Itami, T.

Ozu, M.

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsInternational Compressor Engineering Conference School of Mechanical Engineering1980Some Electrical Observations of Metallic ContactBetween Lubricated Surfaces Under DynamicConditions of Rotary CompressorM. OzuT. ItamiFollow this and additional works at: https://docs.lib.purdue.edu/icecThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlOzu, M. and Itami, T., "Some Electrical Observations of Metallic Contact Between Lubricated Surfaces Under Dynamic Conditions ofRotary Compressor" (1980). International Compressor Engineering Conference. Paper 315.https://docs.lib.purdue.edu/icec/315SOME ELECTRICAL OBSERVATIONS OF METALLIC CONTACT BETWEEN LUBRICATED SURFACES UNDER DYNAMIC CONDITIONS OF ROTARY COMPRESSOR Masao Ozu, Tsugio Itami Toshiba Corporation, Fuji Works 336, Tadehara, Fuji City, Shizuoka, Japan ABSTRAC'I' This thesis describes study of rotary compressor lubrication under dynamic conditions. The degree of lubrication can be determined by metallic contact that consists of measuring the electrical re-sistance between lubricated surfaces. (Bowden, 1971) The measurements of elec-trical resistance between inside of roller and eccentricity, outside of roller and nose of blade, lower surface of roller and inside flat wall of sub-bearing, showed very important information on determining the extent to which fluid lubrication is present between these surfaces. Our obser-vations have convinced us that it is im-possible to absolutely avoid the metallic contact. This makes it essential that the metallic contact be carefully estimated for effective designing of compressors. INTRODUCTION The effectiveness of a lubrication system in a compressor, especially in a rotary machine is essential. The compression chamber of a rotary compressor needs a lubrication system to ensure durability and performance. Usually, clearances between compression elements are designed as small as 10 ~m to 20 ~m. Larger clear-ances would decrease volumetric efficiency, while smaller clearances may result in less durability of the compressor as heavy metallic contact of the elements can not be avoided. Therefore, it is necessary to find the best clearances and to design an effective lubrication system. ilowever, lubrication conditions in a compression chamber have not been suffi-ciently studied, especially under dynamic conditions. We have tried to solve the problem with a method of electrical resistance between the lubricated surfaces. For the test models, we used high side case hermetic rotary compressors. lOS LUBRICATION OF ROTARY COMPRESSOR Fig. l.Cross section of compressor model ~~~::...jli_:-_·J:;:; roller I I Gas pressure inside compression the chamber forces the lubrication oil out through the clearances because the sliding surfaces that form the chamber. In general, the lubrication system of the rolling piston rotary compressor has been a high side case system which keeps a discharge pressure inside the compressor housing. The system has its merits. The compressor must be equipped with an accumulator t~ prevent refrigerant liquid inside the evaporator from flowing into the compreE'-sion chamber. The system has the following merits. (1) It has a higher volumetric efficiency. (2) It separates oil from discharge gas easilv. (3) Diffe~ence in pressure between the compression chamber and the com-pressor housing maintains a smooth flow of oil through the clearances. (4) Refrigerant iiquid inside the com-pressor housing does not flow into the compression chamber. A test model is shown in Fig. 1. First, the centrifugal oil pump feeds oil to most of the sliding parts. Second, the differ-ence in pressure feeds sufficient oil to most clearances between sliding surfaces, such as the lower surface of the roller and inside flat wall of the sub-bearing, the upper surface of the roller and inside flat wall of the main-bearing, the slot