UNDERSTANDING DIFFERENCE BETWEEN HEAVY SPOT AND HIGH SPOT IN DYNAMIC B

[Consultant]

Dear members,

I need to understand more on DIFFERENCE BETWEEN HEAVY SPOT AND HIGH SPOT IN DYNAMIC BALANCING.

Share your views, please.

S.T.ANBUKKANI
mdi-india@consultant.com [sig][/sig]

 

[VibrationSpecialist]

Dear Consultant,

The Heavy Spot is the angular location of the imbalance (unbalance) vector at a specific lateral location on the shaft or the rotating element.

The High Spot is the angular location on the shaft or the rotating element directly under the vibration sensor at that point in time when the shaft makes its closest approach to that probe. In other words, it is the location on the shaft surface under the vibration probe at the positive peak of the vibration.

Now at a very slow speed (slow roll) where the dynamic forces still considered negligible, the High Spot and the Heavy Spot are on the same angular location. This means the probe will sense the maximum shaft approach at the same angular location of the unbalance.

As the dynamic forces become more pronounced, then the shaft will approach the probe with phase difference (lagging). This phase become at 90 when the shaft speed is the first resonant frequency.

Take Care. ;-)

 

[dgallagher]

A definable relationship exists between the heavy spot and the high spot which depends upon the rotor critical speed. If a rotor never experiences a critical speed throughout its operating range the heavy spot will always be the exerting force resulting in the high spot coinciding with the heavy spot, just as a weight being swung on a string exerts a displacing force on the rotation axis.

As the rotor surpasses the first critical speed the heavy spot and high spot split apart until the they are separated by about 180 . This phenomenon occurs because the rotor now rotates around its mass centerline instead of its geometric centerline, forcing the high spot to be the location of maximum displacement. If the rotor continues to increase its speed and experiences another critical speed the high spot rotates another 180 until the high spot coincides with the heavy spot. This 180 shift in the high spot's relationship to the heavy spot continues as subsequent critical speeds are surpassed.

For additional information about the relationship between critical speeds and the associated phase shift, see my Application Note, System Response (F=MA)@

http://www.reliabilitydirect.com/appnotes/fma.html.

For additional information on dynamic balancing check out

http://www.reliabilitydirect.com/appnotes/balance.html