Vibration Monitoring Introduction
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What is Vibration?
Vibration is the back and forth movement of an object. It is illustrated by a mass suspended by a spring. Apply a force to the mass and release it, the mass will oscillate in a sinusoid as shown below in Figure 24.
Its amplitude describes the movement, which is its distance away from the neutral position. Also by its period, which is the time to complete one cycle. The Frequency of a vibration is 1/Period & is measured in Hertz (cycles/sec) or in cpm (cycles/minute).
We can think of machines as a number of complex spring mass systems, excited by a number of applied forces. The main units of measurement for vibration are described below. Both Metric and American measurement systems are widely used.
Displacement – Distance the object has moved away from the neutral position [Metric – Microns (0.001mm), American - Mills (0.001”)].
Velocity – Speed the object moves relative to the neutral position [Metric – mm/sec, American – in/sec]
Acceleration – Rate of speed increase relative to the neutral position [Metric – g’s or m/sec2, American g’s]
Vibration Amplitude & Vibration Time Waveform
A simple sinusoidal vibration oscillation is described by its amplitude & period. Typically the actual vibration movement of a machine is much more complex as shown below.
To monitor we need simple Overall Vibration Parameters to describe the complex Time Waveform. The most widely used overall parameters are RMS & Peak. Peak is the maximum amplitude measured from the neutral point. RMS is a measure of the vibration energy. The most useful Overall Vibration Parameter is RMS Velocity Vibration and is widely used to assess the severity of an equipment defect.
Vibration Transducers convert vibration to an electrical voltage. Accelerometers are the most widely used vibration transducers. They have a mass supported by a Piezoelectric crystal. The force from movement of the mass generates a voltage output. This signal is typically amplified to reduce signal loss in cables.
Vibration Instruments - There are many simple instruments designed to measure RMS Velocity Vibrations vibration velocity in mm/sec. Many instruments also measure acceleration or ultrasonic parameters, which are higher frequency vibration parameters useful for bearing and lubrication monitoring. These instruments are ideal for equipment vibration severity checks.
Measurement is typically carried out by:-
- Attaching the accelerometers to the machine
- Using a permanent magnet
- Or by a hand held probe (aprox. 0.5 - 1kg force)
- Waiting for the measurement to stabilise
- Pressing a hold button
- Recording the parameter
For a wide range of equipment vibration monitoring can be performed successfully using simple vibration meters. There are also more advanced vibration instruments available that can be used to analyse complex vibration waveforms to help determine the cause of difficult defects.
The figure below shows how specific components produce vibrations at specific frequencies. These combine together to produce a complex vibration waveform. The more advanced vibration analysis equipment can split the complex waveform up to discover these different frequencies and link them to specific components or equipment defects.
Vibration Data Collection Systems
As well as manual paper systems there is a wide range of software available to store and display vibration information on computers. Vibration Data Collectors are used to connect to a computer and upload a Collection Route and after collection to download the vibration and other information back to the computer. Data collectors collect overall vibration parameters but most also collect more complex vibration information as well.
Data Sources for Data Collectors
- Manual input process data from gauges or panels using data collector keyboard
- Portable accelerometers (many different accelerometers are available)
- Fixed accelerometers (requires appropriate cable)
- From fixed proximity probes that measure displacement (requires appropriate cable)
- From portable temperature sensors (requires appropriate cable)
- Other sensor or panel outputs with voltage output (requires appropriate cable)
- The data collector can modify collected signals by filtering and other techniques to form different parameters
Information that can be Stored in a Data Collector
- Date and time of measurement
- Overall parameters (Magnitude) eg RMS Acceleration Vibration
- Time Waveform data (list of measurements recorded at a specified time interval)
- Spectrum of frequency domain data (FFT analysis)
- Inspection codes (if loaded)
Data Collection - Before going to site for data collection, get a copy of the previous CM report. A copy of the previous CM report assists with the following issues. Understanding of the current machines defects so specific 5 senses observation can be made relating to these problems. More attention can be paid to any parameter changes for these machines, which may indicate a deterioration or improvement in machine condition. You can be prepared to facilitate simple maintenance actions for these machines, such as greasing or checking and tightening of loose bolts. You can be prepared to carry out more detailed checks with simple temperature meters or with an assisted listening instrument. You can follow-up with other personnel with their progress on actions recommended in the report.