surfaces of the cylinder and side surfaces of the blade. Therefore, the sliding surfaces get sufficiently lubricated, making the compression chamber gas-tight. MEASUREMENT OF METALLIC CONTACT BETWEEN LUBRICATED SURFACES Oil Film Observation Two methods - optical and electrical - have been generally used in observing oil film between lubricated surfaces. The electri-cal method may be further divided into (1) measurement of electrostatic capacity, and (2) measurement of electrical resistance. Electrical resistance of oil film between lubricated surfaces can be measured when the two surfaces areelectri-fied with direct current. Differences in resistance caused by metallic contact between sliding surfaces, can be analyzed from the out-put voltage. Electrical Properties of Oil Film Lubricated film includes hydrodynamic oil film, absorption oil film, and boundary lubrication film. Electrical resistance of refrigeration machine oil is 107- 1013 ~-em when it is mixed in solution with refrigerant 22. When the difference in potential between sliding surfaces grows beyond a certain limit, it causes an electric breakdown of the oil film, and influences wear and tear of sliding surfaces. Magnitude ofelectri-cal breakdown by 0.1 Vis said to be 20 A, which is close to the size of an oil molecule (Kawamura, 1979). Accordingly, we selected a potential difference of 0.1 V in our experimentation. Measurement of Metallic Contacts between Lubricated Surfaces, that is, Measuring Potential Drop across Oil Film When a pair of sliding surfaces are electrically insulated from other elements, the potential drop across the oil film can be measured in a running compressor, keep-ing the performances constant. We tried to apply this original principle. Before the surface of a sliding element was finished, it was drilled and insulated electrodes were fitted into the holes. 106 The whole surface was then finished to the normal standard. The resistance of the oil film between sliding surfaces is that value among an electrode (X), its opposite sliding surface (Y) and the surface fitted with electrode (Z) (For example; see Fig. 2). A sub-bearing fitted with electrodes, and the potential drop measurement system are shown in Figs. 2, 3. Fig. 2. Sub-bearing fitted with electrode, roller (Y) sub-bearing (Z) 0.1 v Fig. 3, Potential drop measurement system, slidirtg surfaces In order to measure resistance, the inserted part and number of electrodes were as follows. Table 1. Inserted part and number of electrodes Symbol Combination of Inserted part Number of sliding surfaces electrodes A inside of roller maximum loaded 2 and eccentricity surface of eccentricity --B outside of roller nOse of blade 3 and eccentricity c lower surface of inside flat 8 roller and inside wall of sub-flat wall of bearing sub-:-bearing When perfect metallic contact arises, the potential drop are as shown in Fig. 4, Fig. 4 Perfect metallic contact. :Jl k 1 revolution A B c potential 0 J drop V - 0.1 llil nn THEORETICAL CONSIDERATIONS OF METALLIC CONTACT Inside of Roller and Eccentricity (A) Fig. Sa. Main force which load a roller. cylinder roller eccentricity The main forces which load a roller are (F6p) the force of gas pressure difference in the cylinder, (Fb) the force of the blade, and (Fe) the centrifugal force o£ the roller. The force of pressure is much larger than the others. The force act as a radial load. The sliding surfaces between A must form a hudrodynamic oil film because the surfaces are filled with oil fed from the centrifugal oil pump. Fig. Sb. Mechanism of journal bearin£• When a roller experiences a force of gas pressure load, calculation of lubrication between A is based on journal bearing theory as follows. Oil film pressure of journal bearing is shown as Reynolds equation. (1) where xoor·B, hooc(l+scos8), sooe/c x"" circumferential co-ordinates, r == shaft radius, 8 ==bearing angle, z"" co-o:::dinates of width, p"" oil