Mechanical Engineer''''s Reference Book 2011 Part 9 pptx

Lubricants (oils and greases) 9/11 Engine type Oil qualily Conditionis API US Military test procsdurci CCMC specilicalions Addilive Ireatment level - Gasoline I Diesel High Low 1 Low Hiah Severe High Low Nil 1 Law High I SHPD = Super High Pnrlormance Diesel PQ = Passenger (Car) Diesel Figure 9.6 Approximate relationship between classifications and test procedures pressure pumps have been developed. Additionally, systems have to provide incrseased power densities, more accurate response, better reliability and increased safety. Their use in numerically controlled machine tools and other advanced control systems creates the need for enhanced filtration. Full Bow filters as fine as 1-10 pm retention capability are now to be found in many hydraulic systems. With the trend toward higher pressures in hydraulic systems the loads on unbalanced pump and motor components become greater and this, coupled with the need for closer fits to contain the higher pressures, can introduce acute lubrication problems. Pumps, one of the main centres of wear, can be made smaller if they can run at higher speeds or higher pressures, but this is only possible with adequate lubrication. For this reason, a fluid with good lubrication properties is used so that ‘hydraulics’ is now almost synonymous with ‘oil hydraulics’ in genera1 industrial applications. Mineral oils are inexpensive and readily obtainable while their viscosity can be matched to a particular job. The hydraulic oil must provide adequate lubrication in the diverse operating conditions associated with the components of the various systems. It must function over an extended temperature range and sometimes under boundary conditions. It will be expected to provide a long, trouble-free service life; its chemical stability must therefore be high. Its wear-resisting properties must be capable of handling the high loads in hydraulic pumps. Additionally, the oil must protect metal surfaces from corrosion and it must both resist emulsification and rapidly release entrained air that, on circulation, would produce foam. Mineral oil alone, no matter how high its quality, cannot adequately carry out all the duties outlined above and hence the majority of hydraulic oils have their natural properties enhanced by the incorporation of four different types of additives. These are: an anti-oxidant, an anti-wear agent, a foam-inhibitor and an anti-corrosion additive. For machines in which accurate control is paramount, or where the range of operating temperatures is wide - or both - oils will be formulated to include a VI improving additive as well. 9.2.6.1 Viscosity Probably the most important single property of a hydraulic oil is its viscosity. The most suitable viscosity for a hydraulic system is determined by the needs of the pump and the circuit; too low a viscosity induces back-leakage and lowers the pumping efficiency, while too high a viscosity can cause overheating, pump starvation and possibly cavitation. 9.2.6.2 Viscosity Index It is desirable that a fluid’s viscosity stays within the pump manufacturer’s stipulated viscosity limits, in order to accom- modate the normal variations of operating temperature. An oil’s viscosity falls as temperature rises; certain oils, however, are less sensitive than others to changes of temperatures. and these are said to have a higher VI. Hydraulic oils are formulated from base oils of inherently high VI, to minimize changes of viscosity in the period from start-up to steady running and while circulating between the cold and hot parts of a system. 3.5- 3.0- / arise; a high figure indicates a high level of compatibility. This system has been superseded by the more accurate Seal Com- patibility Index (SCI), in which the percentage volume swell of a ‘standard‘ nitrile rubber is determined after an immersion test in hot oil. PRESSURE, ATMOSPHERES ABS Figure 9.6 9.2.6.3 Effects of pressure Pressure has the effect of increasing an oil’s viscosity. While in many industrial systems the working pressures are not high enough to cause problems in this respect, the trend towards higher pressures in equipment is requiring the effect to be accommodated at the design stage. Reactions to pressure are much the same as reactions to temperature, in that an oil of high VI is less affected than one of low VI. A typical hydraulic oil’s viscosity is doubled when its pressure is raised from atmospheric to 35 000 kPa (Figure 9.6). 9.2.6.4 Air in the oil In a system that is poorly designed or badly operated, air may become entrained in the oil and thus cause spongy and noisy operation. The reservoir provides an opportunity for air to be released from the oil instead of accumulating within the hydraulic system. Air comes to the surface as bubbles, and if the resultant foam were to become excessive it could escape through vents and cause loss of oil. In hydraulic oils, foaming is minimized by the incorporation of foam-breaking additives. The type and dosage of such agents must be carefully selected, because although they promote the collapse of surface foam they may tend to retard the rate of air release from the body of the oil. 9.2.6.5 Oxidation stability Hydraulic oils need to be of the highest oxidation stability, particularly for high-temperature operations, because oxidation causes sludges and lacquer formation. In hydraylic oils, a high level of oxidation stability is ensured by the use of base oils of excellent quality, augmented by a very effective combination of oxidation inhibitors. A very approximate guide to an oil’s compatibility with rubbers commonly used for seals and hoses is given by the Aniline Point, which indicates the degree of swelling likely to 9.2.6.6 Fire-resistant fluids Where fire is a hazard, or could be extremely damaging, fire-resistant hydraulic fluids are needed. They are referred to as ‘fire resistant’ (FR) so that users should be under no illusions about their properties. FR fluids do not extinguish fires: they resist combustion or prevent the spread of flame. They are not necessarily fireproof, since