Batteries - Ensure data collector batteries are adequately charged &/or spare batteries are available before going to site. Proper battery care is the most important part of maintaining a data collector. You may need spare batteries if you use a data collector heavily (6 hours operation). Changing battery packs on-site should not be problem.
Self-discharge occurs naturally with rechargeable batteries and depends upon cell temperature. If you do not use batteries often (once a month for example), or store batteries as spares/stock, a battery cycling program is necessary. You should cycle (discharge/charge) spares, which could lie unused for weeks, once a month. Ensure batteries are labelled or numbered so you can identify a specific battery that is starting to show poor performance.
Final check of data collector before going to site
- Check that the relevant machines are likely to be operating (phone if remote from your location)
- Review any special entry or safety requirements such as specific PPE, entry authorisation, sign-on systems, permits or JSA’s (Job Safety Analysis)
- Turn data collector on and ensure the accelerometer is connected
- Check the battery indicator is showing fully charged
- Ensure the correct route is loaded
- Select the first point in the route and check that the machine details are as expected
- Press store and confirm that movement of the attached accelerometer gives a logical vibration indication on the data collector screen and cable or connector movement does not effect measurements
- Press cancel
- Turn off the data collector
Collection Routes - A Collection Route is the specifications for a group of specific measurement and inspection tasks. This is usually to collect information from a group of machines at a set frequency, typically trigger by a planning & scheduling system. A Route Contains:
- Machine names
- Measurement point order
- Measurement locations
- Measurement specifications
A data collector measurement point screen display is shown in Figure 30 above. A manual collection route is usually documented on one or more sheets of paper or is made up out of a number of trend sheets.
Collecting the Right Data - The most significant data collecting error you can make, is to collect vibration from the wrong machine or the wrong location on a machine. This may not be an issue if you are experienced in an area but at some time a less experienced personnel will likely have to collect the route. The first requirement is to be able to use text on a data collector screen or manual route document to locate the machine to be monitored. Sometimes machines are not well labelled & different local groups may call the same machine different names. Where there are multiple identical machines they are usually numbered in the same direction across the whole plant. The best way to locate machines is by using diagrammatic maps. These can be just quick hand drawn diagrams or they may be marked up plant layout diagrams.
Finding machine measurement point locations - There are International Standards (ISO) to help identify measurement ‘Point’, ‘Position’ and ‘Direction’.
Measurement ‘Point’ is usually an abbreviated Location description of a machine element and a specific bearing. Machine element abbreviation examples are MTR for motor, PMP for pump and GBX for gearbox. Further abbreviations for bearings are OB for Outboard, IB for Inboard, DE for coupling end or NDE for Non-Drive End meaning opposite end to the coupling. A typical ‘Point’ description is MTR-OB.
Measurement Position and Direction
International Standards specify how equipment bearings measurement positions should be numbered and identified. Each bearing ‘Position’ is given by a number starting at ‘1’ at the outboard end of the machine providing the rotational force (usually a motor). Each bearing is given a number even if it is not to be measured. Each position is also given a measurement direction H, V, A or R (Horizontal, Vertical, Axial or Radial) to indicate the direction the transducer should face. The diagrams on this and the following page illustrate the system. If Non-bearing measurement points, such as a pump casing, are to be measured they are given a text position name (eg. Case) or the number after the final bearing number. If there is any confusion on the position number in a route you can cross-reference to the Point description.
Setting Up Transducer Attachment Locations - There are recognized transducer attachment locations for standard housing arrangements such as those below. Others have to be learnt. Access restrictions and specific machine loading directions may affect the placement location for the accelerometer.
Why specific measurement parameters, point locations and measurement directions are chosen?
The bearings within a machine transfer the loads from the rotating components to the machine housings and so measuring vibration at these locations tells us most about rotational defects. Also bearings are a key source of machine failures, so again are a focus for measurement location.