film pressure, 107 h == oil film shape, c "" radial clearance, u == tangential speed of shaft, e "" magnitude of eccen trici-ty, n ==viscosity of oil, s == ec-centric rate Note; s is determined by Sommerfeld number (JSME, 1975), On the other hand, Sommerfeld number (s) and minimum oil film thickness (ho) are shown as follows. s = (r/c) 2 (nN/Pm) ho"" c (l- s) where Pm == W/ (dQ,) , (2) (3) N == rotary speed, dQ, = loaded area, d == shaft diameter, Q, = loaded wioth; W = magnitude of load If ho is smaller than the summation of inside surface roughness of roller and surface roughness of eccentricity, lubri-cation condition between A is metallic contact. The calculation of metallic contact in a model compressor is shown in Table 2. where W == DH.6Pm/ (dQ,) D ==roller diameter, H ==roller width .6Pm ==mean pressure difference in the cylinder r == 1 • l em 1 C = l . 1 x 10-3 em 1 D == 4 • 52 em, H == 2 • 5 em, 1'. = l . 4 em, N == 60 Hertz, Summation of inside surface roughness of roller and surface roughness eccentricity= 2 .4Rz Table 2. Calculation of metallic contact ~ Running ondition 1 2 Pressure Discharge 18 22 (kgf/cm2 abs) Suction 5 6 Load Pm (kgf/cm2) 17.5 21.5 Temperature of oil 70 95 between A (oC) Viscosity of oil * (kgf.s/cm2) 5.4xlo-8 2,0XlQ-8 Sommerfeld number 0.34 0.10 Minimum oil film 4.4 2.0 thickness (]Jm) Metallic contact no yes * A solution of refrigerant 22 in oil was investigated in the running compressor, and its solution determined the viscosi-ty of oil (See ASHRAE HAND BOOK, 1976) Accordingly the Table 2, metallic contact can not arise at least under 70°C, but it must arise below 95°C. Outside of Roller and Nose of Blade (B) Although the force between B is not large, the condition of oil film is so bad that the lubrication must be continuously metallic contact because the contact between B is extremely small in area. Lower Surface of Roller and Inside Flat Wall of Sub bear~ng (C) The force which loads the sliding surface in a sub-bearing is only the weight of the roller and it is very small. Although the sliding surfaces between C are covered by oil by pressure difference, the clearance does not £arm a hydrodynamic oil film. The surfaces must. be lubricated by absorption oil film. The metallic contact will vary with running conditions. RESULTS OF EXPERIMENTS Metallic Contacts of Compression Chamber A test model ran under regular conditions with both cooling and heat pump running and the metallic contacts are shown in Fig. 6. Metallic contacts between A, B, C with heat pump running are larger than when the cooling is running. When the heat pump is running are as fo1lows. (1) Metallic contact between A arises for a moment at the rate of once or twice per one hun~ied revolutions. (2) That between B arises approx~rnately at the rate of twice per revolution. (3) That between C arises at the rate of once per revolution. Effect of Running Condit~ons In order to clarify the ~ffect of r~nping, we selected heat pump rupning be~~use metallic contacts during heat pump must be worse than when the cooling is run~ing. Sliding surfaces of measuring are shown in Table 3. Table 3. Sliding surfaces of measuring Running condition A B c Starting yes no no Increase of discharge pressure yes no no Increase of temperature yes no yes Decrease of liquid return n(J no yes Increase of viscosity no no yes Overloaded heat pump yes yes yes Fig. 6 Metallic contact in a compression chamber Indoor temperature ~ 21 °C ·rndoar· ~ 2"rc Running Heat temperature condition pump Ol!tdoor temperature~ 7°C Cooling Outdoor · .;; 35°C 'temperature Pressure Discharge = 17.5 Dischq):'ge ~ 16 (kgf/cm2 abs) S\lction = 5 Suction ~ 5,5 .. Temperatut:e ·in a compression 70°C 65°~ chamber 1 revolution I 100 revolutions ,. ~ ~ A potential 0 f lni. .I I drop O.lv Ill .i 1 I Lll .I I I 1 revolution B potential OI ~ Hll!lllllll!!l!llll· !lllll!llllllli!lll l·llll!!l-'llllii!JIIIU!IIHIIII!III 11 drop O.lv . . 