any fluid will even- tually decompose if its temperature rises high enough. Nor are they high-temperature fluids, since in some instances their operating temperatures are lower than those of mineral oils. FR fluids are clearly essential in such applications as electric welding plants, furnace-door actuators, mining machinery, diecasters, forging plant, plastics machinery and theatrical equipment. When leakage occurs in the pressurized parts of a hydraulic system the fluid usually escapes in the form of a high-pressure spray. In the case of mineral oils this spray would catch fire if it were to reach a source of ignition, or would set up a rapid spread of existing flame. FR fluids are therefore formulated to resist the creation of flame from a source of ignition, and to prevent the spread of an existing fire. Four main factors enter into the selection of a fire-resistant fluid: 1. The required degree of fire-resistance 2. Operational behaviour in hydraulic systems (lubrication performance, temperature range and seal compatibility, for example) 3. Consideration of hygiene (toxicological, dermatological and respiratory effects) 4. cost 9.2.6.1 Types of fluid The fluids available cover a range of chemical constituents, physical characteristics and costs, so the user is able to choose the medium that offers the best compromise for operational satisfaction, fire-resistance and cost effectiveness. Four basic types of fluid are available and are shown in Table 9.4. In a fully synthetic FR fluid the fire resistance is due to the chemical nature of the fluid; in the others it is afforded by the Table 9.4 CETOP classifications of fire-resistant hydraulic fluids Class Description HF-A Oil-in-water emulsions containing a maximum of 20% combustible material. These usually contain 95% water Water-in-oil emulsions containing a maximum of 60% combustible material. These usually contain 40-45% water Water-glycol solutions. These usually contain at least 35% water Water-free fluids. These usually refer to fluids containing phosphate esters, other organic esters or synthesized hydrocarbon fluids HF-B HF-C HF-D CETOP: ComitC European des Transmissions Oleohydrauliques et Pneumatiques. Lubricants (oils and greases) 9/13 equipment. Condensation corrosion effect on ferrous metais, fluid-mixing equipment needed, control of microbial infection together with overall maintaining and control of fluid dilution and the disposal of waste fluid must also be considered. Provided such attention is paid to these design and operating features, the cost reductions have proved very beneficial to the overall plant cost effectiveness. presence of water. The other main distinction between the two groups is that the fully synthetic fluids are generally better lubricants and are available for use at operating temperatures up to 150"C, but are less likely to be compatible with the conventional sealing materials and paints than are water-based products. When a water-based fluid makes contact with a flame or aaa hot surface its water component evaporates and forms a steam blanket which displaces oxygen from around the hot area, and this obviates the risk of fire. Water-based products all contain at least 35% water. Because water can be lost by evaporation, they should not be subjected to operating temperatures above about 60°C. Table 9.5 shows a comparison of oil and FR fluids. 9.2.6.8 High water-based hydraulic fluids For a number of years HF-A oil-in-water emulsions have been used as a fire-resistant hydraulic medium for pit props. Concern over maintenance costs and operational life has created interest in a better anti-wear type Buid. Micro- emulsions are known to give better wear protection than the normal oil-in-water emulsions. At the same time the car industi-y, in attempts to reduce Costs especially from leakages on production machinery, has evaluated the potential for using HWBHF in hydraulic systems. As a result, in many parts of industry, not only those where fire-resistant hydraulic fluids are needed, there is a increasing interest in the use of HWBHIF. Such fluids, often referred to as 5/95 fluid (that being the ratio of oil to water), have essentially the same properties as water with the exception of the corrosion characteristics and the boundary lubrication properties which are improved by the oil and other additives. The advantages of this type of fluid are fire resistance, lower fluid cost, no warm-up time, lower power consumption and operating temperatures, reduced spoilage of coolant, less dependence on oil together with reduced transport, storage, handling and disposal costs, and environmental benefits. In considering these benefits the the user should not over- look the constraints in using such fluids. They can be summa- rized as limited wear and corrosion protection (especially with certain metals), increased leakage due to its low viscosity, limited operating temperature range and the need for additional mixing and in-service monitoring faciiities. Because systems are normally not designed for use with this type of fluid, certain aspects should be reviewed with the equipment and fluid suppliers before a decision to use such tluids can be taken. These are compatibility with filters, seals. gaskets, hoses, paints and any non-ferrous metals used in the Table 9.5 Comparison of oil and FR fluids Fire resistance Relative density Viscosity Index Vapour pressure Special seals Special paints RdSt protection Mineral 011 Poor 0.87 High Low NO NO Very good Water-in-oil emulsion Fair 0.94 High High Partly No Good Water- giycol Excellent 1.08 High High Partly Yes Fair Phosphate ester Good 1.14 Low Low Yes Yes Fair 9.2.6.9 Care of hydraulic oils and systems Modern additive-treated oik are so stable that deposits and sludge formation in norma! conditions have been almost eliminated. Consequentiy, the service life of the oils which is affected by oxidation, thermal degradation and moisture is extended. Solid impurities must be continuously removed because hydraulic systems are self-contaminating due to wear of hoses, seals and metal parts. Efforts