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engineering
rotating machinery reliability
Rotating Machinery Reliability For Technicians And Engineers 1st Edition W. Ron Brook - Solutions
A balance run using a trial weight placed at a 6-inch radius has called for a final weight of 210 g. The final weight is to be welded to the face of the fan at a radius of 18 inches using a ⅛-inch welding rod. How much should the steel plate weigh?
A fan and shaft are sent to a shop for bearing replacement. The shop balances the fan in a hard bearing machine operating at 450 rpm. The fan is high speed, being directly driven in the field by a motor at 3,600 rpm. The field engineer has cautioned the shop foreman to paint the final balance
Electric motors generate heat in their rotor. What is the average time for a loaded motor to reach stable temperature?
A new induction motor is put into operation. The initial vibration is acceptable, but after 1 hour of loaded operation, the 1× vibration has increased to an unacceptable level. The motor is immediately uncoupled and operated solo, and the vibration at 1× is now unacceptable as well. What is the
Trial weight runs have been completed, and the final weight calls for approximately 150 g. Using ¼-inch plate steel, what size piece should be cut if a ⅛-inch welding rod is to be used for attaching it? (Readers: Use the “Trial Weight Calculator” in Figure 5.4.)Figuer 5.4 Weight calculations
A trial weight should generate a force equal to 10% of the rotor weight in order to establish the location and amount of imbalance. The rotor weighs 100 lb and has an operating speed of 3,600 rpm. What is the size of the trial weight in ounce-inches?
A newbie is out in the field getting ready to balance a fan. The vibration level is severe and predominantly 1× running speed in the radial direction. A trial weight is placed on the fan, and there is no change in the level or the phase reading. What is the problem?
The same newbie changes the trial and runs the fan for a new reading. The calculations call for the trial weight to be increased from 100 to 400 g. What should the newbie do?
True or False: Phase information is always required to balance a rotor.
True or False: Trial weights can be attached to the outer edge of a fan wheel just using the clamping bolt.
True or False: A rotor that has been installed in the field, experiences a rise in vibration levels and the ends are in phase. Eventually, the vibration levels call for the machine to be taken off line for being above operating specifications. The technician needs to balance this rotor by pushing
What setting should the technician use to capture the maximum vibration at every speed during a coast-down?
What setting can the technician change in the data collector or while viewing the data in the historical database to ascertain whether the reason for a large vibration peak is the proximity to a natural frequency?
True or False: The only thing that can cause a high vibration at 1× running speed is unbalance.
True or False: Balancing a rotor suspected of running near a resonant frequency calls for care when increasing the size of the trial weight indicated by a balance program.
The case study that involved balancing a gas turbine that could not pass through its first critical frequency called for use of the existing proximity probe outputs input into a digital FFT using peak-hold averaging. These results were then put through a no-phase balance procedure that resulted in
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 naturalfrequencies. The amplitudes of the data are all very low. What is the most common problem for
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 have been done to achieve this result?
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 a speed of 622 rpm. What can the reader glean from the vibration levels that were recorded through
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 force that is exciting the end bracket?
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 it was coupled through a fluid coupling, with ball bearings, for speed control, to a boiler feed
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 into a large block of reinforced concrete sitting outside on a thick concrete slab?
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 speed and harmonics of it. What is the diagnosis?
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 tried to alleviate the vibration?
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 speed up, the vibration at the critical frequency continues, and it is severe. What is the diagnosis?
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 onto the shaft anyway, this is okay.
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 three months of running, the bearings are showing “bad” signatures. What is the logical
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 arrival, testing proves that the bearings are cracked. What happened?
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 have a problem tracking, resulting in unstable bearing operation. What is the fix for this
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 feet, and another reading is taken. The laser calculation now calls for 0.100 inch under 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 shaft will operate on what is known as its magnetic center. This can be marked while the motor is
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 feet. The technician reverses the laser head and prism to see if the alignment can be accomplished
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.” Below is a picture of the resulting installation. Will there be a problem? V
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 shapes at this point? Medalion FAS 6,21 SaveRecall BankUP EXPORT PRINT GEN ACQUIRE QUET FIEJECT
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 0:090 0.04 Voltage (V) 0 100 300 200 100 -100 -200 -300 -400
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 -0.5 0.5 0 -0.5 -1 I 0.5 0 -0.5 .1 PHASE A PHASE B PHASE C 0.5 COMMON MODE VOLTAGE (VCM = V
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 Plot Span 10 T 10 -10 100 Rockwell WAVEFORM FROM MORE THAN ONE MACHINE 200 300 400 500 Time in
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 6mV 370us/Div 1.00 ms Input A
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 stripping away the outer cable insulation and then bolting a metal clamp to the backplate of the VFD
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 0:00
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 the running speed of the motor. The resulting vibration signature was captured in the following
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 outboard shaft of the motor. The requested speed can be from a programmed source, or it can be an analog
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 Corrections Probe Seting 0-1000Len PR1301 Condon Cole Good Scale KB Of Of 10 1 12 13 TV 13 Vou Pho
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 specification of a voltage imbalance of 2%?
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 what can the reader say about the current readings? Results Page - Exhaust #4 (01/21/2009 08:45:39 AM)
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 2-9 427477 421000 1.40 Vage1-3 478353 428774 142 142 0.72 Prese S1554 Wit 326 ES KVA D1 Pl 0.515
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 good unit.Current inrush on faulty unit.Other than the time required to achieve full speed, is there
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. Unfortunately, a faulty rotor was the diagnosis. This was a cast rotor. What two tests could have
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