Very critical machines sometimes have vibration measured in three directions at each bearing (V,H&A). Normal machines tend to have one H or V measurement per bearing and one Axial measurement per shaft (3 measurements per shaft)
Horizontal is usually the direction chosen for a single measurement, as the machine is usually less stiff in this direction and gives a higher velocity vibration. If the machine is less stiff vertically this should be selected. Access requirements will also determine between use of H & V and where neither H nor V can be used the direction is specified as R for radial. The axial measurement is usually taken at the coupling end of the shaft but if the axial locating bearing on the machine element is at OB end then this may be used.
Another factor that may affect the spot chosen for measurement is its transmission sensitivity to high frequency vibrations. Lower frequency vibrations indicated by velocity vibration are not sensitive the exact location of the transducer position (with direction the same). 20mm one way or the other from the chosen spot will not have a big effect on a large casing. This may not be the case with acceleration or ultrasonic measurements, as high frequency vibrations reduce (are attenuated) over distance from the source, and especially so through housing interfaces. If vibration can’t be measured directly on the outside of the bearing housing then the transducer location is chosen for the best, most direct transmission path. An example of this is the OB bearing on many motors, where the fan limits direct access to the bearing housing. The direction of the force between the rotating shaft and the bearing will have an influence on where on the bearing housing will give best high frequency measurements. This will have an influence on the chosen transducer location.
Vibration parameters that are typically measured by modern vibration data collectors are listed below.
- Overall RMS Velocity Vibration in mm/sec to trend and assess vibration severity
- A high frequency parameter such as acceleration, demodulated acceleration or contact ultrasonic measurement. These parameters give warning of bearing, friction or other defects
- Process parameters are collected in many situations such as below:
- Pump discharge pressure and/or flow
- Fan damper position or flow
- Motor currents and temperatures
Consistency of vibration trending over time
One of the most important issues for consistency of vibration measurements over time is ensuring the same accelerometer and magnet type is attached to the identical location each time. A system of marking the measurement point locations is required to achieve this. One widely used technique is the white spot system. This requires use of white marking paint bottle with brush or a white paint marker pen. The marking is typically done during a route collection by, after measurement, cleaning the point and applying the paint. The paint has to be touched up on occasions and remarked if machine elements are changed. Inform equipment operators before any machine marking occurs so they are aware of its purpose. Another method is the use of steel disks glued to the measurement location. This is necessary if a magnetically attached transducer is to be used on a non-magnetic surface such as aluminium. The disks are usually made of a magnetic grade stainless steel and can be used with stronger contact flat-based magnets to improve higher frequency vibration measurement. Another alternative is using standard bolt washers. 5-minute epoxy glue can be used and the disk can be held in place with a strong magnet while drying. Surface preparation and cleaning the surfaces to be glued with a good solvent is necessary to ensure the disc will not come off.
Recommended items to carry while data collecting
- Belt mounted pouch to store items
- Note book and 2 pens
- Small torch (focusing type best)
- Small scraper and Cleaning rags
- White marking paint
- Black felt tip marking pen
Optional items to take to site or have available
- Wire brush, Shifting spanner, Screwdriver, 150mm ruler, Feeler gauge
- Temperature meter, assisted listening instrument, strobe light, taco
- Digital camera.
- Oil Sample bottles & sampling tools.
- Multi tool pocketknife (eg. Leatherman)
- Green Electricians tape (for marking operating point on gauges)
- Information tags for identifying the location of defects
- Extra PPE such as Gloves and Goggles
Optional items to take to site with data collectors
- Special connection cables for external transducers and a can of electronics cleaner. Special transducers if required.
Route Data Collection - If the location you are collecting data from has an operator control room or a key operator, contact to tell them you are on-site and ensure you ask about problems with the equipment monitored. Operators are often the best source of equipment defect information, so building these relationships can be very important. When placing the accelerometer onto the measurement point try not to create a high impact. An impact will excite the measurement electronics of the instrument and then it will take longer for the reading to stabilise. This is a problem with magnet-attached transducers as the magnet can create an impact force on placement.
