To many people, a conveyor system is – quite literally – straightforward. It is perceived to merely move along a track in a straight line at a constant speed and direction, transporting products or raw materials from point A to point B. What could be simpler than that?
In fact, industrial conveyor systems can incorporate multiple levels; changes in direction and speed; accommodation for various loads of different weights, shapes, sizes and materials; and be fitted with sophisticated controls and tracking devices, so they can be seriously complex pieces of kit.
However, for specifying any conveyor there are only five essential criteria, according to Florian Kley, global product manager at Interroll (Schweiz). They are: maximum throughput per unit time, geometry of the material to be transported, weight and material of the transported product, control requirements and environmental conditions.
He adds: “Once these requirements are established, users should base their conveyor selection strategies on different key performance considerations.” They might include the drive architecture, for example – centralised or decentralised – or the degree of modularity of the chosen conveyor platform; whether multiple tiers are necessary; how variable are the products to be transported, and the extent of labelling and identification required.
Once the right conveyor concept is chosen, it is time to consider existing space, environment and process conditions. For example, pillars or existing machinery may pose constraints that might require the usage of modules like curves within the system. Normally, a curve is integrated between two conveyor lines. Their interface is crucial to keep things moving; using the same technical supplier for both modules (lines and curves) avoids possible problems during operation.
TYPES OF CONVEYOR
Roller and belt conveyors are the two principal types of package conveyors, according to Dave Bull, head of sales in Northern Europe at Dematic (pictured, left). He adds: “The key criteria are the load and whether or not it is conveyor-able… [If it is], the bottom of the load should touch at least three rollers at any given time. The pitch of the conveyor can influence its cost because there are fewer rollers on a wider pitch. If the load is more problematic – for example, polythene bags – then you would probably opt for a belt conveyor.” Roller conveyors are typically used for packages, workpiece carriers and other units with a stable structure.
Dead roller conveyors, according to SSI Schaefer, are often employed in areas where totes (standard plastic boxes) can be moved manually, such as shipping areas. Gravity roller conveyors descend in the conveying direction, using gravity to propel transport totes.
Conveyor belts, meanwhile, often powered, are used for horizontal transport and for ascending and descending conveyor lines (up to, but not beyond, inclinations of 18°, according to SSI Schaefer).
But how are changes in direction catered for? Ed Brundrett, technical sales engineer at LB Foster Automation, provides a few answers: “You could employ turntables or transfers. You could also have arms that push the product sideways off the conveyor.”
As another example, belt diverters push the transported unit at a right angle to the conveying direction, ending up on a parallel conveyor line at the right or left side. Diversion is carried out by lifting the transport unit and pushing it over to the adjacent line. Depending on the length of the transport units and the characteristics of the bottom side of the belt, the belt diverter is equipped with two or five belt supports. The belt diverter may be retrofitted without problems into an existing live or accumulation roller conveyor. (See also box, p24, for other examples).
Alternatively, conveyors can incorporate curves for changing direction. Dematic’s Bull says there are essentially two types of curve: “You either have a roller bed with tapered rollers, or you can go around a bend with a belt curve. This is a more expensive option, but it does give you a tighter radius.”
To that list Jon Warrilow, design manager of Conveyor Units, adds that curves can also be made using a belt of articulating plastic sections or slats. He states: “The design of the curve is, first and foremost, based around the product to be handled. Belt curves are capable of high speeds, small products and maintain product spacing.” He says that roller curves are cost-effective and adapt to various drive means, including O-ring drive, poly-vee belt drive, round cord drive beneath the rollers, line-shaft, timing belt-driven and chain-driven drives. Belt slat curves are primarily used in food and bottling/canning industries – their benefits being easy wash-down – and they are able to slide under the product if the product is accumulating.”
For SSI Schaefer, curves are used within live or accumulation roller conveyors for turning totes, while guaranteeing a continuous flow. It is also possible to create an accumulation area in curves. By combining different curve segments, it is also possible to build S-curves.
Conveyors are generally designed as low-maintenance and high-availability (more than 99%) systems. Maintenance schedules normally include regular visual and noise testing procedures. The critical issue with regard to the maintenance of curves is belt tension; conventional, friction-driven belt curves frequently present the problem of belt slippage and, as a result, overheating.
On top of this, friction-driven curves are subject to significantly higher belt wear due to the necessarily-high belt tension; that also means that the idler pulleys have a lower service life. This maintenance issue does not exist if a positive-driven curve is used; it has no belt tensioning and belt slippage is technically impossible.
Jon Warrilow, design manager of Conveyor Units, expands on this point: “Different types of conveyor need different service checks and routine maintenance. DC motor roller curves require very little attention, as they have brushless DC motors with a life of 20,000 running hours.
Other components further that trend of reduced maintenance, he adds. Modern motor gearboxes have life-long gearbox oils; line-shaft curve components and drive and jump belts benefit from advances in polymers; sealed-for-life precision bearings also play a role.
Nonetheless, a range of maintenance options are available from suppliers should they be required. Dematic, for example, can place a full residential team on a customer’s site or can simply visit every six months to check for wear and tear.
LB Foster Automation’s technical sales engineer Ed Brundrett concludes: “Maintenance needs will depend on what is going on to the conveyor; whether you are operating in a harsh environment; whether you have something that is potentially going to damage the sides of the conveyors; if people are using forklift trucks to load on to conveyors. In general, we would say that it is probably worth having a look over the conveyor system once or twice a year.”
BOX OUT: Alternatives to curves
Horizontal curves are not the only way that conveyors change direction. Jon Warrilow offers the following as alternatives:
● Ball tables: Require human intervention, but allow boxes to be turned and re-oriented before being ‘pushed’ on to continue their journey.
● Transfers: Automated, so they come with associated controls costs. They also change the boxes’ orientation by 90°. They slow product throughput down and can become a ‘bottleneck’.
● Sorters/wheel diverters: Faster throughput than transfers, as loads continue their forward motion seamlessly. They also maintain product orientation. However, they have a higher capital cost and require a level of automation control to operate.
● Turntables: Automated or manual, these maintain box orientation but slow throughputs down.
● Chutes: If level change before and after the curve can be accommodated, then a chute could be used. These have no maintenance or automation, but require significant level change and can offer no system control during the change of direction.
● Gravity roller or skate wheel curves: All the benefits of a chute but require a much lesser gradient to operate.
● Pushers: Where loads are simply pushed (by a pneumatic or electrical actuator) off a main line. They pose same box orientation and throughput issues as transfers. And they require automation.
● Plough arms: Like the paddles on a pin ball machine, they sweep across and guide product off a main line.