Adhesive wear is classified as mild if it is confined to oxide layers on gear tooth surfaces. However, if oxide layers are disrupted exposing bare metal, a transition from mild wear to severe adhesive wear usually occurs. Severe adhesive wear is termed scuffing. Scuffing has been avoided through proper gear design, lubricant selection, and control of the running-in process.
When new gear units are first put into operation, contact between gear teeth is not optimal because of unavoidable manufacturing inaccuracies. If tribological conditions are favorable, mild adhesive wear occurs during running-in and usually subsides with time, resulting in satisfactory gear life. Wear that occurs during running-in is beneficial if it smoothes tooth surfaces (which increases the specific film thickness) and increases the contact area by removing minor imperfections through local wear. To ensure that the wear rate remains under control, new gearsets should be run-in by operating under a controlled load spectrum, or for at least the first ten hours at one-half load.
The amount of wear considered tolerable depends on expected gear life and requirements for control of noise and vibration. Wear is considered excessive when tooth profiles wear to the extent that high dynamic loads occur, or tooth thickness is reduced to the extent that bending fatigue becomes possible.
Many gears must operate under boundary lubrication conditions where some wear is inevitable due to practical limits of lubricant viscosity, speed, and temperature. Highly loaded, slow-speed (<0.5 m/s), boundary lubricated gears are especially prone to excessive wear.
Nitrided gears have good wear resistance, whereas carburized and through-hardened gears have similar, but lower wear resistance.
The lubricant viscosity has the greatest effect on slow-speed adhesive wear, the high-viscosity lubricants reduce the wear rate significantly, and the sulfur-phosphorus anti-scuff additives are detrimental with very slow-speed (<0.05 m/s) gears, resulting in very high wear rates.
Some gear units operate under ideal conditions with smooth tooth surfaces, high pitch line speed, and thick lubricant films. However, most gears operate between boundary and full-film lubrication regimes, under elasto-hydrodynamic (EHL) conditions. In the EHL regime, with the proper lubricant type and viscosity, the wear rate usually decreases during running-in, and adhesive wear virtually ceases after running-in is completed. The gearset should not suffer an adhesive wear failure with properly maintained (cool, clean, and dry) lubricant.
Methods to Prevent Adhesive Wear:
Use smooth tooth surfaces.
Run-in new gearsets by operating under a controlled load spectrum, or for at least the first ten hours at one-half load.
Drain and flush the lubricant after the first 50 h of operation to remove wear debris from running-in, refill with filtered service lubricant, and install a new filter element.
Use high speeds, otherwise, recognize that highly loaded, slow-speed gears are boundary lubricated and are especially prone to excessive wear; specify nitrided gears and the highest permissible lubricant viscosity for these conditions.
Avoid using lubricants with sulfur-phosphorus anti-scuff additives for very slow-speed gears (<0.05 m/s).
Use an adequate amount of cool, clean, and dry lubricant of the highest viscosity permissible.
Improve cooling to lower gear mesh temperature.
Abrasive Wear
Abrasive wear on gear teeth is usually caused by contamination of the lubricant by hard, sharp-edged particles. Many gear manufacturers do not fully appreciate the significance of clean assembly; it is not uncommon to find sand, machining chips, grinding dust, weld splatter, and other debris. Contamination in gearboxes are built-in, internally generated, ingested through breathers and seals, and inadvertently added during maintenance.
To remove built-in contamination, the gearbox lubricant should be drained and flushed before start-up and again after the first 50 h of operation, refilled with the recommended lubricant, and a new oil-filter element installed.
Internally generated particles are usually wear debris from gears and bearings due to Hertzian fatigue pitting, adhesive wear, and abrasive wear. Wear particles are especially abrasive because they become work-hardened when trapped between gear teeth. Internally generated wear debris can be minimized by using precise surface-hardened gear teeth (with high pitting resistance), smooth surfaces, and high-viscosity lubricants.
Breather vents are used on gearboxes to vent internal pressure, which can occur when air enters through seals and when air within the gearbox expands (or contracts) during the normal heating and cooling of the gear unit. The breather vent should be located in a clean, non-pressurized area, and should have a filter to prevent ingress of airborne contaminants. In especially harsh environments, the gearbox can be completely sealed, and the pressure variation can be accommodated by an expansion chamber with a flexible diaphragm.
All maintenance procedures that involve opening any part of the gearbox or lubrication system must be carefully performed to prevent contamination of the gearbox system.
