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Low noise, high efficiency
Developments in the field of noise-testing equipment for greases, based on customer needs for extended bearing service life and low-noise performance, are ensuring that the required cleanliness level and lubrication conditions are fulfilled, says SKF's Phil Burge.
Noise analysis of grease in bearings can be used to gain a better understanding of bearing life and, most importantly, the lubricant quality with regard to cleanliness and damping characteristics. Today, noise testing is an important technique that helps determine the best grease specification, which is vital to the performance and life expectancy of bearings in many applications.
The use of clean lubricant in rolling bearings is essential to maintain long service life in the bearings themselves and the overall system. Many factors can affect the degree of cleanliness during operation, but clean grease for the initial lubrication, as well as for re-lubrication, is always required. This pertains not only to heavy duty applications where bearing lifespan is a major issue, but also to light load functions where the bearing fatigue life is not at stake; this is because low bearing noise is often required for items such as electric motors and fans. Where fans are expected to run extremely quietly, engineers need to specify bearings with the lowest possible vibration level and absence of disturbing noise. Key to achieving this is to supply clean, low-noise grease.
So how does noise occur? The causes and symptoms are too numerous to go into here but a common example is offered by considering bearings in automotive gearboxes. These bearings are regularly exposed to foreign matter in the gear oil, often originating from the gears, which can cause adverse issues. The particles cause over-rolled damage to the raceways, producing 'dents' and raising the stress level at their edges, leading to surface distress, increased noise and, ultimately, to bearing fatigue failure. Because the film thickness that separates the mating surfaces in the rolling contact is very thin (usually below one micron), any particles in size larger than the film thickness will naturally disturb the smooth running of the bearing. Vibration peaks can be recorded by filtering the short duration effect of the over-rolling of particles from the total signal.
Particle over-rolling can be split into four distinct quality classes: dirty, noisy, clean and quiet. Some greases on the market are classed as 'noisy', sometimes because they actually contain solid additives that produce small permanent dents. In contrast, there are a few greases that can be classified as 'quiet', such as lithium-soap greases that are produced in a clean environment. Such is the significance of these conditions that there are now sophisticated methods of measurement, designed to minimise noise and maximise efficiency in engineering systems.
The purpose of the new SKF grease test rig BeQuiet+ is to determine grease quality in order to develop, improve or identify quiet greases. Using BeQuiet+, comparison of the damping characteristics of different lubricants can be made on a given bearing. Selection of the lubricant with optimum damping characteristics can then be made successfully.
Developments in the field of noise-testing equipment for greases are based on a pressing customer need for extended bearing service life and low-noise performance. To verify that the required cleanliness level and lubrication conditions are fulfilled requires a rigorous testing system; the SKF Grease Test Rig BeQuiet+ enables thorough assessment of lubricant quality with regard to cleanliness and damping characteristics. The BeQuiet+ has been so successful that it has become the standard equipment for grease noise testing. The rig is recommended as a tool for lubricant manufacturers in development activities and for production control. Bearing producers can use the test machine as a means to select the best lubricant on the market, while the end user can verify the lubrication quality in service.
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