should be made to exclude all solid contaminants from the system altogether. Dirt is intro- duced with air, the amount of airborne impurities varying with the environment. The air breather must filter to at least the same degree as the oil filters. It is impossible to generalize about types of filter to be used. Selection depends on the system, the rate of contamination build-up and the space available. However, a common ar- rangement is to have a full-flow filter unit before the pump with a bypass filter at some other convenient part of the system. Many industrial systems working below 13 500 kPa can tolerate particles in the order of 25-50 pm with no serious effects on either valves or pumps. Provided that the system is initially clean and fitted with efficient air filters, metal edge-strainers of 0.127 mm spacing appear to be adequate, although clearances of vane pumps may be below 0.025 mm. It should be remembered that an excessive pressure drop, due to a clogged full-flow fine filter, can do more harm to pumps by cavitation than dirty oil. If flushing is used to clean a new system or after overhaul it should be done with the hydraulic oil itself or one of lighter viscosity and the same quality. As the flushing charge circulates it should pass through an edge-type paper filter of large capacity. It is generally preferable to use a special pump rather than the hydraulic pump system, and the temperature of the oil should be maintained at about 40°C without local overheating. 9.2.7 Machine tools Lubricants are the lifeblood of a machine tool. Without adequate lubrication, spindles would seize, slides could not slide and gears would rapidly distintegrate. However, the reduction of bearing friction, vital though it is, is by no means the only purpose of machine-tool lubrication. Many machines are operated by hydraulic power, and one oil may be required to serve as both lubricant and hydraulic fluid. The lubricant must be of correct viscosity for its application, must protect bearings, gears and other moving parts against corrosion, and, where appropriate, must remove heat to preserve working accuracies and aligments. It may additionally serve to seal the bearings against moisture and contaminating particles. In some machine tools the lubricant also serves the function of a cutting oil, or perhaps needs to be compatible with tlhe cutting oil. In other tools an important property of the lubricant is its ability to separate rapidly and completely from the cutting fluid. Compatibility with the metals, plastics, sealing elements and tube connections used in the machine construction is an important consideration. In machine-tool operations, as in all others, the wisest course for the user is to employ reputable lubricants in the manner recommended by the machine-tool manufacturer and 9/14 Tribology the oil company suppying the product. This policy simplifies the selection and application of machine-tool lubricants. The user can rest assured that all the considerations outlined above have been taken into account by both authorities. The important factors from the point of view of lubrication are the type of component and the conditions under which it operates, rather than the type of machine into which it is incorporated. This explains the essential similarity of lubricating systems in widely differing machines. 9.2.7.1 Bearings As in almost every type of machine, bearings play an important role in the efficient functioning of machine tools. 9.2.7.2 Roller bearings There is friction even in the most highly finished ball or roller bearing. This is due to the slight deformation under load of both the raceway and the rolling components, the presence of the restraining cage, and the ‘slip’ caused by trying to make parts of different diameter rotate at the same speed. In machine tools the majority of rolling bearings are grease- packed for life, or for very long periods, but other means of lubrication are also used (the bearings may be connected to a centralized pressure-oil-feed system for instance). In other cases, oil-mist lubrication may be employed both for spindle bearings and for quill movement. In headstocks and gear- boxes, ball and roller bearings may be lubricated by splash or oil jets. 9.2.7.3 Plain journal bearings Plain bearings are often preferred for relatively low-speed spindles operating under fairly constant loads, and for the spindles of high-speed grinding wheels. These bearings ride on a dynamic ‘wedge’ of lubricating oil. Precision plain bearings are generally operated with very low clearances and therefore require low-viscosity oil to control the rise of temperature. Efficient lubrication is vital if the oil temperature is to be kept within reasonable limits, and some form of automatic circulation system is almost always employed. 9.2.7.4 Multi-wedge bearings The main drawback of the traditional plain bearing is its reliance on a single hydrodynamic wedge of oil, which under certain conditions tends to be unstable. Multi-wedge bearings make use of a number of fixed or rocking pads, spaced at intervals around the journal to create a series of opposed oil wedges. These produce strong radial, stabilizing forces that hold the spindle centrally within the bearing. With the best of these, developed especially for machine tools, deviation of the spindle under maximum load can be held within a few millionths of a centimetre. 9.2.7.5 Hydrostatic bearings To avoid the instabilities of wedge-shaped oils films, a lubricating film can be maintained by the application of pressurized oil (or, occasionally, air) to the bearing. The hydrostatic bearing maintains a continuous film of oil even at zero speed, and induces a strong stabilizing force towards the centre which counteracts any displacement of the shaft or spindle. Disad- vantages include the power required to pressurize the oil and the necessary increase in the size of the filter and circulatory system. 9.2.7.6 Slideways Spindles may be the most difficult machine-tool components to design, but slideways are frequently the most troublesome to lubricate. In a slideway the wedge-type of film lubrication cannot form since, to achieve this, the slideway would need to be tilted. 