Gear dynamics and lubrication

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The specification is based on three elastohydrodynamic parameters: film thickness, the extent of metal contact through the lubricant film, and traction/ slip characteristics. These three performance factors were evaluated as functions of load, speed, slip, temperature and surface roughness. Three different lubricants were tested between two rolling-contact discs with various applied loads, both discs being driven but at variable rates to apply variable slip. Film thickness was measured using a special X-ray technique. Film thickness data were found to be a predictable function of viscosity,

Engel P.A. [)lalzm~k tH 9, t'~r I)uc to Percussive hnp~t'l

137

I:.yre " I . S . U . e t)1%letallurgieal r e c h nlquex ig Wear 1)in,at,sis

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The Role o f Tribology in Industry

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R o b e r t s W.II. Trd)t,hJgy and Indu~tt3 in the UK

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Avery tI.S. I lard I a, illg~

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Tribology education: Status and challenges of the "80s

S h a p i r o ~ . I luld-I ihn Bcarln~ Rcsp,,n~c hi 1)3 fldmlc I uJdlng

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( a m e r o n ,~. [!fltv~'r'.ll~, .|llduMr> Inter.itJtli)n In I uhrlcali+ln I'ducalJon

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Ten lubricants were evaluated against the new specification. They were representative of those now in military use and exhibited a broad range of operational performance even though they had all met one of two similar specifications. Some of the lubricants were shown to perform better than others and better under some conditions. Since all perform adequately in service the specification applies stricter conditions than are met in service and allows the best lubricant for a particular application to be selected.

The Wear Control Handbook: the ASME Centennial Research Dudley D.W., (;laeser ~'.A.. Needelman W.M.. RabinowicL E., Sibley I . B . , Peter~on M.B. and Wlnet 9 , . 0 .

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I . i l l m a n n ~'.l-. ( h a r a c l c r i / a l i o n and R e c o g n i l i t m of ~,t,'ear Pnnlarily Due to ('taCk I t)rnlation and Prupagatlon ~uch as (',intact I atigue and I)elaminJthm

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velocity, load and surface roughness. Measurements of percentage film coverage indicated that typical bearings or gears will tend to operate in either a full film mode or a high metal contact mode of lubrication. The dividing line between these two modes of lubrication occurs only when the surfaces are very smooth and the lubricant viscosity is very high. A high percentage of metal contact will occur if the viscosity is low or the surfaces are rough. Traction is a strong function of surface roughness as well as lubrication type and load. Traction measurements are a good indicator of lubricant behaviour in the contact region.

TRIBOLOGY international February 1981

136.

129

134.

Better selection follows an improved specification for lubricants for engine bearings and gears which has been produced following work at the Battelle Columbus Laboratories. The specification incorporates elastohydrodynamic lubrication factors and, compared with existing specifications, establishes stricter criteria which are relevant to specific conditions and which therefore allow the selection of lubricants that are especially good for particular applications.

Bearing design - historical aspects, present technology and future problems (To be pub-

127

(;ro~/ek A.J. gt't~ I uhrtc;~Ime (hls I)) ( ; r J p h i t c T r e a t m e n t + td Pcl/olcurn I)lstlllatc~ (ASI I I'apcr N,~ 8(I-I USA4)

Willetmel P.A., Mahoney I..R. and Kaadah S.K.l.ubricant De~:radatiun and Wear IV, The I It'cot u f Oxidation on the a,~ear o f Pentacrytllri~, I T e t t a l l c p t a n o a l c (ASI.I Paper Nu.

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Selecting lubricants of Industry

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149.

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Further information: a 113-page report "Determination of Lubricant Selection Based on Elastohydrodynamic Film Thickness and Traction Measurement" by T.A. Dow et al, reference N7914384, paper-copy £8.00. Gear d y n a m i c s and lubrication A greater understanding of film thickness and surface temperature in gear teeth contacts, essential in avoiding pitting and scuffing, and particularly important in high-performance gears, results from an analysis undertaken for NASA. The work should enable the design of gears to be placed on a more secure scientific basis since the researchers feel that gear lubrication has previously been poorly understood. Means of predicting variations in dynamic load, surface temperature and lubricant film thickness along the contacting path of a pair of involute spur gears have been developed. The analysis includes a new procedure for determining the dynamic load between gear teeth contacts having a contact ratio greater than unity and considering