The lubrication system should be carefully maintained and monitored to ensure that the gears receive an adequate amount of cool, clean, and dry lubricant.
For circulating-oil systems, fine filtration removes contamination. Filters as fine as 3 μm significantly increase gear life. A 10 μm online filter and a 3 μm off-line filter are standard equipment for wind-turbine gearboxes.
For oil-bath gearboxes, the lubricant should be changed frequently to remove contamination. Under normal operating conditions, the lubricant should be changed at least every 2500 h of operation, or every six months, whichever occurs first. Alternatively, an off-line filter can be used to filter the oil bath.
For critical gearboxes, a regular program of lubricant monitoring can help prevent gear failures by indicating when maintenance is required. Lubricant monitoring should include spectrographic and ferro-graphic analysis of contamination, along with analysis of acid number, viscosity, and water content.
Methods to Prevent Abrasive Wear:
Remove built-in contamination from new gearboxes by draining and flushing the lubricant before start-up and again after the first 50 h of operation, refill with filtered service lubricant, and install a new filter element.
Minimize internally generated wear debris by using surface-hardened gear teeth, smooth tooth surfaces, and high-viscosity lubricants.
Minimize ingested contamination by maintaining oil-tight seals and using filtered breather vents located in clean, non-pressurized areas.
Perform all maintenance that involves opening the gearbox or lubrication system in a clean environment (if possible) and use good housekeeping procedures to avoid contaminating the gearbox.
Use fine filtration for circulating-oil systems, change or process lubricant to remove water, and include an off-line filter to remove small particles.
Change the lubricant at least every 2500 h or every six months, whichever occurs first, for oil-bath systems, or use an off-line filter to filter the oil bath.
Monitor the lubricant with spectrographic and ferro-graphic analysis together with analysis of acid number, viscosity, and water content.
Polishing Wear
Chemically reactive anti-scuff additives in the lubricant can cause polishing of gear tooth surfaces until they attain a bright mirror finish. Although polished gear teeth may look good, polishing wear is undesirable because it generally reduces gear accuracy by wearing tooth profiles away from their ideal form.
Anti-scuff additives, such as sulfur and phosphorus, are used in lubricants to prevent scuffing. They function by forming iron-sulfide and iron-phosphate films on areas of the gear teeth where high temperatures occur. Ideally, the additives should react only at temperatures where there is a danger of welding. If the rate of reaction is too high, and there is a continuous removal of the surface films caused by a very fine abrasives in the lubricant, polishing wear can be excessive.
Methods to Prevent Polishing Wear:
Use less chemically active anti-scuff additives (e.g. borate).
Remove abrasives from the lubricant by using fine filtration and frequent oil changes.
Fretting Wear
Fretting is localized wear of contacting gear, spline, and bearing surfaces caused by minute vibratory motion. It occurs between contacting surfaces that are pressed together and subjected to cyclic, relative motion of extremely small amplitude. Under these conditions, lubricant squeezes from between the surfaces, and motion of the surfaces is too small to replenish the lubricant. Natural oxide films that typically protect surfaces are disrupted, permitting metal-to-metal contact and causing adhesion of surface asperities.
Fretting commonly occurs in joints that are bolted, keyed, and press-fitted, and in splines and couplings. It could occur on gear teeth and bearing raceways and rollers under specific conditions where the gears and bearings are not rotating and subjected to structure-borne vibrations such as those encountered during transport and in parked wind turbines.
There are two mechanisms of fretting wear:
False Brinelling:
True brinelling is a separate failure mode unrelated to false brinelling, but is discussed here to contrast it with false brinelling. True brinelling occurs in contacts subjected to Hertzian stress high enough to cause permanent plastic deformation of the contacting surfaces. It is characterized by permanent deformation (without loss of material or change of surface texture), which can occur during a single load event. For example, true brinelling of a rolling element bearing frequently occurs when the bearing is not rotating and subjected to an impact load great enough to plastically deform the raceway. Dents in the raceway occur at roller spacing, have raised shoulders, and the original grinding marks are visible microscopically in the bottoms of the dents.
False Brinelling. Fretting begins with an incubation period during which the wear mechanism is mild adhesion confined to the natural oxide layer that covers steel. Wear debris is iron oxide magnetite (Fe3O4), a highly magnetic black powder. Magnetite discolors the lubricant surrounding the contact and forms a black, greasy film. Damage during the incubation period is false brinelling, and it has distinctly different morphology than true brinelling. False brinelling is characterized by dents that do not have raised shoulders. Furthermore, original machining marks within the dents are worn away by mild adhesive wear. Dents are created by the wearing off of pre-existing and continually reforming oxide films. Generally, the wear rate is low, and the damage caused by false brinelling is negligible. False brinelling occurs on gear teeth and bearing components when they are not rotating but oscillating through extremely small angles.
