The Swedish company, Electrolux, has invested around 20 million Euros in assembly lines for dishwashers at its plant in Zarow, Poland. The assembly lines have been operating since 2006 and are being continuously extended. Zero pressure roller conveyors are a major part of the line. They are driven by rollers with integrated 24 V motors in a space-saving all-in-one design for ultra-quite operation. Distributed control of individual conveyor sections brings a number of advantages.
The conveyor systems at the Electrolux plant ensure optimal efficiency for the assembly of dishwashers, from pre-assembly to assembly of the basic unit, testing, packaging and dispatch. The conveyor system needs high availability components and has to support the synchronized assembly without time delays. This is achieved by means of appropriately sized storage sections that act as supply buffers between the assembly stages. No manpower is needed for transferring goods to and from the buffers. A total of 260 conveyor sections, including storage buffers, were initially installed. Moreover, by using fine-grained segmentation with distributed control units Electrolux is able to reduce the minimum batch size from 100 to 20 dishwashers. This extra level of flexibility enables Eletrolux to deliver on demand in order to meet customers' individual requests, for example, for short notice orders, special series and small export batches. Even the assembly stations are seamlessly connected to the conveyor system. At each station operators are able to adjust the speed of the conveyor according to the assembly task. Some stations also have reverse operation which requires drive technology with the appropriate control functions. There are also turning, hub and tilting stations that bring equipment into the perfect position for easier and faster assembly.
Choosing the right drive concept
Traditionally, this type of internal supply chain management system is implemented with chain-driven accumulating roller conveyors with back pressure. This type of implementation, however, does not actively support the desired assembly synchronization and therefore does not assist throughput. This is why Electrolux could not use a simple passive accumulating roller conveyor with a single drive at the start of the conveyor. Neither were conventional central or distributed drive systems desired. On the one hand, rising energy costs are making it increasingly expensive to generate and distribute pressurized air. On the other, systems with pressurized air are noisy. Using central or distributed flange-mounted motors with chains to drive the passive rollers is also inefficient – the design is complex and the chain and cog wheels generate a lot of noise. This is why Electrolux decided to use rollers with integrated motors. They are extremely quiet since the mechanics are reduced to a minimum. They are also more efficient because energy does not have to be redirected through an angle as is the case, for example, with flange-mounted spur-geared drives and pinion drives. In addition, mounting the roller drives is just as easy as mounting a passive roller. Only an additional cable needs to be attached. This ensures fast initial installation and simplifies any future maintenance work. The energy is transferred to the passive rollers via round belts that run in grooves and are therefore also easy to install. An additional PVC covering on the roller drives and passive rollers ensures energy transfer to the goods on the conveyor. It also ensures stable positioning during maintenance work. The result is an extremely quiet and efficient conveyor system.
Silent conveyor system
"It is an extremely quite system", declares Rainer Hofmann, project manager for assembly lines for Electrolux dishwashers. On the new conveyor lines the rollers with integrated motor produce only 50 dB(A) which is barely audible for the workers. Another noise reducing factor is the use of short sections which generate less resonance than longer conveyor sections. Central motors, chains and pressurized air would have significantly increased operating noise. Quiet operating conditions are more than just a bonus – a quiet working environment results in less mistakes and greater efficiency.
24 volt technology with integrated control electronics
The flexible conveyor system was implemented with 24 V DC RollerDrives EC300 as well as passive universal rollers from the Interroll 1700 series. The RollerDrives EC300 with 50mm diameter have an integrated brushless motor, transmission and electronic regulator that can be directly connected to the control electronics to regulate the direction of spin, start, stop and rotational speed. The additional fault notification output sends either a "no fault" or "fault" status message which enables direct connection of ICE IEC 61116.1 conforming controls ((Soft)PLCs). Alternatively, the motor can be controlled via an external control unit (Interroll EC200/300 DriveControl) with an eight-level rotary encoder switch for regulating the speed. The electronic switch for start, stop and changing the direction of motion can be controlled with standard control units which significantly raises the design scope for machine and plant engineers. In this connection, direct connection to the control unit without the need for external electronics is a particular bonus. The number of required components is thereby reduced to a single, space-saving and easy to design-in DC RollerDrive per conveyor section. This ensures easy implementation of the required custom solutions.
Long lifetime
The Interroll RollerDrives EC300 are specially designed for high speed applications with a large number of cycles. They are brushless and offer a proven lifetime of over 20,000 hours operation. The high IP54 protection rating makes them suitable for damp environments and protects the integrated electronics. The motors can be quickly removed for repair and maintenance: remove the hexagonal nut, pull out the plug and that's it. Interroll developed the RollerDrives EC300 for zero line pressure conveyors but they are also used in non-conveyor applications, e.g. in packaging machines. Depending on the required drive train (force-fit or form-fit, number of passive rollers) the side heads are equipped for round belts, chains, toothed belts or poly-v belts or with flanges in the drum casing.
Interroll offers covers such as tubes and coatings in different materials and degrees of hardness. The latest development is a PU tube with a hardness of up to 75 Shore and high abrasion resistance. The sum of all these possibilities makes the Interroll RollerDrives EC300 a flexible system for optimal material flow in complex plants.
Convenient solution: complete kits
Interroll also offers complete solutions for zero line pressure roller conveyors. They consist of RollerDrives configured according to the requirements of the respective application, passive rollers, safe power supplies for around 8 motors as well as individually configured control electronics such as Z-cards (Z = Zero-pressure accumulation) that can be used to control up to four conveyor zones. The control electronics are optimally adjusted according to the motor to ensure long life-time operation. The use of independently controllable sequences for zero line pressure conveyors eliminates the need for centralized control units with complex mechanics or pneumatics. Instead, the distributed units have a much simpler design. Moreover, using a distributed concept means that isolated faults do not result in the shutdown of the entire plant. In addition, the modular concept with no central power supply is a lot more flexible when it comes to restructuring the internal logistics.