Gears:- Wear and Failure

Gears:- Wear and Failure

      Gear failures can be traced to the following underlying causes 

Lubricant-related failures due to contamination, oil film collapse, additive depletion, and use of improper lubricant for the application. The most common failures are due to particle contamination of the lubricant. Dust particles are highly abrasive and can penetrate through the oil film, causing “plowing” wear or ridging on metal surfaces.

Water contamination can cause rust on working surfaces and eventually destroy metal integrity.

To prevent premature failure, gear selection requires careful consideration of the following:

Design related

- Gear tooth geometry,

 -Tooth action,

 -Tooth pressures,

- Construction materials and surface characteristics,

Application related

- lubricant characteristics.

- Operating environment. 

a. Wear.

(1) Adhesive wear.

       New gears contain surface imperfections or roughness that is inherent to the manufacturing process. During the initial run-in period, these imperfections are reduced through wear. Smoothing of the gear surfaces is to be expected. Mild wear will occur even when adequate lubrication is provided, but this wear is limited to the oxide layer of the gear teeth. Mild wear is beneficial because it increases the contact areas and equalizes the load pressures on gear tooth surfaces. Furthermore, the smooth gear surfaces increase the film thickness and improve lubrication.

    The amount of wear that is acceptable depends on the expected life, noise, and vibration of the gear units. Excessive wear is characterized by loss of tooth profile, which results in high loading, and loss of tooth thickness, which may cause bending fatigue , and subsequently tooth damage.

Wear cannot be completely eliminated. Speed, lubricant viscosity, and temperature impose practical limits on gear operating conditions. Gears that are highly loaded, operate at slow speeds, i.e., less than 30 m/min, and rely on boundary lubrication are particularly subject to excessive wear.

Slow-speed adhesive wear is highly dependent upon lubricant viscosity. Higher lubricant viscosities provide significant wear reduction, but viscosities must be carefully selected to prevent overheating

The following guidelines should be observed to minimize the onset of adhesive wear in gear units:

-Gear teeth should have smooth surfaces.

- The run-in period for new gear units should be restricted to half load for the first few hours of Operation , if practicable.

 -The highest speeds possible to be used. High-load, slow-speed gears are boundary  lubricated and are especially prone to excessive wear. For these applications, nitrided   gears should be specified.

-  Lubricants with sulfur-phosphorus additives for very slow-speed gears (less than

   3m/min,).

- The required quantity of cool, clean, and dry lubricant at the highest viscosity permissible.

(2) Abrasive wear . Abrasive wear  is caused by particle contaminants in the lubricant. Particles may originate internally due to poor quality control during the manufacturing process. Particles also may be introduced from the outside during servicing or through inadequate filters, breathers, or seals. Internally generated particles are particularly destructive because they may become work-hardened during compression between the gear teeth

The following guidelines should be observed to prevent abrasive wear in gear units:

-Remove internal contamination from new gearboxes. Drain and flush the lubricant before initial start-up and again after 50 hours of operation. Refill with the manufacturer’s recommended lubricant. Install new filters or breathers.

- Usage of surface-hardened gear teeth, smooth tooth surfaces, and high-viscosity lubricants.

-Maintaining oil-tight seals and use filtered breather vents, preferably located in clean, no  pressurized areas.

-Good housekeeping procedures.

- Fine filtration for circulating-oil systems. Filtration to 3 μm has proven effective in  

  Prolonging gear life.

-Replacement of oil and frequency to be increased.

- Laboratory analysis of lubricants. Analysis may include spectrographic, Ferro graphic, acid number, viscosity, and water content.

-  Removal of abrasives from the lubricant by using fine filtration or by frequent oil changes

(3) Polishing wear . Polishing wear is characterized by a mirror-like finish of the gear teeth. Polishing is caused by ant scuff additives that are too chemically reactive. An excessive reaction rate, coupled with continuous removal of surface films by very fine abrasive particles in the lubricant, may result in excessive polishing wear.

The following guidelines should be observed to prevent polishing wear in gear sets:

-Minimizing of chemically active anti-scuff additives such as borate.

(4) Corrosive wear :This is the wear occurring due to chemical action on account of water admixture in gear case oils , resulting in breakdown of the base stock in oil , thus resulting in a chemical attack , which is by a combination of oxidation , as well as additive stratification – resulting in abrasive wear with pitting.

(5)  Cold working during initial operating periods and  loading conditions results in another type pf work hardening on the  surface. This continues as long as the conditions exist.In case of hardened gears , cold working takes place beneath the surface leading to ripple formation(commonly referred to as cold working / rippling/ ridging ) . These ripples as asperities , entrain oil , thus prevent further deterioration, but cyclic non-uniform loads and fatigue lead to sub terrabean cracking , which is referred to as Case crushing (illustrated under  fatigue failures)

Cold working

Rippling

Ridging

b..Fatigue failure

(1) Pitting. Pitting occurs when fatigue cracks are initiated on the tooth surface or just below the surface. Pits are the result of surface cracks caused by metal-to-metal contact of asperities or defects due to low lubricant film thickness. High-speed gears with smooth surfaces and good film thickness may experience pitting due to subsurface cracks.

These cracks may start at inclusions in the gear materials, which act as stress concentrators, and propagate below and parallel to the tooth surface. Pits are formed when these cracks break through the tooth surface and cause material separation, which is further aided by hydrodynamic pressure of oil being forced into the space conbined with cyclic stresses(hertzian stresses)  generated by operational conditions.

When several pits join, a larger pit (or spall) is formed , detaching metal in pieces.

