Tribology Handbook 2E Episode 9 pptx

C6 Other liauids 1000 800 -50 0 50 100 TEMPERATURE,? Figure 6.6 The viscosity of various water-based mixtures I .o B 6 4 3 2 io-' B 6 4 3 2 1Ci2 For all practical purposes the above fluids may be classed as Newtonian but other fluids, such as water-in-oil emulsions, are non-Newtonian. The viscosity values given for the typical 40% water-in-oil emulsion are for very low shear rates. For this emulsion the viscosity will decrease by 10% at shear rates of about 3000s-' and by 20% at shear rates of about 10 000s-'. C6.6 Plain bearing lubrication e7 Mineral oils and greases are the most suitable lubricants for plain bearings in most applications. Synthetic oils may be required ifsystem temperatures are very high. Water and process fluids can also be used as lubricants in certain applications. The general characteristics of these main classes of lubricants are summarised in Table 7.1. Table 7.7 Choice of lubricant Table 7.2 Methods of liquid lubricant supply ~~ Lubricant Operating range Remarks Mineral All conditions of Wide range of viscosities oils load and available. Potential speed corrosion problems with certain additive oils (e.g. extreme pressure) (see Table 7.9) Synthetic All conditions if Good high and low oils suitable temperature properties. viscosity Costly available Greases Use restricted to Good where sealing operating against dirt and speeds below moisture necessary 1 to 2 m/s and where motion is intermittent Process Depends on May be necessary to fluids properties of avoid contamination of fluid food products, chemicals, etc. Special attention to design and selection of bearing materials The most important property of a lubricant for plain bearings is its viscosity. If the viscosity is too low the bearing will have inadequate load-carrying capacity, whilst if the viscosity is too high the power loss and the operating temperature will be unnecessarily high. Figure 7.1 gives a guide to the value of the minimum allowable viscosity for a range of speeds and loads. It should be noted that these values apply for a fluid at the mean bearing temperature. The viscosity alf mineral oils falls with increasing tempera- ture. The viscosity/temperature characteristics of typical mineral oils are shown in Figure 7.2. The most widely used methods of supplying lubricating oils to plain bearings are listed in Table 7.2 The tubricatinq properties of greases are determined to a large extent by the viscosity of the base oil and the type of thickener used in their manufacture. The section of this handbook on greases summarises the properties of the various types. Additive oils are not required for plain bearing lubrica- tion but other requirements of the system may demand their use. Additives and certain contaminants may create potential corrosion problems. Tables 7.3 and 7.4 give a guide to additive and bearing material requirements, with examples of situations in which problems can arise. Method .f Main characteristics WblY Exampies __ - Hand Non automatic, irregular. Low-speed, High maintenance cost bearings Drip and Non automatic, Journals in oiling. Low initial cost. cheap journal wick adjustable. some machine feed Moderately efficient. tools, axles Cheap Ring and Automatic, reliable. Journals in collar Efficient, fairly cheap. pumps, feed Mainly horizontal blowers, large bearings electric motors Bath and Automatic, reliable, Thrust bearings, splash efficient. bath only. lubri- Oil-tight housing Engines, cation required process general High initial cost machinery, Pressure Automatic. High-speed and feed Positive and adjustable. heavily Reliable and efficient. loaded High initial cost journal and thrust bearings in machine tools, engines and compressors Notes Pressure oil feed : This is usually necessary when the heat dissipation of the bearing housing and its surroundings is not sufficient to restrict its temperature rise to 20°C or less grooves in the bearing housing. Some common arrangements are shown in Figure 7.3 pressure feed from the centre of the bearing satisfactory performance and long life Journal bearings: Oil must be introduced by means of oil Thrust bearings: These must be lubricated by oil bath or by Cleanliness: Cleanliness of the oil supply is essential for C7.1 c7 Plain bearing lubrication Table 7.3 Principal additives and contaminants Problem Occurs in Requirements Oxidation of IC engines Antioxidant