Vibration Transducers, Magnets and Attachment
Relationship between vibration transducer attachment and vibration frequency range is displayed below. The more solid the connection the more sensitive the transducer is to high frequency vibrations. Two magnet types are displayed in Fig 41.
Two pole magnets are best for non-flat surfaces and flat magnets give best higher frequency vibration measurement.
Magnet attachment - Smaller diameter Rare Earth Magnets are better for flat surfaces and small machines. Magnet & mounting location need to be clean to maintain higher frequency vibration transmission. Steel or magnetic stainless steel disks can be glued to a measurement point to improve attachment with a flat magnet or where the attachment position material is non-magnetic (eg. Aluminium housing).
The total height of a magnet, transducer and cable may limit options for positioning where access is tight. A 90Deg cable attachment can improve this (Figure 42). Removing accelerometer by pulling cable should be avoided if possible as this could damage the cable.
Cables for vibration instruments
There is a wide range of cables used for data collectors, transducer signal & measurement (See right). Always ensure the cable connectors are clean and the connectors and cable is undamaged.
Connecting to permanently mounted transducers – Where access is not possible to measure a specific vibration point there is an option to install permanently mounted accelerometers. Many vibration instruments can be setup to connect to a permanently mounted accelerometer. An example is given to the right of a junction box for 4 external accelerometers with BNC output connections. To connect your vibration instrument to this will usually require a special cable.
Other External Inputs (Data Collectors) – Vibration data collectors can usually be setup to measure a wide range of voltage input signals. This could be from a special transducer such as speed, pressure or flow sensor or from a process parameter. Collecting signals from external inputs requires having the correct cables. For complex panel outputs, a diagram/ photo marking outputs may be required.
The information on the data collector setup required to measure the specific signal is stored in the route information and the data collector automatically sets itself up for the measurement.
The quality of the cable connection is very important. In dirty environments, a can of electronic cleaning spray should be used to ensure the connections are kept clean
Vibration Collection Error Sources and Their Effects
The most important concern in data collection is the accuracy of the data collected. If information collected is incorrect, at best, the route will have to be collected again and at worst, incorrect judgements may be made with large financial consequences. This section explains way errors can occur and how they can be avoided.
Measuring the wrong equipment - This can be an easy error to make with equipment that is similar and confusing naming systems but must be avoided. Ensure you have a diagrammatic layout of each area identifying machines. It is also useful to have more detailed text descriptions of equipment location and naming details. If while measuring on a machine, a number of measurement points show change, immediately re-evaluate if it is the correct machine.
Measuring the wrong machine point or direction - Understand the machine numbering and direction conventions. Look for the transducer location spot markers and repaint these if becoming obscured. Marking systems are not always perfect, so you must know how to find the standard transducer locations on the standard machine housing arrangements. If a measurement is changed from the previous reading, immediately re-evaluate if you have selected to correct transducer location. If you are not absolutely certain that the correct location has been selected, make a note indicating the uncertainty on the trend sheet.
Poor cable connections & cable movement - The electrical signals produced by transducers have quite small voltages and currents. Small changes in the electrical resistance of connectors & connecting cables can substantially affect measured values. Connectors should be regularly cleaned with electronic cleaning spray and connectors and cables checked for damage. Some connectors have sealing ‘O’ rings that should be inspected. It is important to check cables and connectors before going onto site.
Connect the standard transducer and measure overall Velocity Vibration data while moving the cable and wiggling connector. Ensure that the cables and connectors movements do not significantly affect the measured values.
Regularly check for cable effects while collecting data. Some causes of these effects are:
- Damaged cable
- Damaged connectors
- Moisture or dirt entry
- Ineffective shielding (Figure 45)
Poor accelerometer to magnet attachment - Both the accelerometer and magnet connection surfaces should be correctly machined and free of damage. The faces should be cleaned, a connection fluid added and tightly coupled by hand. Coupling tension should be regularly checked.
Figure 46 – Magnet to Accelerometer Attachment
Poor magnet to machine attachment - A strong connection is required between the machine and the magnet with a good vibration transmission path between the vibration energy source and the accelerometer. Both the machine point and the magnet have to be clean. A flat magnet base requires a flat surface. A slight sliding action on attachment is recommended to ensure it is corrected seated.
