PM, NDT, Vibration Flashcards
Types of maintenance (4)
breakdown
preventive
predictive
proactive
This maintenance requires planning
Preventive
This maintenance predicts problems before they happen and work continuously
Predictive (uses vibration, heat, pressure, flow monitoring, oil sampling)
This takes predictive maintenance one step further, also called condition based monitoring
Proactive (identifies and corrects root causes)
Other variations of maintenance (4)
total productive
preventive engineering
reliability engineering
productive maintenance
This helps determine the urgency and extent of activity needed
Priority system - high priority for things that will shut down the plant, next level is for machines that keep whole departments running, and last is for auxiliary equipment etc.
The number of vibrations that happen in a particular unit of time
Frequency (cpm or cps)
1 Hz = 1 cps = 60 cpm
The distance of a vibrating object from it’s original rest position. The total movement above and below the axis
Displacement (important for less than 600 rpm)
The distance from one extreme to another (when displacement is at it’s maximum). Indicates how much a machine is vibrating
Amplitude (thousandths of an inch or mils)
Detects slow moving equipment less than 600 rpm
Speed and direction combined. The rate of change of displacement for a given time. Max occurs on neutral axis
Velocity (inches per second)
Detects problems with frequencies of 600-600 000 cpm
When an object increases velocity, measures in g’s force
Acceleration (max occurs at top of sign wave)
Detects high frequency
Acceleration value
9.8 meters per second or 32 feet per second
When vibrations act together moving in the same direction at the same time
in phase (phase angle zero)
When vibrations have different rhythms or directions
out of phase
Spike-like shock vibrations with extremely high frequency (400 000 to millions of cpm)
Pulse vibrations (indicate very early deterioration of bearings or gears)
This device converts the mechanical energy of the vibration into an electrical signal
transducer (pickups)
The shape of the waveform made by the vibration. This indicates the nature and origin of the vibration problem
Characteristic (analyzed from transducer)
Two types of velocity transducers
moving coil (seismic) - depends on a moving coil of fine wire and a permanent magnet
piezoelectric - no moving parts, electricity generated by transducer by vibration pressure upon ceramic material or crystal
Acceleration transducer that produces a voltage proportional to the acceleration of the vibration. Widely used for vibration measurement (most common)
Accelerometers - generate their own power, use an amplifier to increase signal strength
Displacement transducer that detect vibrations from clearances in a journal and bearing housing. Uses eddy current probe, does not generate it’s own power
Non-contact transducer (proximity transducer gap from shaft 0.020-0.060)
Sometimes mounted 90 degrees to each other
types of transducer mounting (5) from best to worst
- threaded stud mount (most accurate)
- magnetic pickup mount
- epoxy
- handheld
- shaft rider
Where are vibration measurements taken. Referred to as x, y, z axes
at the bearing or as near as possible to it (horizontal/radial/axial positions)
The most common cause of machinery vibration, the unequal distribution of weight of a rotating body about its centreline
unbalance (same frequency as machine 1 x rpm, phase stable)
The bearing with the highest frequency is most likely the _____ ______
problem component
This vibration is most commonly found in machine foundations at base bolts or anchors, cracks in machine bases. It should be eliminated first
Looseness (2 x rpm)
an unequal balance of weight of a rotating part around it’s axis
unbalance (1 x rpm)
four types of unbalance
static
couple
quasi-static
dynamic (hardest to correct)
unbalance caused when the shafts weight is evenly distributed along it’s length, but unevenly from side to side. The weighted centreline and shaft axis are parallel but misaligned along their length
static unbalance (detected by suspending arbor between two knife edges or rollers)
unbalance where there is extra weight at the end of each shaft. The weights are equal but one opposite sides of the shaft (6 and 12 o’clock)
Couple unbalance (detected when the shaft is rotated and a 180 degree phase angle is indicated between the ends of the rotor)
combination of static and couple unbalance
quasi-static
the most common type of unbalance, usually requires a computer to sort them out.
dynamic unbalance (couple or twist)
The oldest and most common form of maintenance inspection.
visual (shine light 5-45 degrees to flawed surface toward you)
This method of inspection is used to detect small cracks or discontinuities which cannot be seen by visual inspection. Can be used metals, ceramics, plastics, glass etc.