9.2.7.7 Plain slideways Plain slideways are preferred in the majority of applications. Only a thin film of lubricant is present, so its properties - especially its viscosity, adhesion and extreme-pressure characteristics - are of vital importance. If lubrication breaks down intermittently, a condition is created known a ‘stick-slip’ which affects surface finish, causes vibration and chatter and makes close limits difficult to hold. Special adhesive additives are incorporated into the lubricant to provide good bonding of the oil film to the sliding surfaces which helps to overcome the problems of table and slideway lubrication. On long traverses, oil may be fed through grooves in the underside of the slideway. 9.2.7.8 Hydrostatic slideways The use of hydrostatic slideways - in which pressurized oil or air is employed - completely eliminates stick-slip and reduces friction to very low values; but there are disadvantages in the form of higher costs and greater complication. 9.2.7.9 Ball and roller slideways These are expensive but, in precision applications, they offer the low friction and lack of play that are characteristic of the more usual rolling journal bearings. Lubrication is usually effected by grease or an adhesive oil. 9.2.7.10 Leadscrews and nuts The lubrication of leadscrews is similar in essence to that of slideways, but in some instances may. be more critical. This is especially so when pre-load is applied to eliminate play and improve machining accuracy, since it also tends to squeeze out the lubricant. Leadscrews and slideways often utilize the same lubricants. If the screw is to operate under high unit stresses - due to pre-load or actual working loads - an extreme-pressure oil should be used. 9.2.7.11 Recirculating-ball leadscrews This type was developed to avoid stick-up in heavily loaded leadscrews. It employs a screw and nut of special form, with bearing balls running between them. When the balls run off one end of the nut they return through an external channel to the other end. Such bearings are usually grease-packed for life. 9.2.7.12 Gears The meshing teeth of spur, bevel, helical and similar involute gears are separated by a relatively thick hydrodynamic wedge of lubricating oil, provided that the rotational speed is high enough and the load light enough so as not to squeeze out the lubricant. With high loads or at low speeds, wear takes place if the oil is not able to maintain a lubricating film under extreme conditions. Machine-tool gears can be lubricated by oil-spray, mist, splash or cascade. Sealed oil baths are commonly used, or the gears may be lubricated by part of a larger circulatory system. Lubricants (ails and greases) 9/15 filter, suitable sprays, jets or other distribution devices, and return piping. The most recent designs tend to eiiminate wick feeds and siphon lubrication. Although filtration is sometimes omitted with non-critical ball and roller bearings, it is essential for most gears and for precision bearings of every kind. Magnetic and gauze filters are often used together. To prevent wear of highly finished bearings surfaces the lubricant must contain no particle as large as the bearing clearance. Circulatory systems are generally interlocked electrically or mechanically with the machine drive, so that the machine cannot be started until oil is flowing to the gears and main bearings. Interlocks also ensure that lubrication is maintained as long as the machine is running. Oil sight-glasses at key points in the system permit visual observations of oil flow. 9.2.7.13 Hydraulics The use of hydraulic systems for the setting, operation and control of machine tools has increased significantly. Hydraulic mechanisms being interlinked with electronic controls andor feedbacks control systems. In machine tools, hydraulic systems have the advantage of providing stepless and vibra- tionless transfer of power. They are particularly suitable for the linear movement of tables and slideways, to which a hydraulic piston may be directly coupled. One of the most important features for hydraulic oil is a viscosit y/temperature relationship that gives the best compromise of low viscosity (for easy cold starting) and minimum loss of viscosity at high temperatures (to avoid back-leakage and pumping losses). A high degree of oxidation stability is required to withstand high temperatures and aeration in hydraulic systems. An oil needs excellent anti-wear characteristics to combat the effects of high rubbing speeds and loads that occur in hydraulic pumps, especially in those of the vane type. In the reservoir. the oil must release entrained air readily withoul causing excessive foaming, which can lead to oil starvation. 9.2.7.14 Tramp oil ‘Tramp oil’ is caused when neat slideway, gear, hydraulic and spindle lubricants leak into wster-based cutting fluids and can cause problems such as: Machine deposits @ Reduced bacterial resistance of cutting fluids and subse- quent reduction in the fluid life Reduced surface finish quality of work pieces Corrosion of machine surfaces All these problems directly affect production efficiency. Re- cent developments have led to the introduction of synthetic Lubricants that are fully compatible with all types of water- based cutting fluids. so helping the user to achieve maximum machine output. 9.2.7.15 Lubrication and lubricants The components of a hydraulic system are continuously lubricated by the hydraulic fluid, which must, of course, be suitable for this purpose. Many ball and roller bearings are grease- packed for iife, or need attention at lengthy intervals. Most lubrication points, however, need regular replenishment if the machine is to function satisfactorily. This is particularly true of parts suujected to high temperatures. With the large machines, the number of lubricating points or the quantities of lubricants involved make any manual lubrication system impracticable or completely uneconomic. Consequently, automatic lubrication systems are often employed. Automatic lubrication systems may be divided broadly into two types: circulatory and ‘one-shot’ total-loss. These cover, respectively, those components using relatively large amounts of oil. which can be cooled, purified and recirculated, and those in which oil or grease is used once only and then lost. Both arrangements may be used for different parts of the same machine or installatiox. 