Superfinishing Motor Vehicle Ring and Pinion Gears
By : Lane Winkelmann ,
Abstract
By: L. Winkelmann, J. Holland and R. Nanning, REM Chemicals, Inc.
Today, the motor vehicle market is focusing on “lubed for life” differentials requiring no service for the life of the vehicle. Still, differentials are prone to develop problems of one sort or another since they are used to transmit a heavy torque through a right angle. One weak point in the differential is the ring and pinion gearset. As such, a proper break-in period is essential to attain the required service life. Break-in is an attempt to smooth the contact surfaces of the gears and bearings through controlled or limited metal-to-metal contact. The roughness of the contact surfaces is reduced during this process until a lower and relatively stable surface roughness is advantageous, but irreversible metallurgical and lubricant damage occurs since break-in always results in stress raisers, metal debris and an extreme temperature spoke. Break-in and its negative effects can be eliminated with chemically accelerated vibratory finishing. When this method is used to superfinish ground (AGMA Q10) or lapped (AGMA Q8) ring and pinion gearsets to less than 10 min Ra, the life of the lubricant, bearings and gears is significantly increased. Just a few years ago, this technology was considered impractical for high production volume OEM ring and pinion gearsets due to lengthy processing times. This superfinishing technology also had difficulties preserving the geometry of rough lapped gears, which required more stock removal than finely ground aerospace (AGMA Q12+). As a result, the transmission error of these gears was increased leading to unacceptable noise. The superfinishing technology in this paper overcomes these obstacles and meets the needs of the motor vehicle industry. Gear metrology, contact patterns, transmissions error and actual performance data for superfinished gearsets will be presented along with the superfinishing process.
Introduction
Break-In Process
Vehicular differentials are apt to develop problems of one sort or another since they are used to trans- mit a heavy torque through a right angle. A differential consists of a ring gear, pinion gear, side gears, spider gears, and bearings. See Figure 1. The spi- ral bevel or hypoid gearsets can be a weak point in the differential since they need to withstand large sliding pressures and shock loading. Over the years, many improvements have been made to dif- ferentials such that now many require no mainte- nance (i.e., “lubed for life”). New ring and pinion
gears are not normally ground after carburization, but rather are lapped at the factory and maintained as a matched gearset. Lapping partially corrects the distortion which occurs during carburization, and therefore somewhat reduces the operating temperature, wear and noise. It is impractical, how- ever, to perform the lapping under the same loads as those which are experienced under actual driving conditions. Therefore, ring and pinion gearsets must always go through a “break-in” cycle, which is professed by car experts as the magic potion for preventing future failure. As one expert puts it, im- proper break-in results in a differential lasting 90,000 miles, and proper break-in results in a differ- ential lasting, 180,000 miles.
Figure 1. Exploded view of a differential pointing out the various parts discussed in this paper.
| Key | |||||
| 1. Ring | 2 Pinion | 3 Shims | 4 Housing | 5 Side gears | 6 Spider gears |
Break-in is an attempt to create a smooth surface on the contact surfaces of the gears and bearings through controlled or limited metal-to-metal con- tact. The roughness of the contact surfaces changes during this process until a lower and rela- tively stable surface roughness is reached. During the break-in cycle, it is hoped that the lubrication of the ring and pinion gearset is maintained. In fact, this is vital to the life of the differential. During the initial start-up of the break-in cycle, an oil film is formedonthesurfacebetweenthegearteeth. This film is referred to as Hydrodynamic or Full Fluid Film Lubrication, which completely separates the ring from the pinion so that there is no metal-to-metal contact. As the speed of the ring and pinion in- creases, the hydrodynamic layer thickens as well. As a load, however, is placed on the gearset, the hy- drodynamic layer decreases. At the same time, the temperature rises and the viscosity of the lubricant decreases, which further decreases the film thick- ness. As the load and/or temperature continue to increase, the lubricant film becomes too thin to pro- vide total separation. Contact between the peak as- perities occurs, which results in higher frictional forces and the concomitant temperature rise. This is referred to as the Boundary Lubrication or Thin Film Lubrication regime. The break-in process is an attempt to maintain the temperature low enough to provide boundary lubrication until the peak asper- ities are worn away leaving the lower and relatively stable surface roughness on the contact surfaces.
In order to understand the shortcomings and mis- conceptions concerning break-in, it is worthwhile to briefly examine what advice the experts are giving to the end user. Although every expert has his or her own recipe for break-in, the following is fairly typical:
All new ring & pinion sets run hot until they are “broken in” and in some situations they can run hot enough to break down the gear oil and dam- age the gear set. Some of those situations are:
S Towing
S Tall tires
S Heavy loads
S High numeric gear ratios (4.56 & up) S Motorhomes
New gears are lapped at the factory but some are lapped more than others and even with lap- ping they are still not lapped under the same pressures that driving creates. The loads generated while driving force any microscopic high spots on the gear teeth back into the surface of the metal. This is called ”work hardening”. Work hardening is similar to forging in the way that it compresses the metal molecules into a very compact and hard formation. This can only be accomplished if the metal surfaces are lubricated and the temperature is not hot enough to change the molecular structure due to the heat alone. If the temperature of the metal gets hot enough to change the molecular structure it will soften the surface instead of hardening it. This may seem like a balancing act but it all happens easily & passively as long as the oil keeps the gear cool while it is breaking in. All new gear sets require a break-in period to prevent damage from overheating. Usually 500 miles will break-in the gears but until then, you want a cool rear end. The greatest damage results when a new ring & pinion has been run for several miles during the first 500 miles and the oil is very hot. Any heavy use or overloading at this time will cause irreparable damage to the gear set. So all this means keeping your rear end cool.