A 2 pole magnet can be used on more rounded surfaces. A slight twisting or sliding attachment is recommended to ensure each pole is well attached
A final check should be made by gently holding the top of the accelerometer & trying to wobble it in a circular pattern to ensure attachment strength is good in all directions.
Inadequate measurement stabilization - When an accelerometer is attached, it requires a short time for the reading to stabilise. Longer if attached with an impact (place edge of the magnet down first). Look at the instrument reading and ensure it is stable before recording the value.
Most data collectors display a trend line or a bar to observe the current measured value. Do not press store until the measured value is stable.
Fluctuating vibrations - Vibrations in some situations can vary in a slow cyclic or other pattern. Record vibrations when vibrations are at the greatest magnitude.
Change in background vibrations - Large vibrations from nearby equipment (eg. Crusher or materials chute) can often affect vibration levels on the machine being measured. If these background vibrations vary, collect vibrations when the background interference is lowest. Note the presence, level and source of the background vibrations to assist in future trend analysis.
Vibration signal high amplitude clipping - High frequency, high amplitude vibration signals can cause the accelerometer amplifier to saturate and become overloaded. An accelerometer has an acceleration value over which it will not measure correctly. Some causes of such high amplitude vibration that causing accelerometer overload are severe pump cavitation, fluid release, impacts from loose or reciprocating parts and even gear mesh noise. If you record a very high value measure readings away from this point to check that the original measure was sensible.
This problem can be checked if you have someone who can do a vibration spectrum analysis or time waveform of the vibration. The overload problem will produce truncated waveforms and large ‘ski-slope’ spectrums (see below). The example shows how sensor overloading destroyed the frequency spectrum on a gearbox. If high acceleration amplitudes or impacts are expected then the spectrum should be check to ensure it has no significant ‘Ski Slope’ (see lower spectrum)
Change in transducer or magnet type being used - Different magnet designs and transducer designs have different frequency responses. This can have a marked effect on high frequency parameters such as acceleration and ultrasonic noise. Where a change in accelerometer to one with a higher frequency response is made, it will often give a significant step change in amplitude of high frequency parameters.
Change from a flat magnet to a hand held probe can have the same effect (Figue 48). The same is possible with changes to a lower frequency response accelerometer with Velocity Vibration amplitudes but is not a likely.
Transducer temperature - Temperature has an effect on the electrical characteristics of an accelerometer (see diagram). If equipment measured is very hot, try to minimise the sensor contact time. A thermally insulating pad can be placed between the accelerometer and the magnet to minimise the effect. A 5% increase in acceleration amplitude at 120oC would be typical Touching a transducer after contact with a hot surface can cause burns. Care is required and gloves are required if the high temperature is normal.
Effects of cable length - The portable accelerometers typically used with data collectors have internal amplifiers and so cable length should not be a major factor. Where very long cable length (>50meters) are used for permanent accelerometers there may be an influence on outputs. The same is the case at much shorter lengths when non-internally amplified accelerometers are used.
Damaged transducer - A transducer that is damaged internally can produce substantial errors. If a transducer consistently measures Velocity Vibrations that are significantly different than that felt by the hand, then compare the readings measured by another vibration instrument.
Electrical interference - The signal from a vibration transducer to the vibration instrument is very small and can be effected by magnetic and electric fields. The electrical shielding on cables is supposed to eliminate this effect. If you notice unusual measurement variations around electric motors or similar electrical equipment, the transducer cable shielding may be damaged and may require replacement.
Accelerometer contact force and direction - The contact force between an accelerometer and the machine can have a significant effect on measurements, especially high frequency vibrations. If very high frequency vibrations are important for monitoring, the machine may be drilled and tapped for stud mounting of an accelerometer. With very high vibrations the momentum of a transducer may overcome magnetic force and cause transducer movement & measurement error. If this is suspected, keep hand on magnet. If a handheld accelerometer probe is used then it is important to hold the probe steadily onto the equipment with a reasonable force (0.5 to 1 kg force). Typically the probe is held in the direction indicated (H, V, R or A) and usually perpendicular (90o) to the surface. If that is not possible then the details should be recorded and the same angle should be maintained for each future measurement.
