Rotor BalancingHG 4 (Chapter 8)

INTRO

• Mechanical imbalance is one of the most common causes of machinery vibration

• Static, or standing, imbalance is the condition in which there is more weight on one side of a centerline than the other

• A rotor maybe in perfect static balance, but not be in a balanced state when rotating at high speed.

SOURCES OF VIBRATION CAUSED BY MECHANICAL IMBALANCETwo major sources:

1. Assembly Errors2. Incorrect key length guesses during balancing

ASSEMBLY ERRORS

Potential errors include:

1. Relative placement of each part’s center of rotation2. Location of the shaft relative to the bore3. Cocked rotors

ASSEMBLY ERRORS - Center of Rotation • All rotors should be balanced on a shaft having a diameter as nearly

the same as possible as the shaft on which it will be assembled

• For best results, balance the rotor on its own shaft rather than on a balancing shaft

• This may require some rotors to be balanced in an overhung position, a procedure the balancing shop often wishes to avoid

ASSEMBLY ERRORS - Locating Position of Shaft Relative to Bore

• If the operator removes the rotor from the balancing shaft without marking the point of bore and shaft contact, it may not be in the same position when reassembled

• This often shifts the rotor by several mils, thus causing an imbalance

ASSEMBLY ERRORS - Locating Position of Shaft Relative to BoreMethod:

1. Permanently mark the location of the contact point between the bore and the shaft during balancing

2. Use the mark when the equipment is reassembled in the plant 3. For end-clamped rotors, slide the bore on the horizontal shaft,

rotating both until the mark is at the 12 o’clock position, then clamp it in place

ASSEMBLY ERRORS – Cocked Rotors

• If a rotor is cocked on a shaft in a position different from the one in which it was originally balanced, an imbalanced assembly will result

• For very narrow rotors, If the rotor is slightly cocked, the small axial distance between the two very large centrifugal forces causes an appreciable couple imbalance

ASSEMBLY ERRORS – Cocked Rotors

How to prevent?

1. Tighten each setscrew gradually—first one, then the other, and back again—so that the rotor is aligned evenly

2. On flange-mounted rotors, clean the mating surfaces and the bolt holes

ASSEMBLY ERRORS – Other

Other assembly errors can cause vibration:

1. Variances in bolt weights when one bolt is replaced by one of a different length or material

2. For setscrews that are 90 degrees apart, the tightening sequence may not be the same at final assembly as during balancing mark which was tightened first

KEY LENGHT

When balancing a keyed-shaft rotor:

• One half of the key’s weight is assumed to be part of the shaft’s male portion

• The other half is considered to be part of the female portion that is coupled to it

KEY LENGHT

To prevent an imbalance:

1. Do not allow the balance operator to guess the key length2. It is strongly suggested that the actual key length be recorded on a

tag that is attached to the rotor3. The tag should be attached in such a way that another device

cannot be attached until the balance operator removes the tag

THEORY OF IMBALANCE

• Imbalance is the condition in which there is more weight on one side of a centerline than on the other

• This condition results in unnecessary vibration can be corrected by the addition of counterweights

TYPES OF IMBALANCE

1. Static2. Dynamic3. Couple4. Dynamic imbalance combinations of static & couple

TYPES OF IMBALANCE - Static

• Static imbalance is single-plane imbalance acting through the center of gravity of the rotor, perpendicular to the shaft axis• Also can be separated into two separate single-plane imbalances,

each acting in-phase or at the same angular relationship to each other• The only force involved is weight• When rotation occurs, static imbalance translates into a centrifugal

force

TYPES OF IMBALANCE - Dynamic

• Dynamic imbalance is any imbalance resolved to at least two correction planes (i.e., planes in which a balancing correction is made by adding or removing weight)• The two imbalances do not have to be equal in magnitude and any

particular angular reference to each other• The primary components of dynamic imbalance include the number

of points of imbalance, the amount of imbalance, the phase relationships, and the rotor speed

DYNAMIC – Points of Imbalance

• The number of imbalance points on the rotor can be more than one point of imbalance within a rotor assembly

• Especially in rotor assemblies with more than one rotating element, such as a three-rotor fan or multi-stage pump

DYNAMIC – Amount of Imbalance

• The amplitude of each point of imbalance must be known to resolve dynamic balance problems

• Most dynamic balancing machines are able to isolate and define the specific amount of imbalance at each point on the rotor

DYNAMIC – Phase Relationship

• Balancing instruments isolate each point of imbalance and determine their phase relationship

• Plotting each point of imbalance on a polar plot - a circular display of the shaft end - does this

• Each point of imbalance is located on the polar plot as a specific radial, ranging from 0 to 360 degrees

DYNAMIC – Rotor Speed

• Most rotating elements are balanced at their normal running speed

• They may be out of balance at some speeds that are not included in the balancing solution

TYPES OF IMBALANCE - Coupled

• Coupled imbalance is caused by two equal non-colinear imbalance forces that oppose each other angularly

• Pure couple imbalance occurs if this same rotor is revolved at an appreciable speed

TYPES OF IMBALANCE - Dynamic Imbalance Combinations of Static and CoupleExample of case:

• Rotor that has only one imbalance in a single plane and not at the rotor’s center of gravity but is off to one side• This force to one side of the rotor causes translation (parallel motion

caused by pure static imbalance) and also the rotor rotate or wobble end-over-end as from a couple• Such a force would create a combination of both static and couple

imbalance

BALANCING

• Imbalance is the main source in about 40% of the excessive vibration situations

• Before a part can be balanced with the vibration analyzer, certain conditions must be met:

1. The vibration must be caused by mechanical imbalance, and2. Weight corrections can be made on the rotating component

• To calculate imbalance units, simply multiply the amount of imbalance by the radius at which it is acting

BALANCING – In-Place Balancing

• The process of balancing a part without taking it out of the machine is called in-place balancing

• This technique eliminates costly and time consuming disassembly

• Also prevents the possibility of damage to the rotor

BALANCING – Single-Plane Vs. Two-Plane Balancing• Disc-shaped rotating part usually can be balanced in one correction

plane only

• Parts that have appreciable width require two-plane balancing

• The narrower the rotor, the less the chance for a large couple component and therefore the greater the possibility of getting by with a single-plane balance

BALANCING – Precision Balancing

• The driving force for providing this service is that many large mills and refineries have started doing their own precision balancing to tolerances considerably closer than those used by the original-equipment manufacturer

• Example: ISO for process plant machinery calls for a G6.3 level of balancing in its balancing guide. This was a calculated based on a rotor running free in space with a restraint vibration of 6.3 mm/sec (0.25 in./sec) vibration velocity

TECHNIQUES USING PHASE SHIFT

Method:

1. Determine how the rotor is vibrating vertically by comparing ‘‘vertical only’’ readings with each other

2. Determine how the rotor is vibrating horizontally3. If the rotor is shaking horizontally and vertically and the phase

differences are relatively similar, then the source of vibration is likely to be imbalance

4. Be sure that other l x rpm sources (e.g., bent shaft, eccentric armature, misaligned coupling) are not at fault

BALANCING STANDARDS

• The ISO has published standards for acceptable limits for residual imbalance in various classifications of rotor assemblies

• Balancing standards are given in ounce-inches or pound-inches per pound of rotor weight or the equivalent in metric units (gram-millimeters per kilogram)

• Most balancing standards are based on a residual imbalance (imbalance of any kind that remains after balancing) and do not include multi-plane imbalance

BALANCING STANDARDS