Dye penetrant - Liquid penetrates flaw due to capillary action. A developer is applied which highlights the flaw. Does NOT detect sub-surface flaws
3 types of dye penetrant
Water washable - least sensitive, not good for corrosion present. Can be wiped or rinsed off with water
Post-emulsifiable - highly fluorescent, difficult to wash away
Solvent removable - where water is not permissible. High accuracy even on contaminated defects, requires solvent to remove excess. Highly flammable
Machine parts that are to be inspected by this method must be easily magnetized. Detects subsurface flaws not greater than 1/4” (6mm)
Magnetic particle inspection. Must be transverse to the magnetic field to produce flux leakage. Ferromagnetic material only. Used fine iron oxide powder (dry=dyed or wet=fluorescent)
This inspection uses electromagnetic techniques, most effective for locating irregularities near the surface of the specimen, even through a thick layer of paint. Not used for preventive maintenance
eddy-current - ideal for conductive materials that are non-magnetic
AC power
This inspection uses a high-frequency sound waves over 20MHz to detect discontinuities viewed on a oscilloscope or CRT
Ultrasonic inspection (immersion and contact type, contact more portable) Tests thickness over 1/8". Needs couplant on the surface. Surface must be uniform
This inspection can reveal internal or surface defects even when they are covered or hidden by other machine parts
Radiographic inspection (x-ray)
measures the weight of contaminants per litre or fluid
gravimetric level analysis
Assesses the amount and type of contamination. Determines the concentrations over a period and accurately predicts the lubricant failure before it occurs
spectrographic oil analysis
determines quantity, distribution, and appearance by observing (thought a microscope) an oil sample as it is suspended over a magnetic field
ferromagnetic analysis
indicates how much of the oil additives have been depleted
total base number tests
determines how much the oil is breaking down into a harmful acidic compound
total acid number tests
reveals the amount of oil degradation occurring
infrared tests (10% increase in viscosity can have serious consequences)
water content of this amount is allowable in most cases
0.5%
these are liquids used to make excess oily penetrant on the surface of a workpiece water-washable
emulsifiers
oil-based -function by diffusion
water-based - liquid concentrates
2 types of yokes for magnetizing purposes
permanent magnet
electromagnetic
Three main tools for remote visual inspection
videoimagescope (flexible probe, mini colour tv camera, 1/4” probe - up to 65 feet long)
fiberscope (live image in the eyepiece, length up to 20 feet, field of view 10-100 degrees)
borescope (lengths up to 90”)
4 types of developers
Dry developer - power like substance brushed or sprayed on
Wet aqueous - water based, sprayed, dipped or poured
Wet non-aqueous - drying not necessary, evaporates rapidly
Film type - plastic powder and solvent
3 types of waves in ultrasonic testing
longitudinal - particles vibrate in same direction as the wave
transverse/ shear - particles vibrate at right angles to direction of wave
surface waves - detects surface cracks
4 ultrasonic testing methods
resonant frequency - ring test
transit time - determines thickness by time
intensity method - measures amplitude, tests sheet metal
intensity and transit time - most common, measures amplitude and time (used when you can’t reach both sides of workpiece)
2 penetrating waves of radiography
X-ray - no residual radiation, can be used on all materials (through 20 inches of steel)
Gamma ray - no electrical power, radiation hazard, cannot be used on all materials
Safe distance of 50 feet in all directions
cobalt 60, iridium 192
This detects the thermal radiation (emissivity) of an object
thermography (measures heat being emitted)
a graphic depiction of a vibration cycle
sine wave
the time relationship between two related events and is measured of expressed in degrees
phase
When taking phase readings keep accelerometers facing the ____ direction
same
In this frequency the system amplifies the energy from the forcing function frequency causing large amplitude. It destroys machines
resonant (occurs in any system not kept out of natural frequency by at least 20%)
the time required to complete one cycle of a periodic signal
period
occurs when the rotor rotates at or near its natural frequency
critical speed (should operate 20-30% outside of rotors critical speed)
These transducers are used for general vibration measurements and balancing. They generate their own power. Larger than other transducers
Velocity transducers (made with a coil of wire fixed inside transducer case)
HVA measurements meaning
transducer mounted horizontal, vertical, axial at the bearing of machine
When an oil sample is removed from a point in a circulating system it’s called a
live zone sample (only collect when machine is running at normal operating temp, pressure, speed, load)
Where should you not collect oil samples from
dead pipe legs where fluid is not moving, or after filters or separators
Oil sample 18/16/13 means
18 - particles greater than 4 microns
16 - particles greater than 6 microns
13 - particles greater than 14 microns
A strobe lights primary purpose in balancing is to check
phase angle
misalignments frequency
2 x rpm
Belt drive frequency
3 x rpm
doubling the rpm _____ the force caused by unbalance
quadruples (weight and rotational speed determine amount of vibration)
How much should a trial weight be
sufficient to change the amplitude of the vibration by 30%
how thick is a penetrameter shim
2% of the thickness of the specimen being radiographed
One of the best uses for x-rays
checking an engine cylinder head for cracks
ultrasonic thickness measurements are accurate up to this
1%
The most predominant frequency with the highest spike is called
predominant or fundamental frequency
three measurements of amplitude
displacement, velocity, acceleration
The total distance travelled is called this kind of displacement
peak to peak (measured in mils)
the frequency at which an object or system naturally vibrates when a forcing function has been introduced
natural frequency
the normal standard for the vibration of a particular machine
baseline
When using a strobe light the part causing the vibration appears to do what
stand still (measures phase angle too)
oil whirl occurs at what frequencies
45-50% of shaft rotation
amplitude that is similar in vertical and horizontal directions are peculiar to
imbalance
Where is oil analysis more effective than vibration analysis
hydraulic systems, reciprocating equipment
a thermal imagining device produces an image called a
thermogram
If the required balance correction is at a single point on the rotor is it called
single plane balancing
When assemblies of significant length require two weights at opposing ends of the rotor this is known as
dual plane (two plane) balancing
the distance that the heavy spot moves past the pickup before the flash occurs is called
phase lag
the angle of the phase lag is called _____ and the spot where the line remains is called the
flash angle
phase angle
If the vibration pickup is moved from horizontal to vertical and you get a 90 degree phase shift it is usually caused by
imbalance
if you get a 180 degree phase shift the problem is probably caused by
misalignment
trial weight formula.
Force= 1.77xRxWx(rpm/1000)^2
Force in lbs
Radius in inches
Weight in oz
Force is 10% of the weight. If supported between bearings force is 10% of half the weight. If overhung force is 10% of full weight
The sum of additional weights is always _____ than the original correction weight
greater
separate single plane balancing when the rotor length to diameter ratio is large is called
two-plane balancing
4 reason for imbalance
wear
buildup
cracks
fatigue
how is displacement measured
mils
how is velocity measured
inches per second (ips)
how is acceleration measured
g’se force (9.8m/s or 32 feet per second)
whats the most expensive method of measuring vibration
vibration analyzers (measure vibration from all their characteristics, and separate frequencies)
how is amplitude measured
thousandths of an inch
