Too many bearings are being scrapped when they could be remanufactured to cut costs and boost sustainability, comments Phil Burge, communication manager for SKF
There’s no denying that bearings take a beating every day, sometimes 24/7, and it’s true to say that even the best components will not last forever. Nevertheless, there are opportunities for companies to increase efficiency by remanufacturing bearings, not least because the long lead times that can be required when ordering replacement bearings can lead
to unnecessary downtime and damage to plant profitability. With sustainability a rising priority, the scrapping of ‘end of life’ bearings is becoming a less viable option.
The aircraft industry is one of a number of industries that have set the pace. Here, it is common practice to remove rolling bearings during maintenance intervals and rework them for extended service life.
But, how can more engineers and plant operators take advantage of this option? To understand what is and isn’t viable, engineers first need to understand the different stages of wear that occur during the life of a bearing and which types of wear can be addressed by remanufacturing.
Not all bearings or bearing components can be reworked, it depends on the type of fatigue that has wrought the damage. A clear distinction, for example, will be made between subsurface-initiated fatigue and surface-initiated fatigue. Subsurface-initiated fatigue describes the shear stresses that appear cyclically immediately below the load carrying surface of the ring(s) and rolling elements. These stresses cause cracks underneath the surface that gradually extend to the surface and, as the rolling elements pass over the cracks, fragments of the material break away – a process known as ‘spalling’. When a bearing raceway is damaged by subsurface-initiated fatigue it is not considered for reworking, but where there is surface initiated fatigue to the bearing raceways caused by dirt or debris, raceways can often be restored by honing or grinding.
The process of assessing a bearing for remanufacture begins with disassembly and cleaning, and is then followed by both a visual and a microscopic inspection, after which ring hardness is measured. A dimensional inspection completes the assessment phase, which is then concluded by the submission of a report and a recommendation for treatment.
Recommendation for reworking is categorised into classes, from Class I – Reclassification, which encompasses all of the operations of inspection described above and any appropriate additional work such as buffing, polishing, grinding of scratches and grooves, and demagnetisation; through to Class IV. Each stage adds further levels of component replacement and refurbishment to the remanufacturing process.
Depending on the findings of the initial assessment, reworking can typically result in the grinding or polishing of bearing side faces, bore diameter and outside diameter, while a nickel or chrome plating may be applied to allow the surfaces to be reground or polished to their original dimensions. This can result in a surface finish that is even better than that specified by the original blueprint. Bearings are then typically refitted with new rolling elements, while cages are inspected for cracks and potentially remanufactured by replating. With all issues assessed and addressed, the bearing is then reassembled.
When larger bearings are considered for remanufacture the option is also there to execute a deeper grinding of inner and outer ring raceways, while further machining methods such as hard turning may also be applied. Superficial damage is removed, and the stressed material volume is modified.
The option to remanufacture and not only restore but exceed the original quality specifications of rolling bearings presents a significant opportunity to increase efficiency. Cost-benefit analysis has shown that remanufacturing can exact a saving of as much as 80% against the cost of a new bearing. In terms of sustainability, the argument in favour of remanufacturing is as robust as the refurbished bearings themselves – SKF estimates that the remanufacturing of 100kg of used bearings leads to a reduction of about 350 kg of CO2 emissions.
SKF’s Remanufacturing Services employ the same high-quality materials, methods and machinery to rework bearings as those that are used in the original manufacture. With this in mind it’s clear to see how remanufacturing bearings can extend the operating life cycle of bearings, while significantly reducing costs and environmental impact.
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