Troubleshooter’s Guide to Gear Drives

The function of the industrial gear drive is to reliably transmit torque and rotary motion between a prime mover and a driven piece of equipment. The gears are used to transmit motion and power from o...

December 1, 2006 | By Lloyd (Tex) Leugner

Gears transmit motion and force by means of successively engaging machined projections or gear teeth. The smaller component of the pair is called the pinion, the larger, the gear. When the pinion is on the driving shaft, the gear set acts as a speed reducer; when the gear drives the pinion, the result is a speed multiplier.

Gear types

The basic gear type is the spur gear or straight tooth gear, with teeth cut parallel to the gear axis. Spur gears transmit power in applications using parallel shafts (see Figure 1).

When spur gears are used, the teeth mesh along their entire length, creating sudden shifts in load from one tooth to the next, often causing unacceptable noise and vibration.

This problem can be overcome by the helical gear, which has teeth cut at an angle to the centre of rotation, so that the load is transferred progressively along the length of the tooth, from one edge of the gear to the other.

When shafts are not parallel and the drive is transmitted at a 90 angle, the common gear type used is the bevel gear, with teeth cut on a sloping gear face, rather than parallel to the shaft.

The spiral bevel gear has teeth cut at an angle to the plane of rotation, which like the helical gear, transmits power at an angle and reduces noise and vibration.

The hypoid gear resembles a spiral bevel gear, except that the pinion is offset so that its axis does not intersect the gear axis. These gear sets are used in automobile differentials. The offset of the axis of hypoid gears introduces additional sliding between the gear teeth, which when combined with high loads, requires a high-quality oil with extreme pressure (EP) additives.

A worm gear consists of a spirally grooved screw moving against a toothed wheel. In this gear type, where the load is transmitted across sliding, rather than rolling surfaces, compounded or EP oils are necessary to provide effective lubrication (see Figure 2).

The herringbone gear has teeth cut at an angle on the gear face, but the teeth are cut at right angles to each other on both the pinion and gear faces. This provides gear drives which eliminate excessive shaft end play, while ensuring low noise and vibration of the gear set (see Figure 3).

Meshing

As gear teeth mesh, they roll and slide over each other. The first contact is between a point near the root of a driving tooth and a point at the tip of the driven tooth. As contact progresses, this rolling and sliding action continues as each tooth contacts the next.

The sliding is always away from the pitch line on the driving teeth and always toward it on the driven teeth, while rolling is continuous throughout gear meshing. This combination of sliding and rolling occurs on all meshing gear teeth regardless of type, although some gear types, such as worm, hypoid and spiral bevel experience additional radial and sideways sliding.

Given this combination of continuous sliding and rolling contact between gear teeth, it is easy to understand that fully 50% of gear failures are caused by improper lubrication, incorrect gear loading or some condition related to lubrication or loading, such as temperature (see Table 1).

Elastohydrodynamic lubrication

Machine elements such as anti-friction bearings and gear drives operate under elastohydrodynamic lubrication (EHL) conditions. EHL is a condition which results when the surfaces in relative motion are defined as having a low degree of conformity and high contact pressures.

In machine applications such as gears, the surfaces in motion trap the lubricant film under extreme pressure in the convergent zone as rotation occurs and the oil’s viscosity can actually increase to the point where the lubricant forms a pseudo-solid film. This creates a condition where the ‘almost’ solid film separates the two surfaces.

So long as the operating conditions, such as speeds, loads and temperatures are not exceeded for the particular application, surface contact may never occur due to this remarkable characteristic of lubricants. The gear teeth contact surfaces may actually deform elastically before this pseudo-solid oil film breaks, thus the term elastohydrodynamic lubrication (see Figure 4).

In some gear drive applications, loads or speeds may be such that the EHL oil film may become ruptured when peaks of the metal surfaces (called asperities) come into contact. In these severe gear load applications, lubricants with extreme pressure additives are required.

Various chlorine, potassium borate and sulphur-phosphorus compounds are used primarily as extreme pressure additives, with sulphur-phosphorus being the most common.

Under these boundary lubrication conditions, most EP additives are activated by a dramatic temperature increase when asperity contact occurs and react with the metal surface to form a eutectic, sacrificial film. The film formed as a result of the EP additive reaction has a lower melting point than that of the surface of the gear tooth, thus the term ‘eutectic’, and this film protects the gear tooth surface from further damage or welding of asperities (see Figure 5).

Extreme pressure additives, depending upon their type and application, may have some limitations. For example, sulphur phosphorus additives may be too chemically reactive, causing ‘polishing’ wear, particularly in slow-speed gear applications (less than 10 ft/min.). These same additives may cause corrosive pitting in yellow metals, such as used in phosphor-bronze worm wheels, particularly at temperatures in excess of 60C (140F).

