Allowable Surface Durability in Grease Lubricated Gears

8/22/2019 Allowable Surface Durability in Grease Lubricated Gears

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Allowable Surface DurabClity in Grease Lubricated Gears@KEIGO FUKUNAGA

Mitsubishi Electric CorporationFukuoka, Jap an

Ex/)cri~~lenlralesul~s re presented for grease lubricaled gears.Firsl, the oil-Jilvtl formatio ?ls we re measured , usin g a power cir-cii~l(~lingn r muchine. The same tests were also performed ruitltgear oils. The oil-film ormation for grease was unstable comparedlo oil. Th e res~alts how h a t tile normal load a1 which lhe oil-film

Sr,~-rttulion fg re c~ se ecomes insufficient w as a bout 60 percent of1lrc~1fir il. In addition, a life lest was carried ou t. Surface dama geo f g e c ~ reeth was scoring rather than spalling. T he gear lije wasclo.sely related to the normal load. The results were also comparedwilI~ G M A sta nd ard s. I1 was suggested tltat lhe allowable surfacer111.rcabililyof (L grechse lubricaled gear was 0.9 GP a in herlzianPressi~rewhich was (about 60 percent that of a n oil lubricaled gear.The (&(L were then used lo develop a design for grease lubricatedgears.

INTRODUCTIONGrease, a half solid liquid oil, is comprised of a thickener

which acts as a net, dispersed in a base oil. Grease used ina shielded bearing will not flow with gravity. However, ifan external force is applied to the grease, the thickenerwhich acts as a net, is broken and the grease softens andbegins to flow. Consequently, it has been considered thatthe lubricating characteristics of grease and gear oil, be-tween gear tooth surfaces, work in the same way. Thus,until now, grease lubricated gear design methods have de-penclcd on oil lubricated gear design methods.- ..I here ar e many reports on the surface durability of geartceth ( I ) , ( Z ) , but the lubricants examined in those studiesar c gcnwally industrial oils. Within the last ten years, elec-tric machines, such as gearmotors, in Japan have changed

In gear design, there are two important factors: surfacedurability and tooth strength. As regards allowable surfacedurability, one of the most important factors is the oil-filmbetween the gear teeth surfaces. T h e oil-film prevents directcontact of tooth surfaces and helps avoid spalling, pittingand scoring damage. Accordingly, the oil-film can improvethe performance of th e gear unit and extend its lifetime.

In view of the above problems, the present paper is con-cerned with the surface durability of grease lubricated gears.The oil-film formation between gear tooth surfaces wasmeasured and a life test was conducted. After consideringresults and data, the allowable surface durability in greaselubricated gears was determine d.

EXPERIMENTAL METHODSExperimental apparatus

T h e experimental apparatu s was the I.A.E. power cir-culating gear testing machine with a center distance of82.55 mm and a circulating power of 260 kW. Th e motorhad a constant speed of 1800 rpm and was rotated by aY - A starter which lowered the shock load to the gearteeth.

Gear speed could be changed by exchanging V-belts andV-pulleys. Before all experiments, an intermediate ba r cor-responding to the pair of gears was twisted, so that thetorques of the load works were applied to the test gears.

Two differing quantities of oil and grease were first placedin the gear case for respective experiments. After normalforce ( P n )and pitch velocity ( V p ) were decided, the moto rwas rotated at the desired speed.

- ~I.rom using oil lubricants to using grease lubricants. Unfor- Test Gearstunatcly, however, the mechanism of grease lubricated gears Table 1 shows the test gear dimensions. The se dimensionsis little understood and there are few available reports on are similar to the gear units found in electrical machines.this s~ ~bj ec t3). T h e only difference between the oil-film formation mea-surement and the life test was the difference in the gearsurface hardnesses. T h e gear surface roughness before t heexperi ment was about 2 km in R -. Total gear profile erro r

Final manuscri~tapprovedDecember 15,1987 and alignm ent erro r were between about JIS grad e 1 an d 2.454


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lutions until operations were stopped. Life factor C1 is 1.0for surface durability of gear teeth a tN = 1 x lo7 n AGMA2 15.01 (1986). In this report, the allowable surface dura-bility of grease lubricated gears can be determined byN L 1 x 10' revolutions.

RESULTSOil-film FormationCharacteristics of Grease and Oil Bath Lubrication

Figures 4 and 5 show typical temperature 0u of lubricantand typical average recording voltage Eab In the oil bathlubric;ttion indicated in Fig. 4, sufficient oil-film formationoccurred. Enb decreases continuously with an increase ofoperating time. Also, E,b is a sufficiently large value after8 to 16 minutes. On the other hand, Fig. 5 shows that, inthe initial operating period, the quality of oil-film formationis good. However, E,b decreases rapidly as time progresses.After a period of imperfect oil-film generation, the oil-filmformation again improves.

