Industries that need to move loose bulk materials between their many plant processes are (or should be) familiar with the concept of tubular drag conveying as it applies to their operations. What they may not know is there are different conveyor types within this product category.
The notion that chain-type tubular drag conveyors are more robust than their cable-driven cousins, which should never even be considered for the same applications, is a relatively easy concept for potential users to understand. However, recognizing the important differences between different chain offerings can be a little more challenging—especially for plants that decide to experiment with more than one chain type and end up learning, the hard way, what works best and why. A little education can go a long way toward avoiding these difficult lessons and the resulting consequences, which can include excessive maintenance and prolonged downtime.
Chain-type tubular drag conveyors fall into two basic categories: those that use round-link chains and those that use engineered chains. The round-link variety is a utilitarian design that employs a mass-produced chain made from round rod. This material is cut, formed into loops, and welded together at the ends. Flights (sometimes called discs or pucks) can either be molded or bolted onto the chain. If all of the proper operating conditions exist, chains with this grade of materials and construction may be sufficient for extremely low-duty, low-risk commercial applications.
But for the vast majority of industrial environments, the brutal truth is that round-link chains aren’t good enough—not by a longshot. Initial price is almost always considered to be the biggest advantage to using these types of chains, but how advantageous is it when you consider the big picture?
Tubular drag conveyors in the other category incorporate engineered chains that are specially designed for use in heavy-duty material handling equipment. These chains can have an open- or closed-architecture design. The latter are precision-machined and assembled by compressing elastomeric washers into the hinge point of the chain, which minimizes the opportunity for material presence in that crucial area (an important consideration when handling certain materials).
Each of these chain categories features a unique design philosophy and construction. As a result, there is some variation between them in terms of operation and reliability. Overall, the performance and serviceability of engineered chains is superior, resulting in a much lower longer-term cost of owning, operating and maintaining a conveyor that uses them instead of round-link chains.
Digging deeper, here are several specific problems that should eliminate round-link chains as an option for most industrial applications—followed by an explanation of how engineered chains can solve each problem.
Problem 1: Round-link chains require dramatically longer start-up time.
It’s true that round-link chains are more flexible and enable more complex circuits with back-to-back bends that are out of phase with each other. But it’s also true that this kind of flexibility is rarely needed and doesn’t warrant what is perhaps the most significant downside to using round-link chains: they are impossible to install without inadvertently introducing a significant amount of unwanted twist in the chain. Consequently, for the first several days after the installation of a conveyor with a round-link chain, the start-up procedure automatically requires close monitoring of the inevitable accumulation of twisted chain. When that occurs, it is imperative that the conveyor be stopped immediately, before these wads of chain climb onto the drive sprocket and cause derailment—or worse, a catastrophic equipment failure. At each occurrence, the master link must be located, the chain parted, and the chain untwisted. This procedure must be repeated several times, and depending on the length of the conveyor, can take a day or two to remove enough twist that it no longer accumulates.
Solution: While too much twist is problematic, especially with round-link chains, some twist is often desirable and even necessary for certain conveyor routings to work. Engineered chains are designed with this capability in mind. Minimum distances vary from one chain to the next, and it is important to observe those distances when designing circuits so that the chain can articulate freely. This minor limitation is, by far, outweighed by the relative reluctance of engineered chains to twist. It is exceedingly easier to control them and prevent undesired twisting during chain installation, which shortens start-up time and eliminates the aforementioned monitoring, plus the associated labor costs. This comparative ease of installation is particularly important in high-production environments with short service windows, where users need the ability to replace chains very quickly and don’t have that option with round-link chains.
Problem 2: Round-link chains bring an increased likelihood of chain failure and other maintenance-related headaches.
As mentioned previously, construction of round-link chains inherently includes a welded seam where the two ends of the round rod are pinched together. This seam is the most common point of failure in a chain that’s not very robust to begin with. Operators must also bump the conveyor or go searching for a master link when round-link chains require maintenance.
Solution: The design and construction of an engineered chain eliminate the troublesome seam that exists in a round-link chain. In addition, of the many differentiating features of engineered chains, the most appealing to plant maintenance personnel is the fact that each link is individually serviceable. This allows the chain to be parted and serviced wherever it is most convenient.
Problem 3: Round-link chains result in less efficient operation with excessive spillage.
Conveyors that use round-link chains are notorious for spilling material from one flight to the next, because this chain design often leads to flight tipping. Plants often install wiper flights in an attempt to counter this phenomenon, but this is merely a stopgap measure compared to alleviating the issue with a better option.
Solution: Due to their longer pitch, engineered chains naturally hold flights more firmly and keep them perpendicular to the carry surface. This allows a conveyor to operate more efficiently and retain more material than if it were equipped with a round-link chain.
Problem 4: Round-link chains require the “over use” of roller turns.
One advantage of tubular drag conveyors is their ability to change directions. Due to the weakness of round-link chains, however, they must use roller turns in the corners to reduce the load on the chain if the configuration requires bends or any kind of conveying length. Roller turns are often placed in the side of the conveyor that carries the material, also called the “carrying” run. Unfortunately, this results in material flowing through the mechanical assembly, where it can damage the material being conveyed, cause excessive wear on the assembly itself, or create material build-up, which is difficult to remove.
Solution: Engineered chains do not require the mechanical turn sections, as the chains are designed with enough strength to withstand the loads. This effectively eliminates any performance issues associated with material entering the conveyor’s mechanical assembly.
Problem 5: Round-link chains are prone to less positive engagement, uneven wear distribution, and derailment if any slack develops.
Compared to engineered chains, round-link chains have a much shorter and narrower target, which can compromise the equipment’s positive engagement and ability to wear evenly. Also, conveyors with round-link chains usually include a spring-loaded chain tensioning mechanism. These devices are necessary to keep short-pitch chains engaged with the sprockets that drive them, but if any slack develops, derailment often occurs.
Solution: An engineered chain does not require a tensioning mechanism, and its drive sprocket teeth can be wider and longer for more positive engagement and even wear distribution by virtue of having more contact surface. Greater engagement also makes the conveyor more slack-tolerant and allows the use of a simple jack-screw-type take-up mechanism. For the sake of operating in the ideal slack-free, un-tensioned condition, most users prefer the occasional adjustment to the accelerated wear associated with round-link chains.
Problem 6: Round-link chains experience more joint separation under heavy stress.
Tubular drag conveyors with round-link chains are often supplied with an outer casing constructed of light-gauge tubing that is joined together using compression couplings. These couplings are susceptible to separation and failure in anything beyond an extremely light-duty application.
Solution: Conveyors that use engineered chains generally have outer casings that consist of heavy schedule-40 pipe and are joined using weld-on flanges that are cut from heavy plate stock. The flanges are a structural component of the conveyor and are designed to endure stress levels that are common to industrial environments. These conveyors can also be tugged into a position to accommodate unanticipated misalignment issues during installation, without compromising joint integrity.
Conclusion
While both varieties of chain-type tubular drag conveyors often compete for the same work, clearly they shouldn’t. A thorough examination and comparison of the two chain types reveals a different design mentality and vastly disparate calibers of equipment—and not all conveyor manufacturers offer the engineered-chain option. With this knowledge in mind, the only question when specifying a conveying solution is this: Is a round-link chain good enough for your application, or is your process important enough to demand the robust build and reliable performance of a conveyor with an engineered chain? When you consider all of the factors involved in the decision, the answer should be obvious.
Eric Smith is technical director – systems group for Hapman and has nearly 20 years of mechanical conveying design and application experience. For more information, contact him at [email protected] For more information on Hapman, call 800-427-6260 or visit www.hapman.com.
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