Identifying, Correcting & Rejecting Bad Data
Measurement Fluctuations
Regular sinusoidal variation in vibration amplitude - If there are two vibrations in a machine that are almost the same frequency, but not quite, a vibration ‘Beat’ will occur. These vibrations go in and out of phase, which varies the amplitude. This is not usually dangerous but should be noted. If the variation is slow, measure vibration when the amplitude is highest. Sometimes variable speed machines can also produce vibration variations, if the speed control system is slightly unstable. Note this if suspected.
Irregular variation in vibration amplitude - Check magnet or transducer connection to the machine. If due to high vibration or surface shape, reposition magnet or stabilise with force applied by your hand. Check cable and cable connections by manipulating and seeing the effect on vibration output. Clean connections and if still a problem, minimise cable movement while collecting data, note issue and organise to get cable replaced.
Check for presence, level and possible source of the background vibrations that could be affecting vibration amplitudes. Large vibrations from nearby equipment can be a cause eg. Crusher or materials chute. Collect vibrations when the background interference is lowest and note the issue.
If acceleration levels are very high and there is sharp amplitude jumps, there may be transducer overload. Try placing rubber material under the transducer.
Measurement Changed from Previous Value –
- Check if wrong machine, wrong point or wrong direction has been measured.
- Check transducer and magnet attachment and double check measurement
- Comparing measured values to 5 senses (hand) measurements
- If measured value is low and variation is less the 30%, treat it as normal.
Measurement lower than expected - It is unusual for equipment vibrations to reduce, without a repair or other changes to the causes of the vibration eg greasing, realignment, machine balance etc. Check for evidence of maintenance. Other issues that can effect vibrations are load changes, speed changes, process variations or nearby machines not running. These are also relevant for the item below.
Measurement higher than expected - Check for possible causes of higher vibration by 5 senses inspections. More detailed 5 senses inspections to be performed if the increase is more than 100%, the original value was high, a trend (or other pattern) is recognised or a defect was already identified.
Completion of a Route Report
Function of the route report – Is to:
- To communicate urgent issues and the required follow-up actions to interested persons.
- To communicate issues not directly relevant to machines monitored to interested persons.
- To identified extent of monitoring completed to interested persons.
- To use as a prompt of items the review carefully for the next monitoring of this route.
Use of prepared sheet formats for route report - An A4 Generic route report sheet is shown on the following page and is in typical machine/comment format. A photocopier is used if multiple copies are required. A generic example is shown in Figure 50.
Inspection Report REPORT NAME: DATE
DISTRIBUTION:
No. Equip. Name Defect Description Comment
Submitted by: Telephone:
Mobile Fax:
On-Site Vibration Diagnostic Approaches
In this section some additional vibration diagnostic method are documented.
Below is a diagnostic method for an increase in RMS Velocity vibration.
- Check the pattern of increase across the machine and find the location with greatest increase.
- Axial increase may indication misalignment or other axial forces
- An increase each side of a coupling may indication misalignment or a coupling defect (same for V belts)
- Radial locations next to an impellor may indicate balance
- Check if there is any evidence that machine elements have been changed or adjustments made.
- May be alignment or mounting issues.
- Carry out standard external looseness checks
- Check around problem area for higher RMS velocity vibrations
- Ensure there is not an external source for the vibration
- Use chosen listening technique for bearings with an increase, comparing to other bearings and other similar machines
- Measure the bearing and motor temperatures and compare to historic levels if baseline temperatures are available
- Has this or similar machines shown this symptom pattern before?
- Set severity for the defect (eg. Ok, Moderate, Warning & Severe)
- Determine if the next stage in diagnostics is required and what the next step will be.