Fretting Corrosion:
Wear debris from false brinelling accumulates in the oil meniscus surrounding the contact. If the amount is sufficient to dam lubricant and prevent it from reaching the contact, the lubricating regime changes from boundary lubrication to unlubricated. When the lubricant within the contact is depleted by oxidation, the wear rate increases dramatically until breaking through the natural oxide layer, at which time strong welds form between the asperities of the parent iron components, and damage escalates to fretting corrosion. Relative motion breaks strongly welded asperities and generates extremely small wear particles, which oxidize to form iron-oxide hematite (α-Fe2O3), a fine nonmagnetic powder with a reddish-brown color of cocoa. Relative motion breaks strongly welded asperities and generates extremely small wear particles, which oxidize to form iron-oxide hematite (α-Fe2O3), a fine nonmagnetic powder with a reddish-brown color of cocoa.
The wear debris is hard and abrasive with the same composition as jeweler’s rouge, and polishing wear (fine scale abrasion) is frequently found around the periphery of a fretting corrosion scar. Hematite discolors the lubricant surrounding the contact and forms rouge-colored paste. Usually, the wear scar is discolored with black or reddish films. Fretting corrosion damages gear and bearing surfaces by forming ruts along lines of contact. During operation, damaged gears and bearings could generate a sharp, hammering noise as the wear scars pass through the Hertzian contacts. Pits from fretting corrosion create local stress concentrations, leading to macro-pitting and fatigue crack initiation, which, if in high tensile stress areas, could propagate to failure. Generally, fretting corrosion significantly reduces fatigue strength. If bearing fits are inadequate to stop relative motion between the inner ring and shaft, and between the outer ring and housing, fretting corrosion could develop at these interfaces
Methods to Prevent Fretting Wear:
Stop the vibration, rotate the components to entrain fresh oil, or both.
Ensure angular motion for reciprocating systems, such as yaw drives and actuators, is sufficient to wipe fresh lubricant into the contact.
Avoid parking wind turbines for extended periods.
Avoid dithering of wind turbine blades; vary pitch angle enough to entrain fresh oil and pitch blades frequently.
Ensure adequate interference fit between shafts and couplings, gears, bearing rings, and other interference-fit components.
Use case hardening (nitriding is best), carburizing, and physical vapor deposition (PVD) hard coatings to obtain adhesion resistant surfaces.
Use cold work, case hardening, and shot peening to induce compressive residual stresses.
Use lubricant with antiwear additives.
Use oil rather than grease, and use a high pressure jet to flood the contact and flush away wear debris.
Store the gearbox in a vibration-free environment.
Support the gearbox on vibration isolators.
Ship the gearbox with shafts locked to prevent dithering motion.
Ship the gearbox filled with oil.
Ship the gearbox on an air-ride truck.
Electrical Discharge
Gear teeth and rolling element bearings could be damaged if faulty insulation, induction effects, and improper grounding allows electric current to pass through the gears and bearings. Electrical discharge damage is caused by electric arc discharge across the oil film between the active flanks of mating gear teeth and between bearing rolling elements and raceways. Wind turbine-generator bearings are especially vulnerable to electrical discharge damage. Electric current can originate from many sources including:
Generators, especially wind-turbine generators.
Electric motors, especially variable-frequency drives (VFDs).
Electric clutches and instrumentation.
Accumulation of static charge and subsequent discharge.
During electric-resistance welding on, or near, the gearbox if the path to ground is not properly made around the gears rather than through them.
During lightning strikes on wind turbines.
An electric arc produces temperatures high enough to melt the contacting surfaces of gear teeth and bearings. Microscopically, the damage appears as small, hemispherical craters. Edges of the craters are smooth and might be surrounded by burned and fused metal in the form of rounded particles, which were once molten. A section taken transversely through the craters and acid etched could reveal austenitized and rehardened areas in white, bordered by tempered areas in black. Sometimes microcracks are found near the craters. Overall, damage to gear teeth and bearings is proportional to the number and size of points of arcing. Depending on its extent, electrical discharge damage can be destructive to gear teeth and bearings. Associated microcracking can lead to subsequent Hertzian fatigue and bending fatigue. If electrical discharge damage is found on gears, all associated bearings should be examined for similar damage. Frequently, electric discharge damage creates a periodic pattern on bearing raceways called “fluting,” which usually makes the bearing excessively noisy.
