Drag-chain conveyors can get a bad reputation for the amount of maintenance they require. While it’s true that even the best chain conveyors need more attention than belt conveyors, it’s a misconception that all chain-style conveyors require excessive time and attention. There are well-designed drag systems on the market, our SMART Conveyor™ being the prime example.
So what makes for a poorly designed drag conveyor and a well-designed one? And how do these designs relate to maintenance? To answer these questions, let’s address some common problems.
Problem: Uneven Wear in Twin-Chain Conveyors
From time to time, we hear concerns from prospective customers about uneven chain wear in dual-chain conveyors. They’ve tried such conveyors before, and one chain has always worn much faster than the other. To a certain degree, we expect this to happen. There’s no way to prevent uneven wear entirely, as it isn’t possible to ensure all the conditions that contribute to it will be precisely the same on both chains. Yet while we can’t prevent it, we can minimize it with a good design.
Good design starts with sound metallurgy. This is almost impossible for us to control, though. The slightest variation in heat treatment from one batch to the next or in the process of tab welding changes everything. (Tabs on our chains are robotically welded before heat treatment.) We do our best to specify metallurgy, always striving for the best strength-to-weight ratio, and we check for quality issues, but we understand the reality of the market.We lose control once we install them and hand them over to a client. And let’s not forget that the client wants to pay as little as possible for the chains.
The second thing you should know about uneven wear is that it’s most often associated with uneven loading. If you drop incoming material into the conveyor so it collects on one side of the trough, one chain will do more work than the other as it pulls the material along. Look at the sprockets if you need to confirm that you’re loading the conveyor unevenly. You’ll see the same, uneven wear as in the chains.
The best way to ensure you’re loading the chains evenly is to specify as narrow a conveyor as possible. If the load in front of the paddle is deep, you have a better chance it is also even. If your current conveyor is running too fast, it can allow more material on one side as well. Slow it down, and fill the pocket. You will then have a more evenly loaded set of chains.
Problem: Overloading the Chain
Overloading the chain is another common problem. We calculate (and have empirical evidence to support) chain loads on every conveyor we build. We have proven that you get adequate and relatively even chain life if you stay well inside the working load envelope. It is important to note that we have established our chain working loads based upon the manufacturer’s ultimate strengths. We do not accept the various wild claims of working loads. We use a percentage of ultimate strength as our guide. When you overload the chain, the chain life decreases, sometimes drastically.
Our practice of calculating the chain load stands in stark contrast to other manufacturers—the company that built the conveyors a failed U.S. pellet plant, for example. Sometime during the short life of this operation, we visited and calculated the chain load on one of the conveyors. We examined the chain; added up the weight of the chain, paddles, and frames; and accounted for friction. With that information, we discovered the chains were running 900 percent above their working load. In other words, the conveyor could safely handle only a tenth of the material it was expected to carry!
This pellet manufacturer essentially failed at startup because the conveyors continually broke down. We aren’t saying the chains snapped every time the conveyors started, of course—as long as you’re under a chain’s ultimate strength, it will perform for a short time—but the chains never lasted long.
Waiting Too Long
Related to the problem of overloading the chain is wearing it beyond the manufacturer’s limits. We’ve discovered such wear many times upon inspecting chains our customers believed were severely out of alignment. Customers often don’t understand that the chains’ published limits are important due to how the manufacturer makes them. During the manufacturing process, the company case-harden the chains, not through-harden them, so they retain some ability to withstand impacts. Thus, when a customer runs a chain so long that it wears past the case hardening, wear exponentially accelerates, as it has worn into the soft metal.
Problem: Too Much Wear
Excessive wear is yet another complaint associated with chain conveyors. Inspect virtually any chain or paddle conveyor, and you’ll find a common design flaw: the chains and paddles drag along the sidewalls and floor of the conveyor. This is problematic because the design increases the friction load (which decreasing efficiency) and internal wear. We refer to such machines as “dumb conveyors,” especially when they’re little more than steel boxes and chains.
Of course, the average guy at the average fab shop doesn’t consider friction and wear. We do, however. We support the chain assemblies on low-friction UHMW strips the entire length of our SMART Conveyors™ so the chains don’t touch the floors. The only steel-on-steel contact in our conveyors occurs at the sprockets. Even when other manufacturers get less “dumb” by adding a UHMW floor liner, they miss the fact that they’re supporting the system in the wrong place.
Wear in the average “dumb conveyor” is further exacerbated by a process we call scalping. Scalping occurs when the chain flexes to climb over material in the conveyor trough, usually at a chute and often at an inlet (material can build anywhere material catches inside the conveyor). It happens at every paddle when the paddle touches the floor no matter whether there’s a wear liner or not. It’s is an issue because it works the chain’s pins in the bushings—wearing them—every time the chain goes over the material. And pin wear is an issue because it’s the failure of this component that virtually always causes the chain to break in real-world applications.
Scalping isn’t an issue in SMART Conveyors™, though. We don’t let the paddles contact the floor, and the channels through which the chains run capture the chains, preventing them from flexing over the material. This protects the pins and bushings, so they flex at the sprockets and curves only.
Wear is also at times associated with a lack of controls. Few mills have good controls schemes that ensure the conveyors run only when needed and at the minimum speed required. We’ve seen many mills where the conveyors run through lunches and breaks. We’ve even seen mills run them at night just in case someone would be there cleaning!
Wear occurs at all times when the conveyor is running—the wear on the chain’s pin and bushing is relative to how many times these components flex. So, letting the conveyor run when it doesn’t need to and letting it run faster than it needs to leads to unnecessary wear. Shutting the conveyor down and running it as slow as possible (while staying within the chain’s safe working load) extends the chain’s life.
Other maintenance woes arise from how easily chain conveyors can receive damage. To this, two points can be made. First, most conveyors are designed without fail points. Stresses the average conveyor encounters when it meets an obstruction are primarily transferred to the chains and shafts. There’s nothing that gives way to reduce the forces on critical components. Thus, the chains are shafts are more likely to break when something gets into the conveyor that shouldn’t be there.
In contrast, SMART Conveyors™ have multiple fail points to protect the chains and shafts, the first of which is the paddle. Our paddles are designed with “fingers” or “piano keys” that flex when they encounter an obstruction and thereby limit the stress applied to the rest of the system. Beyond the fingers, the paddles themselves flex and absorb stress. When the paddles flex too much, the paddle frame then breaks. Better to lose a paddle than a chain!
The second point regarding damage is that no conveyor, no matter how well made, can avoid damage if it’s fed material it’s not designed to handle, particularly if that material is too large. Companies must screen their materials before the materials enter the conveyors if they want to avoid damage.
Systematic prevention thus proves the key to avoiding headaches related to chain conveyor maintenance. While changing the oil regularly and checking the chains for wear are essential tasks, you can prevent many problems by installing a conveyor designed to avoid a cacophony of maintenance issues. Design is crucial. And a well-thought-out design is what we’ve developed.