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engineering
rotating machinery reliability

Questions and Answers of Rotating Machinery Reliability

A technician is performing an impact analysis with a portable data collector. No matter how hard he strikes the object with the proper impact tool, the data are not identifying any clearly defined
What are the two most important factors when preparing to impact a machine or part for natural frequency identification?
The tall vertical pump example was successfully brought down to acceptable vibration levels by placing a long 2- × 4-inch beam between the top of the motor and a cinder block wall. What else could
The coast-down data recorded below were taken with the motor operating at 892 rpm and the pump operating at 862 rpm via an eddycurrent drive, and the peak average was continued until the pump reached
True or False: A successful impact analysis of a motor end bracket can be accomplished with the rotor at rest in sleeve bearings.
True or False: The rotor can be turning very slowly in sleeve bearings for an accurate motor end bracket impact analysis.
A motor, operating on 60 Hz power, has high vibration in the axial direction on the end brackets. The vibration frequency is 7,200 cpm. The motor is only running at 1,180 rpm. What is the excitation
The case history in this section of the book discussed a large two-pole motor, operating on a 60 Hz-power, sleeve-bearing machine with high axial vibration at 7,200 cpm. The motor was 2,500 hp, and
Question 9 indicated high vibration on the sleeve bearing and bracket in the axial vibration. What damage would this have caused?
End bracket resonances are subject to changes in support stiffness. What could change the total equivalent support stiffness on a largehorsepower motor that is bolted to a steel sole plate imbedded
True or False: Unbalance due to a bent shaft can always be corrected by hanging a sufficient amount of weight to overcome the unbalance.
True or False: A worn sleeve bearing will usually be indicated by harmonics of a fundamental frequency that is less than half the shaft speed.
A brand-new sleeve bearing has been installed in a machine. The vibration levels on restart of the machine are not within tolerance, and the predominant peaks are at exactly one-half the operating
A sleeve bearing machine on a closed loop oil pumped lubrication system has bearing clearances that are close to specification, yet there is a vibration at exactly 1/2 operating speed. What can be
A sleeve-bearing machine is running up to speed, and as it passes through a known critical frequency, vibration at the critical frequency starts to occur, and even though the machine continues to
True or False: Brand-new ball bearings are being fitted to an existing pump shaft. The shaft is measured and found to be 0.002 inch over specification. Because we are looking to shrink the bearing
A reconditioned motor with new ball bearings is placed on a truck for delivery. The shaft turns freely, indicating that the bearings are properly installed. The motor gets to the customer, and after
True or False: A bearing that shows signs of brinelling but no sign of surface deformation or corrosion can yield a full life.
A motor has large rotors mounted onto its dual output shafts. A design engineer has calculated the static rotor weight to be within the specifications for bearing loading. The motor ships, and on
What is the calculation for coupling hub shrink fit?
Large rotating machinery has historically been fitted with sleeve bearings. Manufacturers have switched to ball bearings to lower cost. The clearances in these bearings can be so large that the balls
What is the typical number of harmonics of running speed in a spectrum taken on a machine operating within satisfactory limits?
A technician is laser aligning a motor-generator set. The first set of readings calls for 0.025 inch under all four feet. The next set of readings calls for 0.050-inch shims to be added to all four
True or False: When checking for soft foot on a large, heavy machine resting on I-beam rails, it is always advisable to use the one-foot-at-atime method called for by the laser equipment manual.
Alignment of a motor with sleeve bearings is critical to long life. The following true/false questions are all related to the alignment of a sleeve bearing motor:a. True or False: An electric motor
What is required when necking down foot bolts to eliminate a motor foot that is bolt bound?
A technician has just taken their first set of readings on an alignment job. The solution calls for the removal of 0.100 inch from the back feet of the motor. There are no shims under the motor back
What is the radial alignment adjustment for a machine train with a 3-ft spool piece as part of the coupling?
True or False: Thermal alignment numbers are always available from the manufacturer.
True or False: The technician should never need to check exhaust piping, equipment supports with torsional pivots, and so on, because shaft alignment is all that is necessary.
True or False: Belt tension should be very tight. The tighter the better.
How many shims are too many under any one foot?
What valuable analysis should be completed when a machine or motor under test is getting ready to be shut down?
True or False: The motor design engineer is not responsible for the frequencies of the rigid-body modes.
