the service magaine of the prÜftechnik roup the service magaine of the prÜftechnik roup gear mesh...

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1 the service magazine of the PRÜFTECHNIK Group The Jungfraujoch in the Swiss Alps is a must-see attraction. The Jungfraujoch is a particular favorite of tourists from the Far East. To see Jungfraujoch, you can now go via modern or historic railways. However it is annoying, if on the way to this fantastic place, noises suddenly oc- cur in one of the railcars. But how dangerous these noises are, was a question to be answered using measurement technology. Or better yet, what can be done to eliminate this noise? A diagnostic order that is undertaken with glee. Fig. 2 shows a rack and pinion drive found in historic railcars. Acceler- ometers were mounted to characteristic measurement points of several drives, and a microphone installed inside the train. For data acquisition a mobile on- line CMS was temporarily installed in the train, to enable real-time measurement evaluation and diagnosis during the mea- suring run. Then the run began with an addition- ally coupled, fully filled water truck to simulate the passenger mass. It was apparent that the train changed gear in narrow curve sections both during the ascent and the descent, which needed to be considered as part of the evaluation. Condition Monitoring Service Temporary noise on Jungfrau train transmission Dr. Edwin Becker, Ismaning, Germany Gear drives are in widespread use. They are used to regulate the speeds of the driving and driven machines and to transmit torque. Germany, with around 8 billion, is a global market leader in gears and gearboxes. One of the rea- sons for this is that manufacturers in Germany collaborated to carry out early research, under the German Research Association for Power Transmission En- gineering (FVA). Initial FVA studies on the usefulness of condition monitoring in gears appeared in the 1990s. At that time, measurement techniques were not as advanced as is required for meaning- ful gearbox diagnostics. This has dra- matically improved in the meantime, such as the wind industry, for example, demonstrates. Here condition monitor- ing systems from PRÜFTECHNIK have also ensured that deviations in running and operating behavior of gears and rolling bearings are detected at an early stage, so that industry gearboxes are no longer one of the main causes of plant shutdowns. However, this sector also showed that particle detection is further condition monitoring measure required for obtaining reliable state information in slowly rotating gearbox components. Telediagnose no. 16 is devoted to gears. PRÜFTECHNIK topic PRÜFTECHNIK for gear transmissions No. 16 topic: Gear transmission In this issue: Temporary noise on Jungfrau train transmission Calculate meshing frequencies Typical measurement locations and ac- ceptance criteria for vibration behavior in gears Investigate gear transmission core us- ing bending stress measurements of gear tooth root Level 3 analysis of planetary gear Vibration analysis of a double helical highspeed running gearbox Structural analysis on gearbox housing Determine root causes of tooth and shaft breakages The noise and vibration measurement clearly indicated that the dominant noise occurred only when the generator was descending at a speed of 24 km/h. And from the sound pattern we could deduce that gearing noise was the likely cause. The next step was to find out the gear stage responsible, using frequen- Figure 1: A historic train of the Jungfrau Railway in the Swiss Alps Photo courtesy of Julian Ryf.

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Page 1: the service magaine of the PRÜFTECHNIK roup the service magaine of the PRÜFTECHNIK roup Gear mesh frequency occurrence is typical for gear drives, whether straight, helical, or double

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the service magazine of the PRÜFTECHNIK Group

The Jungfraujoch in the Swiss Alps is a must-see attraction. The Jungfraujoch is a particular favorite of tourists from the Far East. To see Jungfraujoch, you can now go via modern or historic railways. However it is annoying, if on the way to this fantastic place, noises suddenly oc-cur in one of the railcars.

But how dangerous these noises are, was a question to be answered using measurement technology. Or better yet, what can be done to eliminate this noise?

A diagnostic order that is undertaken with glee. Fig. 2 shows a rack and pinion drive found in historic railcars. Acceler-ometers were mounted to characteristic measurement points of several drives, and a microphone installed inside the train. For data acquisition a mobile on-line CMS was temporarily installed in the train, to enable real-time measurement evaluation and diagnosis during the mea-suring run.