lubricant Steam turbines additives Compressors High-speed gearboxes Scuffiing Gearboxes Extreme- Cam pressure mechanisms additive Deposit formation IC engines Dispersant Compressors additives Excessive wear General of lubricated surfaces Antiwear additives Water IC engines Good contamination Steam turbines demulsification Compressors properties. Turbine- quality oils may be required Dirt particle IC engines Dispersant contamination Industrial plant additives Weak organic IC engines Acid neutraliser acid contamination Strong mineral Diesel engines Acid neutraliser acid Process fluids contamination Rusting IC engines Rust inhibitor Turbines Industrial plant General Plain journal bearings Surface speed, Mean pressure, where u = ndn, ms-' 7 = -, kNm-* n = shaft speed, s-' I = bearing width, m d =shaft diameter, m W Id W = load, kN Minimum allowable viscosity qmin., cP, may be read directly Plain thrust bearings Surface speed, u = nDn, ms-' Mean pressure 7 = KW, kNrn-' ID where n = shaft speed, s-' I = width of bearing ring, m D = mean pad diameter, m W = thrust load, kN (3 Minimum allowable viscosity qrhrust = qmin, Surface speed, ft/min 100 1000 000 1 00 10 11 0.1 1 .o 10.0 3' Surface speed, mls 1 .o 1.01 1.001 3 Figure 7.1 Lubricant wiscosity for plain bearings C7.2 Plain bearing lubrication e7 Table 7.4 Resistance to corrosion of bearing metals Maximum Additive or contaminant Strong mineral Synthetic acids oil Lrnperature, Extreme-pressure Weak organic “C additive Antioxidant acidc Lead-base white metal 130 Good Good Moderatelpoor Fair Good Tin-base whi1:e metal 130 Good Good Excellent Very good Good Copper-lead (without overlay) 170 Good Good Poor Fair Good Lead-bronze (without overlay) 180 Good with good Good Poor Moderate Good quality bronze Aluminium-tin alloy 170 Good Good Good Fair Good Silver 180 Sulphur-contain- Good Good-except Moderate Good ing additives tor sulphur must not be used Phosp hor-bronze 220 Depends on Good Fair Fair Good quality of bronze. Sulphur- ised additives can intensifv corrosion Copper-lead or lead-bronze 170 Good Good Good Moderate Good with suitable overlay Note: corrosion of bearing metals is a complex subject. The above offers a general guide. Special care is required with extreme-pressure lubricants; if in doubt refer to bearing or lubricants supplier. 10 000 5000 3000 2000 1000 500 300 - 200 i ‘;i 100 5! .o 50 E 0 0 4- ._ > 5 30 20 10 7 5 41- -1 0 Temperature, ‘C Figure 7.2 Typical viscosity,’temperature characteristics of mineral oik Bearing temp era ture Lubricant supply rate should be sufficient to restrict the temperature rise through the bearing to less than 20°C. A working estimate of the mean bearing temperature, is given by ebenring = 6svpply + 20, “c Dynamic and Kinematic Viscosity Dynamic Viscosity, q (cP) = Density X Kinematic Viscosity (cst) Viscosity classification grades are usually expressed in terms of Kinematic Viscosities. c7.3 c7 PI a i n bea ri ng I u b ri cat i on Rotation Unidirectional Reversible Housing Unit Split Fixed Variable Unitlsplit I Direction of load AXIAL* W GROOVE @ W AXIAL* GROOVES w CIRCUMFERENTIAL GROOVE CIRCUMFERENTIAL GROOVE * At moderate speeds oil holes may be substituted if l/d does not exceed 1. Note: the load-carrying capacity of bearings with circumferential grooves is somewhat lower than with axial grooves owing to the effect of the groove on pressure generation. Figure 7.3 Oil grooves in journal bearings c7.4 ~~ Rolling bearing lubrication C8 SELECTION OF THE LUBRICANT Table 8.7 General guide for choosing between grease and oil lubrication Factor affecting the choice 1Jse grease Use oil Temperature Up to 120°C-with special greases or short Up to bulk oil temperature of 90°C OF bearing temperature of 200°C-these temperatures may be exceeded with special oils relubrication intervals up to 20O/22O0C Speed factor. IJp to dn factors of 300 000/350 000 (depending Up to dn factors of 450 000/500 000 (drpending on type of bearing) on design) Load Low to moderate All loads up to maximum Bearing design Not for asymmetrical spherical roller thrust All types bearings Housing design Relatively simple Long periods without Yes, depends on operating conditions, especially No More complex seals and feeding devices necessary attention temperature Central oil supply for No-cannot transfer heat efficiently or operate Yes other machine elements hydraulic systems Lowest torque When properly