Page 113 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 85

Interference diagram
0 Coupling dynamic torques
Dynamic gear loads
0 Harmonic torque loads for reciprocating machinery
Torsional vibrations
a Shaft stresses
3. Transient Torsional Analyses
a Start-up time
Stress versus time
0 Cumulative fatigue
0 Allowable number off starts
4. Impeller and Blade Analyses
Natural frequencies
Mode shapes
Interference diagram
Experimental shaker tests or modal analysis
5. Pulsation Analyses
Acoustic resonances
Mode shapes
Shaking forces
Surge effects

Lateral Critical Speed Analysis

The most common design audits are the lateral and torsional critical speed audits
since they potentially offer the most benefits. Experience indicates that many sys-
tems have been installed with critical speeds in the running speed range and have run
successfully for years before troubles are encountered. This sometimes is difficult to
understand, but a design audit that considers the entire range of possible values for
the shaft unbalances and bearing and seal parameters will usually indicate the possi-
bility of a problem.
A lateral critical speed audit should include these calculations:

1. Critical speed map
2. Undamped natural frequencies and mode shapes
3. Bearing stiffness and damping properties
4. Seal stiffness and damping properties
5. Rotor response to unbalance
6. Pedestal and foundation effects on response
7. Rotor statbility

The first step in performing a lateral critical speed analysis is to model the shaft
with sufficient detail and number of masses to accurately simulate the rotor respons-
es through its speed range. An accurate shaft drawing giving the dimensions,
weights, and centers of gravity of all added masses is needed to develop the model.