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FEATURE


BEARINGS, SEALS & GASKETS


THE PRECISE SOLUTION FOR MEDICAL DEVICES


Precision bearings are critical components within a huge variety of medical devices – from power tools and ventilators to heart pumps. With high reliability essential,


Alexander Bloos, sales and application engineer at HQW


Precision, examines what designers need to consider in order to specify the right bearing for their application


C


omponent reliability is essential for medical devices. With applications including surgical power tools, ventilators and heart


pumps, precision bearings are among the critical components on which the safety of treated patients often directly depends. As a result, the specification and selection of precisely fitting rolling bearings is highly important. Every component selected for use in medical


products needs to meet performance and reliability requirements, even after years of use under difficult ambient conditions and operation at high loads. But in addition to the special requirements of the application, designers must also consider factors such as corrosion resistance, lubrication, and the materials used for the ball bearings. Corrosion resistance, for example, can be a


critical factor for surgical tools and medical devices that require repeat sterilisation. It is therefore essential that the bearings used in the devices can withstand the aggressive chemicals used in the sterilisation process as well as the +140˚C temperatures which are encountered in the autoclaves. As a result, care must be taken when selecting the material for the rolling elements, rings and cage. Good results have been achieved with the high-performance AMS5898 / X30CrMoN15-1 (Martensitic Stainless Steel), for example, which


16 DESIGN SOLUTIONS MAY 2023


has a high chromium content of 14-16% and thus provides a considerably higher level of corrosion resistance than standard rolling bearing steels.


LUBRICATION


The type and quantity of lubricant also has a significant affect on the characteristics and service life of the application. For bearing systems that rely on grease lubrication, the lubrication must be biocompatible so that damage to the patient’s health is averted even if the lubricant comes into contact with tissue. It is often advisable to specify an NSF-certified H1/H2 lubricant. H1 lubricants are classified as safe for human consumption in small quantities or very few parts per million. H2 lubricants are regarded as suitable for use in foodstuffs but are not specifically food-grade products. Depending on the use of the medical device, biocompatible lubricants in accordance with DIN EN ISO 10993 can also be considered. In the case of heart pumps and other implants, bearing lubrication may not be possible at all. However, precision bearings produced from special materials can ensure reliable performance and a long service life even without lubrication. This is also true in cases where other liquids are in direct contact with, or even flow through, the ball bearing.


MATERIALS The correct choice of suitable ball and ring materials and the right product design also ensure that the high-precision bearings – and consequently the medical devices – have a long service life. One material that can be used to improve the life and wear resistance of a bearing is AMS5898 / X30CrMoN15-1 – a martensitic, through-hardened and corrosion-resistant steel. With the ability to withstand the aggressive


chemicals used in the sterilisation process, this material is ideal for surgical tools. Silicon nitride is one of the ball materials used in medical applications, and this offers excellent performance, even in applications with poor lubrication or where lubrication is not possible. Bearings made of stainless steel X65Cr13 / 1.4037 have also proven successful in medical technology applications and are suitable for use in breathing apparatus, for example. They combine good load carrying capacity with


a high fatigue strength and stability.


CAGE DESIGN The fundamental purpose of the bearing cage is to maintain a uniform distance between the rolling elements, in order to prevent contact between them, and thus ensure an even distribution of load within the bearing. The cages can also be designed in such a way as to reduce torque and minimise the generation of heat. This requires the selection of a cage material that exhibits the greatest possible degree of wear resistance in order to optimise lifetime, with good options including high-performance plastics such as Polyetheretherketone (PEEK). This material is also certified by NSF International as compatible with the human body. With their low mass, corrosion-resistant


properties and low friction, these materials also reduce wear and heat generation, meaning that the bearings can be operated at higher speeds and the service life of the grease is extended. A pre-requisite for any medical application


is the resistance to repeated serialisation and radiolucency characteristics. In terms of ventilators, Torlon cages offer a long service life at an affordable price, while cages made of PEEK provide the ideal solution for heart pumps – as the material is biocompatible, it is also an option for applications that remain in the human body for more than 24 hours.


TRACEABILITY In addition to factors such as material and lubrication, which contribute directly to the reliability of precision bearings, documentation on components is also a key aspect governing use in the medical sector. The demands on the quality assurance and traceability of devices and components far exceed comparable PPAP (Production Part Approval) processes that are commonly encountered in other sectors. The bearing system must be supplied with the


relevant component documentation in accordance with the ISO 9001 quality management standard and this must be retained for at least 15 years. The documentation should also include certificates issued in accordance with BS EN 10204:2004 including Type 3.1 with traceability to raw material source. With this European standard on inspection certificates for steel (and other)


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