Then the run began with an addition-ally coupled, fully filled water truck to simulate the passenger mass. It was apparent that the train changed gear in narrow curve sections both during the ascent and the descent, which needed to be considered as part of the evaluation.

Condition Monitoring Service

Temporary noise on Jungfrau train transmissionDr. Edwin Becker, Ismaning, Germany

Gear drives are in widespread use. They are used to regulate the speeds of the driving and driven machines and to transmit torque. Germany, with around € 8 billion, is a global market leader in gears and gearboxes. One of the rea-sons for this is that manufacturers in Germany collaborated to carry out early research, under the German Research Association for Power Transmission En-gineering (FVA). Initial FVA studies on the usefulness of condition monitoring in gears appeared in the 1990s. At that time, measurement techniques were not as advanced as is required for meaning-ful gearbox diagnostics. This has dra-

matically improved in the meantime, such as the wind industry, for example, demonstrates. Here condition monitor-ing systems from PRÜFTECHNIK have also ensured that deviations in running and operating behavior of gears and rolling bearings are detected at an early stage, so that industry gearboxes are no longer one of the main causes of plant shutdowns. However, this sector also showed that particle detection is further condition monitoring measure required for obtaining reliable state information in slowly rotating gearbox components. Telediagnose no. 16 is devoted to gears.

PRÜFTECHNIK topic

PRÜFTECHNIK for gear transmissions

No. 16 topic: Gear transmission

In this issue:

Temporary noise on Jungfrau train transmission

Calculate meshing frequencies

Typical measurement locations and ac-ceptance criteria for vibration behavior in gears

Investigate gear transmission core us-ing bending stress measurements of gear tooth root

Level 3 analysis of planetary gear

Vibration analysis of a double helical highspeed running gearbox

Structural analysis on gearbox housing

Determine root causes of tooth and shaft breakages

The noise and vibration measurement clearly indicated that the dominant noise occurred only when the generator was descending at a speed of 24 km/h. And

from the sound pattern we could deduce that gearing noise was the likely cause.

The next step was to find out the gear stage responsible, using frequen-

Figure 1: A historic train of the Jungfrau Railway in the Swiss Alps

Photo courtesy of Julian Ryf.

Page 2: the service magaine of the PRÜFTECHNIK roup the service magaine of the PRÜFTECHNIK roup Gear mesh frequency occurrence is typical for gear drives, whether straight, helical, or double

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the service magazine of the PRÜFTECHNIK Group

cies analysis and excitation frequencies calculation. The acoustically dominant frequency 1197 Hz was quickly identi-fied. Unfortunately, the number of teeth was not documented anywhere, and the gearbox manufacturer no longer existed.

The Jungfrau Railway technicians quickly presented a solution. A spare standby gearbox was opened at Eiger-

Figure 2: Rack and pinion gear with mounted accelerometer Figure 3: Open replacement gearbox

gletscher railway station office. Subse-quently associated teeth were counted (Figure 3). The respective meshing fre-quencies were calculated, and the ab-normal gear stage identified. That same evening a targeted inspection of the abnormal gear stage took place in the depot, via a small inspection hole cover. A non-uniform wear was identified. Since

the bearing vibrations were without ab-normalities, a manufacturing defect in the initial production phase was the conclusion.

A gear manufacturer then got the order to rebuild the gear in the affected gear stage. Regrinding of the affected gear stage, was all that was required.

Dictionary of terms

Did you know?The two main factors that determine

acoustic gearing excitation in gear mesh-ing frequency, are the contact impact between pinion and wheel on gearing start and the change in overall gearing stiffness when traveling through the sup-porting tooth pairs. The pitch circle mo-mentum has an additional effect, based on the change in direction of the sliding movement in the pitch point and the fric-tion momentum in non-hydrodynamic supports, which is dependent on the surface quality, the lubrication conditions and the sliding velocity level in the gear mesh.