'I'he differences between grease and oil bath lubricatedgears seem to be as follows: initially,E,b in grease lubricationis large, as in oil bath lubrication, because sufficient greaseexists in the gear tooth spaces. When the gears start torevolve, they form a channel in the grease. Consequently,-Obessens because grease does not exist near the gear toothsurfaces.

Figure 6 shows the model used to investigate the supplyof grease for the gear tooth surfaces described above. Greasecarried by rotation and retained in the teeth spaces existsncar the gear contact point. Two forces act on the grease.On e force pulls in the direction of the contact and the othe r,the contact pressure of gear teeth, counters this pullingforce, and pushes the grease. As grease is thicker than oil,

ModuleNumber o f teethCutter pressure angleAdd. modification factorPitch circle diameter'rip circle diameterCenter distanceOperating pressure angleOperating pitch circlek ~ c e i dthLength of line o f actionCon c~ct atioMaterialHcat treatmentHardness of T ipFinishing

Mn mmz

a o d egX

Do mmDk mm

A mmd egDb mmB mmL mm

E

H RC

2.531 1 3 50.' 2 0 --;is

77.583.5 91.6

82.5520.09577.547 1 87.553

1012.248

1.66S45C I S45C

High. freq. ind.hardening45.5-47 (53-56)*Ground byReishauer geargrinder

it seems that the rate of grease returned is greater in quan-tity. Accordingly, one tooth surface contacts another be-cause of a lack of lubricant, causing the tem peratu re of thegear tooth and the surrounding area to rise abruptly. Thebase oil viscosity is reduced, as is the oil-film thickness; thus,

Time minFig. 4--Variation of oil-film formation with time for gear oil

Time minFig. &Variation of oil-film formatlon with time for grease

R O L L I N G GREASE

* Lifetime test Fig. &Profile for starvation of grease


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Allowable Surface Durability in Grease Lubricated Gears 45 7

TABLE-LUBRICATION

SC: scoring, S.SC: slight scoringSP: spalling

TABLE-LOAD CONDITIONSND RESULTS

GREASE

LOADSTAGE

A- 123

B- 123456

C- l2345

Load EffectFigure 7 shows the relationship between Pn and E,b clar-

ified in this study. Here, the temperature of the lubricantwill also have a significant effect on the oil-film formation.E,b was unstable and 0, increased in the initial operating Normal load pn kN/tntime shown in Figs. 4 and 5. However, E,b became stablebut 0, continued to increase after 8 to 16 minutes. Then, Fig. 7-Relationship between Pnand Eab tor gear oil

LUBRICATIONILColourThickenerBase oil viscosity cStAdditiveDropping pointTimken OKConsistencyOil separation Wt %Work stabilityAmbient Temperature "CNLGL Grade

to generate an oil-film becomes difficult. Finally, this hightemperature melts the channeled grease. The grease base 25:'oil, holding the thickener which acts as a net, begins to ru n.As the base oil turns into an oil-film between th e gear toothsurfaces, after a certain amount of time passes, the oil-film 20-formation improves.

On the other h and, in Figs. 4 and 5 , we can see that theE,b ampli tude in grease lubrication is larger than that of oil > ' 5 -lubrication and that oil-film formation is also unstable. As ndescribed above, it seems logical that channelling, higher L=; 10-temperatures, usion and formation of an oil-film with thegrease would reoccur.

JIS GradeSpecific gravity 1514OCKeinematic Viscosity cStAdditiveViscosity indexTimken OKFlash point "CBase oil typeAGMA Grade

Pale (yellowish)brownLithium soap

133114 at 40/100 OCnon-lead EP additive

198over 20 kg370 at 25 "C16.7 at 100C & 30 hrs

418 after 100 000- 0- 120

0

vPmls

6.19

3.19

1.59

JIS Grade 2. No. 20.897

154114.7 at 401100CSn-P EP additive

95over 25 kg

226Mineral4 EP

I I I I -VP mfs Grease Oil1.59 a -3.196.19 0 0

*,aa "8?@'.

