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PII Oct-Nov 2013

FOOD INDUSTRY FOCUS Materials and coatings increase operating life of bearings Dr Steve Lacey, Engineering Manager at Schaeffler UK, discusses In the food and beverage industries, production is typically based on highly automated, fast moving processes and systems, where every second of production counts. In these operating environments, unscheduled production downtime is a crime and so processes, systems and individual machine components such as bearings must be energy efficient, cost effective and capable of withstanding harsh, often highly corrosive conditions. When selecting bearings for these applications, robust, reliable bearings are often required, with corrosion protection, sealing and lubrication for continuous operation. New materials and surface coatings for bearings are constantly being developed by suppliers, which can provide even higher levels of protection and even longer operating lives. Bearings to suit every application A wide range of bearings are available to suit most types of food and beverage applications. Schaeffler, for example, offers deep groove ball bearings, radial insert ball bearings, plain bearings, housed units, track rollers, slewing rings, ballscrews and linear guidance systems. For harsh food and beverage environments, bearings should fulfill several key principles: compact design; maintenance-free operation; reliable, long operating life; modular components and sub-systems matched precisely to each other; and minimum number of interfaces through functional integration where possible. Special materials and coatings While standard, catalogue bearings may guarantee the customer optimum performance and a long service life, in many food and beverage environments, a standard bearing material or coating may need to be upgraded in order to prevent wear or corrosion of the bearing and to improve the service life. Coatings can be applied to the surfaces of rolling bearing components without forming a material bond, for example, by diffusion between the coating and base material. In many cases, it is sufficient to coat only part of the bearing or one of the parts that come into rolling contact. A good bearing supplier will be able to advise the customer when a coating is appropriate for a particular bearing and what type should be used in certain applications. As well as offering special corrosion-resistant coatings, some suppliers can also provide corrosion-resistant materials for rolling bearings that enable their products to operate in corrosive environments. While standard steel bearing materials provide satisfactory corrosion resistance in many applications, for higher performance requirements, highly corrosion-resistant, nitrogen alloyed martensitic HNS (high nitrogen) steels are recommended. These types of steels provide maximum corrosion resistance under extreme operating conditions, including dry running applications or when bearings come into contact with aggressive media such as water, salt spray, acids and cleaning chemicals. Schaeffler’s own Cronitect® steel, for example, is a highgrade martensitic hardening steel that is based on the consistent refinement of high-grade corrosion-resistant the importance of selecting a special surface coating or corrosionresistant material for rolling, plain and linear bearings operating in steels. Through its unique composition and new thermochemical surface layer treatment process, this material achieves an extremely high hardness. Even after 600 hours of salt spray testing in accordance with DIN 50021 SS, the material shows no signs of corrosion. The heat treatment process optimises the surface zone and core properties of the base material. This produces a hard, wear-resistant surface zone with very high corrosion-resistance and a tough core. The material is therefore suitable for a wide range of industrial applications, particularly food and beverage processing such as filling plants, fish processing and poultry production. Bearings can be provided that combine conventional stainless steels such as AISI440C for the rolling elements with corrosion-resistant rings and bearing seals. In these configurations, the focus is on extended life under insufficient lubrication and on emergency operating features. These types of materials can significantly increase the availability of machinery, providing extended maintenance intervals. For example, bottling plants would typically need to replace all their AISI440C rolling bearings that come into contact with cleaning detergents, every four months. With Cronitect® bearings, this interval can be doubled. In a recent cam control solution for a bottle capping machine company replaced its original solution – a standard stud type track roller immersed in an oil bath – with a roller stud, Cronitect outer ring, corrosion-resistant steel balls and an HRS seal. This improved solution requires no lubricating oil, has extremely high resistance to corrosion and aggressive media, while presenting no risk of contamination of foodstuffs. This has resulted in improved hygiene, reduced maintenance costs and higher machine availability. Ceramics and plastics Other special bearing materials are available from suppliers, including ceramic rolling bearing components, which provide excellent characteristics for certain applications. Silicon nitride balls, for example, are light and offer a longer operating life than standard steel ball elements. High performance plastics such as PEEK can also be used for bearing components operating in corrosive environments. PEEK is resistant to high temperatures and, depending on the load, is suitable for operating temperatures of up to 250°C. The wear resistance is strongly influenced by the filler materials and is significantly better than that of the standard material PA66 with glass fibre reinforcement that is often used for bearing cages. Plastic & stainless steel housed bearing units Housed bearing units (e.g. flanged, take-up and plummer block designs), which are commonly found in food and beverage processing plants, can also be protected with various coatings in order to prolong their operating life. The housings and insert bearings can be corrosion protected and the housings themselves can also be made from a corrosion-resistant (stainless) sheet steel or plastic (e.g. glass fibre reinforced PBT). Plastic housings are highly resistant to moisture, UV radiation, bacterial and fungal attacks, as well as many chemical media. These bearing units are used predominantly as locating bearings, but are also suitable as non-locating bearings under low loads and speeds. Track rollers Due to their numerous technical advantages, track rollers generate added economic benefits for machine operators, for example, in bottling and filling plants. Track rollers require little installation space and operate reliably with low wear. They also have large grease reservoirs and effective sealing, which means relubrication is seldom required. Linear guidance systems As well as rotary bearings, linear bearings and guidance systems are commonly required in food and beverage production. Examples include shaft, track roller and monorail guidance systems; linear recirculating ball bearing units; miniature guidance systems; linear actuators and linear tables; ballscrews and planetary roller screws. Some suppliers now offer linear accessories in the form of ‘kits’ that include a variety of linear components such as front and additional wiper seals, sealing strips and long term lubrication units. These types of accessories help to protect the guideways from contamination, providing long relubrication intervals, ensuring that lubricant is provided where and when it is needed. Slewing rings In bottling and filling plants, larger bearings are often required. The load carrying capacity of large slewing rings is needed to provide secure bearing supports for filling machines, star wheel transfer units or bottle rinsing machines. These damp, aggressive operating environments require highly reliable slewing rings. Schaeffler (UK) Ltd, Sutton Coldfield, West Midlands Can be contacted on: Tel: 0121 313 5870 E-mail: info.uk@schaeffler.com Web: www.schaeffler.co.uk harsh food and beverage environments. Deep groove ball bearings in a bottle labelling machine Radial insert bearing protected against corrosion by Corrotect® Ceramic balls make rolling bearings lighter & more durable Dynamic linear ball bearing & guideway assemblies High performance capacity slewing rings 44 Process Industry Informe r October-November 2013


PII Oct-Nov 2013
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