Paper explains role of vibration

A technical white paper entitled The Role of Vibration Monitoring in Predictive Maintenance has been written by Dr Steve Lacey, engineering manager at Schaeffler (UK) and offers engineers advice and guidance on the condition monitoring of rolling bearings

As companies look to reduce costs, maintenance can often become a casualty. However, as equipment and machinery becomes more complex and automated, the need for a properly structured and funded maintenance strategy is more important than ever.

Undertaking thorough risk assessments across the business can help companies identify how critical existing machines are to their overall operation, which helps to determine the potential return on investment of a properly funded maintenance strategy.

Dr Lacey says: "Rolling bearings are a critical component used extensively in rotating equipment and machinery.When they fail unexpectedly this can result in a catastrophic failure with high associated repair and replacement costs. Vibrationbased condition monitoring can be used to detect and diagnose machine faults and form the basis of a predictive maintenance strategy." As well as providing the reader with information on the basic approaches to the various types of maintenance strategy (reactive, preventive and predictive), the paper provides guidance on how to set up and identify the criticality of assets in a business, including potential return on investment from implementing a predictive maintenance regime. It then discusses the technical benefits of using condition monitoring systems and techniques to support the predictive maintenance strategy.

This includes vibration monitoring, which can be used to detect early signs of failure of rolling bearings.

Detailed analysis of the various rolling bearing vibration monitoring techniques are appraised. Sections are included on frequency spectrum, envelope spectrum, Cepstrum analysis, bearing characteristic frequencies, typical bearing defects, variable compliance and bearing speed ratio.

Dr Lacey comments: "Rolling bearings generate characteristic vibration frequencies that can combine to give complex vibration spectra, which at times may be difficult to interpret other than to an experienced vibration analyst. However, with rolling bearings, characteristic vibration signatures are often generated in the form of modulation of the fundamental bearing frequencies. This can be used to our advantage and vibration condition monitoring software is designed to identify these features and provide an early warning to an impending problem. This usually takes the form of signal demodulation and the envelope spectrum, which indicates early deterioration of the rolling contact surfaces." The final chapter of the paper considers various real-life scenarios of rolling bearing vibration monitoring, including detailed studies of a 250kW electric motor; an impact crusher drive shaft; a 2MW generator on a test bed; a vertical impact crusher; and the gearbox of a wind turbine.

Wind power, for example, is a rapidly growing form of renewable energy in many parts of the world and wind turbines are set to play a key role in future energy supply. In the UK, there is increasing interest in placing wind turbines offshore, which provides advantages such as improved wind conditions and reduced planning restrictions. However, the environment in which such turbines have to operate is more demanding, requiring a higher degree of integrity and reliability if costs are to be minimised.

As Dr Lacey puts it: "Due to the remote location of these offshore wind farms, accessibility for maintenance is difficult and so it is critical that faults are detected early and consequential damage reduced or avoided and that repair costs are minimised.

This results in shorter downtimes and cuts revenue losses. Detecting bearing damage early can also mean the difference between replacing the wind turbine gearbox at a cost of around €250,000, compared with a bearing replacement cost of just €5000." Wind turbine gearboxes are subject to high dynamic loads, and because of the changing wind conditions the load spectrum varies greatly, consisting of high peak loads and low load operating conditions. The high static safety required for maximum load means that bearings with high load carrying capacity are required. On the other hand, when there is little wind, loads are low and this can lead to bearing damage due to sliding of the rolling element set. As a result, many field-operating failures originate from gearbox bearing failure.Misalignment, poor lubrication and maintenance also contribute towards this trend.