Tooth root bending stress measure-ments using strain gages are the most direct measurement method to analyze gear meshing and acting toothing forces.

PreviewIssue 17 of telediagnose.com deals with the topic of rolling bearings:

– Experiences of bearing diagnosis in the ultrasonic range

– Detection of current passage in a paper machine

– Vibration control analyses to verify the non-locating bearing function

– Changing the design-related intrinsic vibrations in a pump bearing

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the service magazine of the PRÜFTECHNIK Group

Gear mesh frequency occurrence is typical for gear drives, whether straight, helical, or double helical gear stages. The meshing frequency can be calculated by multiplying the number of teeth on the pinion and/or on the gear with the respective rotation frequency. Figure 1 shows a cement mill girth gear drive with three gearbox meshing frequencies and a mill stage gear mesh frequency.

In the absence of tooth numbers, with some experience simple gearbox meshing frequencies can be reverse engineered from frequency spectra or order spectra. Or you can use open gearbox inspections to count the teeth.

For multi-stage gearboxes, the gearbox manufacturer should be contacted, and based on the type designation and serial number the number of teeth can be sup-plied. Some gearbox manufacturers have documented the numbers of teeth on the nameplate, in the operating instructions, or in the drawing.

Condition Monitoring Fundamentals

Meshing frequencies calculationThe amplitudes of the gear mesh fre-

quencies and the respective rotational frequencies are the important variables related to gear vibration diagnosis.

Figure 2 illustrates the importance of gear meshing forces due to load. Gear tooth root bending stress can be eas-ily identified with the light-optical stress analysis, Hertzian stress. And is an im-portant dimensioning factor for running gears. Using such photoelastic analysis, the viability of involued tooth profile can be studied at early stage for particular application. These figures demonstrate the complexity of gearing vibrations. In the diagnosis it is necessary to dis-tinguish between machine vibrations in mm/s and high-frequency structure-borne vibrations in m/s2. In normal run-ning industrial gearboxes, the vibration velocity spectra should be evaluated with approximately 4 harmonics of the gear mesh frequency. For high speed rotating gears, accelerations are to be used. If the

Figure 1: Drive system with several meshing frequencies

Figure 2: Gear meshing effects and view of a photoelastic investigation on ‘compacted’ gear meshing.

Meshing frequency of a simple spur or bevel gear stage:

fz = T • fn

T = Number of teeth of the gear under considerationfn = Rotational frequency of the respective shaft

gear mesh wear over the tooth height is to be evaluated, ac-celeration envelope spectrum are particularly useful.

Meshing frequencies in planetary gearboxes re-quire special calculations. PRÜFTECHNIK has included the corresponding calculation formulas in the gearbox editor of the OMNITREND® Center Software.

Figure 3:Some typical gear types

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0.5 m/s2

0.3 m/s2

12 m/s2

7.5 m/s2

10 Hz - 1000 Hz

If a new gear is brought into operation, the running, vibration, and operating performances should be checked after commissioning. This also helps the gear-box manufacturer, since unrecognized foreign influences increase the risk of damage, particularly to the gear mesh-ing. As the the thickness of the oil film between the contacting tooth surfaces is only a few microns, so the question arises that which measurement points and which acceptance criteria are to be used for gears?

There is no general answer to this. For main gears in wind turbine drive trains, for example, ISO 10816-21 provides rec-ommendations regarding measurement locations to be used. Figure 1 illustrates the measurement locations on one such

Condition Monitoring Knowledge

Typical measurement locations and acceptance criteria for vibration behavior in gear drives

wind turbine gearbox for continuous condition monitoring.

Measurements must take place in the three measurement directions, radial vertical, radial horizontal, and axial, as a minimum. ISO 10816-21 also stipu-lates that both vibration acceleration and vibration velocity are to be used as measurement variables. Another special feature of this 2015 standard, is that at least 20% rated torque is required, to be able to make representative vibration evaluations for this gear.

