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Wednesday, May 12, 2010

Alignment of Pumps and Drivers

Outside of serious unbalance of pump components, there is no single contributor of poor mechanical performance more significant than poor alignment. Incorrect alignment between a pump and its driver can cause:
  • Extreme heat in couplings
  • Extreme wear in gear couplings and fatigue in dry element couplings
  • Cracked shafts and totally failed shafts, with failure due to reverse bending fatigue transverse to the shaft axis initiating at the change of section between the large end of the coupling hub taper and the shaft.
  • Preload on bearing (evident by an elliptical and flattened orbit resembling a deflated beach ball); pure asymmetry of vertical and horizontal vibration can be misleading because the bearing spring constants could vary greatly in the kyy (vertical) and the kxx (horizontal) axis.
  • Bearing failures plus thrust transmission through the coupling, which can be totally locked (axial vibration checks across the coupling; that is, at each adjacent machine, will generally confirm this condition).
Significant changes in the cold non-running alignment of a pump and driver can take place if the temperature rise in machine is different and if the piping imposes forces on the pump.

Therefore, alignment under actual operating conditions must be predicted or, if unknown confirmed by instrumentation. In other case, an allowance must be made in the initial cold alignment to component for changes in alignment from cold idle to hot running.

The face and rim method has a sensitivity advantage when the diameter of a coupling exceeds the indicator open of reverse indicator bracket tooling. This is rare, as the pump will generally have in spacer coupling and the reach of the reverse indicators can be increased by clamping onto the driver or the gear could not be rotated, as it seems unlikely that the pump could not be rotated. In order to compensate for the measuring surface’s out being circular or smooth, both shafts should be rotated together when using this method.

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