Vibration Analysis And Balancing Flashcards
What does vibration analysis refer to?
detecting, measuring, and analyzing the vibration in a machine and determine if its within normal parameters
What are some possible sources of vibration?
misalignment, imbalance, bearing damage, gear problems, cracked welds, resonance
What conditions can be captured using thermography?
overloaded electrical circuits, loose connections, frictional heating of bearings
What do thermography detect?
the thermal radiation (emissivity) of an object
What is a forcing function?
The force that an object vibrates in response to (also known as an exciting force); these functions and their signatures indicate the cause of vibration.
What would be caused by a change in either the mass or the stiffness of an object?
a change in the vibratory signature of that object
What are the 3 terms used to describe vibration?
frequency, amplitude, and phase
What is frequency, and how is it expressed?
frequency is how often an event happens in a a unit of time or in one cycle, expressed by cycles per second (cps, Hz), cycles per minute (cpm), or orders
What are orders?
multiples of the operating speed
What is the total distance a vibrating part moves either side of its neutral axis know as?
displacement
How is displacement measured on a sine wave?
from peak to peak
What are the 3 measurements of amplitude? Which is the best indicator of overall machine condition?
displacement, velocity, acceleration, velocity is the best indicator of overall machine condition
In what range are velocity readings most consistent?
between 600 cpm and 60,000 cpm
What is velocity measured in?
in/sec or mm/sec
What is displacement measured in?
mils pk to pk, or microns pk to pk
What is acceleration measured in?
g’s force (32 feet/sec squared)
Which measurement of amplitude is best for lower frequencies (below 600 cpm)
displacement
Which measurement of amplitude is best for higher frequencies (over 60,000 cpm)?
acceleration
What is the term used to indicate the position of a vibrating part at a given instant with reference to a fixed point or another vibrating part?
phase
What is the term used to describe a frequency at which an object naturally vibrates when a forcing function is introduced?
natural frequency
What condition can take place when a machine runs at one of its critical speeds, and rpm equal to its critical speed, or an rpm equal to its natural frequency (cpm)?
resonance
What is the critical speed of a shaft or rotor?
when the shaft or rotor rotates at or near its natural frequency
How far outside a rotor’s critical speed should a machine run?
20-30%
On a signature, the most prominent frequency known as the:
dominant or fundamental frequency
What type of vibration corresponds to a frequency that is a whole multiple of the rotating speed of the machine? Ones that are not whole multiples?
synchronous vibration, non-synchronous
Frequencies that occur at less than the rotating speed of the machine are known as:
sub-synchronous
What is a harmonic?
A multiple or sub-multiple of the fundamental frequency.
What are 3 types of vibration meters?
vibration pencils, simple data trap, vibration analyzers with FFT capabilities
What are the three basic categories of transducers?
displacement transducers, velocity transducers, acceleration transducers (accelerometers)
Which type of transducer requires a power source?
displacement transducers
Which transducer is capable of measuring all 3 types of amplitude? What material is often used in these?
accelerometer, they often use piezoelectric materials
What are the 4 methods of mounting transducers, and which method is the best?
stud mount (best), magnetic pickup mount, hand-held probe, shaft rider accessory
What are HVA measurements?
positions the transducers are mounted in, horizontal, vertical, and axial
What can be done to avoid errors when determining the rpm of a machine using a strobe light?
locate something like a keyway or bolt or other mark that you know there is only one of
What is done to ensure consistency when taking readings from a machine?
always take readings from the same place
What is the effect on phase if vibration is caused by imbalance?
phase does not change
What would an axial vibration reading of 50% or more of the radial be a good indicator of?
misalignment
If you turn off the power to the motor and notice an immediate drop in the amplitude of the vibration, what is this indicative of?
an electrical problem
What are the characteristics of a gear problem?
they show up at high frequencies, will be synchronous
What are possible characteristics of a bearing problem?
high frequency, non-synchronous
When would oil whirl normally occur in vibration analysis?
usually sub-synchronous, at about 40-50% of shaft rotation
When would mechanical looseness be identified?
when there’s a dominant frequency of 2 x RPM, and vibrations at other frequencies such as 1/2 and 4 x RPM
When is vibration from belts the greatest?
in the direction of belt tension
Where would an imbalance situation from a missing impeller blade show up?
at 1 x RPM
What would be suspected if there is high vibration on the axial plane?
angular misalignment or a bent shaft
What temperature condition would cause sludge in the oil of a machine?
too cold
What are the 4 products of oxidation in oil?
gum, varnish, sludge, acids
When should an oil sample be taken from a machine?
at normal operating, temperature, pressure, speed and load
What’s an oil sample called that’s taken from a point in a circulating system?
a live zone sample
What particle sizes are used in an ISO cleanliness standard in reference to machine oil analysis?
4, 6, and 14 microns
What machine condition is defined as “the unequal distribution of weight of a rotating body about its centreline”?
imbalance
When does eccentricity appear on a vibration signature?
1 x RPM
Why is it important not to lose the keys that come with sheaves, pulleys, hubs, gears, or shafts?
They may have been balanced with that/those key(s)
What is it called if the required balance correction is at a single point on the rotor?
single plane balancing
What type of vibration is caused by extra weight unevenly distributed along the length of the shaft, across the width of the shaft, and/or and different angles about the radius?
dynamic imbalance
What is normally required to diagnose the forces and vibrations from dynamic imbalance?
a computer
How is static imbalance (shaft’s weight unevenly distributed from side to side) detected when the shaft/rotor is suspended between two knife edges or rollers?
the heavy side of the rotor settles at the bottom of the rotation
What is balancing?
Trying to find out where the heavy spot is and attaching or welding weights in the location opposite to that heavy spot.
If balance weight has to be split, what would the sum of the two weights be in relation to the original correction?
the sum would be a larger value
What are 4 ways a machine may become unbalanced?
corrosion and abrasion, distortion, eccentricity, bent shaft
How does doubling the rotor speed affect the force caused by imbalance?
it quadruples it
How is phase affected with couple imbalance?
there is a 180 deg phase difference between rotor ends
