Vibration Analysis Flashcards
What is vibration?
Vibration in rotating equipment is the result of any force that moves the shaft from the reference. Vibration is any type of movement
What is the vibration signature?
It is a graphical depiction of vibration frequency
What is vibration a response to?
It is a response to some forcing function, or exciting force
What are the three terms used to describe vibration?
Frequency, amplitude, phase
Define frequency
It is how often an event happens in the unit of time or in one cycle
What units of measure are used with frequency?
Cycles per second (cps, also same as hertz)
Cycles per minute (cpm, same as rpm)
Orders- multiples of the operating speed for example how often the event occurs with each revolution
What is predominant frequency?
It is the highest amplitude
What is the key to identifying the problem with vibration
Knowing the fundamental frequency of the vibration is the key
What is amplitude?
Amplitude indicate how much machine is vibrating
What are the three measurements of amplitude?
Displacement, velocity, acceleration
What is displacement?
Displacement is the total movement above and below the axis
The total displacement is also known as peak to Peak. How is this expressed?
It is expressed in mils (.001”)
When is displacement most often used?
Below 600 CPM or 1000rpm
What is velocity in relationship to amplitude
Velocity is the rate of change of displacement and is measured as the maximum velocity
How was velocity measured?
It is in inches per second and it is located in the middle of the sign wave as it crosses the line
Where are velocity readings most consistent?
Between 600 CPM and 60,000 CPM. They are the best indicator because of the wide speed range
What is acceleration?
Acceleration is the rate of change in velocity and it’s measured in G Force. Maximum acceleration occurs at the very top of the sine wave or bottom
How often does maximum acceleration occur in one sine wave?
Twice. At the very top, and very bottom of the sine wave
What is the acceleration method of amplitude best used on?
Very high frequency readings. Above 60,000 cpm
What is phase?
It is the relationship between displacement, velocity, and acceleration. Phase is the time relationship between two events, and is expressed in degrees
How should accelerometers face when taking phase readings?
The accelerometers need to be facing in the same direction
What is the deciding factor in which amplitude reading to use?
Frequency
Displacement - low
Velocity - low to fast
Acceleration - very fast
Describe natural frequency
The frequency at which an object or system naturally vibrates when a forcing function has been introduced
What affects natural frequency?
It is affected by size, material, length, and shape of an object
What is resonant frequency
When at resonant frequency, the object amplifies the energy from the forcing function frequency causing large amplitude.
How is resonance avoided?
By keeping the forcing functions frequency at least 20% outside the natural frequency range
What is the critical speed?
When a rotor rotates at, or close to its natural frequency. They should operate 20-30% outside of the rotors critical speed. Some machines
What is a harmonic?
It is a multiple or sub multiple of the fundamental frequency
What is synchronous vibration?
Synchronous vibrations correspond to the frequency that is a whole multiple of the rotating speed of the machine. These are phase locked to the rpm of the machine
What is nonsynchronous vibration?
Sub synchronous vibrations are frequencies that occurs at less than the rotating speed of the machine
Describe baseline
A baseline is a normal standard for the vibration of a particular machine. It is used as a reference to the normal condition
What is a vibration pencil?
It is a diagnostic tool used for precautionary measurements it is not an analyzer, but able to give some indication of an increase in vibration.
What are the three types of transducers
Displacement transducers, velocity transducers, and acceleration transducers also known as accelerometers
Which of the proximity probes does not generate its own power?
Displacement transducers. For the displacements transducer which uses DC electrical signals the shaft must be a conductor
Describe velocity transducers
They generate their own power. The faster the motion of the vibration the larger the voltage generated. They are very fragile
Describing acceleration transducer or accelerometer
They generate their own power but often use an amplifier to increase signal strength. They are the most common transducer. They use a piezoelectric material as the sensing component
How does a Piezoelectric material work?
When strained or compressed by and external force it creates an electrical current proportional to the force
What does accelerometer measure?
Although it is meant to measure acceleration, the analyzer is capable of transforming a signal into either displacement or velocity. They can operate over a wide frequency or temperature range and is not affected by magnetic fields
What are the different ways to mount transducers?
Stud mount, magnetic pick up mount, handheld probe, shaft rider accessory
Which of the best methods of mounting a transducer?
The stud mount is the best, and the magnetic pick up mount is good also. The shaft rider is permanently installed in the bearing housing so It can monitor the shaft.
What are the different ways you must collect information with the transducers?
Horizontal, vertical, and axial positions. These are referred to as HVA measurements
They should be read as close to the bearing as is possible
How can you avoid errors in determining the RPM while using a strobe light?
