Troubleshooter’s Guide to Vibration (June 01, 2007)

The application of vibration analysis should be part of any proactive total equipment management program. A regularly scheduled vibration analysis program can be carried out by in-house vibration technicians or by local contractors who are familiar with the plant’s equipment and its processes. Various instruments can be used to determine vibration frequencies and pinpoint machinery problems.

There are several common vibration frequencies, and we began to deal with them in Part 2 of the Troubleshooter’s Guide to Vibration (Part 1 appeared in MRO’s Feb. 2007 issue and Part 2 in the April issue). These frequencies include rotor frequencies, rolling element bearing fault frequencies and sleeve bearing fault frequencies.

This issue, Part 3 examines frequencies involving gear drives, belt drives, chain drives, fan blades and pump impellers, as well as resonant frequencies. Part 4, next issue, will conclude this series with a focus on frequencies related to electrical equipment.

4. Gear Drive Problem Frequencies: Gear mesh frequencies will be very high and can be calculated by multiplying the number of gear teeth X the rpm of the shaft.

When diagnosing a multiple gear train, calculate the gear ratio between the drive and driven gear(s). Then use this ratio to calculate the speed of the driven gears. Most gearbox manufacturers will provide this information and may also provide the various frequencies within the system.

Several typical gear drive faults are illustrated in Figures 11 and 12.

5. Belt Drive Frequencies: The primary belt frequency is equal to [3.142 X pulley rpm X pitch diameter] divided by belt length.

Common belt drive problems include misalignment, mismatched belts and belt resonance. A loose or bouncing belt in a multiple V-belt set may indicate a resonance problem and can be located easily by using a strobe light.

To eliminate resonance, change the belt frequency by using single-banded belts when replacing multiple belt sets. Always replace the entire set and never use belt dressing to correct a slipping belt problem.

Keep in mind that belt frequency is always sub-synchronous energy, less than shaft speed. Analysts frequently mistake belt frequencies as an unbalance problem, since they see a frequency in the spectrum close to the 1 X turning speed frequency.

Another concern is that of belt strand resonance. As a drive belt is released from the sheave as it rotates, the belt actually snaps out of the sheave — it’s like a guitar string being plucked. This can excite a natural frequency in a drive belt’s steel strand. If the resonant frequency is equal to or close to the shaft turning speed frequency, a vibration will result that may actually destroy the drive system.

Belts which have been over-tightened to reduce slippage can cause premature bearing failure, drive shaft deflections, worn sheaves, an increase in the possibility of resonance, and can seriously overheat the drive itself — both belts and sheaves.

Belt temperature must not exceed 60C (140F). The temperature can be measured with a handheld infrared temperature monitor or drive temperatures can be measured using infrared thermography.

The general rule for belt tension is: Press down on the belt firmly and measure the deflection. The deflection should be 1/64 in. for every inch of sheave centre-to-centre distance. A more accurate measurement can be obtained with the use of a very inexpensive device called a deflection gauge.

If, after the belt tension has been adjusted, the belts continue to slip or squeal, replace the belt and/or replace the sheaves if there is any evidence of wear. Belts work properly only when they are dry and free of foreign material.

Also inspect the sheaves for correct alignment, both offset or angular. Use a string or a straight edge. A belt or belt set’s life expectancy should be about 25,000 hours and sheaves should be measured and considered for replacement after five sets of belts have been used.

As with other rotating machinery, it is advisable to take vibration readings axially both horizontally and vertically on each sheave if possible.

V-belts used in potentially explosive applications should be grounded to prevent static electricity. These belts should have six megohms resistance or less and should be of the static-conducting type. Grounding V-belts, with the use of a grounding brush, will also reduce the possibility of static electrical discharge through the bearings.

Several common belt drive problems are illustrated in Figure 13.

6. Fan Blade and Pump Impeller Frequencies: Blade pass frequencies occur as each blade on a fan or compressor rotor delivers its contribution to the process. The blades on a pump impeller create a frequency as the blade passes the outlet port.

Blade or vane pass frequencies (BPF) are inherent in pumps, fans and compressors and normally do not pose a problem. However, a large-amplitude BPF with harmonics will be generated in pumps if the vane-to-diffuser gaps are unequal, if the impeller wear ring seizes on the shaft, or if welds fail. High BPF can also be caused by abrupt or sharp bends in piping or ducts, or by obstructions that disturb flow.

Flow turbulence is caused by variations in compressor pressure or velocity, causing low-frequency vibrations, usually at 50 CPM (cycles per min.) to 2,000 CPM, while excessive turbulence or surging can excite random high frequencies.

Cavitation indicates voids in the pump inlet, generating noise combined with random high-frequency energy superimposed with blade pass frequency harmonics. Impeller vanes will erode as a result and vibrations generated by unbalance will also be generated.

Wherever possible, pump, fan and compressor inlets must be unrestricted and flooded in order to reduce unwanted hydraulic and aerodynamic forces (see Figure 14).

7. Chain Pass Frequency is equal to the number of sprocket teeth X rpm. Chain drives will indicate that a problem exists if this frequency appears on the vibration spectrum.

Chain or sprocket misalignment, bearing problems, eccentric sprockets or resonance should be investigated.

8. Resonant Frequencies: Resonance is the excitation of the natural frequency of a system (or the excitation of a natural frequency of a component within the system). Put another way, resonance is a condition where the forcing function frequency coincides with a system’s resonant or natural frequency. It is one of the most common problems associated with today’s industrial machinery.

