Faults associated with Spur and helical gearing

SLOGGER

Hammer like action between teeth caused by variations in pitch or torsional vibration , may be negated by nodal drives

PITTING

The mechanism for pitting is poorly understood . One theory is that it starts below the surface and parallel to it . When extended to surface oil under hydrodynamic pressure is forced in.
Shown above is the deformation of the surface due to the rolling action of one tooth over the other. Subsurface hertzian stresses are formed which run parallel to the surface.

Most severe on the pitch line or just below it , but may also be found on the dedendum of both driving and driven teeth and is dependant largely on finish
There is experimental evidence which suggests that pitting occurs only where there is a low ratio of slide to roll. With worm and most hypoid gears, excessive side slide tends to wear away high spots before true pitting would occur.
With spur and bevel gears , as each tooth passes through the centre of the mesh , the entire load is momentarily concentrated on the pitch line. If the area along the pitch line has already started to pit , this concentration of load on the roughened surfaces of spur gears is quite likely to increase the pitting progressively until the tooth surfaces are destroyed or severely damaged.
On the other hand , with helical , herringbone and spiral gears , there is less likely hood of destructive pitting . This is because each tooth during the mesh makes contact along a slanting line which extends from root to tip. This line cuts across the pitch line , and therefore, though pitting may have roughened the area along the pitch line, the line of contact always extends beyond this roughened surface, and thus the load is carried on undamaged root and tip areas. Under these such circumstances , pitting may cease as soon as the few, isolated high spots along the pitch line have been removed.

INITIAL

New gears suffer initial pitting , these can disappear on the teeth as they work harden. Normal wear polishes out pits . Further problems can be avoided by proper running in Back to back loading of the gearbox on opposite hand sets can allow the gearbox to be run in using high torque , low speeds.
Initial pitting is attributed to local overstressing caused by asperities and profile irregularities (see diagram above). The teeth of new gears may have variations in smoothness. Although these variations may be too small to break through the oil film , yet they may be large enough to affect gear operation. In addition , there may be variations in the hardness of the surface metal. When smoothness or hardness is non- uniform across the tooth , the distribution of load is also non- uniform. Thus as the teeth pass through the mesh, the load is concentrated on local high spots or hard spots.
The running in process reduces asperity's , and profile irregularities , surface stress becomes more uniform and pitting is arrested .
Profile inaccuracy can lead to root pitting and more rarely pitting on the addendum.

INCIPIENT/( corrective )

Most commonly found on wide-faced gear teeth, because of the difficulties in obtaining true and uniform contact across the entire width of the teeth. On routine inspection pitting may be found . This is incipient pitting and requires close monitoring, unlike initial pitting which is only found on the maiden voyages, and can occur at any time during the life of the gearbox.
Careful monitoring will determine whether this is an isolated case or whether it will lead to progressive pitting with potentially destructive results.
Classic causes are overloading and misalignment ( similar to progressive, and different to initial which is caused by high spots. ) .If found then the gear case should be regularly inspected and the cause ascertained and removed.
The pitting , once the cause has been eliminated should polish out.

PROGRESSIVE

May occur were initial or incipient pitting has not been arrested . However progressive pitting may be mistaken for initial but the pitting is not caused by asperities. Progressive pitting may halt or may continue to destroy the face.
Alternately it may halt , lie dormant then restart.
Most progressive pitting is wider in scope than initial pitting with branching fatigue cracks extending deep into the metal. Progressive pitting is followed by DESTRUCTIVE pitting which rapidly leads to failure

MICROPITTING

Fine attrition of the dedendum surface with a distinct wear step at the pitch line . Mating teeth may wear to a conformable shape and operate as so without problem. May be regarded as a form of wear . However , secondary pits may occur increasing roughness to an unacceptable point.
Development of wear steps, is not fully understood but may be associated with superimposed vibration - say from propeller or main engine. If the tooth surface is poor or if overloading occurs pitting proceeds reducing load bearing surface eventually destroying the tooth.

SPALLING

Deep scallop shaped pieces of metal are removed , possible causes are overloading but is more generally seen as a surface hardening process failure. It is caused by the same mechanism as pitting and flaking. Subsurface cracks form below the surface following the lines of hertzian stress. These may be joined to the surface by cracks formed due to the deformation of the surface under load. Oil forced in to these cracks under hydrostatic pressure enters the subsurface cracks were its non compressibility causes the crack to expand, were it joins other surface cracks and the piece detaches .

