Axle Hypoid Gears Winkelmann et al. reported a number of … · 2019-07-26 · the rubbing energy is insufficient, the superfinishing process will be unable to remove the conversion

ABSTRACTThe benefits gained by superfinishing rear-axle hypoidgearsets are now well documented. Friction, wear andoperating temperature are significantly reduced. The mainimpediment to commercially implementing this process,however, is that it increases manufacturing cost in terms ofprocess speed, work in process and labor. The cost ofsuperfinishing can be significantly reduced by employing anewly developed and fully automatable drag finishingprocess.

The most primitive form of drag finishing occurred whenRoman soldiers dragged their armor through sandy fields forobtaining a mirror-like appearance. Today's drag finisher, ofcourse, is much more sophisticated. A circular turret islocated above a circular bowl containing loose, ceramicmedia. Parts are attached to multiple rotating spindles on theturret, which in turn are immersed in the media in the bowlbelow, and are dragged through the media. This generates ahigh flow of media over the gearsets. By a judicious choice ofmedia and a chemical accelerator, the hypoid gearsets can besuperfinished in less than five minutes to an Ra of less than0.15µm, while maintaining gear geometry. The turret speed,spindle speed and direction of rotation and depth in the mediaare controlled with a programmable logic controller. Theangle of the spindle can also be adjusted. A novel fixture hasbeen developed that adds quick change capability of gearsets(a matter of seconds), and facilitates automation.

BackgroundThe rear axle is the final stage in the drivetrain of a vehicle,and is responsible for converting engine power into usefulpropulsive force. A major component of the rear axle is thering and pinion gearset. Typically, ring and pinion gears are

manufactured by machining, hardening and sometimesshotpeening. Since the hardening step introduces geometricaldistortion, the gears must be lapped or ground to obtain anacceptable contact pattern and noise level. Lapped gearsetsmust be maintained as a matched set. Because both lappedand ground gears still have a rough surface, a break-inprocess is recommended before operating the vehicle undernormal conditions. Break-in is an attempt to smooth thecontact surfaces of the gears through controlled or limitedmetal-to-metal contact. The roughness of the contact surfacesis reduced during this process until a lower and relativelystable surface roughness is reached. Even when the break-inprocedures are followed and a lower surface roughness isachieved, irreversible metallurgical and lubricant damageoften occurs. Break-in always results in stress raisers, metaldebris and an extreme temperature spike, which ultimatelycan lead to the premature failure of the rear axle.

Winkelmann et al. reported a number of studies thatdocument the improvement in rear axle efficiency by usingchemically accelerated vibratory finishing, henceforwardreferred to as superfinishing.1 In another paper, Winkelmannet al. showed that superfinishing can successfully superfinishring and pinion gearsets to a low 0.1 µm Ra without affectingthe gear geometry such that the contact pattern and noise,vibration and harshness (NVH) were acceptable.2 Thistechnology was used commercially to solve premature wearand noise problems of problematic bus and recreationalvehicle ring and pinion gearsets, and is currently used on anumber of production vehicles. Superfinishing usingvibratory machines, however, is not easily automated, andrequires a finishing time between 45 - 90 minutes dependingon the initial roughness and the desired final surface finish.This method is limited to specialized production settings anddoes not meet the current demands of the automotiveindustry. Therefore, a much faster process that is compatible

High Speed, Automatable Superfinishing of Rear-Axle Hypoid Gears

2012-01-0164Published

04/16/2012

Michael Frechette, Gary Sroka and Matthew BellREM Surface Engineering

Copyright © 2012 SAE International

doi:10.4271/2012-01-0164

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http://dx.doi.org/10.4271/2012-01-0164

with a high volume, Just-In-Time (JIT) cellularmanufacturing environment is desired.

This paper discusses a recent development in thesuperfinishing of hypoid ring and pinion gearsets resulting inconsiderably shorter cycles. A novel fixturing method is alsointroduced to help make this new method fully automatable.

