In-situ Balance of Overhung Fan
Case History Number 9
Imbalance exists to some degree in
all machines, and is characterised by vibration at a frequency of once per revolution. In
the absence of high resolution analysis equipment, imbalance is usually first to get the
blame for excessive once per revolution vibration. A state of imbalance occurs when the
centre of mass of a rotating system does not coincide with the centre of rotation.
Imbalance can be caused by a number of things including incorrect assembly, material
build-up, rotor sag, thermal growth and component loss.
This case history shows how in-situ balance correction can be performed without the need
of a costly machine outage. Vibration data collected from the motor of an overhung fan
assembly indicated the worst recorded once per revolution vibration to be at the motor
drive end horizontal direction with an amplitude of 30 mm/s rms as shown in Figure 1.
Figure
1 - High once per revolution vibration.
The amplitude of the once per
revolution vibration, along with its angular position from a known reference, was plotted
on a vector chart as 'Ov'. The machine was brought to rest and a mass of 30 grams added to
the outer radius of the fan at an arbitrary position. The machine was brought up to speed
and the new amplitude and phase angle of the once per revolution vibration plotted on the
vector chart as 'O+T'. From these two vectors we could determine the vector change caused
by the trial weight, this was plotted on the vector chart as 'Tv'. From these values the
mass of the correction weight was calculated and its angular position from the trial
weight found, see Figure 2.
Figure 2 - Vector chart
indicating vibration levels and phase.
The correction weight required was
30 grams and would have to be positioned 35° from the trial weight position in the
direction of the machines rotation . Once the correction weight was fitted, the trial
weight was removed and the machine run up to full speed. Vibration data recorded indicated
that the source of the imbalance had been correctly located and corrected as depicted in
Figure 3.
Figure 3 - Imbalance reduced to
1.6 mm/s rms
In the event of machine failure it was estimated that
repair costs would be £700 with losses to production potentially being many
hundreds of pounds.
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