a variable stiffness along the line of action. The analysis is based on the most recent theories on film thickness and traction in elastohydrodynamic contacts. The results include two design charts for directly estimating lubricant film thickness. A computer program has been developed from the analysis of gear dynamics, lubricant film thickness, equilibrium temperature and flash temperature. A series of solutions simulate gears of different gear ratio, diametral pitch and face width, subjected to widely varying operating conditions. Dynamic loading is covered in the first part of the report. The dynamic load distribution is plotted as a function of the contact position along the lines of action for speeds below, near and above the resonant frequency of the system. The effects of tip relief and teeth profile errors on the dynamic load are included in the presentation. Dynamic response is expressed by a dynamic load ratio defined as the ratio of the maximum dynamic load along the contacting path to the static load. The ratio is plotted as a function of speed with the damping and contact ratios as parameters. Lubrication performance, the distribution of equilibrium temperature, film thickness and total flash temperature along the contacting path for gears operating at speeds below and above the resonant speed are set out in the second section. Also presented are plots of the minimum film thickness and maximum total flash temperature as functions of width, outside radius and diametral pitch. The effect of tip relief on lubrication performance is also presented. The third section contains two design charts for directly estimating the equilibrium temperature of the gear for a known distribution of heat flux and convective heat transfer coefficient. Further information: a 138-page report “Thermal Elastohydrodynamic Lubrication of Spur Gears” by K.L. Wang and H.S. Cheng, reference T80 2293, available as paper-copy g9.50.*

*TRC, Omington, Kent BR5 3RF, UK. Remittances payable to Department of Industry Microfiche copies f 1.61 including vat.

Bearing wear life More accurate wear life equations than have existed so far for flight control bearings have been developed following research by the Kaman Aerospace Corporation. Strictly speaking, the equations are only valid for one manufacturer’s helicopter flight control bearing in the test conditions. Data are presented, however, which indicate that the wear life equations are suitable for cautious general use. They represent a significant first step in improved bearing selection. The bearings under consideration generally consist of a ball mounted within a metallic shell fitted with a low friction wear surface. The configuration permits motion in one, two or three axes. Conditions of use that were investigated included appropriate ranges of static radial load, cyclic radial load, angle of ball oscillation, phase angle between cyclic radial load and axial load, and contamination. A statistically designed test programme was performed to determine the effect on bearing liner wear life of these variables and their combinations. Wear life equations were derived from the results and their validity tested by running further tests using different conditions. The equations allow a designer to calculate wear and radial play with a statistically accepted confidence level. However, the predictive equations may not be within three times the standard error of the estimate for the very short lives of bearings contaminated by water. The test data appear to support a corollary of the incremental wear theory, namely, that total wear is independent of the sequence with which various parts of a duty cycle are performed. Further information: a 183-page report “Design Criteria for Dry Lubricated Flight Control Bearings” by E.J. Nagy, reference AD-A071 322, available as paper-copy %12.50.*

Lubricating titanium alloys Titanium and many of its alloys are notoriously difficult to lubricate, with high and erratic friction during sliding giving rise to severe wear conditions. Recent NASA work shows that self-lubricating plasma sprayed coatings containing silver may provide the solution. This development renders

titanium and its alloys suitable for many applications involving sliding contact, eg jet engine components, from which they have been excluded despite their high strength and excellent creep and corrosion resistance. The investigation has involved sliding titanium alloy pins against plasmasprayed coatings of pure silver and three composite formulations for long periods at high temperature. Friction coefficients and wear rates of both pins and coatings were much lower than in earlier investigations in which chemical conversion coatings and hard coatings were used. The coatings were applied using commercial apparatus, with an arc current of 350 A at 24 V at a target distance of 100 to 120mm. The coatings investigated were pure silver (PS109), 50% silver-SO% Nichrome (PS108), 35% silver-35% Nichrome30% calcium fluoride (l’s1 06) and 30% silver-30% Nichrome-25% calcium fluoride-15% glass (PSlOl). All the coatings were plasma sprayed to a thickness of 0.17mm on a Nichrome bond coat, itself plasma sprayed to a thickness of 0.07mm. Good coating adhesion was obtained on the titanium alloy substrate. Wear tests were carried out using a rotating disc and a hemispherically tipped titanium alloy pin. The pin had a load of 250 g and was in sliding contact with the coated disc at a velocity of 130 mm/s. A temperature of 430°C was chosen for the tests as being a representative extreme temperature for titanium alloy sliding contact applications (Fig 1). Fig 2 shows comparative results for a hard coating and for uncoated titanium alloy. It can be seen that PSlOl (silver-Nichrome-calcium fluoride-glass) combines low friction coefficient with low pin wear and coating wear. The wear surfaces were smooth and the friction coefficient was steady at 0.2. The 0.17 mm silver coating initially produced a very low friction coefficient but this rose sharply after 5 h. Further tests with 0.02 and 0.07 mm thick silver coatings gave much better results but plastic deformation seems to be a problem with all but the thinnest pure silver coating. Further information: a 1S-page report “Plasma-Sprayed Coatings for Lubrication of a Titanium Alloy in Air at 430°C” by H.E. Sliney and D.W. Wisander, reference T795 101 available as paper-copy g2.00.*