Slewing bearings are large bearings which enable rotation between components in large structural systems. These bearings are used for direction purposes on wind turbines, solar tracking devices, cranes and excavators, machining tables, train bogies, tunnelling machines and so on.
WIRE BEARINGS
Iraundi wire bearings were originally used in machine-tool, medical care, defense and aerospace sectors -in turrets and satellites- where space, weight and resistance to corrosion are prime factors.
These bearings are typically produced using aluminium rings and steel rolling elements including wires. They are 60-70% lighter than similarly sized, normal bearings.
Nowadays, IRAUNDI wire bearings may also be used in turntables, radars, antennas and machine tool applications. A light alloy IRAUNDI wire bearing can be made corrosion-proof and maintenance-free by using appropriate materials for the rings and rolling elements. Further, the corrosion behaviour of the aluminium bearing rings can be improved by surface treatment using the anodizing processes.
BEARING TYPE SELECTION
Each type of slewing bearing has its inherent particular characteristics, which make it more or less suitable for different applications and operating conditions.
As can be seen, the most important aspects when selecting a particular type of bearing are as follows:
• Accuracy
Bearings with greater contact surface between the rolling element and the raceway have an increased stiffness, which in turn leads to greater accuracy in the positioning. In this regard, the crossed roller bearings are more rigid and accurate than the four-point contact ball bearings.
• Vibrations
For applications which are subjected to vibrations, generally preloaded four-point contact ball bearings are used, although the crossed cylindrical roller bearings are usually suitable as well.
• Operating or running speed
Due to the smaller contact area between the rolling element and raceway, the four-point contact ball bearings generate less friction than the crossed cylindrical roller bearings, and therefore overheat less. Accordingly, the former having a higher speed capacity and being able to reach speeds of approximately 4 m/s. On the other hand, in the cross cylindrical roller bearings this value is reduced to 1.5 m/s in continuous movement or 2 m/s in short movements, due to the fact of having a greater contact surface.
• Rotation resistance
Rotation resistance is mainly influenced by the following aspects: the rolling resistance of the rolling elements, friction of the spacers, grease, internal clearance or bearing preload, friction of the seals, lubrication, deformation of the support structures and so on. A very high resistant torque due for example to a high preload value, which may be conducive to the overheating of the bearing. And also, to correctly estimate the resistant torque of the bearing is critical so as size the actuator which drives same. Due to the complexity of the phenomenon, these formulas are basically experimental formulas in nature and which may be conducive to resistant torque errors of up to ± 25%. In the absence of more accurate data, the drive system must provide an actuating torque of an approximate value of double than of the rotation resistance value calculated using such formulas.
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