packed can be lower than oil on For lowest torques use a circulating system with which the grease is based scavenge pumps or oil mist Dirty conditiom ~ Yes-proper design prevents entry of con- Yes, if circulating system with filtration taminants * dn factor (bearing bore (mni) x speed (revimin)). Note: for large bearings (> 65 mm bore) use nd, (d, is the arithmetic mean of outer diameter and bore (mm) ) G R EASE LUI B RICATION Grease sellection The principle factors governing the selection of greases for rolling bearings are speed, temperature, load, environ- ment and method of application. Guides to the selection of a suitable grease taking account of the above factors are given in Tables 8.2 and 8.3. The appropriate maximum speeds for grease lubrication of a given bearing type are given in Figure 8.1. The life required from the grease is also obviously important and Figure 8.2 gives a guide to the variation of grease operating life with percentage speed rating and tempera- ture for a high-quality lithium hydroxystearate grease as derived from ]Figure 8.1. (These greases give the highest speed ratings.) When shock loading and/or high operating temperatures tend to shake the grease out of the covers into the bearing, a grease of a harder consistency should be chosen, e.g. a no. 3 grease instead of a no. 2 grease. Note: it should be recognised that the curves in Figures 8.1 and 8.2 can only be a guide. Considerable variations in life are possible depending on precise details of the application, e.g. vibration, air flow across the bcearing, clearances, etc. Table 8.2 The effect of the method of appli- cation on the choice of a suitable grade of grease System NLGI grade no. Air pressure 0 to 2 depending on type up to 3 Pressure-guns or mechanical lubricators Compression cups up to 5 Centralised lubrication 2 or below (a) Systems with separate metering Normally 1 or 2 valves (b) Spring return systems I (c) Systems with multi-delivery 3 pumps C8.1 C8 Rolling bearing lubrication Table 8.3 The effect of environmental conditions on the choice of a suitable type of grease Sbeed maximum Tvbical service temberatur~ /A NLGI hercentage no. maximum for ljpe of grease grade recommended Environment Maximum Minimum Base Oil uiscosi~ Comments (approximate values) grease) "C OF "C "F Lithium Lithium 2 { :P Multi-purpose, not advised at max. speeds or max. temperatures for bearings above 65 mm bore or on up to 140 cSt at 100°F vertical shafts For max. speeds recom- mended where vibration loads occur at high speeds Lithium EP 1 75 Wetordry 90 195 -15 5 Recommended for roll-neck 70 160) - 15 } 14.5 cSt at 210°F bearingsand heavily-load- Wetordry 9o 195 ed taper-roller bearings Lithium EP 2 Calcium 1,2and3 50 Wetordry 60 140 -10 14 140cSt at 100°F (conventional) ~~ Calcium EP 1 and 2 50 Wetordry 60 140 -5 25 14.5 cSt at 210°F Sodium 3 75/100 Dry 80 175 -30 -22 30cSt at 100°F Sometimes contains 20% (conventional) calcium Clay 50 Wetordry 200 390 10 50 550 cSt at 100°F Clay 100 Wetordry 135 275 -30 -22 Up to 140 cSt at 100°F Clay 100 Wet or dry 120 248 -55 -67 12 cSt at 100°F Based on synthetic esters Silicone/lithiurn 75 Wet or dry 200 390 -40 -40 150 cSt at 25°C Not advised for conditions where sliding occurs at high speed and load 200 10 000 8000 S 6000 5 Li 4000 0- L 50 3000 LrY 2500 % 5 LL 1500 a * w 20 c $ 100 5000 f w 0 2000 4 6 m 1000 I 0 20 40 60 80 100 120 140 BEARING BORE, rnrn Figure 8.1 Approximate maximum speeds for grease lubrication. (Basic diagram for calculating bearing speed ratings) .Vultipl_v bearing speed from Figure 8.1 bv this, factor to get /he maximum speed for each Qpe of bearing Bearing gpe - Cage centred on As Figure 8.1 3 Pressed cages 1.5-1.75 x centred on -0 rolling elements E & Machined cages 1.75-2.0 2.; centred on 'i: m 5 5 Machined cages 1.25-2.0 2 centredon outer race m inner race .z .e rolling elements Taper- and spherical- roller 0.5 bearings Bearings mounted in 0.75 adjacent pairs Bearings on vertical shafts 0.75 Bearings with rotating outer 0.5 races and fixed inner races C8.2 Rolling bearing lubrication roo 000 50 ooal z W- 20000 Lb d g 10000 : 2 a 8 5000 W r a :: 2000 OI 1300 1000 500 C8 Method of lubrication Rolling bearings may be lubricated with grease by a lubrication system as described in other sections of the handbook or may be packed with grease on assembly. Packing ball and roller bearings with grease (a) The grease should