Pruftechnik devices use Shock Pulse Method in order to define bearings conditions. This method is based on testing the shock impulses, emitted during metal-metal contact in bearings. Two parameters of shock impulses, carpet value and max. value, are used to estimation of baring condition. Both parameters and difference between them are changing while condition of bearing is getting worse. The device measures the carpet value (noise) and max value of shock pulse. Analysis of these values usually allows on early detection of bearing damage, especially when we make trend analysis using PC software.
Expansion of damaged can be detected with huge efficiency by using the diagnostics software TIPTREND or OMNITREND.
Shock impulse is mostly influed by RPM of bearing elements and that's why measuring device includes the rotating speed and internal diameter of tested bearing, and then readings can be standardized.
Many bearings cannot be measured directly using manual hand-probes. This is a basic weakness of all multimeters and data collectors, which use hand-probes for measurements. In such cases bearing must be prepared for measurement, using the industrial accelerometers on bearings. Such accelerometers can be connected in simple way to measuring device, enabling access to hard to get measuring points/places.
What is the shock impulse ?
Shock impulse is an impulse of short duration, generated by bearing system as a result of mechanical impacts. Mechanical impacts occur in all rotating bearings and are caused by irregular surface of bearing race and rotating elements. Value of shock impulse depends on impact speed and surface size.
Where you can find shock impulses?
The most popular place when shock impulse occur is bearing, especially in the area of reciprocal interaction between weighted rotating element and bearing race.
How to estimate shock impulse ?
In order to estimate the shock impulses, simple scale was developed. Below scale indicates, in three categories, size of damage (from good to poor):
In that way, there is no need to use complicated terms and parameters, to estimate bearing condition.
Since rough bearing surface is continuously being increased and decreased during operating, measured parameters might suddenly increase or decrease for short period of time. That's why, single quick measurement of bearing is often less useful or reliable than trending and monitoring carried out in long-term.
Since many parameters influe on shock impulse measurement, so results received from analyzers of different manufactures might slightly differ.
Rough bearing surface generate a very quick series of small shock impulses, which all together form so called carpet of impulses generated by bearing. It might be heard as noise, when you put screwdriver to ear in order to "listen to" the bearing. Value of impulses is expressed by carpet value (decibel). Carpet value provides with important information about bearing lubrication, correctness of assembly, way of bearing weight (e.g. due to misalignment).
Carpet value is directly connected with thickness of grease layer in the field of reciprocal rolling interaction. Low carpet value occurs, when rolling element surfaces are completely or nearly completely separated from each other by grease layer, see below:
Carpet value does not support with information about bearing damage
Picture below shows, how carpet value increases:
Carpet value is helpful while analyzing of reasons of acceptable or poor bearing condition.
Bearing damage cause emission of single strong shock impulses in random pulse duration (see picture below). The maximal value measured on bearing, was called max value (max. decibel value).
Maximal value is used to determine the bearing opera-ting state and existing damage.
On the drawing carpet value is low, when rotating elements and bearing race are separated from each other by grease layer.
On the drawing we may see, that during bearing life the condition was getting worse from good to poor.