Very careful honing with a carborundum stone can be helpful but care should be taken not to alter the tooth profile
Cracking , flaking and spalling often indicate incorrect heat treatment ; or in the case of ground gears, faulty grinding.
Most often found in case hardened or surface hardened gears but may also appear on work hardening gears such as phosphor bronze. It can be seen that pitting, flaking and spalling are all related , the mechanics of failure is the same in each case and only the size of metal loss is different .

FLAKING

Caused by heavy overloading or over stressing the subsurface of the metal and is a surface hardened phenomena. The heavy compressive or shearing action on the subsurface can exceed the yield point stress of the metal and large flakes may break away.
Can be caused by insufficient depth of surface hardening
Rippling of the subsurface may also occur caused by plastic flow.
Similar formation to pitting but has a much increased length/breadth to depth ratio . On hardened gears , flaking or spalling may be accelerated by abnormal heat hardening strains which decrease resistance to sub surface shearing forces . Heavy loads on worm gears may subject fairly large areas of tooth surface to greater stresses than the metal is able to permanently carry. Sub surface fatigue takes place which results in damage to the bronze gear-tooth surface . This condition is often referred to as worm wheel pitting.

SCUFFING/ (WEAR)

This type of failure - caused by the local breakdown of oil film as the surfaces slide over each other during mating and disengaging - led to the development of EP additives. It was also found that increasing oil viscosity was beneficial. With oil film breakdown, very high tempo are generated and welding of local high spots occurs Similar to that occurring with microseizure). These are then torn apart.
It is most prevalent at the tips and the root were relative sliding is at its greatest .
Were the oil film thickness is greater than 3x the CLA values of the surface finish scuffing is unlikely to occur.
Evidence shows that onset occurs when a tempo related to the lubricant an surface material exceeds a flash point.
Scuffing is definitely due to failure of the oil film to carry the load, either because the operating conditions are abnormally severe, or because of incorrect oil selection. In either event , the thick wedge type film gives way to the microscopically thin , boundary type lubrication which in turn lacks sufficient film strength. to protect the gear teeth from excessively friction and the plastic flow of the 'skin surface' of the metal.
Under conditions of wedge film lubrication, failure of the film would occur where the combined film- forming effect of both rolling and sliding is least, namely the pitch line. Therefore , with fluid film lubrication , seizure would first occur near the pitch line and plastic flow would then tend to wipe the metal over onto the tooth areas that are in contact during the second half of mesh ( interval of recession ) . Scuffing in the areas above the pitch line on driving gears and below the pitch line on driven results.

Where operating conditions are more severe and boundary lubrication is resultant , the entire surface of the tooth will be scuffed .Pressure welding and plastic flow then takes place during the intervals of approach as well as recession , and surface destruction will extend from root to tips of the teeth of both gears . Even though scuffing is the initial cause of failure , severe damage may eventually bring about abrasion and scratching.
It should be noted that EP additives are very soluble in water ,hence, care should be taken when putting this oil through a purifier.
During the interval of approach , the direction of sliding on the contact surfaces is toward the pitch line on the driven gear and away from pitch line on this pinion. At the pitch line the direction of slide reverses , so that during the interval of recession it is still toward the pitch line on the driven gear and still away from the pitch line on the pinion.. Thus , when surfaces scuff, weld and flow under pressure , the direction of slide always tend to wipe the metal on the metal on the driven teeth towards the pitch line and away from the pitch line on the pinion teeth.

May spread to whole tooth , a feather being formed over the tooth tip . If occurs at one end of the tooth this can indicated a misalignment.
Poor surface finish and overloading are the prime causes normally found in softer materials of the wheel.
Use of scuff resistant materials , better surface finish , chemical cleaning and thermo chemical treatments can help, as can surface coatings
Light honing plus more attention to oil viscosity and tempo may help.
Experimental evidence show that scuffing or scoring resistance is raised by increasing the pressure angle, increasing tooth depth or when possible increasing the helix angle and providing tip relief. Scuff resistant metal combinations may be used, better surface finish, chemical and thermo-chemical treatments, surface coatings can all help to increase scoring resistance

Incipient scuffing

If the surface finish is poor , contact between the asperities can be made through the almost rigid ( i.e. very high viscosity caused by the very high pressures ) oil film, generating heat causing the tempo of the gears to rise, reducing inlet oil viscosity and reducing oil film thickness . Some materials when supplied with the correct lubricant quickly polish out incipient scuffing . With harder gears this process takes longer . The risk is high due during the running in period but minimised by chemically active EP oils or with a surface treatment such as phosphating

