Vibration Condition Monitoring

Brian McLauchlan SI TAFE NSW 2007

Vibration Condition Monitoring

Machine Faults

The faults discussed here are typical of most rotating machinery. Reciprocating machinery is far more complex but still amenable to vibration condition monitoring. With rotating machinery the vibration can be linked to machine components relatively easily and consistently across many machine types and the link is consistently related to machine RPM.

Reciprocating machinery is vastly more complex as it involves crankshafts, eccentrics, swashplates or similar mechanisms to go from reciprocating to rotating motion. Associated valving, camshafts, fuel injection, and other components develop vibration that may be very complex and related to how many cylinders are active on the machine. The analysis of a reciprocating machine is generally unique to that particular diesel engine, air compressor, refrigerant compressor or other machine. Other features such as two or four cycle engines, variation in active cylinders, variable valve timing, and so on affect the analysis. The approach to this type of machinery requires considerable experience so that only rotating machinery is considered from this point.

The faults are listed in their approximate order of frequency in industrial plants. Even with bearings, it is frequently excessive loading caused by unbalance and/or misalignment which hastens failure.

Unbalance

Any rotor will have some potential unbalance. The extra expense of balancing to a higher grade may be justified when cost of downtime, spare parts, stripping, rebalancing and reassembly are considered.

Unbalance may be a single heavy spot but is more likely to be two heavy spots somewhere along the length of the rotor. If both are on the same side then static unbalance will be observed. (the rotor will tend to always stop with the heavy side down)

If the heavy spots are on opposite sides at each end and with about the same magnitude then the rotor may be statically balanced but dynamically unbalanced. This situation gives "couple unbalance"

Other unbalance between these two conditions are probably the most common situation encountered.

Because balance is probably the most common fault it is important to consider the relative cost of higher grade balancing before assembly against the cost of early overhaul. In many cases the extra cost of higher grade balancing is marginal compared to overall balancing cost. (ie shutdown, disassembly, rotor transport, balancing, reassembly, restart costs must be added to determine the total cost involved)

Dominant fault vibration at 1x shaft speed.

Misalignment / Bent Shaft

When two machine shafts are to be coupled together (eg a motor driving a gearbox) it is important that the shafts are aligned so that the two centre lines are very close to identical. Alignment is often determined by laser alignment tools which can achieve high accuracy relatively quickly. Shafts can alignment errors which cause:

Parallel misalignment: centre lines of shafts are parallel but are offset vertically, horizontally or a combination of the two.

Angular: centre lines of shafts are not parallel

Combined: the most general situation where there is a combination of parallel and angular misalignment

Dominant fault vibration at 1x shaft speed.

Sometimes at 2x shaft speed and occasionally 3 and 4 times.

Mechanical Looseness

Components may become loose on a shaft or housing. This fault could also include soft foot and loose Journal Bearings.

Dominant fault vibration at 2x shaft speed.

Sometimes loose journal bearings show up ½ or 1/3 x shaft rpm.

Bearings

Journal Bearings

Not used on all machines but still sufficiently common to be of concern. Plain bearings have much bigger clearance than rolling bearings and as such actual shaft displacement relative to the bearing is often important for vibration determination. Oil film Whirl or Whip are typical faults which are mostly seen on bearings of high speed machines.

Dominant fault vibration at 42%-48% x shaft speed.

Rolling Element Bearings

Many machines use rolling bearings (ball, roller, needle roller and tapered roller bearings). Rolling element bearings may have a number of reasons for failure but basically the failure will appear as damage to the outer race, inner race, ball or roller or bearing cage.

Dominant fault vibrations are related to bearing component frequencies. These will be different for each type of bearing design.

Early fault indications are also seen at high frequencies (2- 60kHz)

It is important to inspect every failed bearingto determine the reason for failure. Good maintenance practice is not just about timely diagnosis and repair but also about designing out problems. Most bearing manufacturers will provide failure recognition information to assist in understanding the reason for failure.

Soft Foot

A machine is generally provided with ‘feet’ or mounting points which are used to bolt down to floor slab or mounting frame. A minimum of three feet is generally required for stability and four is common. Soft foot occurs when one of the mounting feet has not been able to be tightened up rigidly and the machine is able to vibrate within this looseness. Soft foot can be caused by loss of mounting shims or packing, loss of grout or some other reason for loss of rigidity at the mounting point.

Dominant fault vibration often at 1x shaft speed but sometimes 2 and 3 times.

Motor faults

A great deal of machinery in industrial plant is driven by electric motors. Although generally extremely reliable, electric motors can develop faults which can lead to vibration. Typically these faults relate mechanical faults such as loose rotor bars. It is possible of course to also monitor motor current and detect faults from irregular current variations. Often these faults can be confirmed by switching off the power. (the vibration should then dissappear)

Dominant fault vibration is often at 1x shaft speed and 1 or 2 times the synchronous frequency. See motor fault references for more detail.