INTRODUCTIONOnly a very brief description of superfinishing will bepresented here since it is discussed in detail elsewhere.3,4,5The process is carried out in vibratory bowls or tubs. Aproprietary active chemistry is used in the vibratory machinein conjunction with high density, non-abrasive ceramicmedia. When introduced into the machine, this activechemistry produces a stable, soft conversion coating on thesurface of the metal gears being processed. The rubbingmotion across the gears developed by the machine and mediaeffectively wipes the conversion coating off the asperitypeaks of the gears surfaces, but leaves the valleys untouched.(No finishing occurs where media is unable to contact orrub.) The conversion coating is continually re-formed andrubbed off during this stage producing a surface smoothingmechanism. This process is continued in the vibratorymachine until the surfaces of the gears are free of asperities.

The rate at which a gear is superfinished is a function of boththe rate of formation of the conversion coating and the rate ofmechanical removal of the conversion coating by the mediarubbing. The rubbing energy must be sufficient to remove theconversion coating. In conventional chemically acceleratedvibratory finishing, the rate of superfinishing is limited by therate of rubbing and in some cases by the rubbing energy. Ifthe rubbing energy is insufficient, the superfinishing processwill be unable to remove the conversion coating.

Consequently, there are two approaches for increasing therate of finishing. (1) Select a machine that provides a higherrate of rubbing than a conventional vibratory machine. (2)Select a machine that has a higher rubbing energy.Fortunately, the drag finishing machine has both of thesefeatures, and thus a significantly more aggressive chemistrycan be used than for conventional vibratory finishing. Adescription of the drag finishing machine is given byChmielewski.6

The concept of drag finishing goes back centuries to whenfarmers first pulled plows through their fields. Although thefarmers' intention was not to deburr or polish their plows,dragging the plow through the abrasive soil did just that.

The initial thesis work of drag finishing's developer focusedon the need for a machine that would increase performancelevels over conventional mass-finishing equipment. The

“think-tank” design people knew that pressure and velocitywere responsible for the work of mass finishing; therefore,drag finishing was the natural result.

As you visualize a plow cutting through a field, you canappreciate the significance of these physical elements. It isthe magnitude of these two elements that provides dragfinishing an advantage. Subsequent experience taught thedeveloper that these two elements, in concert with properlyprescribed media and compounds, allow drag finishing toachieve performance levels as high as 40 times faster thanconventional levels.

Drag finishing uses a circular bowl containing stationary,loose finishing media and a circular rotating turret above thebowl with multiple rotating part-fixture stations. (Ten stationsare average.) Fixture stations rotate about their own axes,similar to a planetary system.

Parts are attached singularly or in groups to part-fixturestations around the perimeter and under the main turret. Therotating turret is then lowered so the parts are submergedand “dragged” through the stationary loose media.

Programmed sequences such as rpm, depth and rotationaldirection provide mechanical repeatability that assuresuniformity cycle to cycle. By combining the right media andcompound with the physical action of the drag finisher, youcan mechanize processes that previously only could be donemanually. Refined surfaces as low as one to two Ra(Roughness average) are consistently achievable.

The energy of rubbing in a drag finishing machine is muchgreater than that achieved in conventional vibratory machines(i.e., Circular vibrator) as shown in Figure 1.



Figure 1. Material removal on steel using ceramic mediaweights and machine types. 7

ExperimentalAn exemplary test was carried out to ensure that the noveldrag finishing technique would be as beneficial as thought.The test gears were subsequently subjected to a rigorousSingle Flank Testing to determine the effect of superfinishingon noise, vibration and harshness.

Ring & Pinion GearsetsThe ring and pinion hypoid gearsets used in this test weremachined, case carburized, shotpeened and lapped. Thesurface roughness of the gears was measured using aHommel T-1000 profilometer with a TKZ-100 pick-uphaving a 5-µm radius stylus. A 0.8 mm cut-off filter and 1.5mm trace length were used. The starting roughness rangedbetween 1.5 to 2.0 µm Ra, which is considerably higher thantypical production gearsets.

The matched gearsets were mounted on fixtures that can berapidly attached to the drag finisher's spindles. See Figure 2showing the ring and pinion attached to the fixture.

Figure 2. A matched ring and pinion gearset fixturedready for attachment to the spindle of the drag finisher

machine.

