Page 307 - Machinery Component Maintenance

Balancing of Machinery Components 289

dle assembly is within a given tolerance. At this point the principal iner-
tia axis of the shaft essentially coincides with the shaft axis of the bal-
anced cage. Center-drills, guided along the axis of the cage, drill the
shaft centers and thereby provide an axis in the crankshaft about which it
is in balance. The subsequent machining of the crankshaft is carried out
between these centers.
Because material removal is uneven at different parts of the shaft, the
machining operation will introduce some new unbalance. A final balanc-
ing operation is therefore still required. It is generally accomplished by
drilling into the crankshaft counterweights. However, final unbalance
corrections are small and balancing time is significantly shortened. Fur-
thermore, final correction by drilling does not exceed the material avail-
able for it, nor does it reduce the mass of the counterweights to a level
where they no longer perform their proper function, namely to compen-
sate for the opposed masses of the crankshaft.

Testing Balancing Machines

Total verification of all purchase specification requirements may be
possible for a production machine, but usually not for a general purpose
machine, such as a machine in a motor repair shop, because a rotor of the
maximum specified weight or polar moment of inertia may not be avail-
able at the time of acceptance tests. Nevertheless, essential conformance
with the specification may be ascertained by a complete physical inspec-
tion and performance tests with typical workpieces and/or a “proving ro-
tor.” Physical inspection needs to take into account all specified dimen-
sions, features, instrumentation, tooling, and accessories that are listed
in the purchase specification and/or the seller’s proposal. Performance
tests are somewhat more involved and should be witnessed by a represen-
tative of the buyer who is well acquainted with balancing machines and
the particular specification applying to the machine to be tested.

Tests for Production Machines
A production machine is usually purchased for balancing a given part
or parts in large quantities. Acceptance tests, therefore, are generally
performed by running samples of such parts, so that total compliance
with specified indicating accuracy and cycle time can be ascertained un-
der simulated production conditions. At the same time, tooling is
checked for locating accuracy and balance. Additional tests, as described
in the following paragraphs, may then be confined to just the first part
(U,,, Test), since compliance with the specified cycle time may already