Gears

Gears will show faults that generally relate to tooth wear or failure over time. It is possible that a gear has an error in manufacture but this would show up immediately as a fault when the machine is first installed. (for example gear pitch circle eccentric to shaft axis)

Dominant fault vibration at Tooth Mesh Frequency which is equal to shaft speed multiplied by number of teeth.

Shaft Whirl

Resonant shaft vibrations at critical shaft speed. May also be maintained at higher speeds.

Dominant fault vibration at shaft critical speed.

Flow faults

Fluid flow may develop flow noise or instability due to pump blade failure, contaminant build up or other flow problem. These faults are often seen as higher frequency vibration.

Dominant fault vibration at blade pass frequency and often high frequency broad band vibration.

Vibration Condition Monitoring

Introduction to Condition Monitoring

Condition Monitoring is a technique used in maintenance that allows measurement of machine condition while in operation. With continual condition monitoring it is possible to determine the point where a machine is at risk of failure and shutdown before failure. As this technique is predictive it is possible to make the shutdown at the most convenient time to minimise interruption of production or service.

Condition Monitoring is also able to determine the likely component that is about to fail so that appropriate spares can be on hand and an accurate estimate of the time taken for the shutdown will be made.

Vibration Condition Monitoring (VCM) is a one of a range of techniques which are chosen to suit the machinery type and application. Spectroscopic Oil Analysis and Thermal Analysis are the other two widely used methodologies for condition monitoring. Details of this and other techniques are available in maintenance references and websites.

Selection of Machines

The measuring system for condition monitoring should be chosen to suit the machine but before measuring a particular machine it is important to make a financial analysis to determine if the cost is justified.

Without going into detail, the basic idea is to consider the risks involved in machine failure. These include risk to staff of injury, risk of damage to the machine and risk to the business of losses due to lost productivity.

In many cases, the cost to monitor hand held small power tools is far beyond the cost to replace so that many of these will have basic preventive maintenance and standard electrical checks but no condition monitoring.

At the other end of the scale, a large power station steam turbine will have continuous monitoring from permanently installed transducers to keep track of condition and manage the whole maintenance program with a long term view.

Measuring System

As with any vibration measurement, the instrumentation must be carefully chosen to ensure accurate, repeatable and reliable measurements.

An extensive knowledge of the machinery to be monitored is necessary to ensure that instrumentation is correctly chosen. All choices are related to machine running speed as vibration is directly related to shaft speed. Knowing the running speed and the mounted components allows determination of the maximum frequencies that are liable to be encountered.

Transducer - for many machine vibration measurements the transducer of choice is the piezoelectric accelerometer.

These accelerometers are compact, robust and have a very wide frequency and amplitude range. Measurement is usually made of vibration velocity as experience has shown this to be the best indicator of machine condition.

The other likely transducer is a displacement transducer such an inductive or capacitive type. These are used when the absolute motion of a shaft relative to a bearing is required. This is usually only necessary with a plain bearing and is often required in large turbines.

Specialist transducers may be used for specific tasks such as early condition assessment of rolling element bearings. These transducers are usually provided with a dedicated instrumentation system for the specific task.

Transducer mounting – to ensure consistent and accurate readings transducer mounting points should be clearly marked and properly prepared for repeatable measurements. Careful account should be taken of the effect on frequency response of different mounting methods

Conditioning - the conditioning amplifier must match or exceed the frequency range of the accelerometer. It may have a low pass filter fitted to limit passing information at frequencies in the resonance range of the transducer.

Readout/Analysis - the vibration signal may be passed to an RMS readout for routine monitoring or to an FFT analyser for fault analysis.

Calibration – instruments must read consistently over a long period so frequent and easy calibration should be included as part of the measuring system.

Data Gathering

Once the instrumentation system has been chosen the measuring points on each machine should be selected and set up

Measurement Points

The measurement points are always taken to be at the machine bearings to ensure consistent readings. Measurements taken on machine casings, covers and other structures may give inconsistent readings and cannot be directly compared to international standards.

The conventional means of labelling the measurements start at bearing 1, the non-driving end of the prime mover (eg electric motor). Each bearing is then numbered consecutively following the drive line of the machine. Tacho and phase pickups can be located at a convenient position for monitoring.

Machine Operation

To ensure that readings are consistent it is important that successive measurements are made under the same loading conditions.

