* Unbalancing of Rotors-2
Unbalancing of Rotors-2
HOW MUCH OF AN UNBALANCE IS ACCEPTABLE
ANSI (AMERICAN NATIONAL STANDARDS INSTITUTE) gives the answer to the above question.
Standard for Balance Quality of Rotating Rigid Bodies is ANSI S 2.19-1975. These standards make recommendations to the permissible residual unbalance as a function of the maximum service speed of a particular rotor. ANSI standards assigns balance quality “grades” to different rotors of various types, sizes, and service speeds with the help of experimental and experience based data taken from various sources.
SYMBOL USED FOR BALANCE QUALITY—-G
MAGNITUDE OF BALANCE QUALITY
Balance quality “G” is equal to the product of the permissible residual specific unbalance ‘e’(called eper) and the rotational speed (w) of the rotor.
G =eper x ω = Constant
eper = permissible residual specific unbalance per unit mass = e
ω = angular velocity
This constant G is a balance quality grade. Balance Grades are used to specify the allowable residual unbalance for rotating machinery. The ISO 1940 standard defines balance grades for different classes of machinery. A rotor named as G4.2 will vibrate at 4.2 mm/s.
G = e x ω mm/s
UNITS OF MAGNITUDE OF BALANCE QUALITY —-mm/s
Specific unbalance is unbalance per unit mass of the rotor
e = U/ M
where “U” is in g mm and “M” is in g.
By substitution, we get
G = (U / M) x ω
On substitution, G = U ( 2 x π x RPM )/ (60 M)
On simplification, we get unbalance
U = 9.54 x M x G
Undesirable Effects of Unbalancing
- Causes vibration
- Produces noise
- Decreases machine life
- Bearings life is reduced
- Creates unsafe working environment
- Causes increase in wear and tear
- Increase in repair and maintenance
- Does undesirable psychological effects
Methods of Removing/decreasing unbalancing
It is achieved by
- Adding mass
- Removing mass
- By changing the position of mass
A balance machine help to correct the unbalance. It measures the existing vibration and gives the corrected method to be applied.
F = U x ω²
e = U /m
Where F = Force due to Unbalance
U = Unbalance
ω = Angular Frequency
e = Eccentricity
m = mass