from:An Analytical Approach to Solving Motor Vibration Problems
page8:
Forcing Frequency Response Vibration
Weak Motor Base:
If the motor is sitting on a fabricated steel base, such as a slide base, then the possibility exists that the vibration which is measured at the motor is greatly influenced by a base which itself is vibrating. Ideally the base should be stiff enough to meet the “Massive Foundation” criteria defined by API 541 [1]. Essentially, this requires that support vibration near the motor feet to be less than 30% of the vibration measured at the motor bearing. To test for a weak base, measure and plot horizontal vibration at ground level, at bottom, middle, and top of the base, and at the motor bearing. Plotted, this information would look like Fig. 11, for a motor sitting on a weak base. In this particular example, had the motor been on a rigid base, the vibration at the bearing would have been closer to .25 mils rather than the measured 2.50 mils. |