Often, extremely low ambient temperatures can also affect the efficiency of sulphur phosphorus EP additives because the reaction rate may be too slow. In these cases, potassium borate or fatty acid additives may provide protection, however potassium borate is sensitive to water contamination.

These examples illustrate the great care that must be taken when selecting, applying and caring for gear lubricants. It also illustrates that the lubricant type and additive compounds used must be considered when troubleshooting gear drive problems or analyzing gear failures.

(Editor’s note: A separate guide, Lubricant Selection and Application for Gears, may be read in conjunction with this article. It can be found in the Winter 2006/2007 issue of Industrial Lubrication, a supplement to Machinery & Equipment MRO, and online in the Online Features section at www.mromagazine.com.)

Gear drive troubleshooting

Gear drive systems will operate trouble free for many years so long as they are properly lubricated, kept free of contamination (both internal and external), operating temperatures are not exceeded and they are operated within their designed loading limitations.

The most common gear tooth shape or profile is called an involute. This design ensures that the location of the pitch line for the driven gear is identical to that of the driving gear or pinion.

The relative angular velocity between the two gears is constant for all tooth contacts, a condition called conjugate action or conjugacy. Conjugate action ensures the smooth transfer of rotary motion from one shaft to another through proper gear action.

Whenever abnormal conditions occur, such as shaft misalignment, excessive backlash, inadequate or incorrect lubrication, or excessive loading, the gear material along the tooth pitch line may become worn, pitted or fatigued, in turn changing the tooth profile and affecting conjugacy of the gear drive.

Maintenance personnel and troubleshooters must regul
arly apply predictive maintenance technologies, such as oil and vibration analysis, to monitor gear drive conditions, and regular inspections must include the following routine condition monitoring.

Listen for abnormal noise. Noise inspection must focus on changes in sound. Use an industrial stethoscope or ultrasonic tester to ensure that noise changes are accurately determined and recorded.

Excessive noise may be the result of sudden fluctuations in drive speeds, coupling wear, unbalance or misalignment, worn, damaged or misaligned gear teeth, worn or failed bearings, lack of lubrication, or loose foundation bolts.

Monitor operating temperature. Remember that mineral base gear lubricants will begin to oxidize at temperatures that consistently run at 71C (160F).

Sudden increases in operating temperatures may be the result of low lubricant levels, excessive sludge, varnish or contamination that causes increased oil viscosity, damaged or restricted oil cooler (if equipped), excessive dirt or foreign material covering part or all of the gear case, bearing failure or damaged gear teeth (which should correspond with higher noise levels), overloading, over-greasing of bearings, or too high oil levels.

Monitor temperature differences at all bearing housings. (If one housing is running hotter than the others, this bearing or a gear on the related shaft may be the culprit). Leaking seals might be the result of high temperatures, which in turn, will create lower than normal oil levels.

Inspect air breathers and oil filters. A plugged or restricted breather will cause excessive housing pressures to develop, which in turn may cause seal leakage. Breather caps usually contain wire gauze filters that are intended to restrict the ingestion of dirt and dust. These filters are useless and should be replaced with good-quality filters with absolute ratings of at least 5 microns to prevent airborne particulate from entering the gear housing.

Oil filters should be fitted with pressure gauges so that pressure differential can be determined and monitored. This will help assure that oil filters are replaced at the correct intervals. If water or particulate contamination is excessive, side stream or kidney filter systems can be installed that will remove unwanted and excessive levels of contamination.

Cut open and inspect both oil and air filters if the gear box is so equipped. The filter media can be spread on a bench, and using a good quality magnifying glass or microscope, contaminants and wear metals will be obvious. Further testing can be done by passing a magnet under the filter media. All ferrous materials will move on the surface of the filter media as the magnet is moved.

Determine if contamination is present in the gear drive. If contamination is suspected, a sample of oil can be obtained in a glass jar, which is then allowed to sit overnight. When the jar is turned over, any contaminant will remain visible on the bottom surface of the jar.

Water contamination in the lubricant can also be determined very quickly by placing a few drops of oil on a hot plate. If the oil drops crackle or sizzle, there is water present, probably in excess of 1,000 ppm (0.10%), which is detrimental to gear drives.

In many cases, any water, if excessive, will separate from the oil sample in a jar stored overnight. If contamination by dirt, water or wear metals appears to be excessive, laboratory oil analysis should be carried out immediately.

