Hydro Generator Aligning
The objective of alignment is simple: to have all the rotating equipment spinning about a common axis at the
designed center of the stationary equipment. Alignment is achieved by measuring and adjusting the positioning of
the rotational centers of the shafts such that they are co-linear with the machines under normal operating
conditions. Proper shaft alignment is ultimately dictated by the designed centers of rotation of the shafts'
Benefits of correct alignment are reduced noise, heat and vibration with improved bearing and seal
service life. Even moderate misalignment can dramatically reduce bearing service life, but excessive misalignment
even under normal operating conditions can generate large forces that are applied directly to the machine bearings
causing accelerated fatigue and wear of the shaft bearings & seals. In extreme cases, the bending stress
applied to a shaft may cause coupler failure or shaft breakage.
The two components of hydro turbine to rotor shaft
Offset Shaft Misalignment
Offset misalignment is the distance between the shaft centers of rotation measured at the plane of power
transmission from the driving unit to the driven unit. This is typically measured at the coupling center.
The units for this measurement are Mils (where 1 Mil = 0.001”).
A special case of offset alignment is Parallel Misalignment (or Parallel Offset Misalignment), where the
axis of rotation of the 2 pieces of equipment are exactly parallel, but not co-linear.
Angular Shaft Misalignment
Also called “gap” or “face”, angular misalignment is the difference in the slope of one shaft,
usually the moveable machine, as compared to slope of the shaft of the other machine, usually the
stationary machine. The axis of the shafts lie in the same plane, however they are not co-linear. The units
for this measurement are comparable to the measurement of the slope of a roof, Rise/Run. In this case the
rise is measured in Mils (1 Mil = 0.001”), and the run (distance along the shaft) is measured in inches,
giving units for Angular Misalignment of Mils/1”.
In practice, turbine to generator alignment
starts with precision measurements of both offset and angle in order to calculate the required
Additionally, an experienced hydro engineer will take into consideration the effects of any coupler
backlash and avoid shaft alignment pitfalls like "soft foot".
Alan Towers Engineering is qualified to measure, diagnose and correct out-of-alignment problems in a
hydro generator installation.