OH&S

Because all measurements are made with the machine operating it is important that operator safety is carefully considered when locating transducer mounting points. Other OH&S issues should also be accounted for when setting up a measuring program. For example clear access ways, training in emergency procedures if there are product or process leaks, personal safety equipment to be used (earmuffs, glasses, hard hats) and so on. It is useful to contact the company OH&S officer or workcover to get advice on these issues.

Monitoring Program

With all the measurement system set up, the question is how often and what sort of measurement should be made? It is very expensive to measure every point on every bearing of every machine and then carry out an FFT analysis on these measurements.

Apart from the staff costs and data storage requirements, the machines are likely to show very little change in vibration for long periods. What is really important is early detection of vibration changes.

Most machines then are only monitored for RMS vibration for the vertical direction at each bearing. This is a measurement that can be taken rapidly using a portable vibration probe and RMS meter. Data from this measurement is used for detection of problems and then a full FFT analysis is used to diagnose problems when they occur.

Data Recording

Because of the large amount of data involved in machine condition monitoring it is important to develop a logical program of measurement and recording.

Monitoring Route

In a large plant or factory some time is spent determining a monitoring route to make it time effective to move from one machine to another as required for taking measurements.

Machine Measuring Instruction

Each machine should have instructions about where and how to take measurements as well as any other relevant information such as safety requirements. Many portable analysers provide text space to allow this to be all electronic

Trend Analysis

When the data is entered in the analyser it is used to create a trend graph. This could be done internally in a portable system or back in the office when the data is downloaded to a computer.

A trend analysis is a plot of successive vertical vibration measurements. (one plot per bearing) This plot shows when the vibration starts to increase and assists in determination of when action should be taken.

The trend analysis shown has a number of measurements close together (perhaps weekly) to the left of the chart. These measurements are taken with the machine new or newly overhauled and should stabilise quickly. The stable level is termed base level and is then used to set the Alarm level and the Shutdown level for this machine.

The alarm level is reached when vibration exceeds twice the base level.

The Shutdown level is achieved when the vibration exceeds six times the base level.

These are guidelines to start a monitoring program. It may be that experience allows significant variation from these values but initial work should start with these as they are based on wide ranging industrial experience.

Once the base level has been established, monitoring is taken at less frequent intervals which may be once per month or longer. These readings are continued until a level is reached that goes above alarm level.

At this point, monitoring intervals are reduced to perhaps fortnightly or weekly and plans made to do a full FFT analysis at every measurement point.

With the thorough analysis completed and diagnosis made, plans can be made for shutdown and repair.

Regular monitoring is continued however to make sure that the machine does not reach the shutdown level before repair. the shutdown level is an indicator that vibration is high enough to risk component failure and/or machine damage.

Once the machine is repaired it can be restarted and the monitoring again follow the same pattern as described above.

Note that the Alarm Level and Shutdown levels should be set for each machine individually. It is possible that, say, four otherwise identical machines will have differing levels of base level vibration. This is primarily due to a combination of manufacturing tolerances making any machine an assembly that may be tight, loose or anywhere in between.

Machine Maintenance Database

With all the data gathered it is important that the information is kept in some sort of a database so that ongoing intelligence can be gathered about each machine. This allows more appropriate vibration levels to be set for alarm and shutdown, appropriate measurement intervals to be determined for each machine and provides data input to maintenance redesign and machine improvement for reliability.

Machine Fault Analysis

The overall vibration signal is measured to determine if vibration levels are acceptable. If vibration levels are not acceptable a more detailed analysis is necessary to determine the source of the excessive vibration.

Measurements

To determine what fault is present will require a more detailed measurement of machine vibration. Recall that the overall measurement is generally measured as Vertical vibration at each bearing when the machine is at normal load and speed.

It is necessary to now measure:

Vertical, Horizontal and Longitudinal vibration at each bearing

Phase at each bearing

RPM of each running shaft

Vibration measurements are made with measuring system requirements similar to the monitoring measurements. An additional requirement is a phase meter.

Phase measurements are generally made with an optical, capacitive or inductive transducer sensing some point on the shaft being measured.

Generally only one phase pick up is needed for all bearings

Fault Analysis

The full vibration analysis described should be taken once the machine has settled in from new or rebuilt to give a set of reference measurements. This analysis is then repeated when a fault is indicated by routine vertical vibration measurements.

Each vibration measurement should be processed by a narrow band FFT analyser so that a detailed frequency spectrum is available.

A diagram of vibration phase for each bearing should be constructed so that the relative phase relationships can be seen for each shaft on a machine.

The spectral analysis allows determination of the frequencies that have increased from the base spectrum and are caused by the fault.

As many faults may cause a similar spectrum, particularly faults at 1x rpm, the phase information is necessary to finally determine which fault is active.

For example unbalance on one side of a rotor will show up as same phase at both bearings whereas couple unbalance will show up as a 180 phase difference.