EquipmentThe specific machine used in this paper is the ROSLER SSA-L Series Mini Drag Finisher. See Figure 3 for a photograph ofthe actual machine with a single ring and pinion gearsetinstalled in its fixture.

A simple schematic of the drag finishing machine showingthe key components is presented in Figure 4.

The specifications of the Drag Finisher are given in Table 1.

Experimental ParametersThe parameters of the drag finisher used to process thehypoid gearsets are given in Table 2.

One gearset was processed for 5.0 minutes, and the othergearset was processed for 7.0 minutes. During the first half ofthe processing cycle the spindle is rotated in the CWdirection. During the second half of the processing cycle, thespindle is rotated in the CCW direction.



Figure 3. Picture of the Drag Finisher with one fixture holding a ring and pinion gearset attached.



Figure 4. Schematic of a drag finisher showing the key components.



Table 1. The technical specifications of the Mini Drag Finisher.

Table 2. The parameters of the drag finisher used to process the two gearsets.



Results & DiscussionThe surface roughness of each ring and pinion was measuredon the lapped region after processing using the sameinstrument and parameters as used for the initialmeasurements. The results are presented in Table 3. Allsurfaces had a ≤ 0.25 Ra. Numerous racing teams havediscovered that superfinished ring and pinion gearsetsfinished to only a 0.25 µm Ra have superior performancecompared to non-superfinished lapped gearsets.

Table 3. Surface roughness of a gearset processed for 5.0minutes and a gearset processed for 7.0 minutes.

The contact pattern of the gearsets processed for 5.0 min. and7.0 min. were determined, and the results are presented inFigure 4. Both contact patterns were deemed acceptable bythe customer.

To determine the effect of superfinishing on NVH, SingleFlank Testing was done for the drive and coast side of thegearset processed for 5.0 minutes. The results are given inFigure 6. The maximum µ-rad recorded for the drive andcoast sides were 31.52 and 34.40, respectively. It waspreviously stated that these gears were abnormally rough,such that smoother starting gears will have no problemmeeting the NVH customer specification. This was recentlyconfirmed by another independent study.

CONCLUSIONS1. The drag finishing process can superfinish ring and piniongearsets to a <0.25 µm Ra in under 5.0 minutes, and evenfaster if the starting Ra is less than 1.5 µm.

2. The superfinished gearsets will meet the customers NVHspecifications.

3. The drag finisher lends itself to full automation byrobotics if desired, and satisfies the need of modernmanufacturing facilities where JIT cells are desired.

Figure 5. Pictures of the drive side and coast side of the gearsets processed for 5.0 minutes and 7.0 minutes.



REFERENCES1. Proceedings of the ASME 2007 International DesignEngineering Technical Conferences & Computers andInformation in Engineering Conference IDETC/CIE 2007September 4-7, 2007, Las Vegas, Nevada, USADETC2007-34860

2. Winkelmann, Lane, Holland, Jerry & Nanning, Russell,REM Chemicals Inc. “Superfinishing Motor Vehicle Ringand Pinion Gears”, AGMA 2004 FTM.

3. Hashimoto, F.F.H., Zhou, R.S., Howlett, K., U.S. Patent5,503,481 (1996) “Bearing Surfaces with Isotropic Finish”(Assigned to REM Chemicals, Inc., Southington, CT.)

4. Michaud, Mark, Winkelmann, Lane, Snidle, Ray, Alanou,Mark, “EFFECT OF SUPERFINISHING ON SCUFFINGRESISTANCE” ASME 2003 Design Engineering TechnicalConference.

5. Sroka, Lane Winkelmann. “Superfinishing Gears- TheState of the Art”, Gear Technology magazine, Nov-Dec 2003

6. Chmielewski, David, Walther Corp. “Is This the Mass-Finishing Answer You Have Been Looking For?”. PF online,May 1997

7. Gillespie, LaRoughx, “Mass Finishing Handbook” firstedition, Page 328, 2006

Figure 6. Single Flank Testing results of the drive (DM01) and coast side (CM01) of the ring and pinion gearset processed for5.0 minutes.

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Axle Hypoid Gears Winkelmann et al. reported a number of … · 2019-07-26 · the rubbing energy is insufficient, the superfinishing process will be unable to remove the conversion