If a resonant frequency is excited by another frequency operating at or near the same speed, a rotating machine or component can destroy itself in a matter of minutes. (Any rotating component that is operating at or near its resonance will be almost impossible to balance, due to its dramatic phase shift.)

Unlike vibration frequencies that appear when a machine is turned on and disappear when it is shut off, resonance or natural frequencies hide within machines and machine components.

Resonance is not a source of vibration, but rather an amplifier of the vibration frequencies produced within a machine or its components. Resonance only becomes a problem when the machine produces forcing frequencies that occur at or near one of the machine’s resonant frequencies.

To reduce the effects of resonance, the forcing vibration frequency can be reduced so that it does not have enough energy to excite the resonance, or the frequency of the resonance can be changed to move it away from the forcing frequency. Resonances result from the values of the machine or system mass, stiffness or damping, and are not generally a function of the operation of the machine or system.

A resonant frequency, once excited, can cause large and sudden increases in the amplitude of the vibration. When this occurs, the vibration analyst must ask the following question: “Is the amplitude of vibration high because t
here is something wrong with the frequency seen on the spectrum, or is the forcing frequency merely exciting a resonant frequency at that point?”

Resonance can cause certain machine problems that occur (or recur) and for which many people simply might consider normal wear and tear. The following recurring problems suggest resonant conditions.

* Machine maintenance history — If the machine has a history of higher than normal vibrations and if repeated attempts at balancing or alignment produce limited results, it is possible that resonance is amplifying certain vibration frequencies.

* Speed sensitivity — Changes in the operating speed of rotating machinery will change the forcing frequencies produced by the machine. If a pump that ran successfully at 1,000 rpm now runs badly at 1,200 rpm due to increased production demands, the new speed may be running close to (or at) resonance. The installation of variable-speed drives for electric motor operation is an example of a cause of possible resonance problems occurring in machines that were trouble-free prior to conversion.

* Premature fatigue and cracks — The repeated cracking or breaks in welds in piping, bases and machine foundations is a strong characteristic of resonant conditions.

Identifying resonance can be done by using several methods, among them, hand feeling the machine for vibration. If a machine is experiencing specific failures, say a repeated bearing failure, hand feel other components during operation, such as the base or the piping. A resonant component could be exciting the bearing frequency.

Another technique is to use the ‘coast down’ test. A sudden decrease in vibration amplitude as the speed decreases might indicate a possible resonance at running speed. A strobe light can be used to measure changes in the phase relationship between the shaft position and the vibration spectrum.

A bump test may be used to impact a resonant component. When impacted, the component will vibrate at its resonant frequencies. If these frequencies coincide with a forcing frequency that is occurring during operation, the resonance might be excited.

In addition, a model analysis is a test method whereby a known force is applied to a machine and the responses to the force are measured at various locations on the machine.

Correcting resonance can be done in one of two ways; remove or change the forcing function, or change the resonant frequency. Since resonance is only a problem when the forcing function is equal or close to the resonant frequency, it is best to remove the vibration source because it is usually the easiest and least expensive corrective measure.

Often, the machine base or foundation is braced or stiffened at great expense, only to result in a situation where the vibration caused by unbalance or misalignment still exists.

Simple methods of changing the forcing function are listed below:

1. Increase or decrease the operating speeds or loads. This will ‘move’ the vibration source or the natural frequency (or both).

2. If pump or fan resonance is at the impeller or blade frequency, change the impeller design or the number of blades or impellers.

3. If resonance is equal or close to the frequency generated by a drive coupling, change the coupling type.

4. If resonance is a multiple of the belt frequency on a belt drive, change the sheave size or the belt type.

Machine resonance is directly affected by the machine’s mass, stiffness and damping. Therefore, changing any one of these parameters will change the resonant frequency of the machine, so keep the following points in mind when considering machine or system vibration corrections.

a) Adding stiffness increases resonant frequency.

b) Adding mass decreases resonant frequency.

c) Adding damping (increasing or decreasing damping conditions, such as process flow rates, oil viscosity, operating temperatures, etc.) reduces the amplitude of the resonant frequency.

To conclude, vibration analysis, if utilized correctly and regularly and with proper interpretation, can provide a huge return on investment by providing early recognition of machine problems. Early recognition, in turn, will reduce repair costs and downtime, eliminate catastrophic failures, improve reliability and productivity, extend equipment life and will allow the plant to take advantage of sound planning and scheduling of necessary maintenance.

If used in conjunction with other predictive maintenance technologies, such as oil analysis, the ROI will be awesome!

Machinery & Equipment MRO technical editor Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of maintenance problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca.

Illustrations courtesy of Maintenance Technology International Inc. of Cochrane, Alta., and Technical Associates of Charlotte, N.C. Technical Associates can be reached at 704-333-9011 or at www.technicalassociates.net.

References

The Simplified Handbook of Vibration Analysis, Arthur R. Crawford.

Rotating Machinery Vibration, Maurice L. Adams, Jr.

Introduction To Machinery Analysis and Troubleshooting, 2nd Edition, John S. Mitchell.

Vibration Spectrum Analysis, 2nd Edition, Steve Goldman, P.E.

Part 4 of this report will be published in our September 2007 issue, and will continue the discussion of common vibration frequencies and how to deal with them, with a focus on frequencies related to electrical equipment.