Page 377 - Intro Predictive Maintenance

368 An Introduction to Predictive Maintenance

to five times greater than that resulting simply from tolerances. Therefore, all rotors
should be balanced on a shaft with a diameter as nearly the same as the shaft on which
it will be assembled.

For best results, balance the rotor on its own shaft rather than on a balancing shaft.
This may require some rotors to be balanced in an overhung position, a procedure the
balancing shop often wishes to avoid; however, it is better to use this technique rather
than being forced to make too many balancing shafts. The extra precision balance
attained by using this procedure is well worth the effort.

Method of locating position of shaft relative to bore. Imbalance often results with
rotors that do not incorporate setscrews to locate the shaft relative to the bore (e.g.,
rotors that are end-clamped). In this case, the balancing shaft is usually horizontal.
When the operator slides the rotor on the shaft, gravity causes the rotor’s bore to make
contact at the 12 o’clock position on the top surface of the shaft. In this position, the
rotor is end-clamped in place and then balanced.

If the operator removes the rotor from the balancing shaft without marking the point
of bore and shaft contact, it may not be in the same position when reassembled. This
often shifts the rotor by several mils as compared to the axis on which it was bal-
anced, thus introducing an imbalance. The vibrations that result are usually enough to
spoil what should have been a precision balance and produce a barely acceptable
vibration level. In addition, if the resultant vibration is resonant with some part of the
machine or structure, a more serious vibration could result.

To prevent this type of error, the balancer operators and those who do final assembly
should follow the following procedure: (1) The balancer operator should permanently
mark the location of the contact point between the bore and the shaft during balanc-
ing. (2) When the equipment is reassembled in the plant or the shop, the assembler
should also use this mark. (3) For end-clamped rotors, the assembler should slide the
bore on the horizontal shaft, rotating both until the mark is at the 12 o’clock position
and then clamp it in place.

Cocked rotor. If a rotor is cocked on a shaft in a position different from the one in
which it was originally balanced, an imbalanced assembly will result. If, for example,
a pulley has a wide face that requires more than one setscrew, it could be mounted
on-center but be cocked in a different position than during balancing. This can happen
by reversing the order in which the setscrews are tightened against a straight key
during final mounting as compared to the order in which the setscrews were tightened
on the balancing arbor. This can introduce a pure couple imbalance, which adds to
the small couple imbalance already existing in the rotor and causes unnecessary
vibration.

For very narrow rotors (e.g., disc-shaped pump impellers or pulleys), the distance
between the centrifugal forces of each half may be very small. Nevertheless, a very
high centrifugal force, which is mostly counterbalanced statically (discussed in