- Quick off-line alignment check
- Quick off-line internal looseness, runout & bent shaft check (check with dial indicator)
- Motor (or other) soft foot check
- Used grease sample sparkle check
- Motor rundown check
- Fan impellor clean
- Fan impellor balance (usually contractor)
- Get in a vibration specialist to carry out an analysis
- If possible, determine the cause of the defect and what actions could eliminate the cause in the future
- Can the deterioration rate be reduced by some simple actions, eg. increased lubrication or force reduction (eg reduce V belt tension)
- Assess if a repair is likely to be required within the next 6 to 12 months from the detected defect. If yes do an initial best guess of time to failure (prognosis) and initiate any early planning if required (spares & procedures available?)
- If defect is serious, inform others that may require the information.
Rolling Element Bearing temperature has significantly increased
Over-greasing – If a bearings internal diameter (mm) multiplied by the rpm is less than 20,000 (d x rpm < 20,000) then the bearing can be run with the bearing housing fully packed with grease. For machines that run 1500rpm or higher, this is not the case and especially so for larger diameter bearings (d>100mm). For these bearings, if the quantity of grease is larger than 30% to 50% of the housing volume, the grease can begin to churn (get drawn around by rotation). Larger motors have grease relief ports that the greaser should use to extract excess grease. Some Plummer Blocks have plugs in the side of the housing that can be removed to allow excess grease to escape. Recommended investigation approach listed below.
- If over-greasing is the problem, all vibrations will tend to be normal.
- Check for unusual noises using listening tool
- Check for signs of recent greasing. Ask if bearing has been greased recently.
- Check if there is a grease relief mechanism, if it is working and if it has been used.
- Ask operators to check if the temperature is always high. If it is sometimes significantly cooler in the morning (cooling through the night can stop the churning) it suggests excess grease.
- If excess grease, remove grease from the bearing housing. If this is not possible ensure no new grease is installed and monitor vibration and temperature normally.
Rolling Element Bearing temperature has significantly increased
Increased loading or inadequate lubrication – Large increases in force or friction on a bearing will increase the operating temperature. Acceleration vibration and ultrasonic noise will increase. Velocity vibrations may increase if caused by a loading.
- Check for unusual noises using listening tool.
- Check for loss of lubricant from the bearing (especially if oil lubricated).
- If it is a drive side bearing, also check for symptoms on the drive side bearing on the connected machine. Is the bearing the fixed or floating bearing for the shaft?
- Is there any evidence of a recent alignment?
- If bearing is drive side to a V belt, check for wear on both pulleys (belt OD will be lower than the pulley OD) if visually accessible and that all belts are in place. Check for signs of recent adjustment or anything that would suggest over-tensioning.
- Is there any evidence of other maintenance work being carried out on the equipment?
Never recommend the application of cooling to the external bearing housing for a hot bearing. This can actually add load to a bearing. Adding lubricant is the most conservative advise.
Rolling Element Bearing temperature has significantly increased
Bearing failure – Increase in bearing temperature is the last stage of a bearing failure and is a warning of immanent failure. The symptoms are listed below.
- Acceleration and/or Ultrasonic noise Levels will be Very High
- For higher rpm equipment temperature may be very high
- Velocity vibrations will also be increased
- There should be unusual noises using a listening tool
- There should have been a developing defect detected previously
- If there was no previous defect, check for signs of maintenance activity, overloading and/or for ways that all lubricant could have been lost from the bearing
Rolling Element Bearing temperature has significantly increased
Temperature from another source – Symptoms are listed below.
- All vibrations will tend to be normal.
- Check the temperature of the connecting housings to see if they are higher or similar to the bearing temperature.
- Check the temperature of oil supplies (where there is an oil recirculation system).
- For DS bearings, check the temperature of the rotating shaft (if a non-contact sensor is available)
Motor temperature has significantly increased
Cooling failure – Most motors have a fan on the non-drive end. Some have external fans and some are cooled by water heat exchangers.
- Check the fan is working effectively.
- Check for blocked cooling fins.
Load increase – Motors run hotter if fully loaded.
Electric defects – Electrical defects can reduce the operating efficiency of a motor. This will increase the temperature and sometimes causes localised temperature increases on the motor casing.