not occupy more than one-half to three-quarters of the total available free space in the covers with the bearing packed full. (b) One or more bearings mounted horizontally-com- pletely fill bearings and space between, if more than one, but fill only two-thirds to three-quarters of space in covers. (c) Vertically-mounted bearings-completely fill bear- ing but fill only half of top cover and three-quarters of 20 40 60 80 100 120 140 TEMPERATURE, 'C bottom cover. Figure 8.2 \/ariation of operating life of a high- quality grade 3 /ithiurn hydroxystearate grease with speed and temperature (d) Low/medium speed bearings in dirty environments -comP1etely bearing and covers. Calculation of relubrication interval The relubrication period for ball and roller bearings may be estimated using Figures 8.1 and 8.2. The following is an example in terms of a typical application: Required to know : Approximate relubrication period for the following: Bearing type : Medium series bearing 60 mm bore. Cage : Pressed cage centred on balls. Speed : 950 rev/nnin. Temperature : 120°C [The bearing temperature (not merely the local ambient tem- perature) i.e. either measured or estimated as closely as possible.] Position : Vertical shaft. Grease : Lithium grade 3, Duty : Continuous. From Figure 8.1 : 60 mm bore position on the lower edge of the graph intersects the medium series curve at approxi- mately 3100 revlmin. Factor for pressed cages on balls is about 1.5; thus 3100 x 1.5 = 4650 revlmin. Factor for vertical mounting is 0.75. Thus 4650~0.75 = 3488 sev/min. This is the rnaximum speed rating ( 100%). Now actual speed = 950 rev/min; therefore percentage - 9 50 348% of maximum = -x 100 = 27% (say 25% approxi- mately). In Figure 8.2 the 12D"C vertical line intersects the 25% speed rating curve for the grade 3 lithium grease at approximately 1300 hours, which is the required answer. Relubrication of ball and roller bearings Relubrication may be carried out in two ways, depending on the circumstances : (a) Replenishment, by which is meant the addition of fresh grease to the original charge. (b) Repacking, which normally signifies that the bearing is dismounted and all grease removed and discarded, the bearing then being cleaned and refilled with fresh grease. An alternative, if design permits, is to flush the bearing with fresh grease in situ. (Grease relief valves have been developed for this purpose.) The quantity required per shot is an arbitrary amount. Requirement is only that sufficient grease is injected to disturb the charge in the bearing and to displace same through the seals, or grease relief valves. A guide can be obtained from Dxw W=- 200 where W is quantity (g) and w is width (mm) D is outside diameter (rnm) If grease relief valves are not fitted, the replenishment charge should not exceed 5% of the original charge. After grease has been added to a bearing, the housing vent plug (if fitted) should be left out for a few minutes after start-up in order to allow excess grease to escape. A better method, if conditions allow, is to push some of the static grease in the cover back into the bearing to redistribute the grease throughout the assembly. This method is likely to be unsatisfactory when operating temperatures exceed about 100°C. C8.3 C8 cst 450 260 170 100 90 80 70 60 50 40 30 20 10 9- 8- 6- c OIL LUBRICATION Oil viscosity selection - - - - - - - - - - - - - - - - 1 - - - - - - - - - 7- Rol I i ng bearing I ubrication 50.0 30.0 20.0 13.0 10.0 9.0 8.0 7.0 Generally, when speeds are moderate, the following minimum viscosities at the operating temperatures are recommended : cSt Ball and cylindrical-roller bearings 12 Spherical-roller thrust bearings 32 The oils will generally be HVI or MVI types containing rust and oxidation inhibitors. Oils containing extreme pressure (EP) additives are normally only necessary for bearings where there is appreciable sliding, e.g. taper- roller or spherical-roller bearings, operating under heavy or shock loads, or if required for associated components, e.g. gears. The nomogram, Figure 8.3, shows how to select more precisely the viscosity needed for known bore and speed when the operating temperatures can be estimated. If the operating temperature is not known or cannot be estimated then the manufacturer's advice should be sought. Spherical-roller bearings 20 - - - - : : : : R" 1500 1000 800 600 400 300 200 150 100 90 80 70 60 50 