Effects of imbalance on machine spindles, toolholders and tools
Imbalance
An imbalance produces a centrifugal force during the rotation of the spindle
impeding the smooth running of the tool. This imbalance infl uences the working
process and the life span of the spindle bearings.
The centrifugal force F increases linear with the imbalance U and squared with
the number of revolutions according to the formula below.
Counter balancing
To compensate for unwanted centrifugal forces, the symmetrical distribution
of mass must be restored with the aim of eliminating any centrifugal forces
infl uencing the spindle bearing. Tool holders generally have compensation holes
or areas which assist in directing the total amount of all centrifugal forces
infl uencing the axis towards zero (see DIN ISO 1940).
Eccentricity of center of gravity
Calculation imbalance
Imbalance is a measure, specifying how much unsymmetrical distributed mass
deviates radially from the rotating axis. Imbalance is measured in gmm. The
measure of distance e determines the distance of the center of gravity of an
element to the rotating axis.
Imbalance is calculated as follows:
U = m × r
U = imbalance in gmm
e = eccentricity of center of gravity in µm
m = mass in kg
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The imbalance of a spindle causes its center of gravity to deviate a certain distance
from the rotating axis in direction of the imbalance. This distance is called
rest-eccentricity e or eccentricity of center of gravity. The heavier the weight of
the balance body mass m, the greater the restimbalance U permissible.