Trouble-shooting Gear Drives
Visual Inspection of Gears and Shafts Can Prevent and Diagnose Failures
The function of an industrial power transmission gear drive is to multiply and reliably transmit torque
and rotary motion between a prime mover and the driven equipment. The gear drive is one part of
the power system which has to accommodate certain load characteristics peculiar to a specific
application.
Properly specified, designed, and manufactured components, as well operational considerations,
constitute an equation whose sum is effective and reliable gear drive operation. Operational
considerations include proper loading, installation and support, alignment and maintenance. If any
factor is incorrect or missing, then gear drive components, including the gears and shafts, can exhibit
abnormal distress. Inefficient operation, increased operating expenses, shortened life, or unexpected
catastrophic failure can result.
Careful planning and working with gear drive manufacturers experienced in your applications, will
help ensure that a gear drive system is properly specified, designed, manufactured and installed.
Getting to that point is good start toward ensuring reliable gear operation. However, the other
factors are just as important to this equation. Poor maintenance practices, abrasive or corrosive
contaminants, overloads, increased output demands, over-motoring, etc., can result in poor tooth
contact, pre-loaded bearings, lubrication leakage, etc. -- all of which can lead to distressed gears
and shafts and their associated problems.
A good maintenance program should include a strong emphasis on preventive maintenance including
checking for proper alignment and lubrication, periodic changing of lubricants, and checking that
noise, vibration and temperature parameters are within recommended guidelines.
Additionally, visual inspection of the gears and shafts is also very important. Learning how to read
the telltale signs of distress can lead plant engineers and maintenance personnel to the proper
corrective measures and the prevention of failures. However, if a breakdown should occur, "failure
analysis" based on "reading" the failed part, can help solve the problem in subsequent operations.
The balance of this article will focus on describing distress and failure modes of gears and shafts,
their probable causes, and how to solve the related problems.
Non-uniform Loads
The elastic deflection of gears, shafts, bearings and their supporting structures determines to
a large degree the manner in which mating gears will be aligned to one another under operating loads.
These deflections, along with manufacturing tolerances, result in loads being non-uniformly
distributed across the gear tooth surface, thus stressing some areas of the tooth more highly than
others. In the AGMA rating formulas, this effect is recognized by the incorporation of load
distribution factors. This, along with other "errors" present in the system, can cause tooth surface
distress or tooth fracture.
Gear Distress and Failure Mode Classifications
Distress or failure of gears may be classified into four categories: 1. Surface fatigue (pitting)
2. Wear 3. Plastic flow 4. Breakage. The appearance of the various distress and failure modes
can differ between through-hardened and surface-hardened gear teeth. These differences result from the
different physical characteristics and properties and from the residual stress characteristics
associated with the surface-hardened gearing.
Surface Fatigue
Surface fatigue is the failure of a material as a result of repeated surface or sub- surface
stresses beyond the endurance limit of the material. Figure One indicates the theoretical
mutual Hertzian stresses occurring when a gear and pinion mesh. There are compressive stresses at
the surface, and unidirectional and bi- directional sub-surface shear stresses.
Pitting
Pitting is a form of surface fatigue which may occur soon after a gear drive begins operation.
Pitting types include initial, destructive, or normal.
Initial pitting, or corrective pitting, is caused by local areas of high stress due to uneven
surfaces on the gear tooth. This type of pitting can develop within a relatively short time,
reach a maximum, and, with continued service, polish to a lesser severity. It is most prominent
in through-hardened gearing, but is sometimes seen in surfaced-hardened gears as well.
The shape of a classical pit appears as an arrowhead pointing in the direction of oncoming
contact. The pit is steep on its back side.
For most through-hardened gears, initial pitting is considered normal and no remedial action is
required. Where necessary, initial pitting can be reduced by special tooth finishing means and
sometimes by reducing loads and speeds during the break-in period. In some special, critical
applications, teeth are copper- or silver- plated.
Destructive, or progressive, pitting usually results from surface overload conditions that are not
alleviated by initial pitting. It usually starts below the tooth pitch-line, in the dedendum portion
of the tooth and progressively increases in both size and the number of pits until the surface is
destroyed. Destructive pitting occurs in both through- and surface-hardened gears. Corrective
measures include reduction of the drive loading, improving lubrication type and viscosity, upgrading
the gearing, or increasing the gear drive size.
Normal dedendum pitting manifests itself as small or modest sized pits, covering the entire
dedendum portion of the tooth flanks. Continued operation results in pit rims being worn away with
virtually no further pitting occurring. Normal dedendum pitting results when loads are at or close to
maximum allowable surface loading values. Figure Two shows the tooth appearance in the pitting
phase prior to pit-rim wear.
The dedendums are most vulnerable to this phenomenon because of the preferential orientation of
surface micro-cracks along the tooth profile. The orientation of the cracks in the dedendum of both
the pinion and gear are such that oil is readily trapped in them as the contact rolls over the surface
openings. These then propagate rapidly into pits by hydraulic pressure.
In the addendum, the oil is forced out of the micro-cracks before the contact progresses far
enough to seal off the surface openings. Thus, hydraulic propagations of the crack are almost nil
and few pits are formed in this region.