Determine if oxidation of the gear lubricant is developing. It is often difficult to determine if a gear lubricant is becoming discoloured because gear lubricants are normally very dark in colour after some time in service. However if discolouration is obvious, particularly if the gear drive has been running at high temperatures, oxidation may be occurring and laboratory testing is recommended.

The oil analysis program

If the routine inspection of the gear drive or its lubricant results in evidence of unusual wear, oil thickening or contamination, analysis of an oil sample is absolutely necessary. In fact, all critical gear drives should be included in a regularly scheduled oil analysis program.

Before determining what tests are desirable, it is first necessary for the gear drive operator to understand that any oil analysis program will only be effective as a condition monitoring technique if the following principles are consistently applied.

* Oil samples must be taken when the oil has reached its normal operating temperature and if possible should be taken while the system is in operation.

* The oil sample must be taken at the same location, using the same method each and every time.

* The oil samples should be taken at or near the same operating interval (e.g. every 250 or 500 operating hours). This will ensure that data trending will be accurate.

* The oil samples should be taken using containers approved by (or supplied by) the laboratory carrying out the analysis. Dirty old pop bottles are not acceptable.

Oil analysis technologies vary significantly and each may be specifically designed to determine one or more of three conditions, the condition of the lubricant itself, the condition of machine components (gears or bearings) and contamination levels.

Oil analysis to determine lubricant condition

Although there are many and varied analysis tests available, the tests necessary to determine gear lubricant condition must include viscosity and acid number. Generally, an increase of 10% or more in viscosity is an indication that the lubricant has reached the end of its useful life.

The general rule for acid number result is, if the acid number has doubled that of new oil, the lubricant has reached the end of its useful life. Increases in viscosity and acid levels are definite indicators of oxidation, which suggest that sludge and varnish will develop and flow will be reduced.

Wear particle analysis for machine condition

There are two common techniques used for wear particle analysis. The most common technique is spectroscopic analysis, which is used to indicate the levels of ultra-fine particles in the 5-7 micrometre range (see Figure 6). The next most common is analytical ferrography.

Spectroscopic analysis results are usually reported in parts per million (ppm). These particles include, among others, iron, copper, aluminum or chromium, which indicate the metallurgical makeup of gear or bearing components in a gear drive.

This analysis also indicates the various levels of metallic additives, such as phosphorus and zinc, which indicate that typical extreme-pressure and anti-wear additives may be present in the lubricant. Another element reported may be silicon, which is an indicator of ingested dirt.

It is important for operators of gear drives to understand that these levels of wear metals are the result of wear, not necessarily the cause of wear. These ultra-fine particles will reach certain levels in every lubricated machine and unless there is a dramatic increase in one or more of the elements, or a sudden change in overall wear rates, no action is deemed necessary. In other words, the spectroscopic analysis results reported in ppm are not as important as are changes or increases in the reported numbers.

Remember that this regularly scheduled program is designed to establish wear trends. It is critically important that at least three oil analysis results at the same or similar intervals be carried out in order to establish the trend of gear drive wear rates. It is for these reasons that operators be familiar with the metallurgical makeup of the various components in the gear drives for which they are responsible.

The second technique used for particle analysis is analytical ferrography. Ferrography is a specialized oil analysis technique that can determine the type of cont
aminant or wear particle and indicate its probable or possible source. In addition, in the case of severe wear, it can illustrate the type of damage, such as abrasive, rolling fatigue, cutting or adhesive wear.

Ferrography can illustrate such conditions as inadequate lubrication, excessive heat or contamination, and provides a permanent pictorial record of the condition of machine components, such as gears.

The analysis effectively detects particles in the size range of less than one to about 250 micrometres. Analysis is done using a technique called bichromatic microscopic examination, which uses both reflected and transmitted light sources with green, red and polarized filters to distinguish the size, composition, shape and texture of ferrous, non-ferrous and non-metallic particles (see Figures 7 through 12).

Contamination analysis techniques

The sources of gear drive contamination are many and varied. Moisture entering gear boxes can be caused by condensation, careless high-pressure water washing, or by high humidity, where water condensate will be drawn into the gear box past poorly designed or poorly filtered oil filler caps or dipstick tubes. As little as 250 ppm of water, if left unchecked or trapped near a rolling element support bearing, can reduce bearing life by as much as 50%.

If the gear box is operating at its best operating temperature of 60C (140F), any water or condensation will not evaporate. Many gear lubricants contain sulphur phosphorus EP additives to prevent scuffing and scoring of gear teeth, but sulphur is an aggressive additive that can promote acids in the presence of water.

