Page 291 - Machinery Component Maintenance

Balancing of Machinery Components 273

With the polar moment of inertia known, the maximum speed n (in
rpm) to which the machine can accelerate this rotor may now be com-
puted.

Example:

Machine specification limits Wk2n2 to 3,000 - lo6 lb ft2n2. The rotor
has a Wk2 of 2750 lb ft2.

nmax = ds = dv 1045rpm
=

To determine the maximum moment of inertia the machine can acceler-
ate to a specific balancing speed, divide the limiting Wk2n2 value by the
square of that speed (n2).

Example:

Machine specification limits Wk2n2 to 3000 . lo6 lb ft2n2.
The maximum rotor Wk2 which can be accelerated to 900 rpm then is:

If the moment of inertia of a given rotor is less than 3,700 lb ft2, it may
be balanced at 900 rpm.
Note: These calculations do not take air resistance and other frictional
losses into account.

Weight-Speed Limitation (Wn2)

The weight-speed limitation stated by a balancing machine supplier for
a given size machine serves (a) to prevent damage to the supports of soft-
bearing machines, and (b) to prevent the hard-bearing machine support
system from operating too closely to its natural frequency and giving
false indications. The stated value of Wn2 is based on the assumption that
the rotors are approximately symmetrical in shape, rigid, and mounted
between the supports.