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BEARING DESIGN Caged Vibrations


Jonathan Newell talks to Schaeffler about the use of solid brass cages in bearings for high resistance to vibration and shock loads in railway gearbox applications.


I


nnovation in bearing technology by automotive and industrial supplier, Schaeffler has resulted in improved vibration resistance using the MPAX cage for cylindrical rolling bearings that offer distinct advantages for the rail sector.


The new MPAX brass cage is a robust, rib-guided, single-piece


solid brass cage, which is extremely resistant to vibration and shock loads. The MPAX cage has a higher load carrying capacity for radial centrifugal forces (higher radial rigidity) than its predecessors and is also suitable for very high speed applications.


RAILWAY GEARBOXES In terms of the rail sector, cylindrical rolling bearings with MPAX cages are particularly well suited to final drive gearboxes.


According to Sally Sillis, Technology Centre Manager at Schaeffler (UK) Ltd, final drives or gear drives in railway


❱ ❱ The brass MPAX bearing cage provides enhanced resistance to vibration effects in the demanding railway industry


CERAMIC INSULATION COATING


Schaeffler has also developed a new ceramic insulation coating for rolling bearings. Applied to the outer ring, the Insutect A J20G coating is a cost effective way of preventing damage to bearings due to the passage of electrical current. The ceramic coating, which is approximately 700µm thick, offers high capacitive resistance, high wear resistance and a breakthrough strength of up to 5,000V DC. The coating helps prolong the life of lubricating grease. The Insutect A coating is applied using the plasma


spray method and sealed. Coating variants are available from Schaeffler to suit a range of different bearing applications such as rail, wind turbines or industrial three-phase electric motors.


vehicles are typically 2–3 stage, with spur or helical gears which are directly mounted on the wheel axle. The transmission transfers the drive power from the traction motor to each driving axle in the train or locomotive. “Rolling bearings in railway gearboxes have to withstand


extreme loads and environmental effects. The main requirements for bearings used in these applications are high speeds & radial accelerations, high loads, high temperatures and high vibration and shock loads from the track and wheels,” she says. In these applications, particular strain is placed on the rolling bearing cage and so cylindrical roller bearings and four- point contact bearings with one-piece cages, guided by the outer ring are the only choice for these conditions, according to Sillis.


With MPAX, Schaeffler has improved the overall cage design by reinforcing the side edges, optimising side edge thickness and optimising the pocket corner radius position, resulting in reduced maximum stress on the pocket corner radii. Fatigue effects occur only when very large forces are applied. The cage is always rib-guided on one ring and the rollers can be removed. Over time, Schaeffler expects the MPAX brass cage to


gradually replace the previous cage types in the railway industry.


TEST PROCESSES I asked Schaeffler how the cage and bearing are tested to validate their long term resistance to vibration in the rail sector. Sillis tells us that the company’s assurance processes begin early in the design.


“Computer modelling is one of the tools used in the design of all Schaeffler rolling bearings. Finite Element Analysis (FEA) is used to evaluate the performance of the cage and bearings when subjected to impact load by applying axial, radial, tangential & centrifugal forces. Several in-house software packages are also used including the well known bearing design & calculation software, BearinX,” she says. The company also uses dynamic rig testing, which includes pulsating tests using a hydraulic impulse test rig operating at >107 cycles to assess the fatigue behaviour of the cage. Torsional & vertical vibration testing is also performed to apply alternating radial loads and alternating speeds as well as centrifugal testing using an in-house test rig to induce cracks.


Climatic & Vibration Testing 2018 Vol 1 No. 2 /// 9


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