SCORING

Should a ferrous particle enter the mesh it can be embedded in a tooth .On mating the particle is heated up by welding, fails in the heat effected zone , and quenches in the copious supply of oil. Some of the oil is carburised , absorbed into the particle , which is now very hard and becomes embedded in a tooth forming a spike. This then gouges a score in the teeth as they mesh until it becomes polished out . If the mark is on the pitch line then a point will form rather than a gouge

ABRASION/ (SCRATCHING )

Caused by foreign abrasive materials entering the mesh May appear as a score from root to tip caused by hard projection on one or more teeth penetrating oil film- this can be referred to as scoring or ridging or may appear as random scratches caused by dirt in the oil.
Another form leads to a highly polished surface and is caused by very fine particles or dust in the oil. The only remedy is careful filtration and honing of bad grooves. Cleanliness is most important. Very heavy abrasion can lead to change in tooth profile.

PLASTIC FLOW

Due to plastic , cold working of the metal which flows ahead of the pressure area building up a wave of metal until by work hardening the ripple resists the flow.
Immediately after this a further wave forms. in extremes, a line of pits form on the crest caused by the subsurface shearing rather than the compressive stress.
Main factors are unsuitable material , overloading and misalignement .
Hypoid and spiral gears are particularly susceptible to this . This type of failure occurs particularly following partial lube oil failure. Plastic flow rather than scuffing occurs. When slight , the rippling effect maybe advantageous acting as oil reservoirs.

Fish scaling

As the flow increases in severity, then the tooth profile alters to a similar condition to that seen due to scuffing.

GALLING.

Very heavy teeth damage due to various reasons, requires new teeth.

BREAKAGE

Has four main causes;
1. overload
2. defective material
3. faulty workmanship.
4. fatigue

Also may be caused by foreign material falling in to the mesh. Checks for cracks should be carried out at regular intervals especially following overload. Checks can be carried out using dye penetrant on magnetic indicators.

CORROSION

The supply of dehumidified air to the crankcase is carried out to prevent corrosion. Corrosion products can lead to the rapid deterioration of the lube oil and lead to sludge formation .
Regular checks should be carried out to ascertain the efficiency of the dehumidifier.

INTERFERENCE WEAR.

This occurs where teeth become too closely engaged .This can occur when fitting new bearings which are incorrectly bored.

PROBLEMS FOUND DURING NORMAL USE.

In normal use, a contact area becomes polished .A wear ridge may form which may move as the bearings become worn, if new bearings are fitted then the position of the ridge will move. Problems may the occur of the teeth slipping off the ridges leading to noise and possibly removing thin shards of metal. Plants running at reduced load, hence reduced tooth bending moment wear in a certain area . Should the plant then be run at full load it may be found that due to the wear the tip relief is now insufficient.

MISALIGNEMENT- Causes problems of overloading at the ends of teeth.
EXCESSIVE BRG CLEARANCE-Pinion moves out of mesh to far, hence load taken on ends of teeth giving excessive bending forces.
INCORRECT TOOTH PROFILE- Pitting and noise PARTIAL SEIZURE OF ROTOR FLEXIBLE COUPLING- Gearing loads transferred to turbine possibly causing to turbine.
If the axial clearance is completely taken up then the main thrust ( as the pinion is locked by the double helical arrangement ) WORM GEAR FAILURES Pitch line pitting as described for spur tooth gearing does not occur in worm gearing. Due to the greater slide in this type of gear than in spur or helical gears, the tendency is for the tooth surface in equalities to be worn away before metal fatigue occurs . Tooth surface failure by abrasion or scoring can occur exactly as in other type gears, but the most commonly encountered worm gear failures are the flaking or scuffing types. Heavy loads on worm gears may subject relatively large areas of tooth profile to stresses sufficient in severity to cause sub surface fatigue and eventual flaking of relatively large pieces of bronze. Flaking on worm wheel gears is frequently heaviest on the ends of the tooth leaving the mesh. Spot tempo in this areas are higher than elsewhere and it is probable that the fatigue resistance of the bronze is lowered as a result . In some cases localized flaking may be due to misalignment. Lubrication failures do occur on worm gears either due to unsuitable oils , incorrect or excessive loading. Scuffing is the usual type of failure and this is influenced by the grain structure of the bronze .

