Reducing and Eliminating Carryback in Mechanical Conveyors

carryback from mechanical conveyor

Conveyor carryback increases operational costs, decreases efficiency, and increases the chance of conveyor-related injuries during clean up. Reducing carryback and tailings should therefore be a goal of every manufacturer who owns a mechanical conveyor system.

Why Address Carryback

By carryback, we refer simply to material that adheres to or embeds into a conveyor belt or chain rather than discharging at the head. The conveyor literally “carries back” the material until it falls off under the conveyor (tailings) or is otherwise removed.

There are two reasons to reduce or eliminate carryback. The first is that it increases profits. The second is that it increases health and safety.

The Cost of Carryback

How reducing and eliminating carryback increases profits should be obvious. In some industries, the cost of lost material itself is expensive. Hemp, for example, makes for expensive waste, even if minimal. Say a conveyor system creates a mere 100 grams of wasted hemp biomass an hour. After 10 hours of operation, 1 kilogram of raw material has gone to waste at a cost of $136 (2019 average price for hemp biomass). The cost is higher when end product is considered.

100 grams is a small amount of waste, however. Increase it to 1 kilogram per hour or more, and the cost grows significantly.

In industries dealing with low-cost material like sawdust or woody biomass, these numbers don’t mean much, of course. At an average of $0.16 per kilogram, companies can afford to lose a few kilograms of woody biomass.

But there are other costs to consider. There’s money spent on labor. Some operations have so many piles of dust from carryback and tailings that they have to pay employees overtime to clean it up. We know of operations where crewmen work around the clock shoveling up sawdust. Companies routinely spend hundreds of thousands of dollars in labor every year on nothing more than shoveling sawdust.

Carryback also increases the frequency of repairs. Material the conveyor carries back often ends back up in the conveyor, where it can cause damage. Costs come by the way of replacement parts and labor.

Efficiency is another area that carryback affects. In a belt conveyor, material that sticks to or which lodges in the belt increases the belt weight. With a heavier belt, the conveyor must use more power to run, which increases operational costs. Damage, whether in a belt or a chain conveyor, will also reduce efficiency.

Health and Safety Related to Carryback

Health and safety is the second major area of concern when it comes to carryback and tailings. Fugitive dust from the conveyor is hazardous to employees’ health (depending on the material; some material like sawdust is a carcinogen in large quantities). It also increases the risk of explosions, fires, and accidents. Tailings also increase the risk that an employee will become injured by the conveyor if they must clean around it as its running. Reports of injuries and deaths related to conveyors are common. Nobody wants to deal with the publicity, healthcare bills, and lawsuits that arise from such an event. And, certainly, nobody wants to inform an employee’s family that such an event has occurred.

Reducing Carryback in Rubber Belt Conveyors

Carryback is an issue associated with all conveyors. For rubber belt conveyors, specifically, a number of systems have been created to reduce it. These systems can be categorized as contact and noncontact types.

Contact systems include scrapers, brushes, ploughs, and beater bars. The term “scraper” most commonly refers to a blade-based system in which a polyurethane or metal-tipped blade is pressed against the conveyor belt to scrape off material. Primary scrapers (prelceaner scrapers) are placed immediately after the head roller. These remove 60-70 percent of the material clinging to the belt. Secondary scrapers are placed close behind these and remove finer materials. These scrapers can increase the removal to 90 percent. Tertiary scrapers are uncommon.

Secondary and tertiary contact systems sometimes use a brush to clean the belt rather than a blade. In systems with grooved, chevrons, or cleats, brushes may be used.

Ploughs are essentially scrapers placed on the noncontact side of the belt. Their purpose is to scrape off material that might get onto this side of the belt. Unlike scrapers, ploughs are not pressed tight against belt, but they are restrained from moving horizontally.

Beater bars are placed on the inner side of a conveyor belt. These systems involve a cylinder equipped with bars that, as their name suggests, beat the inner surface of the conveyor belt, causing material to loosen and fall off.

Another system used to address carryback is a belt turnover. A belt turnover system uses rollers to twist the belt as it returns to the head roller so that the inner belt surface faces the floor and the outer surface (with the carryback material) faces up. At the head roller, the system untwists the belt. While this doesn’t eliminate carryback, it prevents it from dropping on return rollers under the conveyor. Tailings thus drop off in fewer areas and are more manageable.

Noncontact systems[1] are mostly used in the food industry and are not addressed in this article.

Eliminating Carryback in Chain Conveyors

Managing carryback doesn’t eliminate waste, however. Besides the fact that there will be carryback, downsides of belt systems include:

  • Scraper systems can be troublesome to maintain to keep them running optimally. There are solutions for this, but getting the pressure and angle in the sweet spot requires vigilance (which many maintenance personnel lack).
  • They require more energy than other conveyor systems.
    • Scrapers save energy compared to not having scrapers, but they decrease overall efficiency by adding friction, and they wear the belt.
    • They’re less efficient than drag chain systems.

To better reduce carryback, a company will need to change the type of conveyance system it uses. Pneumatic systems will eliminate carryback, but they aren’t right for every application, and they require large amounts of power. A well-designed, enclosed drag conveyor system is the best choice.

We stress a well-designed drag conveyor because there are many drag conveyors on the market that look no different than they did in the 1800s. They’re what we call “dumb conveyors”: a chain with flights that runs through a box. Material gets trapped in the chains of these conveyors and falls off to form piles, which, if not managed, can become massive.

Therefore, simply switching to a chain conveyor won’t eliminate carryback. It takes Chain Conveyor 2.0: the SMART Conveyor.

SMART Conveyors virtually eliminate carryback by keeping the chains out of the material path and by fully enclosing the chain/paddle assembly. The chains run in troughs down the sides of the conveyors instead of the material path in the trough. Material therefore doesn’t lodge in the chains as in old-fashioned chain systems. By enclosing the conveyors, the minimal amount of material that does stick to the paddles is carried back through the system to be dumped in the next pass.

A related benefit in SMART Conveyors is that they are dust tight[2]. Benefits to using an enclosed, dust-tight conveyor include reducing the risk of dust-related explosions, protecting workers from breathing dust, and keeping the workplace clean. Belt conveyors are notoriously bad at containing dust even when enclosed. And open chain conveyors, obviously, do nothing to contain it. To eliminate dust, companies must, as with addressing carryback, opt for a different system.

To learn more about SMART Conveyors from Biomass Engineering & Equipment, click here.

[1] Examples of noncontact systems include modified deluge system, a steam system, and an antibacterial soap-and-water system.

[2] We can design our SMART Conveyors to comply with IP ratings, but we are using the term “dust tight” more generally in this article. Our standard SMART Conveyors, while they control dust, do not meet IP ratings for a “dust-tight” machine.