Three types of movement or deviation can occur between two shafts, see Figure 12.5, namely:


Figure 12.5 Types of misalignment

• Axial misalignment, end float, where the shaft centre lines are in alignment although the axial position is incorrect and axial movement may be possible.

• Angular misalignment, where the centre lines of the respec­tive shafts are not parallel.

The deviations can occur singly or in combinations. Also the in­dividual deviations can change with operating conditions. Atyp­ical changing condition is from cold to running temperature con­ditions. Thermal growth causes machine centre heights to increase slightly as they warm up.

High temperature fans may be centreline mounted to avoid thermal growth of the fan casing, and imposing strain on the connection ductwork. It might also lead to loss of clearance be­tween the fan inlet cone and the impeller. However, the motor driving a centreline mounted fan is usually foot-mounted and may itself have thermal growth. In this situation motors are mounted low so that the growth expands the centreline height of the motor into near perfect alignment.

In large machines changes in ambient temperature or sunshine can affect the alignment. The thermal growth phenomenon can be further complicated when the drive and non-drive ends of a machine expand at differing rates. Not only does the radial alignment change, but also the angular alignment. Accurate on-line measurement is necessary to check for this condition.

Suppliers of couplings provide information relating to the maxi­mum permissible deviations, usually stated for each individual

*^ma [1]^mo J

Mk =Mi



Axial deviation as % of max. permissible

Figure 12.6 Permissible angular misalignment as function of axial deviation and radial misalignment for a particular size of double-diaphragm spacer cou­pling


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