TABLE OF CONTENTS What are the limits for actual balancing operations?
Precise Manufacturing and balancing is important.
Ten microinches (0.25 micron) of mass eccentricity is about the practical limit of measurement with a standard balancer. For special applications we can reach 4 to 5 microinches (0.1 micron). Be careful machines can measure to lower indicated levels than that, but the repeatability issue will get you, as if you run the rotor again you will not necessarily get the same reading.
In balancing we use numbers that are much smaller than those used in most machining operations. A balance tolerance may well be equivalent to ten microinches when the surface finish on the journals is 60 microinches or more. If the machine is capable of measuring to 4 microinches then the difference between two consecutive readings can be 4 microinches. A micrometer may be able to measure to 0.0001" (0.0025 mm) but repeated measurements with multiple operators (and possibly multiple calibrated micrometers) will give a range of values that can vary by up to 0.001" (0.025 mm). Resolution is not the same as accuracy or repeatability.
With any measurement system dealing in units of 100 microinches (2.5 micron) or lower there are inherent measuring problems. As an example consider the 'simple' measurement of a shaft journal diameter. First, it is temperature dependent since the shaft (like all metals) exhibits thermal growth. Second, there is a question of whether the journal is truly circular (it could be oval and...