Another suspected cause of pitting is hydrogen embrittlement of metal due to water contamination of the lubricant.

Pitting can also be caused by foreign particle contamination of lubricant. These particles create surface stress concentration points that reduce lubricant film thickness and promote pitting.

Evidence suggests that pitting occurs only where there is a low ratio of slide to roll. Worm and most hypoid gears, excessive side slide tends to wear away high spots before true pitting would occur.

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- sometimes referred to as beam bending fatigue failure.

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.

Case crushing is one more pitting failure occurring in hardened gears- characterized by severe longitudinal cracks, originating below the hardened surface and propagating upwards

The following guidelines should be observed to minimize the onset of pitting in gear units:

- Reduce contact stresses through load reduction or by optimizing gear geometry.

-Steel should be properly heat-treated to high hardness. Carburizing is preferable.

-Gear teeth should have smooth surfaces produced by grinding or honing.

-Usage of  proper quantities of cool, clean, and dry lubricant with the required viscosity.

(2) Micro pitting. Micro pitting occurs on surface-hardened gears and is characterized by extremely small pits approximately 10 μm  deep. Micro pitted metal has a frosted or a gray appearance. This condition generally appears on rough surfaces and is exacerbated by use of low-viscosity lubricants.

Slow-speed gears are also prone to micro pitting due to thin lubricant films.

 Micro pitting may be sporadic and may stop when good lubrication conditions are restored following run-in. Maintaining inadequate lubricant film thickness is the most important factor influencing the formation of micro pitting.

Higher-speed operation and smooth gear tooth surfaces also hinder formation of micro pitting.

The following guidelines should be observed to reduce the onset of micro pitting in gear units:

- Usage of gears with smooth tooth surfaces produced by careful grinding or honing.

- Using of the correct amount of cool, clean, and dry lubricant with the highest viscosity permissible for the application

- Usage of high speeds as permitted and if possible.

- Usage of carburized steel with proper carbon content in the surface layers, with proper surface treatment.

c. Scuffing and Scoring.

Scuffing is defined as localized damage caused by the occurrence of solid-phase welding between sliding surfaces.

Scoring is defined as the formation of severe scratches in the direction of sliding.

Scoring may be caused by local solid-phase welding or abrasion, but suggests that minor scoring be considered as scratching.

Gear scuffing is characterized by material transfer between sliding tooth surfaces. Generally this condition occurs when inadequate lubrication film thickness permits metal to-metal contact between gear teeth. Absence of lubrication, coupled with direct metal contact removes the protective oxide layer on the gear metal, leading to excessive heat generated by friction welds the surfaces at the contact points. As the gears separate, metal is torn and transferred between the teeth.

Scuffing is most likely to occur in new gear sets during the running-in period because the gear teeth have insufficient operating time to develop smooth surfaces.

               Scoring                                 

            Early stages of scoring                                                         – commonly referred to as Frosting

                                                                              

Localized scuffing – leading to scoring

Critical scuffing temperature.

     Mineral oil without anti- scuffing or extreme pressure additives, there is a critical scuffing temperature that is constant regardless of operating conditions. Evidence indicates that beyond the critical temperature, scuffing will occur. Therefore, the critical temperature concept provides a useful method for predicting the onset of scuffing.

The critical scuffing temperature is a function of the gear bulk temperature and the flash temperature and is expressed as:

Tc = Tb + Tf (9-1)

Bulk temperature Tb is the equilibrium temperature of the gears before meshing and the flash temperature

Tf is the instantaneous temperature rise caused by the local frictional heat at the gear teeth meshing point.

The critical scuffing temperature for mineral oils without anti-scuffing or extreme pressure additives increases directly with viscosity and varies from 150 to 300 ºC .

Scuffing resistance can be primarily attributed to  to differences in chemical composition and only indirectly to the beneficial effects of increased film thickness associated with higher viscosity.

Critical temperature equation indicates that scuffing can be controlled by lowering either of the two contributing factors.

The bulk temperature can be controlled by selecting gear geometry and design for the intended application.

The flash temperature can be controlled indirectly by gear tooth smoothness and through lubricant viscosity. Smooth gear tooth surfaces produce less friction and heat while increased viscosity provides greater film thickness, which also reduces frictional heat and results in lower flash temperature

 (b) For synthetics and lubricants containing anti-scuff additives, the critical temperature depends on the

operating conditions and must be determined experimentally for each case.

Anti-scuff additives commonly used are iron sulfide and iron phosphate. These additives react chemically with the protected metal gear surface to form very strong solid films that prevent metal contact under extreme pressure and temperature conditions.

The beneficial effects of extreme pressure additives are enhanced as the temperature increases.

 The following guidelines should be observed to prevent scuffing in gear units:

- Protection of  gear teeth during the running-in period by coating them with iron-manganese phosphate   or plating them with copper or silver. During the first few hours of run-in, new gears should be operated at half load , as practically permitted.

-Usage of high-viscosity lubricants with anti-scuff additives such as sulfur, phosphorus, or borate.

- Cooling design for gear tooth

- Proper optimized gear profile selection for the application with proper mountings.

-Alignment issues with bearings and meshing also plays an important role in wear and tear of gears , by virtue of overloading .

Note: EP additives are very soluble in water ,hence, care should be taken when putting these oils through purifiers.

 Random Failures

Thease type of failures are associated with working on account of various material grades associated with gear manufacture. Though the material specification is standardized, the process occurs at various facilities. Quality controls and testing , though random , there is always a leeway for failures not associated with running. These are seldom but occur during the operating life of the machinery.