FEATURE LINEAR MOTION STEEL: THE KEY TO LINEAR
MOTION BEARING PERFORMANCE Mikael Östman, key account manager
for bearings at specialist steel company Ovako, explains how to choose specialty steel for long-term performance and reliability
many thousands of cycles. They have two main components: a shaft or track that guides the motion, and a nut that grips and slides along the shaft. In a heavily loaded system, material choice can have a significant impact
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on the lifetime and reliability of both the shaft and the nut. During operation, the components experience cyclic loading, and the body of the steel components is designed to distribute and handle the resulting stress. Steel contains tiny non-metallic particles called inclusions and these act as
localised stress raisers. Over thousands of loading cycles, micro-cracks can initiate at the inclusions, eventually propagating, and ultimately lead to fatigue failure. However, designers can avoid this by specifying bearings made from clean steel, particularly when the linear motion bearing is critical to the machine’s function and the bearings will transfer heavy cycling forces. Clean steel is chemically identical to a standard steel grade and has the
same yield strength. However, it is produced under special conditions that control the size, frequency and distribution of the non-metallic inclusion.
inear motion bearings are found widely in industrial equipment such as machining centres, pneumatic handling systems, handling and packaging lines, and other applications that require precise positioning over
ULTRA-CLEAN AND ISOTROPIC Steel cleanness can be seen as a scale starting with standard steel at one end. At the high performance end, engineers can choose ultra-clean bearing quality (BQ) and ultra-clean isotropic quality (IQ) steels. BQ steel has significantly fewer and smaller inclusions than standard
steel and is ideal for bearings. The inclusions in a standard or BQ steel are often long and slender in nature, therefore the fatigue strength can vary, depending on which axis experiences loading. Therefore, when specifying material for rollers and balls, it may be worth
upgrading to IQ steel. It is produced under intensely controlled conditions for smaller and fewer inclusions that are also more regular in shape. As a result, its fatigue strength does not vary depending on the loading axis.
HARDNESS, STRENGTH, CLEANNESS One popular grade produced in isotropic quality (IQ) for high-fatigue applications is 100 Cr6. This is ideal for balls and rollers as it combines high hardness, high strength and high cleanness. While it typically comes in the form of hot rolled bar, we can also provide blanks cut to tight tolerances from fine ground bars. Bearing manufacturers can cold-press these into spheres for ball bearings or use them as blanks for rollers.
STRAIGHTNESS AND CONSISTENCY The same properties of hardness, strength and cleanness are also important for the shaft of linear motion bearings. However, it’s essential that these are consistent along the length of the bar. To ensure precision and minimise noise and vibration, it’s also important to have an extremely tight tolerance for straightness, typically 0.1mm per meter. The 100 Cr6 grade is ideal for shafts, as well as nuts, but typically in
the form of hot rolled annealed bar. Some manufacturers perform their own fine grinding and surface hardening. However, we can also provide these services so that the bar is delivered ready to fit. To achieve consistent quality and performance along the entire
length of the bar, experts at our Hällefors mill in Sweden use induction hardening and tempering rather than a roller furnace to achieve a high level of control. In addition, they perform heat treatment on individual bars to prevent heat radiation between bars from influencing the thermal conditions in the furnace. Individual heat treatment of bars guarantees that every bar we produce will have homogeneous properties along its entire length.
PACKAGING AS A FEATURE While specifying the right grade and cleanness is essential, it’s also important not to overlook the packaging to ensure that the material arrives in pristine condition. Manufacturers can choose whether they want protective oil, plastic
wrapping, wooden crates or pallets; and they might want spacers that prevent bars from coming into contact. When packaging steel blanks, we can even place them onto pallets in a predetermined pattern to meet the needs of an automated materials handling system. Ultimately, specifying a specialty steel for linear motion bearings is more
complex than simply identifying a steel grade and setting the dimensional tolerances. Steel suppliers can provide expertise in setting the right level of cleanness and in helping you to maximise your production efficiency with precision blanks and packaging.
Ovako
ovako.com
16 DESIGN SOLUTIONS NOVEMBER 2021
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