During “break-in” cycle, it is desirable to run the gears under light loading so that the differential runs cool. During this period, it is hoped that the microscopic high spots on the mating gear teeth are flattened and worked back into the surface under actual driving conditions causing work hardening. [1]
Break-In Misconceptions and Problems
Clearly then the differential break-in process only provides a partial solution to curing the problem, and has several serious and often ignored misunderstandings and disadvantages. First, these gears are typically case carburized to a high hardness, and therefore the peak asperities cannot be work hardened to a smooth surface during run-in. Instead the peak asperities are abraded away resulting in metal debris which is always found in used differentials upon inspection. This debris is not only damaging to the gears but also to the bearings. The wind turbine industry, for example, has recommended a 3.0 micron filtration system for its lubricants since it was discovered that even 10 micron particles cause premature gear/bearing failures. [2] If metal debris is a serious problem in filtered lubrication systems, it is not surprising then that it is even more problematic for systems such as differentials where the oil is recirculated by the ring gear and flung over all the parts without being filtered.
Furthermore, at the microscopic level the run-in abrading process consists of micro-cutting, micro-plowing, and micro-cracking, resulting in the creation of stress raisers setting the stage for future fatigue failure through contact fatigue.
Second, many vehicles which experience high loads at relative low gear speeds (e.g., buses, recreational vehicles, and off-highway equipment) are placed in immediate service such that their differentials never have the luxury of going through the recommended break-in process. Such vehicles often experience high loading and shock loading, whereby the operating temperature can increase to a point where the gear oil is non-functional as a result of its reduced viscosity or degradation. Premature gearset failure then occurs.
Third, break-in does little to remove the distressed metal at the surface of gears. Surface stress raisers are caused by heat treatment and residual machining lines, which serve as the initiation points for future wear and fatigue failure.
In spite then of all the advances in gear manufacturing and lubrication, there are still problems with ring and pinion failures. What is needed is a practical way to remove the peak asperities from the gearset without affecting the contact pattern such that both friction and damaging metal debris are reduced and/oreliminated. Thesegearsetswouldrequireno break-in and could be used immediately for high load service.
The Superfinishing Challenge
Chemically accelerated vibratory finishing, henceforward referred to as superfinishing is routinely used on aerospace spiral bevel gears (AGMA Q12+). Since the flanks are typically ground after heat treatment to a 12 min. Ra, little stock needs to be removed to superfinish such gears to a desirable 3.0minRafinalsurfacefinish. Aswaspreviouslyre- ported, these superfinished gears maintain their critical geometrical tolerances. [2] Superfinished aerospace spiral bevel gears experience hydrodynamic or full fluid film lubrication immediately after being placed in service. It is well documented that these gears have significantly lower wear, noise/ vibration, and operating temperature as well as an increased service life. [4] [5] [6]
Likewise, for over ten years, the motorsports industry has utilized superfinishing on high performance ring and pinion gearsets (AGMA Q10) typically ground to a starting 25 min. Ra. These high performance gears are superfinished to approximately 10.0 min. Ra and in certain venues to as low as 1.5 min. These motorsport gears have been widely recognized for their enhanced durability and efficiency throughout the industry.
On the other hand, ring and pinion gears used for motor vehicles (AGMA Q8) are seldom ground in the U.S. after carburization for economic reasons, but instead are lapped and maintained as a matched gear set. Their starting surface roughness typically has a 60 min. Ra in the contact area. In Europe, however, ring and pinion gears are precision ground to a 30 min. Ra after carburization. By grinding to the final net shape, the need to keep the ring and pinion together as a matched set is eliminated. This precision grinding has recently been introduced to the North American market.
Until recently, superfinishing motor vehicle ring and pinion gearsets had two major problems. First, since the starting surface is much rougher than that for ground aerospace gears, much more stock has to be removed to achieve a smooth surface. Since the stock removal was not uniform across the flank, however, there was an increased transmission error resulting in increased noise. Interestingly, the contact pattern did not drift after superfinishing, and so could not be used to flag changes in transmission error. Second, the processing time to superfinish a ring and pinion gearset using chemically accelerated vibratory finishing was too long for it to be commercially viable for high production manufacturing.
