximum surface area between the chain bars and the plastic over which they slide.
Distributing the chain’s weight over the plastic keeps it well within its pressure limits. (The plastic won’t deform under the weight of the chains and paddle assemblies.)
Controlling Wear in Conveyors
Wear, of course, will occur. The question is: on what? Too many manufacturers allow wear to occur right on the structural steel—the floors and side walls. We, however, control the wear using UHMW wear strips. UHMW is designed to resist wear, and it’s the proven material of choice for kinetic contact with steel (steel sliding over a surface). And it’s far better that wear should occur on an easy-to-replace strip than the steel structure and chains.
Now, some of you reading this may wonder why we chose UHMW instead of another plastic. After all, UHMW expands and contracts widely as the temperature fluctuates. Too many of you have seen what happens when you place a sheet of UHMW in the hot sun. It will shear bolts right off assemblies as it expands.
While UHMW’s thermal properties can be challenging, they aren’t impossible to deal with. We make our wear strips in short-enough sections, so expansion isn’t a problem, and we install the mountings with expansion and contraction in mind.
We also chose UHMW for the wear strips because it’s a good compromise of cost and wear properties. In other places, we use standard polyethylene, and in other places, we use nylons. UHMW is simply the best for this application.
Additional Benefits of UHMW Wear Strips
Drag conveyors have traditionally been designed with the chain running on the floor of the conveyor (single-chain conveyors) and the paddles dragging on the floor and walls (single- and twin-chain conveyors). As these components run over the conveyor floor and walls, the floor and walls (and paddles and chains) wear. As a result, maintenance crews commonly have to replace their conveyor’s bottom panels whenever they replace chains. To address this issue, manufacturers commonly include abrasion-resistant flooring in their conveyor designs.
Doing this, however, addresses the symptom rather than the underlying problem. Why not simply eliminate the cause of this excessive wear?**
Because we use plastic as wear material under the chain, we can lift the paddle off the floor by increasing the thickness of the wear strips. By doing this, we further decrease friction and wear.
We also use these wear strips to line the walls of the conveyor. This keeps the paddles from touching the walls, and it controls wear. (We use angle-shaped wear strips, not flat, plastic bars. See the image below.) This has the added benefit of enabling us to attach the strips to the conveyor on the side of the strip rather than on top, so the chain doesn’t encounter additional friction and wear were it to run over exposed bolts.
Essentially all drag conveyor designs we’ve seen—both single-chain and twin-chain designs—fail to address the friction and wear caused by chains and paddles within the conveyor system. Drag conveyors are too often inefficiently designed and suck up operating dollars.
We addressed common design flaws by running our chains over UHMW wear strips. These strips let us decrease friction further and wear by keeping the paddles off the floors and walls. The result is an extremely efficient conveyor that provides long-term cost efficiency and fewer maintenance headaches.
Stay tuned for Part II of this blog series—Plastics in Conveyor Paddles.
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*This statement is most fully true in horizontal conveyor sections. In inclined sections, the weight is split between the vertical, downward force of gravity and the chain pull force.
**Due to the abrasive nature of woody biomass and other materials like aggregates, we include AR flooring standard in certain models of our SMART Conveyors™. Our point in this article is that AR floors aren’t necessary to deal with wear from internal components.