When a small reducer fails, in most plants the usual reaction is to replace it without even opening it up to see what happened. But, when a large or critical unit is involved, the inspection and evaluation can be incredibly intimidating.
Where should the inspection start? Is the foundation and grouting important? Are those pits on the teeth something to worry about—or is the gear good for another 10 years? What sort of a contact pattern is acceptable and when is it a warning of problems? Is that oil the correct viscosity? Is it contaminated?
In gear design, there is a combination of rolling and sliding motions. At first contact between two teeth, the motion is mostly sliding but as the two pitch circles become closer and closer, more and more rolling occurs. When the pitch circles intersect, and the teeth are on the centerline between the two shafts, the contact is all rolling. Then, as the teeth go out of mesh, there is progressively more and more sliding.
Gear drives and reducer units are designed using some basic rules. The bearings are based on a certain L10 life, while the teeth have to withstand the operating fatigue stresses. These stresses are complex and provisions have to be made for cantilever loading, similar to a beam in bending; Hertzian fatigue loading of the contact surface, similar to a rolling element bearing; plus sliding friction and the lubrication demands of a pair of surfaces that involves both sliding and rolling. This sounds confusing, but evaluation and failure analysis of gear drives and reducers isn’t terribly difficult—if some simple rules are followed.
Starting the inspection
With the unit running, conduct a general inspection of the area around the gears or reducer, including support structure, bolting, foundation block, baseplate and grout. (These elements need to be in good condition so they can help resist forces that distort the housing and cause excessive gear and bearing wear.) Be sure to conduct a detailed vibration analysis and:
For an enclosed reducer…
For an open gearset…
At about the same time:
With this data, you’ll have a good start on the overall evaluation and an insight into any impending problems. Now, it’s time for visual inspection and evaluation of the gears.
Understanding unit metallurgy
The first step should be to determine the tooth hardness and understand the metallurgy. Most industrial gears used today in North America and Europe are case hardened (also called surface hardened) to somewhere between HRC 54 and HRC 63, and they should never show measurable wear or pitting. However, large gears, open gears sets and many older reducers will have through hardened teeth with hardness values anywhere from BHN 140 to about BHN 400.
Further confusing the issue is the fact that from the 1960s through the 1980s, some manufacturers supplied larger reducers with case hardened pinions and through hardened gears. Additionally, while most open gears are relatively soft, some manufacturers made their products from medium carbon steel and then flame hardened the teeth to about HRC 36.
If the tooth’s hardness is above HRC 40, treat it as though it were case hardened. If you don’t have a hardness tester, use a fine file and try to cut the top of the teeth. If the file just skids, you know that the teeth are hardened.
The next step is to understand the contact pattern during operation. The ideal is to have a contact pattern completely across each tooth that was uniform all around each gear. This design is based on full contact, but sometimes there are machining or assembly errors and other times there is distortion of the housing. Consequently, tooth stress can increase tremendously (see Fig. 1.)
Inspecting through hardened gears
A difficult part of your analysis comes when you try to convince people that pitting of a through hardened gear is not only nothing to be overly concerned about, but that it also can be used as a predictive tool.
There have been many articles written about the various forms of through hardened gear wear and pitting. In them, you will see terms like polishing, corrective pitting, destructive pitting and normal dedendum wear. Unfortunately they all mean that the gear is wearing out. From an operational and maintenance viewpoint, what’s most important is determining the rate of wear.
Through hardened gear pitting…
Through hardened gears rarely break teeth. Because of the high local contact stresses, they usually wear and pit. The aforementioned damage classifications refer to the pitting rates. Through hardened gears usually are:
Normal dedendum wear is fine pitting seen in the dedendum of teeth. It occurs after millions of load cycles when a minimal oil film and sliding contact put the tooth surface into tension. The result is minor cracking and pitting and slow removal of the dedendum surface.
Destructive pitting, like that shown in Fig. 3, happens when the lubricant is grossly overloaded and large or sharp pits develop. The result is a noisy and rough gear in serious trouble with rapidly increasing damage. If the pits are relatively small and well rounded, they can support the lubrication film and the gear will last a long time. At the other extreme, large irregular pits destroy the lubrication film and sharp, linear pits can cause formidable stress concentrations.
In through hardened gears, corrective pitting and normal dedendum wear result in slow and measurable tooth deterioration—which typically allows for a relatively long and predictable life. Destructive pitting, though, will rapidly grow rougher and noisier and may result in a catastrophic failure. Early in the gear’s life, it may be difficult to determine if the wear is corrective or destructive, but with corrective pitting the wear rate rapidly drops off.
Rolling and peening…
The most common other damage seen on through hardened gears occurs when the teeth are so heavily loaded that plastic deformation occurs. This is commonly called rolling, where metal is rolled or pushed up the active faces of the teeth (see Fig. 4), and peening, where the shape of the tooth is hammered irregularly until it is no longer an involute curve. In both rolling and peening, the tooth form is slowly destroyed and both mechanisms show that either the gear is very heavily loaded or there is poor lubrication.
The amount of allowable wear depends on the possible consequences of a failure. If it is not a critical application, through hardened gears are frequently run until the pitch line thickness is reduced by more than one-third. On the other hand, with a critical mine hoist, the loss may be limited to only 15%.