'Q

PnkN/m

336.4270.0177.5557.0544.3336.5168.7134.4101.0437.4354.0270.0177.5134.4

500100-50

CLLT

-10

aHGPa

1.31.170.951.691.661.310.930.830.721.491.341.170.950.83

PVsTMNls

11.510.38.367.707.545.934.203.763.253.383.042.662.151.87

Nx lo7

11.51.31121.5221121.52

FA ILUREDRIVERIDRIVEN

SC, SPISCSC, SPISC, SPSC, SPISCSC, SPISC, SPSC, SPISC, SPSC, SPISC, SP

S.SC/S.SCnon1S.SCnonlnon

SC, SPISC, SPSC, SPISC, SP

SCISCS.SC/S.SC

nonlnon


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the value at which oil-film formation becomes stahle is dum and the dedmdum. In stage 0-5. we can see linear0. = 53 - 64(:, and is plotted as% n Fig. 7. 'The vertical damage caused by scoring on the lonth surface. However.line on the leli shows the average electric resistance ( R r ) the damage is only slight, hecause the grinding marks re-hetwern the gear t ~ x ~ t ht~rfaces alculated from formula main on the pitch p i n t and addendum part.[I ]. In line with this fiaure, one can conclude that: Fienre 9 shows the tooth oroliles of the eear measured

I . Regardless of the kind of lubricant, thez nd theoil-lilm formation both decrease ;IS Pn increases. L. S.Akin (7) showed in the followit~g ormula that therelationship among the minimtun oil-film thickness(H.,.) hetween gear tooth st~rfaces,ubricant viscosity(9,) and normal load (Pn ) holds:

~. ,,after the experiment at load condition R. Normal load (Pn )increases accordingly from the upper left (B-5) to upperright (R-4) and to bottom left (R-3) and to bottom right(R-I). In load slage 8-5. gear wear rises in the area thatcontacts during the gear teeth change from one pair contactto two pairs. I t seems that the tip of the driver gear, andthe rm>t and tio of the driven gear wear out in stage R-4.,, -The gear tooth wear is slight in the range of


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Allowable Surface Durability in Grease Lubricated Gears

face, the concavity at the pitch point of the driver gear isdue to the sliding that occurs toward the gear tip and root .The convexity at the pitch point of the driven gear is dueto the sliding that occurs toward the pitch point.

Accordingly, from the discussions above, we can concludethat the gear tooth has surface damage to the tooth profile,as shown in (B-5), at the beginning of operat ion. As timeproceeds it seems that the tooth profile changes from(B-4) to (B-3) to (B-1).Wear Life

For the bending strength of gear teeth, we can decidefrom the allowable bending stress using the load, whetherthe gear teeth will break o r not. T he allowable surface du-rability of the gear tooth, however, is different from itsbending strength. Even if more or less spalling, wear orscoring damage occurs, the gear uni t can be operated con-tinuously because one can predict that the surface damagewill not progress.

In many results of this study, gear tooth wear progressedthrough scoring damage. Hence, let us consider the allow-able surface durability of the gear tooth from the point ofview of the wear life. In this paper, as there is little designdata concerning grease lubricated gears from a practicalstandpoint, the wear life will be considered using a specificwear rate.

As shown in Fig. 10, we assume the tooth profile had astra ight line described by the dotted line. 6 mm: the geartooth mean wear depth .L mm: the gear contact length. Th esummation (Ws) of specific wear rate per tooth surface ofeach gear is given by Eq. [3]. Where i = 1 for the driverand i = 2 for the driven, B mm in tooth width, N; in driverrevolution, S mm in contact length to load direction.

Figure 11 shows the relationship between Ws and P V s Tshown in Table 3. It indicates more or less the amount ofscatter. However, it is apparent that Ws is proportional toP V s T when Ws is larger than 5 x 10- MN/s, and in theregion of P V s T < 4 MN/s the rate of decrease of Ws islarger.

Figure 12 illustrates the relationship between Ws and Pn.It can be clearly seen in this figure that Ws decreases re-markably near Pn < 200 kN/m.

By comparison of Fig. 1 and Fig. 12, t can be concludedthat: In grease lubricated gears, the scatter of data repre-sented by Pn is smaller than that of data represented byP V s T. Therefore, the experimental data suggests that the

Fig. 1O-Calcuiation of specific wear rate Ws

F.1=Fluid IubricationB.1 =Boundary lubricationI

Fig. 11-Relationship between Ws and P V s T

6 1 IF.1 =F luid lub ricationB.1=Boundary lubrication- 1

I I I10" 10-j2 1 0 IWS mrr?(mrn~)

Flg. 12-Relationship between Ws and Pn

surface durability in grease lubricated gears is closely relatedto Pn.

