Unlike reactive maintenance, predictive testing methods provide valuable insight into whether your shaker may experience problems in the near future. Implementing a predictive maintenance test plan allows you to plan maintenance within the context of tightly packed test schedules and to reduce the risk of unplanned downtime and equipment failure. To do this, keeping a series of reference plots for a shaker is critical. These reference plots also allow you to see which of the many tests are putting the most stress on the shaker system.
All shakers produce different characterization plots. It is important to keep a reference set and copies of the same performance throughout the life of the shaker system to ensure that your shaker performs at its best under your operating conditions. When reviewing the latest plots, with reference to the previous ones, differences can be looked at and implications can be drawn.
Recording a character of a shaker
When considering the performance of a shaker system, several aspects must be taken into account in order to properly create the different characterization plots.
- Test Setup: Always use the same equipment setup. If there are any changes, run the same test with the old setup and again with the new, noting any differences due to the control equipment.
- Run the shaker in the vertical axis
- Ensure that the shaker runs from cold
- Fit a triaxial accelerometer to the centre insert of the shaker’s armature
- Vibration Profile: Set a profile in the controller for a 5 mm peak-to-peak constant displacement crossing into a 2 gn constant acceleration sine sweep across the entire frequency range of the shaker. Use peak control with a 1 octave per minute sweep rate.
- Recording: Record the drive, control and the cross axis in both directions.
- Define the highest frequency that can be run with the control position, by looking at the drive. Ensure that the drive does not rise past the nominal level seen at 200 Hz
- Run a high-level sweep at 20% of displacement, velocity and acceleration through the frequency range as defined previously. Record the drive, control, cross axis in both directions and the total harmonic distortion plotted through the frequency range. Use the same measurement technique for the analysis of distortion
Once these plots have been taken, they should be kept in a file and repeated at sensible intervals through the vibration test system life. These intervals should be determined by the usage of the shaker and the practical level of testing that can be performed. As a guide Hottinger Brüel & Kjær LDS would recommend taking these plots at monthly periods, or after any significant high-force testing that has occurred.
These plots can reveal several things, mostly about the health of the armature and suspension. Armature problems can mean either a failure in the coil, or within the frame. Changes to these plots over time will indicate that these parts are becoming worn or old. However, it does not necessarily mean that they need to be replaced.
The fact that the plots are changing will indicate that they need to be monitored and possibly inspected visually. The speed of change indicates whether a failure is imminent. Changes tend to happen slowly to start with and then accelerate just before failure.
What to look for in the shaker plots
Ideally the plot will be flat, if it is not flat then it could indicate that there is a control problem. Nevertheless, there is very little that can be determined from the control plot alone.
This diagram should look the same as the previous 2 g drive plot in terms of level, shape and resonant frequencies.
The most important information is the first resonant frequency that can be seen on the drive plot. This frequency should be plotted over time to easily detect changes. Normally, with any unused armature, there is an initial change, that is, the frequency drops by 2 – 5%. It then stabilizes with minor further changes until the armature begins to fail. Large changes may indicate impending armature failure.
The shape of the drive plot should be the same as before. It is an indication of a problem if the curve has more ‘bumps’ or ‘dips’ than before. Large changes associated with a drop in resonant frequency indicate an impending armature failure.
The base level should be the same as before. If it is not, this may indicate either a control problem or a field loss.
Cross axis plot
This is the most difficult to interpret plot as it will vary with the temperature of the shaker suspension. To interpret this, you should look at the base level of the cross axis and see that it is approximately the same as before.
Any peaks that occur should be logged in frequency and level and plotted through time. Hottinger Brüel & Kjær LDS would recommend taking 4 peaks to plot for any shaker, normally the highest, but spread through the frequency range, this may mean taking lower peaks at the lower frequencies.
Large changes mean that the suspension is getting worn and should be visually inspected.
Although this is a different measurement to the cross axis, the same way of looking at it occurs. Take the previous plots and look at the base level shape to the curve.
Check to make sure that the new plot matches this, while also checking the peaks are not changing primarily in level, but also in frequency. Changes in the base level or increased peak levels can indicate either a problem with the armature or the suspension. Refer to the other plots to identify the problem.
Other factors can cause problems that would look like a failing armature, these are:
- Loose accelerometer cables
- Decoupling, either under the insert or under the accelerometer
- Anything loose on the armature, inserts, screws, etc.
- Excessive noise pick-up
- As with any test procedure over time, consistency is the most important thing, ensuring that as many things as possible are kept the same
HBK maintenance plans suit your needs
Hottinger Brüel & Kjær’s service agreements and preventative maintenance plans provide you with peace of mind that your investment is operating at optimal capacity, allowing you to concentrate on other business-critical activities. All plans include support and annual service maintenance, covering inspection, testing and verification or adjustment of critical components.