Through hardened gear predictive monitoring…
One positive point about through hardened gear wear is that the wear can be easily monitored. Using a gear tooth micrometer as a predictive tool and periodically taking measurements at several points across the teeth and around the circumference, the charted wear rate can be used in planning for gear replacement, evaluating lubricants, etc.
Inspecting surface (case) hardened gears
Most of the confusion in evaluating gears occurs because surface (case) hardened gears, unlike through hardened gears, can tolerate almost no surface damage. On a surface hardened gear, even small pits—those that would be ignored on a through hardened unit—can be indications of a looming disaster. Thus, Best Practices demand that the evaluation of surface hardened gears has to be very different from that of through hardened components.
The case on a surface hardened gear is much harder and much stronger than the softer core. These gears are almost always machined to much closer tolerances than through hardened gears—and the hardened surfaces tend not to wear. Consequently, damage to that hard external “shell” frequently is difficult to see, making surface hardened gears and reducers far more difficult to inspect than through hardened ones. Because the hardened case is not very ductile:
Most surface hardened gears have relatively thick cases from carburizing or carbonitriding, but some have very thin nitrided cases.
The internal inspection of surface hardened gears should begin with a VERY careful inspection of the teeth and the contact patterns. The teeth should look like new and show no surface damage other than mild polishing—using a bright light for the close visual inspection of the teeth is strongly recommended. Because of the fine surface finish, determining the contact pattern on surface hardened gear teeth can be difficult at times. That is why you may have to look at them from several angles.
One note of caution: Some folks run carbon paper through a gear set or use bluing to check contact patterns. We have no problem doing this, but there are many times when it is misleading because the dynamic forces on a reducer or gear set in operation cause deflection of the housing or the machine base. In these applications, the static contact check is almost useless and may lead to a false impression as to the contact pattern. There is no substitute for that previously mentioned careful visual inspection.
While broken teeth clearly are the most serious problem, because of the relative weakness of the softer core material, any tooth that has substantial surface deterioration may be at risk of breakage. The following section describes some common forms of tooth damage and the dangers they present.
Pitting and micropitting…
The most common surface damage mechanisms are pitting and micropitting. Gear pitting occurs as the result of a combination of Hertzian fatigue forces and surface tension. Any pits on a surface (case) hardened gear are cause for great concern because they show that the tooth loads are far in excess of the design loads. Pitting also is indicative of either serious overloading or metallurgical problems. In addition, once the strong and hard outer layer is penetrated, the remaining core is much weaker and there is a good chance of tooth breakage in the very near future.
Micropitting, a less severe form of surface fatigue damage, sometimes occurs when there is an inadequate lubricant film. It shows up where the high spots of mating gear surfaces create pressures great enough to cause a series of tiny fatigue spalls that look as though the area had been sandblasted.
If the micropitting occurs in bands—and is uniform and well distributed—it indicates that the gears are heavily loaded and there are some machining errors. This type of micropitting, however, poses no real problem. For example, in the pair of 30-yearold case hardened gears from a 900 hp reducer in Fig. 5, there is no perceptible wear except for the bands of micropitting across the teeth. Although this shows that these gears are heavily loaded, after having run for several billion cycles, the degree of micropitting they reveal is not a cause of concern. However, when micropitting is off to one side of a gear, as shown in Fig. 6, it indicates there is excessive misalignment within the unit and a serious chance of catastrophic failure.
The normal progression of damage in a surface hardened tooth with excessive loads and misalignment is that the micropitting eventually yields to pitting, followed rapidly by tooth fracture. By the same token, if the entire active face of the teeth was covered by micropitting, it would indicate that the teeth are extremely heavily loaded, the lubricant film isn’t adequate and there is a substantial likelihood of pitting and eventual catastrophic failure
Other common surface hardened problems…
Poor tooth contact and the eccentric loading that causes pitting and the resultant tooth fracture are the most frequently seen surface hardened gear problems. Several other tooth damage modes have to be recognized, though.
Rippling (see Fig. 7) occurs when the contact loads are so heavy that there is plastic deformation of the hardened case. Inspection of a rippled tooth surface will show surface variations that look like ripples in water and should be watched carefully.
Case crushing is seen when the load on the case is so heavy that the ductile supporting core plastically deforms, can’t support the case and the case fractures. The typical symptom is a crack horizontally across the tooth, indicating the loads are excessive and the tooth is in danger of breaking.
There are many metallurgical defects that can cause gear problems—most of which are beyond the scope of a basic article.
One such defect that is within the scope of this article, however, is the appearance of a single large spall or a few large spalls on well separated teeth (see Fig. 8) This is typically the result of manufacturing problems. If these large surface flaws are well separated, they don’t require immediate action. Nevertheless, they should be carefully monitored, with plans for eventual replacement.
Concluding thoughts Now that you understand the condition of your gears and the severity of any damage, think about the fact that there are always multiple contributors to machine deterioration and equipment failures. Go back, review the installation and, with the help of your PdM findings, you can understand those changes that will most efficiently improve your gear and reducer life. MT