March 2020

Focus on:Seals, Bearings & Lubrication , Controls & Plant March 2020 mag.co.uk Plant & Works Engineering | 31 better bearing life model. To do this, they needed three things. The first was a model of sub-surface fatigue within the material. The second was a model for failure at the surface. The third was data from endurance tests, which SKF could use to calibrate and validate the new model. The company spent two years working on the new model, drawing on decades of experience in materials science and tribology. It required a detailed understanding of the behaviour of bearing surfaces, from their friction characteristics to the way dirt particles affect them under load. An initial concept model was presented as a Generalised Bearing Life Model (GBLM) in 2015 at Hannover Messe. However, this did not cover the modelling of hybrid bearings. For this, SKF still needed to collect large amounts of data, for calibration and validation purposes. This required lots of work. Morales-Espejel said SKF needed data on the operating life of bearings over a wide range of loads and surface conditions, in order to build behaviour curves. For every point on the curves, they needed to test around 30 bearings, knowing that several of them would fail. The company also needed to compare bearings with steel and ceramic rolling elements, and those operating with poor lubrication or in contaminated environments. In all, SKF tested hundreds of bearings. The test programme, and the adaptation of the concept model, required another four years of work at its facilities in the Netherlands and Austria. Tested and approved The company completed the new GBLM for hybrid bearings in mid-2018. It has since been tested and approved by an influential group of SKF’s application engineers. They used prototype versions of the model alongside conventional bearing life estimation techniques and compared outputs to actual results from customer projects. A key feature of the model is that it separates surface fatigue from sub-surface fatigue. It applies classical rolling contact fatigue in the sub-surface region and a new tribologically- dependent surface degradation model for the ceramic-steel raceway interface. The fatigue resistance of the ceramic-steel interface can, in most cases, compensate for the extra stress present in the sub-surface region of the contact. Two variants of the GBLM for hybrid bearings were developed. One will be incorporated into the SKF Bearing Select webtool, which is offered to customers on-line and through dedicated software applications. A second variant, which is more sophisticated and complex, will be used by its application engineers to support customer projects. The new model offers real insight into when the use of hybrid bearings can be justified. Benefits are offered in many typical situations. For instance, when a bearing is heavily loaded, but runs in a clean, well-lubricated environment, sub-surface fatigue is likely to be the ultimate failure mode. Here, a steel bearing may perform better than a hybrid. However, many bearings operate under lighter loads and with a greater likelihood of poor lubrication or contamination. The model will reveal whether a hybrid solution offers a longer life in those applications and will also quantify the difference. Paper proof In a 2019 scientific paper, published in Proceedings of the Institution of Mechanical Engineers, Morales-Espejel highlights that SKF demonstrated how the model was used in four representative real-world applications. These were: a pump bearing; a screw compressor bearing; and two applications of a bearing in an electric motor. For the pump bearing, which ran under poor lubrication conditions, the rating life of a hybrid bearing was eight times that of a steel equivalent. For a screw compressor bearing with contaminated lubricant, the hybrid had 100 times the life of its steel equivalent. The other two cases looked at an electric motor operating in clean, well-lubricated conditions, under two different load regimes. In both cases, the rating life of the hybrid bearing was very similar to the conventional bearing. However, other potential benefits of hybrid technology, such as electrical resistance or a longer grease life, could be decisive when making a final decision. Existing life theory models for hybrid bearings have many shortcomings and cannot precisely model ‘real’ conditions. Accurate prediction of bearing life will however help engineers choose bearings that are better matched to the specific needs of their application. Morales-Espejel says that to SKF’s knowledge, there is no other publicly available life model for hybrid rolling bearings. There are only adaptations of models for classical all-steel rolling bearings, which tend to penalise hybrid bearings because of their higher stress under identical loads. This is despite clear evidence that under ‘real’ conditions, such as low load, high contamination and poor lubrication, hybrid bearings can offer superior performance. Making the new GBLM widely available will help engineers do this in the context of hybrid bearings. Here, the properties of hybrid bearings can be modelled in many scenarios, such as low lubrication, to see whether their use can be justified. Troubleshooting Common Cylinder Creep FPE Seal’s Cylinder Parts Manager Tim Bone helps to trouble shoot a common problem that can occur with hydraulic cylinders of all types. What is causing a hydraulic cylinder to creep faster than expected? Often the cylinder on a hydraulic application can creep. A general assumption for this can be that the problem is due to oil passing over a worn piston seal. However, customers are often telling me, “we have resealed the cylinder several times and it still weeps down”. In many cases, the more likely cause is that the oil is passing across the spoil, rather than the problem being with the seals. A simple way to test this, is to lower the load onto the ground so that the cylinder is not under tension, isolate the power source i.e. the hydraulic unit and relax the pressure in the system. You can then remove the hose, from the cylinder port that the direction of the rod is moving in (i.e. if the rod is extending, then remove the hose from the annular side, if the rod is retracting then remove it from the bore side). Place a high-pressure ball valve in the line, attaching the hose to the opposite side of the ball valve. Make sure that all connecting joints are tight and leak-proof and clean up any spillages. Move the lever of the ball valve to open position. Restart the hydraulic power source and operate the valve block (levers), placing load onto the cylinder concerned. If the cylinder shows signs of dropping again, move the lever of the ball valve to the closed position. If the cylinder stops creeping, this proves that the fault is in the main valve block, as generally the only seals in a valve block are the ones at each end of the spool, to stop oil from leaking externally. Hydraulic spools in valve blocks are a machined precision fit, over time the spools can wear across the surfaces causing oil to pass between the internal galleries. Fitting a standard pilot operated check valve or over centre valve can stop this from happening. If the cylinder continues to creep, then the problem is with the internal seals of the cylinder. Once identified these seals can be easily replaced. w: www.fpeseals.com FPE Seals Ltd | Barrington Way | Darlington | Co Durham | DL1 4WF | United Kingdom