Material handling is a broad subject that involves the transfer of cargo on conveyors during manufacturing, warehousing, and distribution. Conveyors are widely used to move quantities of raw materials, work-in-process, and finished goods. They are often the most cost-effective means of safely transporting large volumes of product, and they lend themselves to automated operation. There are two material handling subcategories in belt conveyors: unit handling and bulk handling. Unit handling deals with discrete items or packages, while bulk handling refers to relatively dry materials in loose bulk form, often transporting massive volumes at high speeds.
We caught up with Todd Swinderman, CEO emeritus at Martin Engineering to get his take on the best way to build conveyors. He discusses conveyors made from fabric or steel cable and having elastomeric, wear-resistant, and friction covers that can be flat or formed into a troughed configuration.
What is the best design approach in creating a conveyor operation?
Swinderman: Because they are so common, conveyors are often considered a commodity, but to design a system that is consistently safe and productive requires close attention to detail and specialized knowledge. Conveyor design programs with the right inputs can deliver accurate power requirements, but typically do not guide how to design a clean and efficient conveyor.
In many applications, the conveyor path is an incline or decline route. The angle of the conveyor is critical to prevent the bulk material from slipping on the belt and flowing backward toward the loading point or causing spillage by rolling downhill. Because the steeper the angle the shorter the conveyor, there is a tendency to use the maximum angle of conveyance from the available handbooks.
Slippage occurs when the coefficient of friction between the material and belt surface is less than the tangent of the angle of incline. If the cargo is spheroidal, wet, or tends to fluidize, the material may not go up the incline at all. Material testing of the coefficient of friction between the belt and bulk material under all likely conditions is important for consistent and reliable transport uphill or downhill. In many parts of the world, a system may work fine in the dry season but operate at a much lower capacity during the rainy season due to the changes in the coefficient of friction between the belt and load.
What are the principles involved in selecting a conveyor?
Swinderman: If a conveyor is purchased on price alone, chances are it will not perform as expected. Suppliers who focus on price are concerned with putting as much cargo as possible on the narrowest and shortest conveyor, with minimum compliance to standards and codes. On the other hand, if the focus is on transporting the material cleanly and safely at design capacity, engineers should design from a view of safety and lowest-cost-of-ownership. It takes an owner willing to invest a little extra upfront to get long-term benefits. Engineers need to understand conveyor design and also how the materials behave on a conveyor belt. Best-in-class business performance correlates closely with safety, reliability, and cleanliness when it comes to conveying.
Usually, a project starts with a general idea of the material to be moved, the design capacity, the conveyor path, and the space for installation. Not understanding the practical conveying limits for these four basic inputs is often the root cause of poor performance, unplanned downtime, safety incidents, and even catastrophic failure.
What’s the first step in designing a conveyor operation?
Swinderman: The first step is investigating the material properties under the conditions they will be conveyed. Detailed material properties should be obtained early in the process. This step is often skipped because of the cost of laboratory testing. But unplanned downtime in bulk material handling can easily cost $10,000 per hour, and $50,000 per hour is not uncommon. Therefore, a single unplanned outage due to improper material evaluation can more than offset the cost of a full set of test results.
Designers need to keep in mind that it’s very likely that capacity requirements will increase over time to meet demand or cost goals, and that the handleability of the bulk material will decrease due to sourcing less expensive materials or natural changes in the raw material body over time. The size and shape of lumps on the belt is an important design limitation as well. Even if the specification calls for a maximum lump size, crushers and screens are often not well maintained, so oversized material may need to be handled on a system that can no longer transport it cleanly and safely at design capacity. Rather than stop production, the belt runs full speed, and chute blockages and spillage result.
How do you select the right belts and conveyor speeds?
Swinderman: Arriving at a workable combination of capacity, belt width, speed, and component life is an iterative process. With a given capacity and material, the belt width and speed options can be compared by determining the cross-sectional area of bulk material a given belt width will support without spillage. Capacity is then the product of belt speed times, the bulk density times, and the cross-sectional area per unit length.
Design manuals will provide a general idea of usable belt widths for the calculated cross-section, and they often make suggestions for safe belt speeds based on dust, noise, and wear. Both of the major conveyor design methodologies, CEMS and DIN recommend derating capacity by 10 to 15% to handle surge loading and reduce spillage. There is a temptation to convey at the high end of the speed range at the maximum angle of conveying ability because a fast belt can be narrower, a steep belt shorter, and therefore the capital cost should be lower.
Once the load and belt width speed is selected, the main components are chosen based on the load and belt weight per unit length and the belt tension. First, the belt tension is determined, the strength of the belt required is established, and a suitable belt construction is selected. Belt strengths are measured in pounds-per-inch of width (PIW) of working load or N/mm of ultimate strength. The main pulley diameters and shafts are selected based on tension, reaction forces, and the available friction between the belt and the driven pulley. The carrying idler spacing and size is then selected based on tension, the load of the belt, and material per unit length, typically with the amount of sag between idlers being limited to 1-2% to reduce edge spillage between idlers.
