balance quality for rigid rotors

Based on present experience, the ISO has categorized the normally available rigid rotors into various groups and have suggested the balancing levels for them in ISO-1940 specifications. The following table gives the level to which a rotor should be balanced.

QUALITY
GRADE 'G' ROTOR TYPES - GENERAL EXAMPLES
G 4000 Crankshaft drives of required mounted slow marine diesel engines with uneven number of cylinders.
G 1600 Crankshaft drives of rigidly mounted on large two cycle engines.
G 630 Crankshaft drives of rigidly mounted large four-cycle engines. Crankshaft drives of elastically mounted marine diesel engines.
G 250 Crankshaft drives of rigidly mounted last four cylinder diesel engines.
G 100 Crankshaft drives of fast diesel engines with six and more cylinder complete engines (gasoline or diesel) for cars, trucks and locomotives.
G 40 Car wheels, wheel rims, wheel sets, drive shaft; crankshaft drives of elastically mounted fast four-cycle engines (gasoline or diesel) with six and more cylinders; crankshaft drives for engines of cars, trucks and locomotive.
G 16 Drive shafts (propeller shafts, cardan shaft) with special requirements; parts of crushing machinery; parts of agriculture machinery. Individual component of engines (gasoline or diesel for cars trucks and locomotive; crankshaft drives of engine with six and more cylinders under special requirements.
G 6.3 Parts of process plant machines. Marine main turbine gears (merchant service), centrifugal drums, fans, assembled air craft gas turbine rotors, fly wheels, pump impellers, machine tool and general machinery parts, normal electrical armatures, individual components of engines under special requirements.
G 2.5 Gas and steam turbines including marine main turbines (merchant service), rigid turbo- generator rotors, turbo-compressors, machine tools drives medium and large electrical armatures with special requirements, small electrical armatures, and turbine driven pumps.
G 1 Tape recorder and photograph (gramophone) drives, grinding machine drives, small electrical armatures with special requirements.
G 0.4 Spindles, discs, and armature of precision grinders, gyroscopes.

what if the balancing fails

As pointed out earlier it should be precisely confirm unbalance is the only problem before implementing the balancing, as there are many different problems which are having similar vibration characteristics to unbalance. Yet, after the confirmation, all attempts to balance have failed, the next recommended step is to check the rotor for repeatability of unbalance readings. We have to simply stop the rotor and remove all trial weights and balance correction weights and rerun the rotor to the condition it was in when original readings were taken. Compare readings taken in this run with the original unbalance readings. They should be the same. These exercise may be done several times to verify the repeatability of the unbalance readings. If the two sets of readings differ significantly in amplitude and/or phase, this may be due to one of the following factors.

1. The actual configuration or shape of the rotor may not have been stabilized when the first set of original unbalance readings was taken. The rotor may have had a temporary 'sag', which has disappeared now that it has had an opportunity to to run.

2. The unbalance condition of the rotor may be changing from one run to the next. For example, a rotor that is loose on the shaft may assume a slightly different position on the shaft each time it is started and stopped. In addition, fans with hollow blades or hollow shafts may have accumulated dirt or water that changes location each time the rotor is started or stopped.

3. If nothing of the above observed, the rotor should be thoroughly examined for possible cracks on the shaft or rotor. If a repeatability check reveals that the rotor is repeating the unbalance readings from run to run some other problem is usually the reason and a more thorough analysis should be carried out to determine the cause.

shop balancing

Shop balancing is always preferred in case of new machines and in case of running machines where high degree of balancing is required. Moreover, some machines such as totally enclosed motors, pumps, compressors and others not easy to balance in-situ because extensive disassembly is required to gain access to the rotor for adding or removing balance weight. In these instances, the machine is disassembled and rotor is balanced on a balancing machine.
In-situ balancing will generally produce better results in terms of vibration. However, balancing on a balancing machine will generally produce a better balance. To clarify, the balancing machine is better at measuring and correcting for unbalance, especially two plane or dynamic unbalance. It can not compensate for field installation factors. Two plane balancing on a balancing machine usually produces better results than two plane balancing in the field because the compounding field installation factors like bearing clearances, support stiffness and resonance response, additional components attached to the rotor like coupling, keys, fasteners etc. and alignment are not present. The primary advantage of balancing machine operations is that the unbalance effect is directly measured. The disturbing factors that can cause 1xrpm vibration are not present, so unbalance can be evaluated without these other complications. Balancing machine operations are not so severely troubled by cross effect because they have very flexible supports or they employ plane separation technique. Balancing can be done by either of the methods described earlier depending upon type of rotor, type of unbalance, normal operating speed of the rotor, etc. Some basics of the balancing machines are explained in the coming sections.

