Dr Steve Lacey, Engineering Manager at Schaeffler UK, discusses the importance of selecting a special surface coating or corrosion-resistant material for rolling, plain and linear bearings operating in harsh food, beverage or packaging environments.
In the food, beverage and packaging 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, Schaeffler offers a wide range of robust, reliable bearings, which can all be corrosion protected, sealed and lubricated for life for continuous operation. Schaeffler is constantly developing new materials and surface coatings for bearings, which provide these high levels of protection and longer operating lives.
Schaeffler’s bearing range for food, beverage and packaging applications includes deep groove ball bearings, radial insert ball bearings, plain bearings, housed units, track rollers, slewing rings and linear guidance systems. All these products are designed to 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 Schaeffler’s standard bearings guarantee the customer optimum performance and a long service life, in many food, beverage and packaging 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. Schaeffler can advise customers when a coating is appropriate and what type should be used in certain applications.
As well as offering special corrosion-resistant coatings such as Corrotect, Schaeffler can also provide a range of corrosion-resistant materials for rolling bearings that 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 – such as Schaeffler’s Cronidur and recently developed Cronitect steels – are also available.
Cronitect provides 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. Cronitect steel is a high-grade martensitic hardening steel that is based on the consistent refinement of high-grade corrosion-resistant steels. Through its composition and new thermo-chemical surface layer treatment process, Cronitect achieves a very high hardness. Even after 600 hours of salt spray testing in accordance with DIN 50021 SS, the material shows no signs of corrosion.
The name Cronitect is derived from the chemical composition of the material (chromium and nitrogen-alloying elements) and from its special heat treatment process. The ‘tect’ suffix is found in other Schaeffler coating products and refers to the material’s excellent corrosion protection. Cronitect’s resistance to corrosion is many times better than that of conventional corrosion-resistant steels.
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. Cronitect is suitable for a wide range of industrial applications, particularly food and beverage processing (e.g. filling plants, fish processing and poultry production), materials handling, consumer products and packaging machinery.
Cronitect can be used across most of Schaeffler’s product range, including rolling bearings, linear guides and plain bearings. Rolling bearings that use Cronitect are available in both sealed and non-sealed versions. When used in conjunction with ceramic rolling elements, Cronitect bearings are particularly suitable for dry running operation.
Bearings can be provided that combine conventional stainless steels such as AISI440C for the rolling elements with Cronitect rings and bearing seals. In these configurations, the focus is on extended life under insufficient lubrication and on emergency operating features.
The use of Cronitect significantly increases 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 4 months. With Cronitect bearings, this interval can be doubled.
In a recent cam control system for a bottle capping machine company, Schaeffler replaced its original approach – 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 system requires no lubricating oil, has very 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 include 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 250degC. 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 and stainless steel-housed bearing units
For applications in the food industry, Schaeffler’s range of housed bearing units (flanged, take-up and plummer block designs) can be protected with various coatings. The housings and insert bearings can be protected using Corrotect and the housings themselves can also be made from a corrosion-resistant (stainless) sheet steel or plastic (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.
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
Schaeffler offers one of the widest ranges of linear motion products in the world, including shaft, track roller and monorail guidance systems; six-, four- and two-row ball guidance systems; linear recirculating ball bearing units; miniature guidance systems; linear actuators and linear tables; ball and planetary roller screws; and a range of linear accessories such as fasteners and connectors. For food and packaging applications, Schaeffler’s KUVE-KIT for linear recirculating ball guidance systems is highly recommended. This finely matched sealing and lubrication concept takes account of all ambient conditions. The kit contains versatile components such as front and additional wiper seals, sealing strips and long-term lubrication units. These accessories protect the guideways from contamination, provide long relubrication intervals, ensuring that lubricant is provided where and when it is needed.
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 can provide four-point contact bearings, crossed roller bearings (heavy or light series) with internal or external teeth, sealed or unsealed, in diameters up to 4m.
For more information on Schaeffler’s range of bearings and linear guidance systems for food, beverage and packaging applications, please email or visit the website at www.schaeffler.co.uk.