9.2.7.16 Circulatory lubrication sysiems The circulatory systems used in association with machine tools are generally conventional in nature, although occasionally their exceptional size creates special problems. The normal installation comprises a storage tank or reservoir, a pump and 9.2.7.17 Loss-lubrication systems There are many kinds of loss-lubrication systems. Most types of linear bearings are necessarily lubricated by this means. An increasingly popular method of lubrication is by automatic or manually operated one-shot lubricators. With these devices a metered quantity of oil or grease is delivered to any number of points from a single reservoir. The operation may be carried out manually, using a hand-pump, or automatically, by means of an electric or hydraulic pump. Mechanical pumps are usually controlled by an electric timer, feeding lubricant at preset intervals, or are linked to a constantly moving part of the machine. On some machines both hand-operated and electrically timed one-shot systems may be in use, the manual system being reserved for those components needing infrequent attention (once a day, for example) while the automatic systems feeds those parts that require lubrication at relatively brief intervals. 9.2.7.18 Manual lubrication Many thousands of smaller or older machines are lubricated by hand, and even the largest need regular refills or topping up to lubricant reservoirs. In some shops the operator may be fully responsible for the lubrication of his own machine, but it is nearly always safer and more economical to make one individual responsible for all lubrication. 9.2.7.19 Rationalizing lubricants To meet the requirements of each of the various components of a machine the manufacturer may need to recommend a number of lubricating oils and greases. It follow5 that, where there are many machines of varying origins, a large number of lubricants may seem to be needed. However, the needs of different machines are rarely so different that slight modifica- tion cannot be made to the specified lubricant schedule. ilt is this approach which forms the basis for BS 5063, from which the data in Table 9.6 have been extracted. This classification implies no quality evaluation of lubricants, but merely gives information as to the categories of lubricants likely to be suitable for particular applicatiocs. A survey of the lubrication requirements, usually carried out by the lubricant supplier, can often be the means of significantly reducing the number of oils and greases in a workshop or factory. The efficiency of lubrication may well be increased, and the economies effected are likely to be substan- tial. Table 9.6 Classification of lubricants Class Type of lubricant Viscosity Typical application Detailed application grade no. (BS 4231) Remarks ~ ~ AN Refined mineral oils 68 CB Highly refined mineral oils 32 (straight or inhibited) with 68 good anti-oxidation performance CC Highly rcfined mineral oils 150 with improvcd loading-carrying 320 ability FX Heavily rcfined mineral oils 10 with superior anti-corrosion 22 anti-oxidation performance G Mineral oils with improved lubricity and tackiness performance, and which prevent stick-slip 68 220 General lubrication Total-loss lubrication Enclosed gears ~ general lubrication Pressure and bath lubrication of enclosed gears and allied bearings of headstocks, fced boxes, carriages, etc. when loads are moderate; gears can be of any typc, other than worm and hypoid Heavily loaded gears and worm gears Spindles Slideways Pressure and bath lubrication of encloscd gears of any type, other than hypoid gears, and allied bearings when loads are high, provided that operating temperature is not abovc 70°C Prcssure and bath lubrication of plain or rolling bearings rotating at high speed Lubrication of all typcs of machine tool plain-bearing slideways; particularly required at low traverse speeds to prevent a discontinuous or intermittent sliding of the table (stick-slip) May be rcplaced by CB 68 CB 32 and CB 68 may be used for flood-lubricated mechanically controlled clutches; CB 32 and CB 68 may be replaced by HM 32 and HM 68 May also be used for manual or centralized lubrication of lcad and feed screws May also be used for applications requiring particularly low-viscosity oils, such as fine mechanisms, hydraulic or hydro-pneumatic mechanisms elcctro-magnetic clutches, air line lubricators and hydrostatic bearings May also be used for the lubrication of all sliding parts - lead and feed screws, cams, ratchets and lightly loaded worm gears with intermittent scrvice; if a lower viscosity is required HG 32 may be used. MM Highly refined mineral oils 32 with superior anti-corrosion, 68 anti-oxidation, and anti-wear perf9rmr;iKc Hydraulic systems Operation of general hydraulic systems May also be used for the lubrication of plain or rolling bearings and all types of gears, normally loaded worm and hypoid gears excepted, HM 3X and HM 68 may replace CB 32 and CB 68, respectively HG Refined mineral oils of HM 32 type with anti-stick-slip properties Combined hydraulic and slideways systems Specific application for machines with combined hydraulic and plain bearings, and lubrication systems where discontinuous or intermittent sliding (stick-slip) at low speed is to he prevented May also he used for the lubrication of slideways, when an oil of this viscosity is required Class Type of lubricant Consistency Typical application Detaikd upplicntion number XM Premium quality multi-purpose 1 Plain and rolling bearings XM 1: Centralized systems greases with superior anti-oxidation 2 and anti-corrosion properties 3 and general greasing of miscellaneous parts XM 2: Dispensed by cup or hand gun or in centralized systems XM 3: Normally used in prcpacked applications such as electric motor bearings Nofe: It is essential that lubricants are compatible with the materials used in the construction of machine tools, and particularly with sealing devices. The grease X is sub-divided into consistency numbers, in accordance with the system proposed by the National Lubricating Grease Institute (NLGI) of the USA. These consistency numbers are related to the worked penetration ranges of the greases as follows: Consistency number Worked penetration range 1 310-340 2 265-295 3 22&250 Worked penetration is determined by the cone-penetration method described in BS 5296. 