Tuesday, June 7, 2011

My Vizag Steel Plant Report



We, the students final year mechanical engineering dept. from Bengal Engineering & Science University,Shibpur,Howrah are very grateful to DGM of  T &DC for allowing us for three weeks training(project work) dated 22/06/2009 to 11/07/2009 in this World class Organization.
We feel proud to take the privilege of doing the project work in this world class organization, Visakhapatnam steel plant.
We are very grateful to DGM(PEM),T.K.ROY(AGM),K.TRINADHA RAO,P Srinivas, Simham  and all other employees for  their invaluable co-operation  and guidance  in completing our project work.



Machine is very vital in our life. When a machine fails or breakdown, the result can be disaster in regard to personal injury and possible loss of life and financial loss. For this reason it is very necessary to detect, identify and correction of machinery problem before use to ensure safety and productive operation.
      A level of unbalance that is acceptable at a low speed is completely unacceptable at a higher speed. This is because the unbalance condition produces centrifugal force, which increases as the speed increases. In fact, the force formula shows that the force caused by unbalance increases but the square of the speed. If the speed is doubled, the force quadruples; if the speed is tripled the force increases by a factor of nine! It is the force that causes vibration of the bearings and  surrounding structure. Prolonged exposure to the vibration results in damage and increased downtime of the machine. Vibration can also be transmitted to adjacent machinery, affecting their accuracy or   performance. Identifying and correcting the mass distribution and thus minimizing the  force and resultant vibration is the technique known as dynamic balancing. . This project report introduce us to use of machinery vibration due to unbalance.

Why Balance?
Vibration caused by unbalance has many well-known effects on the machining process. The most obvious is chatter. One common reaction to chatter is to reduce the spindle speed, which of course reduces the capability of the machining center. On the workpiece, the principal effect is poor surface finish and the inability to hold close tolerances. And on the machine you will get markedly poorer tool life and ultimately, over time, spindle and bearing damage is likely to occur. This latter fact is why many machining center manufacturers recommend balanced tooling and in some cases actually void the spindle warranty if balanced toolholders are not used above a certain speed. The threshold that defines high-speed machining for the purposes of balancing historically has been 8,000 to 10,000 rpm. However, there are a number of variables that affect this. This article will discuss in greater detail the spindle speeds at which toolholder balancing becomes critical.

What is Vibration?
Vibration  is defined as cyclic or oscillating motion of a machine or machine component from its position of rest.

Cause of vibration
1.     Unbalance of rotating components.
2.     Misalignment of coupling , bearing and gears, shaft bend.
3.     Looseness.
4.     Deterioration of rolling element bearings.
5.     Gear wear .
6.     Rubbing
7.     Aerodynamic/hydraulic  problems in fans , blowers and pumps.
8.     Electrical problems (unbalance magnetic forces ) in motors.
9.     Resonance
10.         Eccentricity of rotating components such as v-belt pulleys or gears.

Preventive maintainance
 A program of periodic disassembly , inspection and replacement of worn parts has the distinct advantage of lessening the frequency of breakdown ,  repairs and also permits scheduled shutdown. Under this program each critical machine is shutdown after a specific period of operation and partially or completely dismantled for a throughout inspection and replacement of worn parts-if any. This is carried out in Vishakhapatnam steel plant by power engineering maintainance department (PEM).

Characteristics of vibration
The   characteristics needed to define vibratin include:
·        Frequency
·        Amplitude
·        Phase

it  is defined as number of cycle completed per second(cps) or cycles per minute (cpm).

In other words,  frequency  of a vibration is inverse of period of vibration.
The table below illustrates the vibration frequencies and their likely causes:
Frequency in
Terms of  rpm
Most likely
Other possible causes  and remarks
·       Eccentric journals, gears pr pulleys
·       Misalignment of bent shafts
·       Electrical problem
·       Resonance
Mechanical looseness
·       Misalignment of high axial vibration
·       Reciprocating forces
·       Resonance
Usually a combination of misalignment and excessive axial clearance
Less than 1*rpm
Oil whirl
·       Bad drive belts
·       Background vibration belts
Synchronous  frequency
Electrical problems
·       Eccentric rotor
·       Unequal air gap
·       Unbalance phases in poly phases
Many times rpm
Bad  gears
Aerodynamic forces
Gear tooth times rpm of bad geat
Number of fan blades times rpm
High frequency
Bad antifriction bearings
Cavitaion,recirculation and flow turbulence may be unsteady

The magnitude of vibration or how rough or smooth the machine vibration is, is expressed by it’s vibration amplitude. Vibration amplitude can be expressed and measured as :
It is defined as ‘the position of a vibrating part at a given instant with reference to a fixed point or another vibrating part’. The units of phase are degrees.

The instruments used for detection of machinery vibration are available in wide array of features and capabilities like amplitude and phase measurements. Some of the vibration measuring  devices are:
•Vibration meters
•Vibration analyzers
•Shock pulse meters
•The figure below shows vibration analyzer, which is used to measure amplitude and phase.

                             Figure: OSCILLOSCOPE

The vibration meter  connected with accelerometer, which is used to measure amplitude and a strobe or photoelectric sensor, which is used to measure phase. The accelerometer works on the principle of piezoelectric effect and the strobe works on the principle of photoelectric effect.