Methods to Prevent Fretting Wear:
Provide adequate electrical insulation.
Provide adequate electrical grounding.
Ensure proper welding procedures are enforced.
Scuffing Wear
Scuffing is a localized damage caused by solid-phase welding between sliding surfaces. It is accompanied by the transfer of metal from one surface to another due to welding and tearing. It can occur in any sliding and rolling contact where the oil film is not thick enough to separate the surfaces. The symptoms of scuffing are rough, matte, and torn surfaces. Surface analysis that shows transfer of metal from one surface to the other is proof of scuffing.
Scuffing can occur in gear teeth when they operate in the boundary-lubrication regime. If the lubricant film is insufficient to prevent significant metal-to-metal contact, the contact can break through oxide layers that normally protect gear tooth surfaces, and the bare metal surfaces can weld together. Sliding that occurs between gear teeth results in tearing of the welded junctions, metal transfer, and catastrophic damage. In contrast to Hertzian fatigue and bending fatigue, which only occur after a period of running time, scuffing can occur immediately upon start-up. New gears are most vulnerable to scuffing when their tooth surfaces have not yet been smoothed by running-in.
The basic mechanism of scuffing is not clearly understood, but is generally believed to be caused by intense frictional heating generated by the combination of high sliding velocity and intense surface pressure. Blok’s critical temperature theory is believed to be the best criterion to predict scuffing. It states that scuffing occurs in gear teeth sliding under boundary-lubrication conditions when the maximum contact temperature of the teeth reaches a critical magnitude.
For mineral oils without anti-scuff additives, each combination of oil and rubbing materials has a constant critical scuffing temperature, regardless of operating conditions. The critical scuffing temperature increases with increasing viscosity and ranges from 150 to 300 °C. The increased scuffing resistance of high-viscosity lubricants is believed to be due to differences in chemical composition rather than increases in viscosity. However, a viscosity increase also helps to reduce the risk of scuffing by increasing the lubricant EHL film thickness and reducing the contact temperature generated by metal-to-metal contact.
Critical scuffing temperatures are not constant for synthetic lubricants and lubricants with anti-scuff additives; they must be determined from tests that closely simulate the operating conditions of the gears and with in-situ tests on actual gears. Anti-wear additives such TCP and ZnDTP might be adequate for high-speed, lightly loaded gears that are not subjected to shock loads, whereas slows peed, highly loaded gears subjected to shock loads might require anti-scuff additives such as those containing sulfur and phosphorus, alone or in combination.
Methods to Prevent Scuffing Wear:
Use smooth tooth surfaces produced by grinding, honing, and polishing.
Run-in new gearsets using a series of increasing loads and appropriate speed; as a minimum, run-in new gearsets by operating the first ten hours at one-half load.
Protect gear teeth during the critical running-in period by coating them with iron manganese phosphate or plating them with copper or silver.
Use lubricants with adequate viscosity for the operating conditions.
Use lubricants with anti-scuff additives, such as sulfur, phosphorus, and borate.
Cool the gear teeth by supplying an adequate amount of cool lubricant; use a heat exchanger to cool the lubricant for circulating-oil systems.
Optimize gear tooth geometry by using small teeth, profile shift, and profile modification.
Use accurate gear teeth, rigid gear mountings, good helix alignment, and lead modification to obtain uniform load distribution during operation.
Avoid stainless steel and aluminum and titanium alloys for gears, because they greatly increase the risk of scuffing.
Use nitrided steels for maximum scuffing resistance.
Hertzian Fatigue
There are two lubrication-related gear tooth failure modes caused by Hertzian fatigue are:
Macro-pitting
It is a common failure mode for gear teeth, because they are subjected to high Hertzian stresses and many stress cycles. For example, through-hardened gears are typically designed to withstand Hertzian stresses up to 1.0 GPa, whereas carburized gears are typically designed to withstand Hertzian stresses up to 1.9 GPa. In addition, a given tooth on a pinion revolving at 3600 rpm accumulates over 5 x 106 stress cycles every 24 h.