If no such application-oriented stan-dards are available, ISO 10816-3 is used to evaluate gear vibration behaviors (Fig-ure 3). Here the evaluation variable is the vibration velocity in mm/s only.

PRÜFTECHNIK is familiar with appli-

Figure 3: Vibration parameters for general gears according to ISO 10816-3

Figure 2: Representation of evaluation of gear vibrations according to DIN ISO 10816-21 (vibration velocity v @ 10-1000 Hz; vibration acceleration a @ 0.1 to 10 Hz, a @ 10-2000 Hz.)

cation instances, in which insurance for industrial plants with gears is granted only when the vibration on the gearbox is in the green zone of ISO 10816-3. With regard to the frequency range it must be noted that since the publication of the latest revision, the upper frequency limit of 1000 Hz is no longer to be regarded as mandatory. If a dominant meshing fre-quency, for example, lies slightly above 1000 Hz, the upper frequency range needs to be adjusted.

An American vibration standard, as shown in figure 4 gives the overview for vibration behavior of special gear-boxes for machine tools in great detail, is worthy of mention. Here order related detailed guidelines regarding permissible vibration amplitudes are given.

Figure 4: Representation of evaluation of gear vibrations and machine tools

Figure 1: Typical measurement locations on drive trains with gearbox.

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Condition Monitoring Level 4 - Application

Investigate gear transmission core using bending stress measurements of gear tooth root

Drive technology manufacturers and operators want gearboxes with maxi-mum security and durability. For main drives in bucket wheel excavators, the gear mass is another important criterion. Finally, the gear mass of the bucket wheel boom in relation to the stability of the excavator also considerably influences the service weight of the entire excavator. Manufacturers are thus in the position of having to handle high power levels in confined spaces, which often requires multi-motor drives with power merging

and/or internal power splits within the gearbox.

That such extremely lightweight construction has the disadvantage of elastic deformations of shafts, bearings, gears, and the housing. This has a greater effect, than in other applications and in particular on the gear mesh, must be taken into consideration.

Figure 1 shows the schematic arrangement of one such bucket wheel gear. It consists of two drive trains with bevel gear (1) + (2), the three shaft operating planetary stage (3), the straight-toothed spur gear with turning stage and a common front output gear (5). The load distribution within the drive trains and the resulting four-fold gear mesh in the driven wheel with pinion gears 1 to 4, results from the two three shaft operation planetary stages. This gear design endures acting stresses only when both the power distribution

over all four pinions, as well as the load distribution in the gear output stage, conform to design specifications.

But how can we measure that? Strain gages provide an option. If you use strain gages, sym-metrically to the middle of the tooth and the output gear tooth base. When the four pinions en-gage at each measuring point the deformation gain-proportional measurement signals can be ob-tained. Since tooth deformation is proportional to tooth force. By comparing the magnitude of the voltage amplitude information to the quality of the load distribu-tion, load levels and with similar loads, the power distribution can also be derived. Ideal engage-ment conditions are when all gear meshes, both on the left and the right side of the tooth, incur a 50% stress amplitude. This

would then correspond to a width factor of 1.0 KFß The following percentage volt-age amplitudes and width factors derived from them were found in an example bucket wheel at nominal load:

Toothing forces KFß

Pinion 1: 45% to 55% 1.12Pinion 2: 33% to 67% 1.40Pinion 3: 48% to 52% 1.05Pinion 4: 40% to 60% 1.23

Comparing these metrologically deter-mined width factors with those at a con-struction set width factor of 1.2. We were able to conclude that pinions no. 2 and no. 4 require action. Improvement has been achieved through targeted grind-ing of the pinion teeth. Subsequent gearboxes were equipped with eccentric bushings to execute axis position changes without requiring costly dismantling or regrinding.