Look for such things as keyways or bolts or other marks. if you know there is only one key and you see three when the strobe stops the shaft and the frequency shown on the meter is probably three times the rotating speed
What is shock compared to vibration?
Shock is a disturbance caused by a sudden applied force. Shock the oscillations die before the next impulse where is vibration is a continuous condition
What is a vibration signature?
Frequency spectrum a plot of vibration amplitude against frequency
What is MIMOSA?
Machine info management open systems alliance
What Austin shows up at 2X the RPM?
Misalignment or mechanical looseness (pillow block) often shows up at 2X
A fan with six blades will show up with the frequency of what if there is a blade imbalance?
6X
What is the common problem that shows up at 1X
Imbalance
If imbalance is the cause of the vibration what happens with the phase?
The phase does not change. The amplitude of the vibration will also increase with shaft speed
Where is most of the vibration from imbalance?
Most of the vibration will be in the radio direction however if it is on an overhang rotor it will have high axial readings as well
If there is angular misalignment what will show on the transducer readings?
The axial vibration component is usually about 50% or more of the radial amplitude. The phase readings can also be erratic
How do you check if vibration is an electrical problem?
Observe the changing vibration when you turn the power off to the motor. If you notice in immediate drop in the amplitude of the vibration you have an electrical problem.
How will gear problems look with vibration
They can show very high frequency, but will be synchronous. Damaged gear teeth usually show up as a multiple of the number of the teeth times rpm
What affects the frequency from defective bearings?
Ball diameter, number of balls in the bearing, pitch diameter of the path of the balls, speed of the shaft
What is the formula for the inner race ball pass frequency?
Frequency equals 0.6 X number of rolling elements X rpm
What is the ball pass frequency for the bearings outer race?
Frequency equals .4 X Number of rolling elements X rpm
Where will oil whirl show up in frequency?
Frequencies of about 45% to 50% of the shaft rotation
What is the dominant frequency for mechanical looseness
2X
Where is vibration the greatest on belts?
They are greatest in the direction of belt tension
How can you tell if something is vibrating from resonance
If you add or remove weight, stand on a part, clamp or brace it, if the vibration changes the machine is resonating
Where should oil samples be collected?
They should be collected only when the machine is running at its normal operating temperature pressure speed and load
What is a live zone sample
A sample removed from the point in a circulating system
Where should oil not be collected?
If located at dead legs where the fluid is not moving or after filters or separators
Describe infrared radiation?
It can be sensed as heat. The warmer the object the more infrared radiation is immediate.
What is emissivity
It is the materials ability to emit thermal radiation. The highest emissivity is 1.0 and lowest is 0. Shiny objects are very difficult to get emissivity readings from
What is imbalance
It is the unequal distribution of weight of a rotating body about it centreline
What are the major causes of imbalance
Corrosion and abrasion, distortion, eccentricity , bent shaft, excessive clearances, asymmetry, material build up, temperature
Where does eccentricity and the bent shaft show up on frequency?
1X
What should be done with the key in the balanced motor?
Always use the key that came with it is the correct size necessary to keep the rotor in balance
What are the two types of imbalance?
Static imbalance and dynamic imbalance
What is another name for static balancing?
Single plane balancing
When is a part considered statically balanced?
When it does not rotate on knife edges or stops at a different location each time
How is dynamic imbalance normally diagnosed?
A computer is usually used to diagnose dynamic imbalance
What will the phase angles be in static imbalance
It will always be either in phase or 180° out of phase
Describe coupling imbalance
It means there is extra weight at each end of the shaft extra weights are equal but on opposite sides of the shaft 180° out of phase
Doubling the RPM does what to the force caused by the imbalance
It quadruples the force of the imbalance
What is the strobe light used for in imbalance
It’s primary purpose is to check phase angle.
Before trying to balance a machine what should the phase changes be to tell you it is imbalance?
The phase shift should be 90° 90% of the time.
How do you determine a trial weight size?
The common approach is to use hey wait equal to 10% of the rotor wait supported by one bearing. Eg. A rotor carries 2000lbs, each bearing carries 1000 lbs and 10% is 100 lbs
What is the formula for imperial trial weight?
Force=1.77XRXWX (rpm/1000)squared
What is a vector?
It is a line whose length corresponds to the amount of vibration present and who’s direction corresponds to the phase angle
If the weight needs to be placed on the fan blade that falls in between two blades what will the resulting weight be?
The sum would always be larger value if it has to be split