At loadings currently used for industrial surface-hardened gears, pitting is much less prevalent
than with through-hardened gears.
Spalling
Spalling is a term used to describe a large or massive area where surface material has broken
away from the tooth. Spalling is caused by high- contact stresses possibly associated with proud
areas of the tooth surface. Excessive, internal stresses, from improper heat treatment, can cause
spalling in surface-hardened gears. In through-hardened and softer material, spalling appears to be
a massing of many overlapping or interconnected large pits in one locality. In surface- hardened
gears, it manifests itself as the loss of a single or several large areas of material. Frequently,
the bottom of the spall appears to run along the case-core interface.
Case crushing is another form of spalling associated with heavily-loaded case- hardened gears. It
appears as long, longitudinal cracks on the tooth surface. Material along the cracks may
subsequently breakaway. It often occurs, suddenly, without warning signs, on only one or two teeth
of the pinion or gear. The cracks differ from those of pits in that they not only extend below the hard
case, but most of the depth is in the softer core material. Failure may be due to insufficient case
depth or core hardness, high residual-stresses, or too high loading.
If any type of spalling is evident, the drive manufacturer should be contacted for an application
review and a determination of remedial action.
Wear
Wear describes the loss of material from the contacting surface of a gear. There are varying
degrees of wear ranging from light to moderate to excessive. Degrees of wear are measured in terms of
thousandths of an inch per million, or 10 million, contact cycles.
Degrees of Wear
Polishing, or light, wear is the slow loss of metal at a rate that will little affect gear
performance during the life of the gear. It is a normal, very-slow wear-in process in which asperities
of the contacting surfaces are gradually worn until very fine, smooth, conforming surfaces develop.
Light wear can occur by either abrasive or adhesive mechanisms (see next section) when thin oil-films,
or what is known as boundary lubrication conditions, prevail. Such conditions usually occur on
slow-speed applications.
Moderate wear, or sometimes called normal wear, progresses at a rate slow enough that it will
little affect satisfactory performance of the gears within their expected life. Metal is removed from
the entire tooth surface but generally more from the dedendum areas. The operating pitch-line begins
to show as an unbroken line. Surface-hardened gears, because of their high surface-hardness, manifest
less wear than do through-hardened gears.
Moderate wear is affected by the lubrication regime, the nature of the load, the surface hardness
and roughness, and the contaminants present in the lubricating oil which might promote abrasive or
corrosive wear. Moderate wear is normal and remedial action isn't usually required other than
maintenance of the lubrication system. Filtering and more frequent lube changes are appropriate if
contaminants are present.
Excessive, or Destructive Wear is surface destruction that has changed the tooth shape to such an
extent that meshing action is impaired and the gear life is appreciably shortened. Continued
operation results in still greater wear and may eventually lead to tooth breakage. The occurrence of
such wear early in the operational history, can be caused by excessive loads, contaminated oil or
too light an oil viscosity. It should be noted that excessive wear incurred over a long period of
operational history would be considered an advancement of normal wear from the moderate to the
excessive degree and may not be detrimental to the operation of the gear drive.
Types of Wear
Wear types can be classified into two major categories: abrasive and adhesive. Abrasive wear,
sometimes called cutting wear, occurs when hard particles slide and roll under pressure across the
tooth surface. Sources of hard particles include dirt in the housing, sand or scale from castings,
metal wear particles from gear teeth or bearings, and particle infiltration into the housing during
maintenance and operations. Adhesive wear results from high attractive-forces of the atoms
composing each of two contacting, sliding surfaces. Teeth contact at random asperities and a strong
bond is formed. The junction area grows until a particle is transferred across the contact interface.
In subsequent encounters, the transferred fragment fractures or fatigues away, forming a wear
particle.
Scuffing, is an adhesive wear-type occurring at normal temperatures where smooth burnishes
appearing as radial striations are observed in the direction of sliding on the tooth surfaces. It can
appear where tooth pressures are high and oil films are in the boundary regime and where speeds
are slow enough that high- contact temperatures do not occur. This type of wear can be reduced by
increased oil viscosities where applicable or by reduced load.
Scoring is another type of wear. Also referred to as galling or seizing, scoring is the smearing
and rapid removal of material from the tooth surface resulting from the tearing out of small
particles that become welded together as a result of oil film and high temperature metal-to-metal
contact in the tooth mesh zone. After welding occurs, sliding forces tear the metal from the surface
producing a minute cavity in one surface and a projection on the other. Although microscopic in size
initially, it progresses rapidly.
Scoring most frequently occurs in localized areas on the tooth where high-contact pressure
exists or at the tip or root where sliding velocities and contact temperatures are high.
Scoring is caused by high-contact temperature and pressure, and marginal lubrication. Scoring can
sometimes be prevented by use of more viscous oil or by an extreme pressure-type oil. Localized
high-contact pressure can be relieved by improved finishing of the tooth surface. Sometimes
profile modifications are required on highly-loaded teeth to minimize high- localized
pressures.