Macro-pitting is a fatigue phenomenon that occurs when a fatigue crack initiates either at the surface of the gear tooth or at a small depth below the surface. The crack usually propagates for a short distance in a direction roughly parallel to the tooth surface before turning or branching to the surface. A macro-pit forms when a crack grows to the extent that it separates a piece of surface material. If several macro-pits grow together, the resulting larger macro-pit is referred to as a “spall”. There is no endurance limit for Hertzian fatigue, and macro-pitting occurs even at low stresses if gears operate long enough. Because there is no endurance limit, gear teeth must be designed for a suitable finite lifetime.
Hertzian stress must be kept low and material strength and lubricant specific film thickness high to extend macro-pitting life of a gearset. Several geometric variables, including diameter, face width, number of teeth, pressure angle, and helix angle can be optimized to lower the Hertzian stress.
Material and heat treatment are selected to obtain hard tooth surfaces with high strength. Maximum macro-pitting resistance is obtained with carburized gear teeth, because they have hard surfaces, and carburizing induces beneficial compressive residual stresses that effectively lower the load stresses. Drawbacks with carburized gear teeth are that they require strict manufacturing process control and they must be finished by grinding to achieve high accuracy.
Methods to Prevent Macro-pitting:
Reduce Hertzian stresses by reducing loads and optimizing gear geometry.
Use clean steel, properly heat treated to 58 HRC minimum hardness, preferably by carburizing.
Use smooth tooth surfaces produced by grinding, honing, and polishing.
Use an adequate amount of cool, clean, and dry lubricant of adequate viscosity.
Ensure adequate surface hardness and case depth after final processing for surface-hardened steels.
Micro-pitting
On relatively soft gear tooth surfaces, such as those of through-hardened gears, Hertzian fatigue forms large pits with dimensions on the order of millimeters. With surface-hardened gears (for example, carburized, nitrided, and induction and flame hardened), pitting can occur on a much smaller scale, typically only 10 mm deep. To the naked eye, areas where micro-pitting occurred appear frosted, and “frosting” is a popular term for micro-pitting. Japanese researchers refer to the failure mode as “gray staining,” because the light-scattering properties of micro-pitting give the gear teeth a gray appearance. Scanning electron microscopy (SEM) shows that micro-pitting proceeds by the same fatigue process as classical macro-pitting, except the pits are extremely small.
In many cases, micro-pitting does not cause catastrophic failure; it might occur only in patches and can arrest after the tribological conditions improve after running-in. Mild polishing wear removes micro-pits and smooths tooth surfaces. However, arrest is unpredictable, and micro-pitting generally reduces gear tooth accuracy, increases noise, and can escalate to full scale macro-pitting and other failure modes such as scuffing and bending fatigue.
Methods to Prevent Macro-pitting:
Increase oil film thickness by:
Using the highest practical oil viscosity.
Running gears at high speed if possible.
Cooling gear teeth.
Using synthetic oil if gear tooth temperature is >80 °C.
Reduce surface roughness by:
Avoiding shot-peened flanks unless they are polished after shot peening.
Honing and polishing gear teeth, and running gears against a hard, smooth master.
Making the hardest gear as smooth as possible.
Coating teeth with iron-manganese phosphate, Cu, and Ag to limit adhesion and scuffing risk.
Running-in with a special lubricant without ZnDTP anti-wear additives.
Prefiltering lubricant and using a fine filter (≤6 mm) during run-in.
Keeping oil cool during run-in.
Running-in gears using a series of increasing loads and appropriate speed.
Draining lubricant and flushing gearbox after run-in, changing the filter element, and filling with filtered service lubricant.
Optimize gear geometry by:
Using at least 20 teeth in the pinion for parallel-axis gears.
Using a non-hunting gear ratio.
Using helical gears with axial contact ratio mF ≥2.0.
Using aspect ratio (face width to diameter ratio), ma, ≤1.0 for spur and single-helical gears.
Using aspect ratio (face width to diameter ratio), ma, ≤2.0 for double-helical gears.
Minimizing Hertzian stress by specifying high accuracy and optimizing center distance, face width, pressure angle, and helix angle.
Using profile shift to minimize specific sliding.
Using proper profile and lead modification.
Avoiding tip-to-root interference
Optimize metallurgy by:
Maximizing pinion hardness.
Making pinion 2 HRC points harder than the wheel.
Using ≈ 20% retained austenite.
Optimize lubricant properties by:
Using oil with high micro-pitting resistance as determined by tests on actual gears.
Using oil with low traction coefficient.
Using oil with high pressure-viscosity coefficient.
Avoiding oils with aggressive anti-scuff additives.
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