Motor investigations should include:
- Check evenness of temperature around the motor body
- Is the motor making any strange noises
- Check air flow by hand if flow is external
- Check operational reasons for an increase in motor load (eg conveyor loading)
- Record motor current if there is a gauge on the MCC (Motor Control Center)
- Record nameplate full load current and full load speed.
Gearbox temperature has significantly increased• Lubrication defects will generate friction and temperature
- Increased operating loading will increase temperatures
- Cooling defects will increase temperature
- Gearbox investigations should include:
- Check evenness of temperature around the gearbox casing
- Is the gearbox making any strange noises? Use assisted listening tool.
- Check oil level in the gearbox (too low or too high). Visually check oil for colour, contamination, smell and viscosity. If suspicious take an oil sample if practical.
- If there is a cooling fan check airflow by hand.
- If there is a recirculating oil system, check oil supply, return and oil cooler water temperatures.
- Check operational reasons for an increase in gearbox load (eg conveyor loading)
- Record motor current if there is a gauge on the MCC (Motor Control Centre)
- Record any motor & gearbox nameplate power ratings.
Velocity Vibration Parameter Increase
External Looseness checks - Bolts cracked, missing or loose
- Locate the bearing position of highest vibration increase using vibration history for the machine.
- If history is not available locate the point along the machine with the highest velocity vibration using vibration meter or by hand.
- At this bearing, measure how the vibration reduces down its support structure. A large step reduction will indicate the likely area of looseness.
- Place your finger along the casing bolted joints. Your finger is very sensitive in detecting small differential movements.
- Do a visual check for component movement. Fluid at a loose interface will vary its meniscus and is easy to observe.
- Check all bolts for looseness at plummer block splits, machine casings, support structures, foundations and all bolted interfaces supporting load from the bearing.
- Check machine element connections, interfaces and areas of possible cracking using the finger differential movement method, especially in areas where there is significant vibration change over a short distance.
- Check for damaged, cracked or corroded support plinths.
Velocity Vibration Parameter Increase
Alignment
- Is the velocity increase in the axial direction or on bearings each side of the coupling?
- Look in detail for evidence that a machine item has been moved or replaced.
- Look for corrosion, moisture around machine feet or incorrect shimming practice.
- Look for indications of coupling damage (eg. rubber under coupling) or bearing distress (Ultrasonic noise, etc.) on bearing either side of the coupling.
- Look for bearing temperature increases either side of the coupling
Resonance - For machines connected to pipework or mounted on steel structures with long columns or beams.
- Check by hand if the vibration on adjacent pipework or supports is greater than the machine vibration.
Velocity Vibrations Very High – Vibration increased & above ISO ‘Unusable’ level.
- Confirm the problem is not an external looseness or from an external source.
- Initiate planning for the machines repair.
- Monitor more regularly until the repair can be carried out if an in-service failure is a significant problem.
Noise from V Belt - Belt slipping noise heard by you or on start-up heard by operators.
- Check for damaged, broken or missing belts
- If possible visually check for worn pulleys
- Check for signs of moisture or lubricant getting onto belts or pulleys.
- Check for indications that the belts have been adjusted recently
Noise or temperature from a pump casing – This is usually caused by cavitation, pump fluid supply flow restrictions or blocked discharge.
- Listen to pump casing. Cavitation sounds like someone is shaking a can of nails.
- Record discharge pressure and supply pressure if available
- What is the temperature of supply pipework?
- Check the supply pipework for possible blockages or shut or restricted valves
- Check if there are more than the one pump supplying the discharge lines and if you can observe any differences between the pumps.
- Is a blockage around the pump supply suction strainer likely (rag or plastic bag)
Documenting five senses and other observations
It is very important to record information from your inspection and monitoring so you can initiate the required follow-up actions. This is usually done in a pocket notebook.
You should carry a small notebook or equivalent to record information. If you are using trend sheets to record information, history and observation should be recorded on the sheet if it is likely to be useful in the future. The following information should be recorded.
- Record unusual things for later reference
- Record information that may assist diagnostics
- Record issues that may be of interest to the people responsible for the plant
- Record items of significant condition deterioration
- Record information from site investigations
- Highlight items in the notebook required for later entry into the monitoring report, if required.