Plastic Flow
Plastic flow is the cold working of the tooth surfaces, caused by high-contact stresses and
the rolling and sliding action of the mesh. It is a surface deformation resulting from the
yielding of the surface and subsurface material. It is usually associated with softer gear
materials, although it can occur in heavily loaded case-hardened gears as well.
Cold flow, a plastic flow-type, is indicated by the surface material having been worked
over the tips and the ends of the gear teeth, giving a finned appearance. Sometimes the tooth
tips are heavily rounded-over and a depression appears on the contacting tooth surface.
A gear set change or drive system upgrade is usually required to correct plastic flow wear and
failures. Additionally, most plastic flow wear and failures can be eliminated by reducing the contact
stress, and by increasing the hardness of the contacting surface and subsurface material. Increasing
the accuracy of tooth-to- tooth spacing and reducing profile deviations will give better tooth action
and reduce dynamic loads.
Breakage
Breakage is the ultimate type of gear failure. A gear tooth is a cantilever plate with tensile
stresses on the contact side of the tooth and compressive stresses on the opposite side,. If
the tensile stresses at the critical location are allowed to exceed the endurance strength of the
tooth material, fatigue cracks will eventually develop. With continued operations, the fatigue
cracks will ultimately progress to the point where the tooth will break away from the rim
material.
Classical tooth root fillet fatigue fracture is the most common fatigue breakage- type. The crack
originates at the root fillet on the tensile side of the tooth and slowly progresses to a complete
fracture either along or across the tooth.
Fatigue fractures result from repeated bending stress above the endurance limit of the material.
If the tooth contact pattern appears even across the entire face, system overloads would be
suspect. If the contact pattern is confined or "heavy" in the region of the fracture and at one end
of the tooth, an alignment problem of the gearing would be suspect. When the contact is good,
the system load must be reduced or increased strength rating of the gearing is needed. If "localized
loading" is indicated, gear alignment should be examined or face modifications considered.
Low-cycle fatigue, or impact, fractures are due to a low number of high-load fatigue cycles or
to a single very high load. With lower hardness, more ductile materials, the fracture is course,
fibrous and torn in appearance. With harder, less ductile material, the appearance may be smooth or
silky looking.
In some cases, a single overload may break out a tooth or several teeth. A more common
occurrence is the plastic yielding of a group of teeth in one load-zone from a high-impact load. The
plastic yielding displaces the pitch on this group of teeth with respect to the other teeth on the
gears, thus subjecting them to abnormally high dynamic loads in subsequent operations. These teeth
then develop very rapidly progressing fatigue-cracks which soon lead to tooth breakage.
This type of failure is prevented by protecting the gearing from high-impact or transient
loadings. This may involve the use of fluid, controlled-torque, or resilient couplings in the
connected drive train or it may require better control of the process being performed by the driven
equipment.
Sometimes a severe maldistribution of load on gear teeth can occur from damage to associated
parts. For example, a severe bearing failure can cause the load to shift to one end of the teeth
resulting in breakage. Similarly, fractures can occur from a shaft that is severely bent or
broken.
Shaft Distress and Failure Mode
The performance of a gear set is dependent on the shafting for the gear elements. The shafting
must be rigid enough to prevent excessive deflection that would result in abnormal load
distribution on the gear teeth. The fits between the shafts and the bearings, and between the shaft
and the mounted gears must be proper so that mounted members are not too loose nor too tight.
Either of the later conditions can contribute to shaft failure. The shafts must be strong enough to
resist permanent yield from shock loads and the reverse bending fatigue loads that are superimposed
on the transmitted torsional loads.
The most common type of shaft fracture encountered with gear drives is that resulting from the
alternating bending stresses that occur as the shaft rotates while transmitting a unidirectional torque
load. The fatigue-cracks almost always start at some stress raiser on the shaft such as sharp
cornered fillets, snap-ring grooves, fit corners, fretted areas, key or keyway ends, or tool or stamp
marks. There may other situations where an unexpected dynamic load may be
imposed on the shaft which may be two or three times greater than the operating load.
Summary
This article has described many of the distress and failure modes encountered with mechanical
power transmission gear drives. While described as separate distress entities, it should be noted that
actual observations can reveal several modes being found together. These descriptions should help
you better understand the nomenclature and "read" the telltale signs you see upon opening the cover
of an enclosed gear drive and inspecting the gear sets.
This information should help you initiate any needed corrective measures to ensure reliable,
efficient, and long-life gear drive operations. And, when necessary, this knowledge should help you
communicate information regarding specific problems to the gear drive manufacturer's service
personnel who are familiar with the subtleties of the gear drive design and with the load
characteristics of the particular application.
This article was condensed from the Falk document, Failure Analysis: Gears, Shafts, Bearings and
Seals authored by D.L. Borden and G.A. Holloway of Falk, Milwaukee,
Wisconsin. For a copy of the document, please contact Falk, 1 (800) 440-3255,
Ext. 108. Or for additional information, contact the American Gear Manufacturers Association and
ask for the information sheet American National Standard: Nomenclature of Gear Tooth Failure
Modes, ANSI/AGMA 110.04-1980, Reaffirmed 1989. The American Gear Manufacturer
Association, 1500 King Street, Alexandria, Virginia 22314.
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