Conveyors are automatic systems for moving products and materials between two points. Roller tables perform the same function, but without the automation. Some are built on the ground; others are elevated to bench level. Some even hang parts overhead.
While conveyors certainly have an application in many situations, Lean tends to look to other solutions first. Lean’s use of work cells and flexible stations, as well as the constant rearranging of processes can make conveyors impractical.
Conveyors take considerable space, require a capital investment, add maintenance issues, and are often not compatible with some of the fixtures and ergonomic work areas you might find in Lean companies. Often, conveyors end up being treated like monuments, with processes designed around them, rather than having the conveyors support the process.
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Conveyors and roller tables essentially perform the same function, with automation being the main difference. Conveyors move things on their own. Roller tables need an assist—gravity or pushing. They are both used to move materials from one point to another without any lifting or carrying.
Conveyors and roller tables may be set up as an integral part of a process. The work may slowly move past parts, connecting the movement to the design of the workstation. Short, inclined rollers are also commonly used as a means of moving materials to and from a workstations. For example, you might see rollers built into a station so a material handler can drop bins in from behind, and so an operator can drop empty bins or scrap to the outside of the work area.
In many cases though, conveyors and roller tables are used as a way to link two separate processes. Long roller tables may even connect stations that are quite a distance apart. The problem is that these tables and conveyors can collect work-in-process, adding to inventory. If a conveyor system gets backed up, it generally means that you are overproducing. Time might be better spent on the downstream process than on producing more at the upstream process.
Try to find an alternative to roller tables that limits the amount of WIP that can be present at any given time. Consider a finite number of carts with fixed capacity. These can even serve as pull signals. An empty cart means build more. When no carts are available, stop production.
Another option is to modify conveyors or roller tables so that operators can’t overproduce. Cut out sections of the conveyor belt to limit space, or remove sections of rollers and place inventory on trays. Without the trays, items will fall through.
Those options are just workarounds, though. The real question to ask is why you need the material movers in the first place. Why are your processes so far apart? If the work areas were closer together, there would be no need for a conveyor or roller table. Create flow! Roller tables and conveyors are like big red flags waving in the breeze telling you that you have a problem with your flow.
In most cases, conveyors and roller tables are not sized in a way that helps control inventory. The space on a conveyor tends to allow overproduction. Make sure you have a way to limit the amount of material that can be placed on a conveyor or roller table.