Experimental
Description of Ring and Pinion Gearset
For the majority of work in this study, Yukon DANA 44 ring and pinion gearsets were used. This is a very popular and readily available gearset used in various vehicles since its introduction in 1955. It was also selected since it matched an existing housing of the single flank test rig used in this study. The DANA 44 ring and pinion used in this study consists of a 46-tooth, 10-bolt hole, ring gear and a 26 spline, 13-tooth pinion gear, resulting in a 3.54 gear ratio. The ring gear has a diameter of 8.5 inches, and the pinion has a diameter of 1.376 inches. The pinion offset is 1.50 in. The ring and pinions are a matched set which have been case carburized and lapped for correct contact patterns.
Superfinishing Using Chemically Accelerated Vibratory Finishing
Details of using chemically accelerated vibratory finishing have been published elsewhere. [7] The following is a brief summary of the technique. The superfinishing is produced in vibratory finishing bowls or tubs. An active chemistry is used in the vibratory machine in conjunction with high density, non-abrasive ceramic media. When introduced into the machine, this active chemistry produces a stable, soft conversion coating on the surface of the metal gears being processed. The rubbing motion across the gears developed by the machine and media effectively wipes the conversion coating off the “peaks” of the gears’ surfaces, but leaves the “valleys” untouched. No finishing occurs where media is unable to contact or rub. The conversion coating is continually re-formed and rubbed off during this stage producing a surface smoothing mechanism. This process is continued in the vibratory machine until the surfaces of the gears are free of asperities or until the surface attains the desired level of finish. At this point, the active chemistry is rinsed from the part and the gears are dipped in rust preventive.
Details of Processing Procedure
The Yukon DANA 44 ring and pinion gearsets were fixtured as shown in Figure 2 in an early attempt to maintain the lapped ring and pinion together as a pair. This fixture is rugged and protects the threads on the pinion from damage, and also prevents media from lodging in the tapped holes on the ring gear. Since that time, other proprietary devices have been designed, which have the same features, but are more applicable for handling high production volumes.
The gearsets used in this study were processed in a 3-ft3 vibratory bowl using a newly developed high speed gear finishing process, which has a much higher stock removal rate than that previously available. Two different medias were used to superfinish the gearsets to determine the effect of media size and shape on the uniformity of stock removal, contact pattern and ultimately noise. Descriptions of Media A and Media B are detailed in Figure 3.
In a previous project, Media A had been used to superfinish similar OEM gearsets to a 4.0 min. Ra. Al- though the operating temperature of this gearset was significantly reduced, the customer reported that superfinishing increased the noise level. Therefore, this test was designed to verify whether or not Media A was the root cause of the problem.
Based on in-house process knowledge, it was an- ticipated that Media B would be less likely to alter the gear profile. From the outset of this study though, it was uncertain if it was possible to superfinish gears having a high starting roughness to a 4.0 min. Ra without increasing the noise level. Therefore, Media B was used to process one gearset to a 10 min. Ra, and another gearset to a 4.0 min. Ra.
Figure 2. Fixture used to superfinish and keep the lapped gearsets as a matched pair.
Figure 4: Relative stock removal normalized to unity using Media A and Media B.
Two separate groups of gearsets were processed and analyzed. See Table 1 for details. Group I was tested at the Gear Dynamics and Gear Noise Re- search Laboratory at Ohio State University. Group II was tested at The Gleason Works. Neither laboratory was aware of the processing conditions or the expected outcomes.
Table 1
Uniformity of Stock Removal
Three different experimental methods were used to determine the effect of superfinishing on gear geometry: (1) Direct measurement of stock removal across the flanks of the ring and pinion gearsets; (2) measurements of the contact patterns; and (3) single flank testing.
Direct Measurement of Stock Removal
Figure 4 shows the relative stock removal normal- ized to unity across the flank of the gearsets super- finished using Media A and Media B. From these charts, it is apparent that Media A distorts the profile by removing more stock from the flank of the gear nearer the tip than the root, but does not negatively affect the spiral. Therefore, it is expected that gears superfinished with this media mixture will have a higher transmission error leading to an increase in noise. On the other hand, Media B does not distort the spiral or the profile, but removes stock uniformly from the tip to the root and across the spiral. The small variations seen in the Media B charts are due to slight measurement inaccuracies. Therefore, it is expected that the transmission error will not be increased. This will be shown and discussed in more detail later.
Figure 4: Relative stock removal normalized to unity using Media A and Media B.
Contact Patterns
Group I contact patterns were measured for the four gearsets to ensure proper alignment and positioning. The gearsets were then installed in a DANA44 housing which had been modified for use in the Loaded Bevel Gear Test Rig at the Gear Dynamics and Gear Noise Research Laboratory at Ohio State University. General Motors marking grease was used to coat the gearsets. They were then rotated by hand in both the forward and reverse directions until a clear contact pattern was developed on the ring gear. The contact pattern for each gearset was then checked to determine if the superfinishing had altered or caused a contact pattern to become unacceptable. Though there were some very slight deviations from the baseline contact pattern, all contact patterns before and after superfinishing were found to be acceptable.
Group II gearsets were superfinished identically to those tested at the Gear Dynamics and Gear Noise
Research Laboratory. The contact patterns of before and after superfinished gearsets were measured at The Gleason Works. Again, the laboratory reported no change in the contact patterns after superfinishing. The contact patterns are displayed in Figure 5.