Two rotating shaft shall be aligned such that the shear forces and bending
moments at the coupling are zero.
Final alignment of rotors are obtained by
adjusting the bearings ( for turbine rotors) and by adjusting the motor for
Pumps, fans, etc to give parallelism and concentricity at each pair of
couplings.
The alignment is carried out by measuring
the face gaps and periphery errors between couplings.
Gap measurements, using suitable gauges,
are taken at the top, bottom and two side positions and a repeat set taken with
both shafts rotated through 180 deg. The average of the two sets indicates the
true parallelism of the coupling faces, eliminating any errors due to out of
truth between couplings and shaft.
Concentricity is checked by measuring
between the peripheries of the two couplings, using either dial indicators or
feeler gauges, to a finger attached to one of the coupling. To eliminate
coupling errors, both shafts are turned together and readings taken at the top,
bottom and two side positions.
Calculations using ratios and similar
triangles, taking account of shaft length, distance between bearings and
coupling diameter, determine the adjustment required at the bearings to obtain
shaft alignment.
With solidly –coupled shafts, it is usual
to work to very close tolerances of the order of plus/minus 0.025 mm.