balancing mthods

Before actually attempting to balance a rotor, there are few preliminary factors that need consideration. The most important include:

1. Determining the type of unbalance: This is important in deciding whether a single or two plane approach will be needed. If in doubt, use a two plane approach. A single plane problem can be solved by the with a two plane approach. However, a two plane problem can not be solved with a single plane approach.

As a general guideline the length to diameter (L/D) ratio of a rotor can be useful to determine whether single plane or two plane approach should be adopted for balancing, which is described as follows:

L/D ratio Single Plane Two plane
<0.5> 1000
>0.5 rpm <> 150

2. Calculating the amount of trail weight: Great care must be taken in selecting the size of trial weights. If the weight is too small, no significant change in the unbalance readings will result and a run will have been wasted. On the other hand, if the trial weight is too large for the machine being balanced, the unbalance forces generated by the trial weight may cause extensive damage to the machine.
As a general rule, a trial weight that produces a minimum of a 30% change in amplitude and / or a 30 degree change in phase from the original unbalance readings will help insure satisfactory results. A common approach for selecting a trial weight is to use one that will produce an unbalance force equal to 5-10 % of the rotor weight supported by the bearing.

3. Correction method: Correction can be done either by weight addition or by weight removal. Weight addition is preferred than weight removal for greater accuracy. In large rotors slots for weight addition are given in pre-determined balancing planes. If weight addition is not feasible, great care should be taken when weight removal.

Balancing may be done in-situ (i.e. in installed condition) or in a balancing shop depending on certain factors like unbalance severity, type of rotor, planes of correction, rotor weight, downtime of the machine etc. The above mentioned criteria and balancing types applies to both of these. In shop balancing, measurements and calculations are done by the balancing machine software while in in-situ balancing portable vibration analyzers or polar charts may be used.

balancing types

Balancing can be classified into three types depending upon the number of correction planes used for balancing:

• Single plane balancing
• Two plane balancing
• Multi plane balancing

Single plane balancing:

As the name implies, single plane balancing is the correction of unbalance in one plane to achieve good balance. This method is always applicable for thin disks and can also be applicable to long rotors also if the unbalance is mostly in one plane. The one plane does not need be through the center of gravity. The single plane balancing corrects for static unbalance only. It can do nothing for couple unbalance. Generally "thin" overhung rotors, single impellers, motors and other long rigid rotors with unbalance vibration substantially higher at one end, are balanced with this method.

Two plane balancing:

The primary reason for doing two plane balancing is to correct for a couple unbalance or combination of static and couple unbalance, which are usually the case. This type of unbalance generates a rocking force. The correction is to apply two weights in two planes separated by some axial distance. This creates a counterrocking force; i.e. a couple unbalance is corrected by another couple. For rigid rotors, any combination of static, couple or dynamic unbalance can be corrected by two plane balancing and two planes are selected arbitrarily depending upon the accessibility and feasibility.

Multi plane balancing:

When we balance rotors externally by making corrections in planes other than where the original unbalance exists, these cause internal bending moments in the rotors. For the rotors operating above their critical speeds these internal bending moments cause deflection and change in shape of the rotors at different critical speeds as explained earlier. If unbalance is corrected in the plane where it exists, there would be no internal bending moments and the rotors would not deflect at different critical speeds. This necessitates balancing in more than two planes for rotors, which operate above their critical speeds. Rotors like high speed multistage compressors, large steam turbines, gas turbines, rolls of paper making machines etc come under this class of rotors.

balancing steps

Component balancing:

For shaft rotors, which comprise of more than one component, it is vital to balance all of the major components individually before assembly. This is done because if the rotor is fully assembled, there is no way to know exactly what contribution each component part is making to the total measured unbalance vector. In addition, if a large unbalance exists in one of the major components, within the rotor, the rotor shaft may flex at this point during high speed operation and cause significant damage to the rotating and stationary parts.