9/18 Tribology 9.2.8 Compressors Compressors fall into two basic categories: positive- displacement types, in which air is compressed by the 'squashing' effect of moving components; and dynamic (turbo)-compressors, in which the high velocity of the moving air is converted into pressure. In some compressors the oil lubricates only the bearings, and does not come into contact with the air; in some it serves an important cooling function; in some it is in intimate contact with the oxidizing influence of hot air and with moisture condensed from the air. Clearly, there is no such thing as a typical all-purpose compressor oil: each type subjects the lubricant to a particular set of conditions. In some cases a good engme oil or a turbine-quality oil is suitable, but in others the lubricant must be special compressor oil (Figure 9.7). 9.2.8.1 Quality and safety Over the years the progressive improvements in compressor lubricants have kept pace with developments in compressor technology, and modern oils make an impressive contribution to the performance and longevity of industrial compressors. More recently a high proportion of research has been directed towards greater safety, most notably in respect of fires and explosions within compressors. For a long time the causes of such accidents were a matter of surmise, but it was noticed that the trouble was almost invariably associated with high delivery temperatures and heavy carbon deposits in delivery pipes. Ignition is now thought to be caused by an exothermic (heat-releasing) oxidation reaction with the carbon deposit, which creates temperatures higher than the spontaneous ignition temperature of the absorbed oil. Experience indicates that such deposits are considerably reduced by careful selection of base oils and antioxidation additives. Nevertheless, the use of a top-class oil is no coMpRmwp n ONEROTOR WOROTORS n n Figure 9.7 Compressor types guarantee against trouble if maintenance is neglected. For complete safety, both the oil and the compressor system must enjoy high standards of care. 9.2.8.2 Specifications The recommendations of the International Standards Organi- zation (ISO) covering mineral-oil lubricants for reciprocating compressors are set out in IS0 DP 6521, under the ISO-L- DAA and ISO-L-DAB classifications. These cover applications wherever air-discharge temperatures are, respectively, below and above 160°C For mineral-oil lubricants used in oil-flooded rotary-screw compressors the classifications ISO- L-DAG and DAH cover applications where temperatures are, respectively, below 100°C and in the 100-110°C range. For more severe applications, where synthetic lubricants might be used, the ISO-L-DAC and DAJ specifications cover both reciprocating and oil-flooded rotary-screw requirements. For the general performance of compressor oils there is DIN 51506. This specification defines several levels of performance, of which the most severe - carrying the code letters VD-L - relates to oils for use at air-discharge temperatures of The stringent requirements covering oxidation stability are defined by the test method DIN 51352, Part 2, known as the Pneurop Oxidation Test (POT). This test simulates the oxidizing effects of high temperature, intimate exposure to air, and the presence of iron oxide which acts as catalyst - all factors highly conducive to the chemical breakdown of oil, and the consequent formation of deposits that can lead to fire and explosion. Rotary-screw compressor mineral oils oxidation resistance is assessed in a modified Pneurop oxidation test using iron naphthenate catalyst at 120°C for 1000 h. This is known as the rotary-compressor oxidation test (ROCOT). up to 220°C. 9.2.8.3 Oil characteristics Reciprocating compressors In piston-type compressors the oil serves three functions in addition to the main one of lubricating the bearings and cylinders. It helps to seal the fine clearances around piston rings, piston rods and valves, and thus minimizes blow-by of air (which reduces efficiency and can cause overheating). It contributes to cooling by dissipating heat to the walls of the crankcase and it prevents corrosion that would otherwise be caused by moisture condensing from the compressed air. In small single-acting compressors the oil to bearings and cylinders is splash-fed by flingers, dippers or rings, but the larger and more complex machines have force-feed lubrication systems, some of them augmented by splash-feed. The cylinders of a double-acting compressor cannot be splash- lubricated, of course, because they are not open to the crankcase. Two lubricating systems are therefore necessary - one for the bearings and cross-head slides and one feeding oil directly into the cylinders. In some cases the same oil is used for both purposes, but the feed to the cylinders has to be carefully controlled, because under-lubrication leads to rapid wear and over-lubrication leads to a build-up of carbon deposits in cylinders and on valves. The number and position of cylinder-lubrication points varies according to the size and type of the compressor. Small cylinders may have a single point in the cylinder head, near the inlet valve; larger ones may have two or more. In each case the oil is spread by the sliding of the piston and the turbulence of the air. In the piston-type compressor the very thin oil film has to lubricate the cylinder while it is exposed to the heat of the Lubricants (oils and