From the theory of lubrication, it is well known that fluidlubrication holds for the state in which the specific wear rateis smaller than 1.0 x lo-'* mm3/(mm . N) and boundarylubrication holds for the state in which the specific wear rateis smaller than 1.0 x lo- " to 1.0 x lo-' mm3/(mm .N).These regions are shown respectively as F.l and B.l in Figs.11 and 12. If we assume that these relationships can also applyto the experimental results in this paper, Ws is in the fluidlubrication region for P V s T < 3.5 MN/s or Pn < 160kN/m. From the above, if we can produce a gear unit designedas P V s T < 3.5 MNIs or Pn < 160 kN/m for grease lubricatedgears, it seems that the gear tooth surfaces will be fluid lu-bricated. Thus, it is believed the gear unit can be operatedcontinuously as there is little tooth wear, even if scoring dam-age on the gear tooth surface occurs.Discussion of Allowable Surface Durability

As discussed in section 3.1, (Pn), ' is the load a t which oil-film generation becomes difficult. (Pn),' in grease lubri-


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catcd gears was about 60 percent that of oil bath lubrication.S. Y. Poon (8) eported that the oil-film thickness of greasewas 50 percent to 70 percent that of the base oil of thegrease. Therefore, considering the oil film-formation be-tween the tooth surfaces of gears , if the viscosity of the baseoil is equal to that of the gear oil, the allowable capacity ofthe geiir unit for grease lubrication should be made to beabout 60 perccnt that of oil bath lubrication.

If AGMA 2 15.01 ( 1966) corresponds to oil lubrication,the allowable surface durability of gear teeth has Sac = 1.2to 1.3 Gl'a (= 120 to 134 kgf/mni2) at high frequency in-duction ha rdenin g and a gear surface hardness HRc50. Ifwe propose th a ~he ;~llowable ormal load Pn is 160 kN/m;IS the resul t of wear life in this study, Sac becomes 0.90 GPa( = 9 2 kgf/nim2). This value is 69 percent to 77 percent oft.hc allowable surface durability of gear teeth described byAGMA.

From the above discussion, in this region of exp erimentalconclitions, we can conclude that the allowable surface du-rability of grease lubricated gear teeth should be consideredto be about 70 percent that of oil bath lubrication.

CONCLUSIONS- -I he experiments were carried o ut using sp ur gears closeto tlic dimensions applied in practical gear units to obtaintlic allowable surface durability of grease lubricated gearteeth. Lubricants used in this experiment were NLGI GradeO grease an d I S 0 VG 150 gear oil. We first investigated oil-fill11 formation of grease and oil by the electric resistancemcthocl. Next, a lifetime test was carried out for greaselubricalion. The main results obtained in this study are as

1. The normal load at which the oil-film formation be-comes insufficient is about 336.0 kN/m in oil bath lu-brication. 'That of grease lubrication is about 202.0 kN/m, a value which is about 60 percent that of oil.

2. Surface damage of gear teeth with grease lubricationtook the form of scoring.

3. A method of estimating surfa ce durability with greaselubrication gears is proposed in which a specific wearrate calculated from the change of tooth profile curvewas applied.

4. Considering wear life, the allowable surface durabilityin grease lubricated gears is about Sac = 0.90 GPa.

5. Considering the results of oil-film formation and thelife test, the allowable surface durability in grease lu-bricated gears is about 60 percent that of oil lubricationgears.

REFERENCES( I ) Nakanishi. T.. Ariura, Y ., and Ueno, T ., "Load-Carrying Capacity of the

HB = 340(Harden ed and Temper ed) Gears."T r a m .J p . o c. M ec h. E n g. ,53 ,49 2. pp 1829-1935. C (1987).(2) El B ahloul, A. M. M ., "Wear of g ear o f circular-arc tooth profile; effec tof helix angle and oil viscosity,"Trib. Intl . , 20, 4, pp 205-209 (1987).(3) Christian,J . B., "Grease Lubrication of Helicopter Transmissions," . Lubr.

Eng. , ASLE Tram . , 30, 7, pp 340-350 (1974).(4 ) Lane, T. B.. and Hughes. J . R . , "A study of the oil-film formation ingears by electrical resistance measurements," British Jou r. of A#[. Phys.,

3, 10, pp315-318 (1952).(5) MacConochie, I . 0. and C ameron, A,. "T he Measurement o f Oil-Film

Thickness in Gear Teeth,"J. Bmic Eng., T ram . ASME, 8 2 , 4, pp 29-34(1960).(6 ) Ichimaru, K., Kinoshita, K . , and Hirano, F., "A Study of Oil-Film For-

mation with Dynamic Behavior and Pitting tbr Spur Gears,"J p . Soc.Mech. Eng.. 44 , 388. pp 4322-4331 (1978).

(7 ) Akin, L. S.. "EHD Lubriant Film Thickness Formulas for Power Trans-missions Gears," . Lubr. Technol. , Tram .A S ME , 96.7 , pp 426-436 (1974).

(8) Poon. S. Y . , "An Experimental Study o f Grease in Elastohydrodynamicfollows: Lubrication,"J . LUG.echnol. , ~ran. s ; SME , 94 , 1 , p p 2 7 3 4 ( 1 97 2) .

Allowable Surface Durability in Grease Lubricated Gears

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