Each major rotor component must be individually balanced on a precision ground mandrel (note that expanding mandrels are not acceptable for this purpose). The balance mandrel should be ground between centers to assure concentricity of all diameters throughout its length as well as to assure a good smooth surface. After grinding, the mandrel must be precision balanced. A trial bias weight may be used to raise the observed residual unbalance readout of the balancing machine. The desired balance result is such that no matter at what angular location the bias weight is added, the balance readout is always the same. In this case the residual unbalance of the precision mandrel is as close to zero as possible. The rotor component should always be mounted to the mandrel with an interference fit, never a sliding or loose fit. If the rotor component has a key fit to its shaft, than the balancing mandrel should also have a matching keyway. After each component is shrunk on its mandrel, the axial and radial runouts should be checked to ensure that the mounted impeller or hub is not cocked on its mandrel prior to component balancing. As a general rule, runout should not exceed 0.16 mm/meter of diameter.

Progressive component stack balancing (sequential balancing):

After individual balancing of all major rotor components, the rotor must be progressively stack balanced as each major component is assembled onto the rotor shaft. Progressive or stack balancing is necessary due to the deformation of components during assembly. Components with unequal stiffness in all planes, such as those with single keyways, may deform when shrunk onto the rotor shaft. For such components, considerable deformation and resultant unbalance can occur between mandrel balancing using a light shrink fit and stack balancing on the job shaft with a heavy shrink fit.

Progressive balancing is accomplished by stacking no more than two rotor components at a time onto the rotor shaft. Component axial and radial runouts should be checked against mandrel runouts, as each component is start. In general, the start component runouts should match those runouts recorded with the components on the mandrel.

As each rotor component is start into position and the runouts checked as acceptable, the rotating assembly is to be placed in the balancing machine and trim balanced (if required) as necessary to achieve the balance tolerance. Balance weight correction is to be performed only on the most recently stacked component.

After the rotor is completely stacked, trim balancing, if required at all, should be very small to meet the tolerance of permissible residual unbalance. As a general rule of thumb, the remaining residual unbalance in the rotor should not exceed two times the residual balance tolerance prior to trim balancing.

Balancing_Machine_Drives

Balancing machines typically employ one of following three different drive configurations 
to spin the rotating assembly: 
  • Direct end drive 
  • Wrap-around belt drive 
  • Tangential belt drive 

Direct end drive: 
The direct end drive or universal joint connection is typically used with rotors having 
large moments of inertia or high windage losses. This drive design will transmit high 
torque forces for fast acceleration and safer braking. To attach the drive shaft, the rotor ends must be prepared to accept the U-joint directly or with an adapter. With this design, the drive system becomes a part of the rotor and must be considered in the balancing accuracy of the system. Before this type of the drive can be used to accurately balance a workpiece, the U-joint assembly, itself must be balanced. The desired end result is to be able to rotate the U-joint assembly 180 degrees without any variance in the balancingmachine readout. 

Wrap-around belt drive: 
For rotors weighing below 2250 kilograms and with at least one smooth surface, a wrap- around belt drive works very well. For this drive configuration, 2250 kilograms is about the maximum rotor weights that will allow adequate torque transmission to bring the rotor up to the required balance speed. A 180-degree wrap is required to provide adequate torque transmission to rotate the rotor and keep the applied torque in the vertical plane of the balancing machine pedestal. A belt drive of this type is considerably more accurate then a direct coupled end drive in that the drive mechanism does not influence the workpiece balance. 

Tangential belt drive: 
A tangential belt drive, either under the rotor or in an over-arm configuration, is frequently used in smaller capacity, high volume production oriented balancing machines for rotors weighing less than 450 kilograms. This type of drive configuration is used to bring the rotor up to the required balancing speed and the then the drive arm is moved away from the rotor and the unbalance data is collected.