What is the critical speed map good for?
True or False: A rotating machine with mounting feet that run the entire length of the frame only needs to be shimmed at the four corners.
The plot in question 5 has a peak at 7,200 cpm. It appears to almost disappear when the motor power (60 Hz) is cut for the capture of the coast-down. Why?
Where does this second rigid-body mode appear due to an equivalent support stiffness of 300,000 lb/inch?
Why would anyone design a tuned absorber for a machine?
How does a tuned absorber work?
A customer installed 18 motors, all driven by VFDs. The manufacturer had given the following engineering instruction for installation: “One set of power leads through any one motor conduit.”
When installing a VFD on a motor, what specification, if overlooked, can cause turn-to-turn shorts?
The plot below is a FRF taken from impacting and measuring the same point on a structure. It is also known as a driving-point FRF. What can we expect to see in the resulting modal analysis animated
What is the most important feature of a driving point FRF?
True or False: Only electric motors on variable-frequency drives (VFDs) have electric discharge machining (EDM) damage.
True or False: The damage shown in the image below, due to EDM, takes several years to occur.
The velocity spectrum below is due to EDM damage. There are multiple sidebands in the spectrum that are usually separated by what frequency? D+ 300 R$ Pask 151 100 10202011201 Line
The plot below is voltage and current from the output of a VFD. What can you see in the voltage traces that defines them as a VFD source? Current (A) 200 100 0.073 -100 -200 820 0.079 0.089 2800
The following plot of the voltage output from a VFD illustrates why the total 600 V is always present in the common-mode voltage from the source. What is the common-mode voltage of utility power? 0.5
The following photograph of EDM damage on the outer race of a bearing appears to be smeared. What could be the cause of this pattern? 28 24 20 16 12 8 4 32NDS 64TH5 8 16 24 32 40 48 56.
The following stacked time traces from four separate motors identify one as experiencing impacts in the ball bearing. What is the separation in the impacts in this time waveform? Acceleration in G-s
Name the measurement tool seen in the following photograph. www
Readings taken with this particular model yielded 50 mV/A. Based on this sensitivity, is the motor upon which the following readings were recorded in trouble? -42 42 -78 -114 -150 130 80 12 102 66 30
An OEM assembled drive assemblies for its pumps. The following photograph was taken on one of the manufactured drive cabinets. The power cable between the motor and the drive was grounded by
According to numerous tests performed by me, what is the maximum amount of shaft current that a motor can operate with and not have EDM occur?
True or False: Motor manufacturers will warranty their motors against EDM damage with the addition of shaft grounding brushes.
What can be inferred by the following voltage trace from the same phase A but from two separate power heads? 800000- 700000- 40000- 300000- 200000- www 100000- 40000- -200000- start 033 000 S 0:04
Very high vibration levels were experienced on a drag line when the bucket, full of wet sand, was shifted into high speed for the ascent from the lake. The frequency of this peak is much lower than
A DC motor-drive system has a comparator. The comparator’s job is to compare the existing speed of the motor with the requested speed. The motor speed is transmitted from a tachometer on the
Encoders can have very high tachometer pulse rates, on the order of 1,024 pulses per revolution. What is critical about the placement of an encoder?
The following PdMA EMAX® plot was recorded on the output phases of a variable-frequency drive (VFD). What can the reader infer from this plot? Voltage and Current line Domain eye Condition Code
The industry standard for balanced voltages in three phase systems is 2%. When the output data is taken on a VFD, the voltages are not easily read. What should be the current imbalance based on a
This is the results page of the PDMA EMAX® test of a VFD. Note that the voltage readings will always be erroneous because of the nature of the “chopped” DC that forms the voltage output. But
Below are the power results of a large, 2,500-hp motor. What is the problem? Results Page-Sayerville CoGerColorb#1SM (01/28/2011 10:13:03PM) Vote Pawer Fund Fk Tot R CF THD ww Van 1-2 421956 422364
Two identical fans were tested for motor inrush current. One motor would trip after taking too long to achieve full speed, whereas the other motor exhibited no issues. See below:Current inrush on
Broken rotor bars are not a common fault. What are two main causes of broken rotor bars?
True or False: Broken rotor bars are just as easily identified at rest as during full-load operation of the motor.
True or False: Faulty poured aluminum bars are easily found during static testing.
What can you do with a Reliance Electric loose rotor bar construction on which someone has swedged the bars?
What are the three items that should be verified before condemning a rotor in the field as having bad rotor bars?
I was sent to South Korea to investigate a problem with a two-pole motor. The vibration signatures indicated an increase in 1× vibration as the motor, under load, came up to operating temperature.

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