High Spot Balancing

“Interpreting Polar Plots for Balancing”

Georgia Chapter of the Vibration Institute – 1st Quarter Meeting 2010

Kiran Toram Machinery Diagnostics Services Bently Nevada GE Measurement and Control

What is Polar Plot Display of filtered amplitude and phase angle as a function of speed or time in polar co-ordinates Polar plots are useful in the determination of physical high points for balancing Used in conjunction with Bode plots, they help to identify critical speeds and structural resonances.

Example

CRITICAL SPEED Speeds at which response to unbalance is a maximum

Vance, Rotordynamics of Turbomachinery

N=(K/M)0.50

Traversing through Critical Speed there is 180 degrees change in phase angle

Heavy Spot vs High Spot

High spot is in phase with heavy spot

0°

0 90 180 270

1925

High spot 180 degrees out of phase with heavy spot

270°

* * 2065 3220 * 2100 460 2625 * * 2135 2555 * *2170 2520 * * * 2485* * 2240* 2205 2415 *

180°

90°

360 10 8 6 4 2 0 0 1000 2000 3000 4000 5000

Balancing of an ID Fan vs Balancing a Generator

ID Fan Balancing

Machine Train Information • Bearings 1 and 2 on Motor • Bearings 3 and 4 on ID Fan • Running Speed 897 rpm • Journal Bearings on Fan • Proximity Probes at 45° L & R • Temporary Seismic Transducers • Keyphasor at 45° R

Bearing 3 Relative and Seismic Polar Plot before (45 R)

KPH

3X R = 4.2 mils at 27° 3X S = 2.9 mils at 265°

3X Absolute = 3.65 mils at 343°

Bearing 4 Relative and Seismic Polar Plot before (45 R)

KPH

4X R = 3.63 mils at 46° 4X S = 4.48 mils at 297°

4X Absolute = 4.76 mils at 343°

Trial Weight 3X Absolute = 3.65 mils at 343° 4X Absolute = 4.76 mils at 343°

On Bearing 3 - 11.6 oz at 165° On Bearing 4 – 15.6 oz at 162° Against rotation from Keyphasor

Bearing 3 Relative and Seismic Polar Plot (45 R) after Trial Weight Installation

KPH

TW

3X R = 2.5 mils at 69° 3X S = 1.6 mils at 302°

3X Absolute = 2.0 mils at 29°

Bearing 4...