Figure 1: Schematic design of a bucket wheel gearbox

View from left View from right

Zeroposition

SM shaft midpointEM eccentric midpoint

Direction of rotation output shaft

Wear pattern on output

Eccentric for wear pattern correction

left side right side

leftleftright right

leftrightleftright

(+) SMâ (–) SMá(–) SMá(+) SMâ

(+) SMá(–) SMâ(–) SMâ(+) SMá

SM SM

EM EM

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Changed vibrations and an increase in noise were noticeable on a planetary gear in a hydroelectric power plant. Does this pose an operating risk? What is the cause? How long can it be driven?

Questions that arose in a hydraulic power plant and were forwarded to PRÜFTECHNIK. During on site opera-tion, first the noise level was measured using VIBXPERT® and a class 1 micro-phone (Figure 1). Figure 2 shows an A-weighted sound pressure level in the third octave spectrum. The amplitudes were still tolerable in level. Noticeable that the third-octave spectra was a to-nality close to the calculated gear mesh frequency of the planetary stage. Next, the noise measurements were repeated in the tunnel directly on the bulb turbine and the amplitudes of the running plants were compared with one another. Plant 1 was the loudest. In place of the A-weighted third-octave spectra, next the narrowband noise spectra were assessed (Fig. 4). The assumption that the gear mesh frequency of the planetary stage was the source of noise, was confirmed.

This was followed by housing vibration measurement at characteristic measure-ment locations on the gearbox. Finally, in

Condition Monitoring Service

Level 3 analysis of a planetary gear

planetary gears the same tooth engage-ment frequency acts between the sun and planetary gears, and between planetary gears and the hollow gear.

The vibration amplitudes themselves were in the “green area” of ISO 10816-3. There was therefore no immediate opera-tional risk.

Next followed motion analysis directly on the planetary carrier. Figure 5 shows the contactless measuring displacement sensors mounted on the gearbox inspec-tion cover. Measurement was carried out in radial and axial directions. Although the evaluation showed some axial and ra-dial displacements, these do not however indicate functional disturbances within the sleeve bearing planetary stage with the three planetary gears.

It was suspected that something had changed on the involute, and thus the geometry of the gear teeth.

The operators were advised to carry out unscheduled oil analysis and vibra-tion measurements at shorter intervals.

Six months later, an request arrived to repeat the sound and vibration measure-ments. The sounds were already audible from outside the building and in the tun-nel, the sound had doubled. “Something has changed in the planetary stage gear mesh – the spare set should now be in-stalled” – was our recommendation. After dismantling the engagement lines were clearly visible on almost all tooth flanks. This could only functionally be explained by the passage of current through the tooth flanks.

Making the source of current passage in the hydroelectric power plant locat-able, was anything but simple. When this was finally achieved, it could be elimi-nated. We are not permitted to publish the exact cause here.

Figure 3: Measurements using VIBXPERT® in the tunnel

Figure 1: Microphone mounted with magnetic base

Figure 4: Narrow band frequency spectrum of the noise in the nacelle

Figure 2: Third octave spectrum of the A-weighted sound pressure level

Figure 5: Measuring arrangement for the movement analysis on the planetary carrier and movements re-corded in axial and radial direction of measurement.

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High speed rotating gear drives are widely used in the power plant industry. They regulate the speed of each gen-erator to the speed of, for example, the gas turbine, and transmit the required torque. In the power range above 40 MW single-stage double-helical gear units are usually used. These gearboxes with sleeve bearings can run for years with no problems. In Germany, for some time now the particular situation has arisen, that the operating conditions for such gearboxes have changed as a result of changes in energy policy. Too much wind or sunshine must be limited, or the whole plant will be shut down. For double helical highspeed gearboxes each starting operation means an increased risk of failure, until ideal contact pat-tern states – as shown in Figure 2 – are achieved. The gear mesh need to realign with one another at each start up, and the gears and housing should thermally expand as quickly and uniformly as pos-sible. This may cause vibration limits to be exceeded.