HYPOID GEAR FAILURES

Where hypoid gears are involved , surface failures on teeth may take several different forms . These steel to steel gears are generally so heavily loaded , especially in automotive equipment, that although they are flood lubricated , boundary lubrication is the usual condition. Metal to metal contact is therefore , unavoidable. With the correct lubricating oil in use , the degree of metallic contact and the generation of frictional heat between the rubbing surfaces is minimized . Irrespective of speed, such conditions result in smooth , dull polished or brightly burnished tooth surfaces with negligible wear. When hypoid gears show evidence of unsatisfactory lubrication , the surfaces may have the appearance of being either rippled , ridged , flaked , pitted or scored. The particular surface appearance that developed depends on the type and severity of operating conditions i.e. on the speed of rubbing and the magnitude of loading carried by the working surfaces. Furthermore it depends on the lubricity, film strength and anti weld characteristics of the lubricant.
The working surfaces of hypoid gear teeth sometimes develop fine ripples ( fish scale appearance ) . When this happens , the ripples have the appearance of being formed by the metallic flow which builds up a wave of metal ahead of the pressure area, The appearance of the surface indicates that each wave quickly becomes sufficiently work-hardened to resist further flow , whereupon contact then moves over the hard surface to repeat the process immediately beyond. This results in the formation of small wave-like ripples of work-hardened metal at right angles to the diagonal lines of slide , it seems to occur only at comparatively slow speeds.
As rippling progresses, the continued cold-working of the metal causes sub-surface fatigue cracks to develop, with the result that thin flakes of metal ultimately break loose from the surfaces and drop off. This flaking (spalling) action is more pronounced on the tooth surfaces of the pinion due to the smaller number of teeth among which the load is distributed.
A tooth sometimes appears to have a smooth and very highly polished or burnished surface. Under a microscope , however , the smooth surface may take on a very finely ridged appearance with innumerable short, parallel ridges extending diagonally across the working surface of the tooth i.e. in the direction of the slide. Each ridge appears to be made up of many short ridges added approximately end to end. They do not have the appearance of typical scratches or score marks. The hills and valleys have a smoothly rounded outline.
Either a rippled surface or a burnished surface may develop a ridged appearance with ridges of such size that they can be seen and felt .There is no evidence of gouging or tearing but due to the size of the ridges considerable cold working has occurred. Continuation of this cold working leads to the fatigue point being passed . When this this happens , small cracks develop and minute particles of overstressed metal break loose and drop off., leaving fine pits along the crests of the ridges . This pitting may occur over the entire working surface of a hypoid gear tooth . It , therefore , should not be confused with pitting of spur or helical gear teeth which is due to entirely different causes and occurs only near the pitch line. Continued and extensive pitting eventually results in the removal of considerable areas of metal from the tooth surfaces and extensive flaking.
On hypoid gear teeth , scoring results when particles of metal are displaced or transferred from teeth of one gear to the teeth of the mating gear. It may also be referred to as scuffing or galling, particularly in the advanced stages. Scoring is the final result of a combination of factors i.e. High rubbing . speeds and loads , low film strength and insufficient anti-weld character of the oil. When film strength is lacking, considerable metal-to-metal contact will occur, and if the rubbing speed is high enough , frictional heat at microscopic points of contact will create local welding tempo .If the oil lacks anti-weld ( E.P.) character scoring results.

Normal wear

When gears are of the proper design , construction and hardness, do not operate at excessive loads and are correctly lubricated, a condition of normal wear result. Normal wear over a long period and under conditions of flood lubrication gradually smoothes rubbing surfaces of the teeth and work-hardens them to a polish. As the surfaces become smoother and more work hardened , friction and wear decreases until a condition may be reached where further wear practically ceases. There may be signs of long use , but the metal is peened , rolled and polished to a smooth hard surface. Correct boundary lubrication on hypoid gears results in a smooth , dull matted gear-tooth surface and relatively little wear.

CALCULATION OF MAX TOOTH LOADING.

Severity of tooth contact ; is generally expressed in terms of: Tangential load/face width
Alternately a 'K' factor based on the Hertzian stresses is used
K= W/Fod x mg + 1/mg
W= tangential load on gear teeth ( lb )
(pinion torque/pitch radius of pinion)

Fo = face width of teeth (inches)
d = pitch diameter of pinion ( inches )
mg = gear ratio; pinion speed / wheel speed

In the 1950's 'K' factors for turbine reduction gears were about 35 to 80 for unhardened alloy pinions on carbon steel wheels. With improved materials, heat treatment and manufacture; hobbed, shaved gears can have 'K' factors of:

150 max. for primary reduction ( through hardened )
130 max. for secondary reduction

For hard/soft combinations 240-210 respectively

Certain high performance naval vessels have a 'K' factors of 300

The most common failure is when a tooth or part of a tooth breaks off due to fatigue
Impact failure is rare and generally due to negligence.