Vibration Acceleration Sensor: 
                                      This sensor  operates at frequency below its natural frequency. This is a spring mass system. The piezo-electric effect of quartz is used for the conversion of mechanical effect. In this sensor  piezo-electric ceramic disc are preloaded together with a seismic mass. With this constraction the piezo-electric ceramic disc form the spring in the spring mass system. If this is subjected to vibration, the seismic mass imposes a alternating force on the discs which as a result of piezo effect causes a alternating electric charge. This charge is proportional to acceleration of the vibration. This charge is converted by a built in charge amplifier to a voltage by a oscilloscope.

Vibration Acceleration Sensor

Balancing means improving the mass distribution of a rotating object so that it rotates in it’s bearings without any effect of any centrifugal forces, and the bearings are not subject to excessive periodic forces at the fundamental frequencies.
In general the concept of balancing involves two separate actions.
They are:
•Measuring the unbalance
•Correcting the unbalance
Unbalance Defined
Unbalance is defined as "That condition that exists in a rotor when vibratory force or motion is imparted to its bearings as a result of centrifugal force." Unbalance is caused by an uneven distribution of mass around the axis of rotation of a rotating body. This can result from fixed as well as variable sources. The fixed causes of unbalance result from non-symmetry of design.
               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.

                                                                                                      Principal Inertia Axis 

Depending on the distribution of the unbalance over the length of motor, a distinction is made between two types of unbalance in rigid rotors.
Static Unbalance
Static unbalance arises when the principal inertia axis is displaced parallel to the axis of rotation (see Figure 1). It can be compensated for by either adding or removing material equal in weight to the unbalance amount in a single plane, perpendicular to the axis of rotation. Static unbalance can be measured either on a rotating or non-rotating balancing machine.
Couple unbalance exists when two equal unbalance masses are positioned exactly 180' apart in two planes perpendicular to the axis of rotation. This causes the principal axis of inertia to displace not parallel to but intersecting with, the axis of rotation at the center of gravity (C.G.) of the part (see Figure 2). A couple unbalance only can be corrected with another couple. That is, by applying correction equal and opposite to the original couple. It only can be measured on a rotating-type balancing machine.
Dynamic unbalance is the most commonly occurring type of unbalance. It is the combination of static and couple unbalance. It causes the principal axis of inertia to deflect from the rotational axis both non-parallel to it and not intersecting with it at the C.G.(see Figure 3). Dynamic unbalance only can be corrected in two planes by adding or removing material. Like couple unbalance, it only can be measured on a rotational type balancing machine. Fig:
Types of unbalance-
•Static unbalance
•Couple unbalance
•Dynamic  unbalance

                                   Figure1: Static Unbalance

                                             Figure2: Couple Unbalance

Figure3: Dynamic unbalance

Unbalance always exists when the mass distribution of the rotor with reference to the shaft rotational axis is not symmetrical. The causes of Unbalance can take many forms and can be combined into four groups.
e.g. :- components not symmetric, unmachined surfaces on the rotor, variation in roundness and construction because of coarse tolerances.
e.g.:- blow holes in cast components, non -homogeneous material density, uneven material thickness.
E.g.:-misshaping from welding and casting errors, stress errors caused by work procedures, permanent deformation caused by relieved stress, shrinking after welding or soldering, stress caused by uneven tightening of bolts or screws.
E.g.:-corrosion or erosion of rotor, material built up on impellers, thermal deformation of hot gas exhauster fans, blade fracturing on turbine rotors. Wear on grinding wheels, displacement of rotor parts caused by centrifugal force, general wear.

The figure below shows the external view of the layout and construction of the fan, which has constant service speed.

A:-correction plane
1, 2, 3, 4, 5:-vibration-measuring points.
Single plane balancing by vector method:
Definition of single plane balancing:-
A procedure by which the mass distribution of a rigid rotor is adjusted in order to ensure that the residual static unbalance is within the specified limits’
The following steps summarize the procedure for the single plane vector method of balancing:
Step 1:-Operate the rotor at the normal balancing speed and record the original    (O) unbalance amplitude and phase reading. Record this data as “O”
Step 2:-Using polar graph paper, construct vector representing “O” as shown below.

O=120 ,  O+T=30

Step 3:-Stop the rotor and add a trail weight. Record the amount of trial weight.

Step 4:-Again, operate the rotor at the balancing speed and record the new unbalance amplitude and phase readings. Record this data as “O+T”.
Step 5:-Using polar graph paper, construct vector representing “O+T” using the same scale as shown in figure.

O=120    , O+T=30

Step 6:-construct vector “T” by connecting the end of “O” vector to the end of “O+T” vector.
Step 7:-Measure the length of “T” vector using the same scale as that used for vectors “O” and “O+T”, and calculate the amount of correct weight using the formula.
               Correct weight = trial weight X {“O”/”T”}
Step 8:-Using the protractor, measure the included angle between the “O” and “T” vectors. Shift the balance weight by this angle in the direction opposite the phase shift between “O” and “T”.
Working frequency of the fan =745 rpm

·        Initial run:-Amplitude=4.1mm/sec. {"O”}
·        Add trial weight (Tw)=476 gm at 100°
  • Trial run:-         Amplitude =6.0 mm/sec {“O”+”T”}
                                         Phase =120°   
·       Correction weight:
          Correction weight = trail weight X {"O”/”T”}
·       Correction angle =100°-80°=20°
Therefore  ,  the trail weight 476 gm at 100° is removed and the correction weight 390.32 gm is now added at 20°
·        Correction run :- Amplitude =1.16mm/sec.  

     Thus, the fan is balanced by adopting the vector method of single plane balancing.
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
speed increases, 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.