greases) 9/49 lubricants in general. However. the close association between refrigerant and lubricant does impose certain additional demands on the oil. Oil is unavoidably carried into the circuit with refrigerant discharging from the compressor. In many installations provision is made for removal of this oil. However, several refrigerants, including most of the halogen refrigerants, are miscible with oil and it is difficult to separate the oil which enters the system which therefore circulates with the refrigerant. In either case the behaviour of the oil in cold parts of the systems is importan?: and suitable lubricants have to have low pour point and low wax-forming characteristics. Effects of contamination The conditions imposed on oils by compressors - particularly by the piston type - are remark- ably similar to those imposed by internal combustion engines. One major difference is, of course, that in a compressor no fuel or products of combustion are present to find their way into the oil. Other contaminants are broadly similar. Among these are moisture, airborne dirt, carbon and the products of the oil’s oxidation. Unless steps are taken to combat them, all these pollutants have the effect of shortening the life of both the oil and the compressor, and may even lead to fires and expiosions. Oxidation High temperature and exposure to hot air are two influences that favour the oxidation and carbonization of mineral oil. In a compressor, the oil presents a large surface area to hot air because it is churned and sprayed in a fine mist, so the oxidizing influences are very strong - especially in the high temperatures of the compressor chamber. The degree of oxidation is dependent mainly on temperature and the ability of the oil to resist, so the problem can be minimized by the correct selection of lubricant and by controlling operating factors. In oxidizing, an oil becomes thicker and it deposits carbon and gummy, resinous substances. These accumulate in the piston-ring grooves of reciprocating compressors and in the slots of vane-type units, and as a result they restrict free movement of components and allow air leakages to develop. The deposits also settle in and around the vaives of piston-type compressors, and prevent proper sealing. When leakage develops, the output of compressed air is reduced, and overheating occurs due to the recompression of hot air and the inefficient operation of the compressor. This leads to abnormally high discharge temperatures. Higher temperature leads to increased oxidation and hence incieased formation of deposits, so adequate cooling of compressors is very important. Airborne dirt In the context of industrial compressors, dust is a major consideration. Such compressors have a very high throughput of air, and even in apparently ‘ciean’ atmospheres, the quantity of airborne dirt is sufficient to cause trouble if the compressor is not fitted with an air-intake filter. Many of the airborne particles in an industrial atmosphere are abrasive, and they cause accelerated rates of wear in any compressor with sliding components in the compressor chamber. The dirt passes into the oil, where it may accumulate and contribute very seriously to the carbon deposits in valves and outlet pipes. Another consideration is that dirt in an oil is likely to act as a catalyst, thus encouraging oxidation. Moisture Condensation occurs in all compressors, and the effects are most prominent where cooling takes place - in intercoolers and air-receivers, which therefore have to be drained at frequent intervals. Normally the amount of moisture present in a compression chamber is not sufficient to affect lubrication, but relatively large quantities can have a compressed air. Such conditions are highly conducive to oxidation in poor-quality oils: and may result in the formation of gummy deposits that settle in and around the piston-ring grooves and cause the rings to stick, thereby allowing blow-by to develop. Rotary compressors - vane type The lubrication system of vane-type compressors varies according to the size and output of the unit. Compressors in the small and ‘portable’ group have neither external cooling nor intercooling, because to effect all the necessary cooling the oil is injected copiously into the incoming air stream or directly into the compressor chamber. This method is known as flood lubrication, and the oil is uisually cooled before being recirculated. The oil is carried out of the compression chamber by the air, so it has to be separated from the air; the receiver contains baffles that ‘knock lout’ the droplets of oil, and they fall to the bottom of the receiver. Condensed water is subsequently separated from the oil in a strainer before the oil goes back into circulation. Vane-type pumps of higher-output are water-jacketed and intercooled: the lubricant has virtually no cooling function so it is employed in far sma!ler quantities. In some units the oil is fed only to the bearings, and the cormal leakage lubricates the vanes and the casing. In others, it is fed through drillings in the rotor and perhaps directly into the casing. This, of course, is a total-loss lubrication technique, because the oil passes out with the discharged air. As in reciprocating units, the oil has to lubricate while being subjected to the adverse influence of high temperature. The vanes impose severe demands on the oil’s lubricating powers. At their tips, for example, high rubbing speeds are combined with heavy end-pressure against the casing. Each time a vane is in the extended position (once per revolution) a severe bending load is being applied between it and the side of its slot. The oil must continue to lubricate between them, to allow the vane to slide freely. It must also resist formation of sticky deposits and varnish, which lead to restricte’d movement olf the vanes and hence to blow-by and, in severe c,ases, to broken vanes. Rotary compressors - screw type The lubrication requirements for single-screw type compressors are not severe, but in oil-flooded rotary units the oxidizing conditions are extremely severe