1 revolution c ~k potential OI drop 0.1v 1 1 1 1 1 I I 1 1 II 11 r 1 llllr 1 I. .... _.,_1 .... ~ ·-~-~~··,~-~-·-~f· ~· ·. , .. - " "" .... I " 108 Fig. 7 Effect of restarting after 3 minutes. Between A. Time aftet" 0 sec 40 sec restarting ';;j •rl 0. of '-' 0 <::; 1-< Jill ~~L~~~I:L..J~.WIIItlllll!, il) 'Cl ~ .. .Ll .. ~ ,.,; Fig. 9 Effect of discharge pressure. Between A. Pressure· 17 21 25 kgf/ cm2 abs '";j ·rl 0. ~I ·~ 0 lltJt H f'l r' I nh H Jltldlllli~MH~i. c: 1-< il!'Cl .u 1 i 1 tull·~ ..... !..I.L !.1~.:.1~~ JlU&;ifiiUl!ll•l 2 0 Fig. 8 Effect of starting after 24 hours. Between A. Time after Pressure kgf I cm2 abE Tempera tun 1 starting Discharge Suction oc oj ·.-1 0. 1 second 20 ...., 0 --~ -- p 1-< or ;::'Cl~ 0 • ~ 0 '(ci •.-1 0. 3 minutes 16.8 4.2 40 ...., 0 p 1-< OI il) '"0 i> .L.J '""" 0 • p, a ~· <d•.-1 0. 10 minutes 20.7 5.3 70 .w 0 . p H CI 2::"' ~ ~ ~ 1. Effect of Starting Metallic contact between A when restarting after 3 minutes arises slightly for about 40 seconds. When starting after 24 hours it arises almost at the rate once per revolution. From 3 minutes to 10 minutes after starting, although temperature in the compression chamber, and discharge pressure are increased, metallic contact is clearly reduced because Refrigerant 22 mixing with the oil rapidly decreases and viscosity of oil in reservoir increases. 2. Effect of Discharge Pressure The result is shown in Fig. 9. Metallic contact between A is increased by increase of discharge pressure. 3. Effect of Temperature in the Compression Chamber The results are shown in Fig. 10. Summarily, metallic contact at A arises at the rate once per several hundred revolu-tions when the temperature in the compres-sion chamber is 70°C, at the rate of once per ten revolutions when it is 88°C and at the rate of once per one or two revolution when it is 95°C. revolution 100 t"evolutions ~ 4 ! I .lllliu~ l I I I l l ..... ' 1 ' I I I I I I fj~~~rlli~BO.jlll~dl UI~IJ t. llu i I ,h .1. When it is 95°C metallic contact arises constantly, and the minimum oil film thickness must be equal to the summation of the inside surface roughness of the roller and the surface roughness of the eccentricity. This result almost concides with the consideration. On the other hand, metallic contact between C is nearly a cyclic signal. From the above, we have tried to evaluate metallic contact. Fig. 11 shows the eval-uated model. Fig. 12 illustrates the "ratio of metallic contact- temperature" curve. The most striking results are that ratio of metal-lic contact is almost proportioned to temperature and increases of l0°C double the ratio. Over 85°C, the ratio of metal-lic contact does not increase so much. It seems that the metallic contact is influenced by the extreme pressure proper-ty of oil and distortion of the insioe flat wall of the sub-bearing. 4. Effect of Liquid Return (Flood Back) Fig. 13 showes effect of liquid return. It is seen that the curve is shifted parallel to the right when liquid return changes from violent flood to non-liquid. 109 This fact shows that liquid return breaks down the oil film. Fig. 10 Effect of temperature in a compression chamber. Between A. Tempera tun:. 1 revolution oc ~ .-I "' .,., P< 70 LJ 0 ~:I >:1 H IIJ"' LJ 0 .,_. 0 .-I ctl .,., 0. 88 LJ 0 0 I ,::: ).< IIJ"' ~ ' I .u r; 0 P-; 0 "' .,., 0. ~:I 95 LJ 0 ,::: H I IIJ"' LJ ! 0 P-; 0 Fig. 11 Numerical value of metallic contact I ~1 revolution Potential 0 I drop O.lV --~ ...1\.JL .J\JL m .ig. ""' co .._ r--1 X LJ () «< LJ ,:: 0 () () .,., rl rl ctl LJ II) s '-H 0 0 .,., .u ctl pe: Value 0 0.25 0.50 0.75 1.00 1.2 Effect of temperature in a compression chamber. Between C 60 70 80 90 Temperature in a compression chamber I I I 110 ,_, 100 revolutions -"~+=-..I. l I I u~ . .l Ill ., -. I L ~tl1tt&fu5:l!!~ tU!lhli~tdU.dJ Ill 1 ~"·:•:qr,~~t'U!J .¥1[; 'It '1~ ~~~~~~~~.~-lJ'riiJlti J~J!J][IJ Ew i~ 1'" ' :,lii'iG~: " ' "" I I IJI't,•• ! I II lft l.l'l~ltiHt.. . ' --Fig:. l3 Effect of liguid return and viscosity. Between c ""' co .._ rl .2, '-' () ...,..