10-09-2013, 12:59 PM
Dynamic Balancing
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Abstract
When man invented the wheel, he very quickly learned that if it wasn’t
completely round and if it didn’t rotate evenly about it’s central axis, then
he had a problem!
The wheel would vibrate, causing damage to itself and it’s support
mechanism and in severe cases, be unusable. As the task of
manufacturing a replacement was so huge and time consuming, a method
had to be found to minimize the problem. Research showed that the wheel
and its shaft had to be in a state of balance, i.e. the mass had to be evenly
distributed about the rotating centerline so that the resultant vibration was
at a minimum. This had to be achieved during the manufacturing process
(and perhaps just as importantly, as wear occurred) so that maximum
service life could be achieved from the system.
Causes
The International Standards Organization defines unbalance as:
That condition which exists in a rotor when vibratory force or motion
is imparted to its bearings as a result of centrifugal forces. (Ref.2)
A more popular definition is:
The uneven distribution of mass about a rotor’s rotating centerline.
The key phrase being “rotating centerline” as opposed to “geometric
centerline”. The rotating centerline being defined as the axis about which
the rotor would rotate if not constrained by its bearings. (Also called the
Principle Inertia Axis or PIA). The geometric centerline being the physical
centerline of the rotor. When the two centerlines are coincident, then the
rotor will be in a state of balance. When they are apart, the rotor will be
unbalanced.
Manufacturing - Causes
Many causes are listed as contributing to an unbalance condition,
including material problems such as density, porosity, voids and
blowholes. Fabrication problems such as misshapen castings, eccentric
machining and poor assembly. Distortion problems such as rotational
stresses, aerodynamics and temperature changes. Even inherent rotor
design criteria that cannot be avoided. Many of these occur during
manufacture, others during the operational life of the machine. Whilst
some corrections for eccentricity can be counteracted by balancing, it is a
compromise. Dynamic balancing should not be a substitute for poor
machining or other compromise manufacturing practices.
Assembly - Causes
As previously stated, there are many reasons why unbalance occurs when
a rotor is being fabricated. Principle among these is a stack up of
tolerances. When a well-balanced shaft and a well-balanced rotor are
united, the necessary assembly tolerances can permit radial
displacement, which will produce an out of balance condition. The addition
of keys and keyways adds to the problem. Although an ISO standard does
exist for Shaft and Fitment Key Conventions (Ref.3), in practice, different
manufacturers follow their own procedures. Some use a full key, some a
half key, and some no key at all. Thus, when a unit is assembled and the
permanent key is added, unbalance will often be the result. The modern
balancing tolerances specified by ISO, API, ANSI and others make it
imperative that the conventions listed in the ISO standard be followed.
Failure to do so will mean that the low-level balance tolerances specified
in these standards will be impossible to achieve.
Other Causes
Another cause of unbalance that is not readily apparent, is the difference
between types of rotors.
There are two distinct types - rigid and flexible.
If a rotor is operating within 70% - 75% of its critical speed (the speed at
which resonance occurs, i.e. its natural frequency) it can be considered to
be a flexible rotor. If it is operating below that speed it is considered rigid.
A rigid rotor can be balanced at the two end planes and will stay in
balance when in service. A flexible rotor will require multi-plane balancing.
If a rotor is balanced on a low speed balancing machine assuming it is
rigid and then in service becomes flexible, then unbalance and thus high
vibration, will be the result.
Field Balancing
Many rotors can often be balanced in place, running at their own operating
speed, with minimum disassembly. To balance in place, of course, a basic
requirement is that the rotor has to be accessible to make corrections.
Machines such as fans and blowers are good candidates. Totally enclosed
motor armatures and pump impellers are not.
Production Machines
As their name implies, these are used in the manufacture of rotating
elements. They can be highly automated for high-speed production, often
designed around one part, with automatic drills or mills to remove weight
or welding units to add weight. Or, they can be more general-purpose
machines for low volume production, which are easily adjusted to take a
variety of parts.
In some, the part is mounted horizontally like the example shown below,
whilst in others the part is mounted vertically to simplify loading and
unloading. The instrumentation is normally built in to the balancing
machine.
Maintenance Machines
These machines on the other hand are designed to balance rotors after
maintenance overhauls. They are normally horizontal types with
attachments and tooling designed for easy set up and adjustment. They
must accommodate a wide weight range for versatility and be able to
handle a wide variety of shapes and sizes of rotors. Instrumentation can
be either dedicated or separate. The same instrument that is used for field
balancing can also be used in the balancing machine.
Maintenance type balancing machines are a necessity in repair depots
such as motor rewind shops, pump overhaul facilities and turbine repair
workshops. All industrial rotors that have been overhauled should include
a balance check as the last task before re-assembly.
Conclusions
Everything that rotates needs to be in a state of balance to ensure smooth
running when in operation.
Precision balancing is essential to the manufacture of rotating equipment
and to the repair and renovation of installed machines. As machine
speeds increase, the effects of unbalance become more detrimental.
Modern technology allows for accurate balancing to be performed both in
the field and in the workshop.
Increased time between outages and availability for production is the
prime benefit.