Gear drives operating in these conditions should be subjected to periodic water content analysis as part of the condition monitoring maintenance program and the Karl Fischer water test is the most effective and accurate. Water content should never exceed 500 ppm (0.05%) in an enclosed gear drive.

Another serious contaminant in gear drives is ingested dirt and dust. Dust and dirt will enter the gear drive just as water will and the levels of contamination can be excessive.

The gear drive contamination shown in Figure 13 is referred to as silt and can cause serious damage to gear tooth surfaces or bearings, particularly if the particles become work-hardened.

The best way to monitor contamination of this type is to regularly carry out a particle count as part of the condition monitoring program. Particle counts measure the number and sizes of the contaminants and the results are converted to ISO cleanliness levels. It is generally agreed among gear manufacturers that the required lubricant cleanliness levels, based on ISO Standard 4406: 99, should not exceed the 18/16/13 range. If these levels are exceeded, improved filtration and/or periodic flushing should be considered as a necessary part of the maintenance program.

Detecting gear defects with vibration analysis

Gears generate a meshing frequency equal to the number of teeth on the gear, multiplied by the speed in rpm of the shaft on which the gear is mounted. Unlike bearings, for which a bearing frequency will not appear unless a bearing problem exists, gear mesh frequencies will always be present, even if the gear train is in good condition.

Gear drives with parallel shafts upon which are mounted one gear only will always have only one frequency. Double or multiple gear reduction units that have more than one gear per shaft may have several different gear mesh frequencies, It is for this reason that maintenance personnel should be familiar with the design and construction of the gear drives in service, particularly shaft speeds and the number of teeth on each gear.

It is suggested that files be kept containing this information on every gear drive in the facility. This information should include:

* A sketch of the gear drive, noting the number of shafts, corresponding shaft speeds and the number of teeth on each gear.

* The bearing types and part numbers for all shaft support bearings.

* The marked location of all measurement points (axial, radial and vertical) from which vibration data can be obtained at bearing caps and shafts.

Gear drives are designed to maintain a constant velocity ratio between the gears. This condition, called conjugacy, requires that the normal to the common tangent at the contact point between two gears passes through the pitch point that lies on the centre-to-centre line of the two gears. Any variation in conjugacy may cause high vibrations due to poor machining, contact wear, improper gear backlash or any problem that would cause gear tooth profiles to deviate from their proper geometry.

Gear mesh frequency is often affected by process variables such as changes in loads and/or speeds and this frequency is modulated by sidebands indicating a problem. When sidebands reach half the amplitude of the gear mesh frequency, it indicates that the problem may be severe. Tooth wear may excite a natural or resonant frequency of the gear due to the contact shock created when the tooth contacts a tooth on the opposite gear.

The resonant frequency will have sidebands spaced at shaft speed. Gear sets that are designed with improper gear ratios (where the gear teeth only contact some of the teeth on the driven gear), usually have higher levels of gear wear than gear drives designed with proper ratios, where the gear teeth contact every tooth on the mating gear. The waveform may show high levels of impacting where gear wear is suspected.

Backlash and gear wear are similar in that both conditions can excite the gear’s resonant or natural frequency, however gear wear will appear as a dominant 1 X gear mesh frequency, while backlash will appear as a dominant 2 X gear mesh frequency, possibly with multiple sidebands. As with gear wear, the waveform will show high levels of impacting when excessive backlash is present.

Insufficient backlash, on the other hand, particularly in high-speed gear drives where relative tooth velocity exceeds 180 ft/sec., can cause resonant frequencies caused by escaping oil entrapped between the gear teeth. As the oil escapes, the loud noises generate resonance.

Misaligned gears may show up on the spectrum at a 1 X, 2 X or 3 X gear mesh frequency with sidebands, where the 2 X and 3 X peaks will be higher than the 1 X peak.

Transmission errors are variations in gear conjugacy that will result in gear transmission errors, such as improper tooth spacing, machining errors, tooth deflection or misalignment, any of which may in turn cause worn bearings.

Most transmission errors will show up in spectral data as high amplitudes of gear mesh frequency, often with sidebands around the gear natural frequency.

Broken teeth will excite the gear natural frequency and will cause a pulse at each contact and will appear at the 1 X shaft speed. If a broken tooth (or teeth) is suspected, view the waveform. The high amplitude impacting of the broken tooth will appear at time intervals equal to the shaft turning speed frequency (see Table 2).

Taking vibration measurements on gear drives

For single reduction, parallel shaft gear drives, it is advisable to take low-frequency measurements (for looseness) and high-frequency measurements (for gear condition) at the axial and radial positions as close as possible to the bearings that support the shafts. Radial measurements should be both vertical and horizontal to the shaft.

