Non-contact brakes mean minimal maintenance

Dean Palmer reports on a novel brake design for a shrink-wrapping machine which is helping a paper manufacturing plant to cut downtime and maintenance

By replacing a friction-based tension control system on its shrink wrapping machine with a non-contact hysteresis brake system, paper manufacturing company Iggersund Paperboard, based in Workington, has reduced both paper wastage and maintenance downtime on the production line. The shrink wrapping machine wraps rolls of virgin fibre paperboard weighing up to three tonnes in plastic to protect it during storage and delivery operations. By integrating the non-contact brakes (supplied by ZF Great Britain) into its PLC-based system, Iggersund is able to control the levels of tension at all stages of the wrapping process more accurately and without wear, ensuring that finished product always reaches the customer in perfect condition. The machine consists of a giant spool and a wrapping elevator that coats rolls of paperboard in up to five layers of shrink-wrap plastic. During the Summer of 2002, engineers at Iggersund noticed that the friction brakes used on the system were overheating during operation, causing inconsistent levels of tension and an uneven coverage of plastic on the rolls. The Workington site produces around 190,000 tonnes of paperboard from pulp each year, 40% of which is wrapped for distribution by just one machine. Iggersund’s project engineer Alec Pyne, commented: “We were spending increasing amounts of time maintaining the original system to no satisfactory effect. The plastic wrapping was beginning to undo during storage and transportation operations, which may at first seem like a minor problem, but the machine is used to coat rolls of paperboard up to 1.8m in diameter, 2.3m wide and weighing more than 3 tonnes.” He explained further: “The storage and handling of rolls involves standing them on end using large clamp trucks, making the wrapping, especially the ends of the rolls, one of the most important ways we can avoid both wastage and customer complaints. The potential losses in production time and increased labour involved in re-wrapping the giant rolls led us to look for a more reliable tension control solution.” Pyne told Eureka that, following consultation with Warrington-based drive system specialist Newton Tesla Electric Drives, a new tension control system was designed based on two ZF EBU series hysteresis brakes. These non-contact brakes use a magnetic flux to control the film tension in the machine, providing a reliable control solution which, according to the company, has resulted in more than 35,000 tonnes of paperboard being wrapped so far this year without any rejects or maintenance downtime. Reflecting on the decision to recommend ZF’s hysteresis brakes, Newton Tesla’s managing director George Newton, commented that, “the ZF units could easily be open loop controlled via a PLC and a ZF hysteresis controller to provide the exact levels of resistance required to make the plastic film adhere to the paper rolls. This flexibility of control also allowed the wrapping machine to vary the tension at the crucial start and finishing stages of the process, ensuring that the ends of the rolls were wrapped perfectly every time.” The non-contact operation of the brakes allows for an extremely long service life and avoids the creation of dust particulates, making them ideal for test rigs and production environments where levels of air contamination have to be minimised. ZF’s range of brakes includes a high power series that offers continuous slip powers from 250 to 2,000W and Peak capabilities of twice these figures. Hysteresis clutches are also supplied, which can be used for winding-on applications in cable manufacturing or food packaging industries, covering torque capacities ranging from 0.08Nm up to 520Nm.