__violent, VG56 "' LJ 1'1 0 ---non-liquid, VG56 () () .,., ----violent, VGlOO .-I .-I «< LJ ,---II) f:'i N ~ " '-H " 0 / .-I / 0 ~ •ri " LJ ctl " pe: 60 70 80 90 Temperature in a compression chamber (oC) 5. Effect of Viscosity Effect of viscosity is shown in Fig. 13. Two mineral oils - VG56 and VGlOO -were selected. Ratio of metallic contact between (C) of VGlOO is smaller than that of VG56. Fig. 14 shows the "vis;cosity of oil- ratio of metallic contact" chart. If the viscosity increqses the ratios of both VG56 and VGlOO decrease. The ratio of VG56 is larger than the ratio of VGlOO because the solution of R-22 in VG56 is larger than that in VGlOO. Fig. 7 Effect of restarting after 3 minutes. Between A. Time after 0 sec 40 sec restarting "' .,., 0, ~f LJ 0 .I i!l ~ j L\1 I llLJij I llllllillllll I t:: H II! "0 LJ l; 0 Fig. 9 Effect of discharge pressure. Between A. Pressure 17 21 25 kgf/ cm2 abs "' .,., 0, ..., 0 OI ~~~~JrJUI!' I ·rn~~e ~~~ra111~"'~jm C: H 1!!"0 ~ .w , i 1 tl n~· ..... _ .. i.l_ 1.!~,:. lA~' II!M!;LIIU'- !illt ;r 0 Fig. 8 Effect of starting after 24 hours. Between A. Time after Pressure kgf I cm2 ab Temperatur· 1 starting Discharge Suction oc "' .,., 0, LJ 0 1 second 20 C: H o oJ "0 ::>I ..., rl 0 • P-< 0 "' .,., 0, 3 minutes 16.8 4.2 40 .w 0 t:1 H q oJ '"0 ;o-w rl 0 . p 0 .=i "' .,., 0, 10 minutes 20.7 5.3 70 ~ 2 OI II! '"0 :> .u rl 0 • 1. Effect of Starting Metallic contact between A when restarting after 3 minutes arises slightly for about 40 seconds. When starting after 24 hours it arises almost at the rate once per revolution. From 3 minutes to 10 minutes after starting, although temperature in the compression chamber, and discharge pressure are increased, metallic contact is clearly reduced because Refrigerant 22 mixing with the oil rapidly decreases and viscosity of oil in reservoir increases. 2. Effect of Discharge Pressure The result is shown in Fig. 9. Metallic contact between A is increased by increase of discharge pressure. 3. Effect of Temperature in the Compression Chamber The results are shown in Fig. 10. Summarily, metallic contact at A arises at the rate once per several hundred revolu-tions when the temperature in the compres-sion chamber is 70°C, at the rate of once per ten revolutions when it is 88°C and at the rate of once per one or two revolution when it is 95°C. revolution 100 revolutions ~ I l J I I l L 1 L 1 When it is 95°C metallic contact arises constantly, and the minimum oil film thickness must be equal to the summ~tion of the inside surface roughness of the roller and the surface roughness of the eccentricity. This result almost concides with the consideration. On the other hand, metallic contact between C is nearly a cyclic signal. From the above, we have tried to evaluate metallic contact. Fig. 11 shows the eval-uated model. Fig. 12 illustrates the ~ratio of metallic contact- temperature" curve. The most striking results are that ratio of metal-lic contact is almost proportioned to temperature and increases of l0°C double the ratio. Over 85°C, the ratio of metal-lic contact does not increase so much. It seems that the metallic contact is influenced by the extreme pressure proper-ty of oil and distortion of the insioe flat wall of the sub-bearing. 4. Effect of Liquid Return (Flood Back) Fig. 13 showes effect of liquid return. It is seen that the curve is shifted parallel to the right when liquid return changes from violent flood to non-liquid. 109 

Some Electrical Observations of Metallic Contact Between Lubricated Surfaces Under Dynamic Conditions of Rotary Compressor

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1314&amp;context=icec

Some Electrical Observations of Metallic Contact Between Lubricated Surfaces Under Dynamic Conditions of Rotary Compressor

Abstract

Similar works

Full text

Available Versions

Purdue E-Pubs