For multiple-reduction gear drives, use a high-resolution spectrum analyzer to separate the sideband data. Define each area of interest by using narrow bands of the spectral data and take two radial measurements (horizontal and vertical) and one axial measurement at each bearing location.

Loads will affect the amplitude(s) of the gear mesh frequencies, so it is important to take vibration readings over a long enough period of real time to see all of the spectral data in orde
r to view the effects of load change. Always take all vibration measurements at the same axial and radial locations.

To ensure that amplitudes of vibration frequencies are accurate, use transducers which have the linear response range necessary to monitor the full range of frequencies that may be important. Depending upon design, most good-quality magnetically mounted transducers provide linearity to about 3,000 Hz. This can be increased to about 5,000 Hz by applying special contact adhesives or fluid between the transducer and magnet and between the magnet and the machine surface. (Machine surface vibration monitoring contact areas should be clean and absolutely free of dirt, grease and paint). The best, most accurate method of transducer mounting is to use threaded stud transducer mountings. These will provide linearity to about 12,000 Hz.

For those troubleshooters using only handheld probes, these will only provide linearity of about 2,000 Hz and may not be effective for sensing critical vibrations. Using care and with experience, handheld transducers can be used for frequencies as high as 2,000 Hz, however the probe has a resonance between 800 Hz and 1,500 Hz, so the response is not as linear as when you mount with adhesives. The probe should be electrically isolated in order to avoid ground faults that can introduce errors in vibration spectra.

When taking vibration readings, keep the following points in mind:

* Once mounted, allow the transducer to settle down. Allow 3-5 seconds.

* Movement, however slight, of the transducer to analyzer cable, can affect vibration frequency readings, so avoid excessive cable movement.

* Ensure that there are no loose connections between the transducer to analyzer cable.

* When taking vibration readings and a problem is indicated, carry out an immediate inspection of the gear drive and coupling, base plate and grouting.

Look for problems such as:

* Oil leaks (which might be a symptom of misalignment or bearing failure).

* Deteriorated seals (again, this is an indication of a possible misalignment problem).

* Loose or damaged mounting or hold-down bolts.

* Cracked or damaged cement pad or baseplate.

* Missing or loose shims under mounting bolts.

* Broken or loose components.

* Inspect the drive coupling for problems such as grease or oil leaks, rubber dust, excessive backlash or looseness (depending upon coupling design).

* Determine if the bearing and gear housing temperatures are normal (temperatures higher than 77C (170F) might be considered abnormal).

* Inspect drive belts (if used) for looseness, wear or misalignment.

For establishing vibration alarm levels, analysts should review the ANSI/AGMA 6000-A88 guideline, which has established criteria for defining alarm levels for various gear designs in accordance with their pitch line velocities.

Common gear drive failures and their associated root causes

The best troubleshooters will recognize gear failures during inspections based on the condition of the gears themselves. The photos here are examples of typical gear problems or failures (see Figures 14 through 19 ).

In conclusion, troubleshooting gear problems is not that difficult, so long as the troubleshooter is fully aware of gear drive design, application, operating conditions, condition monitoring techniques and effective preventive maintenance practices applicable to the equipment.

Click here to view Table:1 from this article

Machinery & Equipment MRO’s contributing 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 and lubrication problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca.

References

ASM Handbook, Volume 18, Friction, Lubrication and Wear Technology, October 1992, pp 299-312, 535-545.

American Gear Manufacturers Association Standard, 9005-D94, Industrial Gear Lubrication.

The Practical Handbook of Machinery Lubrication, 3rd Edition, L. Leugner, pp 63-69, 180-217.

American Gear Manufacturers Association Standard, 110-04, Nomenclature of Gear Tooth Failure Modes.

Falk Corporation Service Manual, 108-010, Failure Analysis of Gears and Related Components.

Drives & Seals, A Tribology Handbook, Editor, M.J. Neale, SAE International, pp 17-24.

Component Failures, Maintenance and Repair, A Tribology Handbook, Editor, M.J. Neale, SAE International, pp 18-25, 51-70.

Failure Atlas for Hertz Contact Machine Elements, T.E. Tallian, ASME Press, 1999.

Machinery Analysis and Monitoring, 2nd Edition, John S. Mitchell, Pennwell, pp 214-232.

Vibration Spectrum Analysis, 2nd Edition, Steve Goldman, Industrial Press, pp 87-90, 227-228, 300-314.

Machinery Failure Analysis and Troubleshooting, 2nd Edition, H.P. Bloch and F.K. Geitner, 1994, Gulf Publishing, pp 125-156.