Noise solution

Clean lubricant is an essential ingredient to ensure a long life for rolling bearings. For grease lubrication, many factors can affect cleanliness during operation, but clean grease is always needed for initial lubrication and re-lubrication.

Not all lubricants are the same. Their level of cleanliness can vary, either because they are supplied that way or because they degrade during service. This is when particles within the lubricant can start to affect bearing life.

Even in applications where bearing fatigue life is not at stake (with very low loads, for example), clean greases can still be extremely important – such as by contributing to low bearing noise, which is required for many electric motor applications.

But determining the most appropriate grease for an application – and measuring how it performs there – can be tricky. SKF’s BeQuiet+ test rig has been developed to address this challenge.  BeQuiet+ is a highly automated method for comparing the damping characteristics of different lubricants, using noise analysis. It tests identical bearings, mounted in the same way, to pinpoint a lubricant with the optimum cleanliness and damping characteristics for a particular application.

Noise analysis

There are two main reasons for bearing noise: over-rolling of microscopic particles within the lubricant; and the nature of the lubricant itself.

The mating surfaces of bearings are usually separated by a film of lubricant about 1 micron thick. Any particles within the lubricant larger than this will affect its smooth running, and lead to noise. Vibration peaks caused by an unclean lubricant are recorded by filtering the short duration effect of the over-rolling of particles from the total signal.

Over-rolling of larger particles can cause permanent dents in the raceways, while over-rolling of brittle particles can microscopically damage the bearing surface. If this happens, the overall signal will increase in time – showing that lubricant cleanliness directly affects bearing quality.

Over-rolling can be split into four quality classes, according to the concentration of these particles. If a lubricant is dirty, the hardness and size of particles is such that over-rolling causes permanent damage to the bearing, giving increased overall noise and reduced bearing fatigue life. A noisy lubricant may damage bearing surfaces and increase bearing noise but will not adversely affect bearing fatigue life. Clean lubricants – which constitute the majority of products on the market – will produce some noticeable vibration peaks, but no permanent damage to bearing surfaces. And finally, quiet lubricants have the highest degree of cleanliness, having few particles to cause vibration peaks.

Few lubricants are supplied dirty, but may reach that stage during operation. There are some ‘noisy’ lubricants on the market – usually greases that use particular thickeners or solid additives. An example is calcium-complex greases that contain large particles of calcium salts that can produce small permanent dents.

Most modern greases are clean: polyurea greases, for example, can contain large agglomerates of thickener. These can produce large vibration peaks, but because the hardness of these particles is low, the over-rolling will not give noticeable dents.

Li-soap greases are a rare example of a quiet grease. They are produced in a clean environment, by a process that filters out excess particles and produces a fine soap structure.

The lubricant itself also affects bearing noise, and the most important factor here is its damping ability. Many factors within the lubricant contribute to damping. For oils, the most important are viscosity, base oil type and additives. The damping characteristics of greases can be quite erratic, probably due to the microstructure of the thickeners used.

Quiet performance

SKF’s BeQuiet+ uses noise analysis to compare the damping characteristics of different lubricants. It was specifically upgraded – from the previous BeQuiet instrument – to help lubricant manufacturers develop new formulations with higher damping capability. The tool also helps bearing producers select a grease with the highest damping characteristics.

The advantage of the BeQuiet series is its unique test procedure and degree of automation. The whole process is controlled by dedicated software on a personal computer which also stores all peak data and vibration levels, then evaluates the results and presents them as tabular reports or line charts.

The rig excludes dirt particles that could affect results. Therefore, the same bearing can be used for different tests, without the need for dismounting. This is useful for determining any ‘damaging’ characteristics of dirty lubricants, by following a strict sequence: test reference lubricant; test the ‘sample’ lubricant; and re-test reference lubricant. Similar advantages are found when comparing lubricants in terms of their damping characteristics.

With BeQuiet+ the full procedure consists of repetitive dosing of the lubricant to the test bearing and recording of the vibration level and peaks. Automation helps to reduce operating costs to a minimum – both in terms of bearing consumption and operator time.

A measured dose of grease is injected into the test bearing. It is then ‘run in’ (for a default time of 10s) to distribute the grease. The Peak Detector is then zeroed in preparation for the measurement: a key component of the rig is SKF’s Peak Detection Algorithm, which enables vibration peaks to be singled out from the total bearing vibration signal. The peak reading, over a period of 3s, is then stored, and the program checks if the defined number of peak readings has been collected. After each test, the grease is removed from the bearing by a blast of compressed air – and the cycle starts again.

The rig can also measure the ‘Grease Damping Characteristic’ by relating the noise of the greased bearing in certain frequency bands to that in reference conditions (slightly oiled).

After testing, the peak readings are ranked by the program on a quality scale. Default scales are the SKF BQ scale and the GN scale.

Different levels within each classification – from ‘dirty’ to ‘quiet’ – can be distinguished, allowing further grading in quality. BeQuiet+ determines grease quality in these levels to help develop, improve or identify quiet greases.

It can be seen from the charts that a ‘noisy’ grease has far more peaks that a ‘quiet’ one. The highest class of grease will have few peaks, and these will be towards the lower end of the scale (measured in microns/second). The rig measures which of the peaks are equal to, or less than, 5, 10, 20 and 40 microns/second. The number falling into each category then defines the classification, as below.

GN0: anything worse than GN1

GN1: > 95% of all peaks are ≤ 40μm/s

GN2: > 95% of all peaks are ≤ 20μm/s

GN3: > 95 % of all peaks are ≤ 10μm/s

> 98 % of all peaks are ≤ 20μm/s

100 % of all peaks are ≤ 40μm/s

GN4:> 95 % of all peaks are ≤ 5μm/s

> 98 % of all peaks are ≤10μm/s

100 % of all peaks are ≤ 20μm/s

The BeQuiet+ test rig allows a number of assessments to be made. Firstly, it can determine the quiet running classification of lubricants, in terms of particle over-rolling. It can also identify any permanent bearing damage as a consequence of this. More importantly, it measures damping characteristics – and compares lubricants on this basis.

Noise analysis is a powerful tool that helps to determine lubricant quality and bearing lifetime. This new test rig helps to keep the cost of this testing to a minimum – both in terms of bearing consumption and operator time.