45 40 2.5 2.0 1.8 1.6 1.5 1.4 VISCOSITY - : - : : 7 SI' 2000 1000 80 0 600 500 400 300 200 150 100 90 80 70 60 50 45 To use Figure 8.3, starting with the right-hand portion of the graph for the appropriate bearing bore and speed, determine the viscosity required for the oil at the working temperature. The point of intersection of the horizontal line, which represents this oil viscosity, and the vertical line from the working temperature shows the grade of oil to be selected. If the point of intersection lies between two oils, the thicker oil should be chosen. Examples: Bearing bore d = 60 mm, speed n = 5000 rev/ min (viscosity at working temperature = 6.8 cSt), with working temperature = 65°C. Select oil S 14 (14 cSt at 50°C approx.) Bearing bore d = 340 mm, speed n = 500 rev/ min (viscosity at working temperature = 13.2 cSt), with working temperature = 80°C. Select oil S 38 (38 cSt at 50°C approx.) 100 130 210°F 150 250400 d, mrn 20 30 40 50 60 70 BO 90 100 110 12OoC 50 75100 200300 500 d.mm R"= Redwood No. 1 seconds; S" = Saybolt Universal seconds, SSU E" = degrees Engler CSt = centistokes Figure 8.3 Graph for the selection of oil for roller bearings (permission of the Skefko Ball Bearing Co. Ltd). The graph has been compiled for a viscosity index of 85, which represents a mean value of the variation of the viscosity of the lubricating oil with temperature. Differences for 95 VI oils are negligible C8.4 Rolling bearing lubrication Application of oil to rolling bearings C8 System Conditions Oil levelsloil flow rates Comments Bath/splash Generally used where speeds are low Bearings on horizontal and vertical A limit in dn value of ooo is some- shafts, immerse half lowest rolling element times quoted, but higher values can be accommodated if churning is not Multi-row bearings on vertical shafts, a problem fully immerse bottom row of elements Oil flingers, drip feed Normally as for bath/splash Flow rate dictated by particular Allows use of lower oil lubricators, etc. application; ensure flow is sufficient level if temperature- rise is too high with to allow operation of bearing below desired or recommended maximum bath/splash temperature - generally between 70°C and 90°C Pressure circulating No real limit to dn value Use oil mist where speeds are very high As a guide, use:* 0.6 cm3/min cmz of The oil flow rate has projected area of bearing generally to be de- (0.d. x width) cided by considera- tion of the operating temperature Oil mist No real limit to dn value Almost invariably used for small bore bearings above 50 000 revlmin, but also used at lower speeds As a guide, use:* 0.1 to 0.3 x bearing bore (cm/2.54) x no. of rows-cm3/ hour Larger amounts are required for pre- loaded units, up to 0.6 x bearing bore (cm/2.54) x no. of rows-cm3/ hour In some cases oil-mist lubrication may be combined with an oil bath, the latter acting as a reserve supply which is par- ticularly valuable when high-speed bearings start to run * It must be emphasised that values obtained will be very approximate and that the manufacturer's advice should be sought on the performancc of equipment of a particular type. C8.5 [...]... 48.1 59. 0 0.756 1.006 1.320 1.820 2.350 0.028 0.036 0.036 0.048 0.048 4.80 6.40 7 .90 9. 50 12.70 - 4.80 6.40 7 .90 9. 50 - 4 0.158 0. 197 0.236 0.315 0. 394 ~~ STEEL 40 Cold drawn, seamless, fully annealed 0.132 0.178 0.240 0.303 0.70 0.70 0 .90 0 .90 0.028 0.028 0.036 0.036 0.110 0.130 0.160 0.240 0.300 0.60 0.80 1.00 1.00 1.20 0.024 0.032 0.0 39 0.0 39 0.047 3.35 4.50 6.10 7.10 I S 0 274 0.70 0 .90 0 .90 1.22... guidance under unusual conditions Redwood seconds Centistokes SAE 0°F - 18°C sip 2 10°F 99 'C 0°F - 18°C 2 10°F 99 °C Min Max Min M a x Min Max 75 - 3250 - - - 13100 - - 80 3250* 2 700 1 - 13100 87600 - - 90 - 14 25 - - 66 107 140 - - 25 43 - - 107 1 79 250 41 - - - 1 79 - Min Max Speed Method Of limitation Quanti9 application m/s(ft/min) Dip < 10 ( . 18°C 2 10°F 99 'C 0°F - 18°C 2 10°F 99 °C Min. Max. Min. Max. Min. Max. Min Max. 75 - 3250 - - - 13100 - - 80 3250* 21 700 - - 13100 87600 - - 90 - 14 25. speeds Lithium EP 1 75 Wetordry 90 195 -15 5 Recommended for roll-neck 70 160) - 15 } 14.5 cSt at 210°F bearingsand heavily-load- Wetordry 9o 195 ed taper-roller bearings Lithium. operating temperatures exceed about 100°C. C8.3 C8 cst 450 260 170 100 90 80 70 60 50 40 30 20 10 9- 8- 6- c OIL LUBRICATION Oil viscosity selection - - - - - - -