What can you do about it, asked an operator. The proposal from PRÜFTECH-NIK was to capture the behavior of the shaft vibrations, machine vibrations and structure-borne noise vibrations in these starting processes, continuously and with high resolution. With VIBGUARD® por-table, the measurement procedure was relatively simple. A total of 12 acceler-ometers were mounted on the gearbox housing which incorporated all available shaft vibration signals. The measurement locations are indicated in Figure 1 – a symmetrical arrangement was adhered to. On first start-up measurement was carried out continuously at a very high resolution to see how the gearbox be-haved with respect to shaft vibrations, housing vibrations and also with re-spect to the toothing vibrations, over the planned 1.5 day measurement duration.

Figure 3 shows an example of a wa-terfall spectrum of the vibration velocity in the frequency range 0 – 150 Hz. It was noticeable that there were differ-ences in the vibrational harmonics of the gear mesh frequency between two gear halves. This suggested that despite

Condition Monitoring Service

Vibration analysis of a double helical highspeed running gearbox

constant load an unequal contact pattern between the two tooth halves was occurring. It was there-fore decided to leave the condi-tion monitoring equipment for two more weeks. After about three days, similar vibration spectrum has emerged with multiples of gear mesh frequency. What happened?

The idea came from the system manufacturer. Unequal lubrication conditions within the gear mesh were suspected.

The measurements were repeat-ed a few months later, and demon-strated that after a few hours com-parable vibration patterns were introduced as a result of modifica-tion with respect to the meshing frequencies and their multiples.

Figure 1: Turbo gearbox with ‘classically’ mounted accelerometers: On all four positions shown mea-surements were carried out in the three directions, radial vertical, radial horizontal, and axial.

Figure 2: Correct wear pattern development in double helical gear stages

Figure 3: Exemplary waterfall diagram of ma-chine vibrations in mm/s.

Idling or low partial load

50% load

Full load

Left side BContact ratio 80–90%

Left side BContact ratio 80–90%

Left side BContact ratio 90-100%

Right side AContact ratio 20-40%

Right side AContact ratio 40-80%

Right side AContact ratio 90-100%

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Sometimes it is noticeable that gear-boxes have significantly higher mesh-ing vibrations locally. Then you should bring the housing as an amplifier of meshing vibration into consideration. In such cases, it is worth mapping the gearbox in terms of vibration, and to compare the measurement results with FEM calculation results. Whether the

Compared to the past, today the di-mensioning of gear transmissions takes place with lower reserves. If damage oc-curs, as shown in the pictures, then the first thing you want to know is how high were the respective loads. Torque mea-surement with strain gages and telemetry transmission of measured values from the rotating shaft may give an answer to the question of whether short-term overloads or increased torque vibration stresses have acted.

Contact us and make the most of the extensive PRÜFTECHNIK experience.

Condition Monitoring Service

Determine root causes of tooth and shaft breakages

Special Condition Monitoring

Structural analysis on gearbox housingmeasurement technician works with a mobile VIBXPERT®, a 20-channel por-table VIBGUARD® or a vibro-acoustic camera, is usually only a matter of cost. Development and calculation engineers are happy if they can get results and then synchronize their model. Figure 1 shows some calculation examples of a multi-stage helical-planetary gear. High

Figure 1: Results of computational structural analysis at various excitation frequencies

Figure 1: Break in an intermediate gearbox stage and typical damage pattern for a fatigue fracture

vibration areas are clearly visible in some housings. Therefore no sensors should be installed there as part of the regular condition monitoring.

Fundamentally, however, it is better to lay gear mesh frequencies so that hous-ing excitations are avoided.

Trade fairs, conferences, Seminars

Dates

ImprintPRÜFTECHNIK Condition Monitoring GmbH85737 Ismaning, GermanyTel: 089 99616-0Fax: 089 99616-341Email: [email protected]

www.pruftechnik.com

All trade fairs, seminars and other details from the PRÜFTECHNIK Group can be found on our website at www.pruftechnik.com

With 20 subsidiaries and 70 represen-tative offices PRÜFTECHNIK has a global presence. LI

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