The return rollers are selected based on their spacing, the weight of the belt per unit length, and belt tension. During the selection process, it’s common to return to the initial capacity calculation and vary the belt width or speed to arrive at a workable design. Specification of fabrication and installation tolerances is often overlooked in the design stage, making it difficult to install a conveyor that runs straight and level, minimizing component wear and energy consumption. In operations with many conveyors, standardization of components is often desired, which may affect component selection.
What are some of the other considerations is conveyor design?
Swinderman: While this summary is a brief overview, there are many more design decisions to be made regarding power requirements, components, accessories, loading and discharge chutes, dust control, and other details related to movement and storage of the bulk material.
Lack of adequate access for operation and maintenance is a critical productivity and safety issue. In some cases, fitting conveyors into an existing process can be a needle that’s hard to thread, and a conveyor ends up pushed against a wall or results in low headroom obstructions, making it impossible to operate properly and maintain safety. Even in greenfield projects, conveyors are often placed in tunnels or galleries with access only to one side to keep capital costs low. This basic design mistake makes it less likely that a squeaky bearing on the inaccessible side will be replaced before it fails. Failed bearings are a major cause of belt mistracking and conveyor fires. It can take twice as long to change a hard-to-reach bearing as it would if minimum access for maintenance is allowed, so these problems are often ignored until the belt will no longer operate, resulting in unplanned downtime. The CEMA design manual suggests minimum clearances around the different sections of the conveyor.
What are the safety considerations?
Swinderman: Conveyors have numerous in-running nip points everywhere the belt touches a rolling component. The trend today is to completely guard conveyors and limit access. In designing the structure, consideration should be given to the spacing of supports for modular guarding panels that weigh less than 23 kg for safe handling. Guarding is often interlocked to the conveyor drive in critical areas where frequent inspection, adjustment, cleaning, or maintenance is required. On long conveyors, it may not be practical to interlock all the hazards, so area fencing is often used with controlled access points and video monitoring. More recent developments include RFID sensors worn by workers with corresponding interlocks on the conveyor guards or restricted-access area.
Cleaning up around the conveyor is the source of about a third of all conveyor accidents. Most standards allow the tail pulley and the bottom of the return idlers to be only 300 mm from the floor. If the conveyor was undersized or tends to wander back and forth, spillage in excess of 300 mm deep can accumulate in just minutes. The need to clean while the conveyor is running to maintain production often trumps the need for shutting down and following Lockout/Tagout/Blockout/Testout procedures.
Even if the conveyor is shut down, shoveling and cleaning is the source of many muscular-skeletal injuries due to the awkward positions necessary and the heavy effort required. The unfortunate result of the lack of access and ergonomic considerations in the design stage is the risk of a lost-time injury, permanent disability, and even death. All this can be managed by elevating the conveyor structure to a safe cleaning height and guarding the nip points.
The efforts at safety go beyond just personal safety. Today monitoring of individual conveyor components is becoming more common. It has been standard practice for many decades to monitor key indicators of conveyor performance such as main pulley rotation, main bearing temperature, and belt alignment. You can even use video cameras at load points for detecting plugged chutes. At first thought, these appear to be production-related issues, but in reality, conditions being monitored are for both personal and system safety.
What are some of the principles in keeping a conveyor operation healthy during use?
Swinderman: A significant root cause of the need to clean is carryback – the material that clings to the belt after discharge. This material is often very fine and sticky, meaning it will not flow well in chutes or off of a structure, even at very steep angles. Some of the carrybacks can dislodge on the belt’s return run when it contacts the return idlers. It can then fall to the floor or ground where it must be cleared so it doesn’t build-up to the point where it stops the conveyor from running or becomes a potential fuel for a fire or explosion. Carryback that builds upon structures and rolling components can cause a variety of serious problems, including corrosion, structural overload, and belt mistracking.
Belt cleaners are used on the return run of the conveyor, typically on or near where the belt leaves the discharge pulley, to remove carryback and mitigate these issues. Because of the nature of their job, they are subject to significant wear and abuse, requiring frequent inspection and prompt service when problems are detected. Belt cleaners are most effective when installed near the discharge pulley, and this area is often difficult to inspect and service due to drive components and a general lack of access.
This has led to the automation of belt cleaners so they can be remotely monitored and adjusted. With the development of wireless communication that can deal with the interferences caused by large motors and production equipment, belt cleaners can now be monitored wirelessly, sending signals to a user’s computer and even to a service provider’s smart device to seamlessly schedule maintenance, avoid unplanned downtime, and reduce the frequency of a cleanup crew’s exposure to injury.
Any last thoughts?
Swinderman: There are many design traps to avoid in conveyor belt design. Just as in the children’s fable of the tortoise and the hare, slow and steady wins the race, this time for cleanliness, safety, and profitability. Adding as little as 10% extra to the design cost and designing for the lowest cost of ownership can result in a safe, clean and productive conveyor with an almost immediate return on investment.
Rob Spiegel has covered manufacturing for 19 years, 17 of them for Design News. Other topics he has covered include automation, supply chain technology, alternative energy, and cybersecurity. For 10 years, he was the owner and publisher of the food magazine Chile Pepper.
Article courtesy of Packaging Digest's sister publication Design News.