because fine droplets of oil are mixed intimately with hot compressed air. In some screw-type air compressors the rotors are gear driven and do not make contact. In others, one rotor drives the other. The heaviest contact loads occur where power is transmitted from the female to the male rotor: here the lubricant encounters physical conditions similar to those between mating gear teeth. This arduous combination of circumstances places a great demand on the chemical stability, and !ubricating power, of the oil. Other types Of the remaining designs, only the liquid-piston type delivers pressures of the same order as those just men- tioned. The lobe, centrifugal and axial-flow types, are more accurately termed ‘blowers‘, since they deliver air in large volumes at lower pressures. In all four cases only the ‘external’ parts - bearings, gears or both - require lubrication. There- fore the oil is not called upon to withstand the severe service experienced in reciprocating and vane-type compressors. Where the compressor is coupled to a steam or gas turbine a common circulating oil system is employed. High standards of system cleanliness are necessary to avoid deposit formation in the compressor bearings. Refrigerafion compressors The functions of a refrigerator compressor lubricant are the same as those of compressor [...]... speed of the shaft A similar 9/ 28 Tribology log(load) m I log(load) I1 brinelling limit fatigue limit max speed I Wspeed) ~ Figure 9. 10 Figure 9. 8 log(load) log(load) vailable pressure limit reducing viscosity max speed log(sP=4 Figure 9. 9 similar diagram can be drawn for liquid-lubricated hydrodynamic thrust bearings 9. 3.1.2 Liquid-lubricated, hydrostatic bearings (Figure 9. 9) These bearings have a sizeable... the product PV (see Section 9. 1) An upper limit on speed is determined from temperature limitations 9. 3.1.5 Dry bearings (Figure 9. 12) Similar characteristics apply to these bearings as to partially lubricated contacts, but poorer loadkpeed characteristics are exhibited because of the absence of a lubricant 9. 3.2 Bearing selection charts Figures 9. 13 and 9. 14 are taken from reference 6 and indicate the... Specifications have been rigidly laid down after the most exhaustive tests, and it would be unwise, even foolhardy, to depart from the manufacturers’ recommendations No economic gain would result from the use of cheaper, but less efficient, lubricants 9. 2 .9. 3 Performance standards 9. 2 .9 Turbines 9. 2 .9. 1 Steam Although the properties required of a steam-turbine lubricant are not extreme it is the very long periods... power loss (see Figure 9. 19) 170 I I I I I 9. 4.4.7 Bearimg materials The bearing material must be of adequate hardness and strength to support the load, particularly if there is a fatigue element in the load cycle as would be the case, for example, in I I 160 150 140 X - 130 v1 - g 120 Q - 110 3 I 0 _ ;100 E _ n 90 I 0 I 0.1 0.2 I 0.3 I 0.4 // / b'd I 0.5 I I I I I 0.6 0.7 0.8 0 .9 1 Principles and design... gears Figures 9. 25 and 9. 26 show the lines of tooth contact for spur and helical gears 9/ 42 Tribology Line of action Pitch circle ROTATION Figure 9. 24 Relative sliding i n tooth contact area Spur gear showing lines of t o o t h contact As shown in Table 9. 10, the various types of gears may be divided into groups whose conditions of tooth operation, Le sliding and rolling, are similar 9. 5.1 Methods... to excessive forces on the cage, or unwanted skidding of the rolling elements giving rise to severe wear Figure 9. 11 9. 3 I 4 Partially lubricated bearings (Figure 9. 11) These bearings have a lubricant embedded in thz solid material The former slowly escapes into the contact thus providing a partial level of lubrication At low speeds, the maximum load is dictated by the structural strength of the bearing... indicate the operating characteristics of the bearing types in Section 9. 3.1 Figure 9. 13 gives guidance on the type of bearing which has the maximum load capacity at a given speed and shaft size It is based on a life of 10 000 h for rubbing, rolling and porous metal bearings Longer lives may be obtained at reduced loads Bearing selection 9/ 29 and speeds For the various plain bearings, the width is assumed... ratio ( E ) is then obtained from charts of eccentricity ratio against dimensionless load such as Principles and design of hydrodynamic bearings 9/ 33 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 .9 1 Eccentricity ratio Figure 9. 16 Journal bearing: attitude angle Figure 9. 1 7 and the associated minimum film thickness ( h ) determined where: h,,, = (Cd/2).(1 - E) The oil flow, heat generation, etc are also determined... diameter less than 0.1 m 9. 4.4.2 Surface roughness The roughness of engineering surfaces is usually measured by traversing a stylus and recording the undulations as a root mean square (RMS) or centre line average ( R a ) value Typical Ra values for shafts and bearings are: 9/ 34 Tribology 100 c 0.1 I 0 I I 0.1 0.2 I 1 I I I 0.3 0.4 0.5 0.6 0.7 I 0.8 I 0 .9 1 Eccentricity ratio Figure 9. 17 Journal bearing:... successfully applied to diesel engines where the greater part of the dirt particles are under 2 pm in diameter 9. 2.16 Centrifuging The centrifugal separation of solid impurities is adopted either as an alternative to filtration or combined with it For example, a lubricant circulating system can be cleaned by having fixed-element filters that arrest larger particles, and a centrifuge system that removes the . lubricant. 9. 3.2 Bearing selection charts Figures 9. 13 and 9. 14 are taken from reference 6 and indicate the operating characteristics of the bearing types in Section 9. 3.1. Figure 9. 13 gives. fatigue limit max speed - Wspeed) I Figure 9. 10 log(load) max speed - log(sP=4 Figure 9. 11 9. 3. I .4 Partially lubricated bearings (Figure 9. 11) These bearings have a lubricant embedded. emulsion Fair 0 .94 High High Partly No Good Water- giycol Excellent 1.08 High High Partly Yes Fair Phosphate ester Good 1.14 Low Low Yes Yes Fair 9. 2.6 .9 Care of hydraulic

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