Lubrication and Reliability Handbook 2010 Part 9 ppsx

B7 Lubricant biological deterioration B7.2 Comparison of microbial infections in oil emulsions and straight oils ECONOMICS OF INFECTION The total cost of a problem is rarely concerned with the cost of the petroleum product infected but is made up from some of the following components: 1 Direct cost of replacing spoiled oil or emulsion. 2 Loss of production time during change and con- sequential down-time in associated operations. 3 Direct labour and power costs of change. 4 Disposal costs of spoiled oil or emulsion. 5 Deterioration of product performance particularly: (a) surface finish and corrosion of product in machining; (b) staining and rust spotting in steel rolling; (c) ‘pick-up’ in aluminium rolling. 6 Cost of excessive slime accumulation, e.g. overloading centrifuges, ‘blinding’ grinding wheels, blocking filters. 7 Wear and corrosion of production machinery, blocked pipe-lines, valve and pump failure. 8 Staff problems due to smell and possibly health. ANTI-MICROBIAL MEASURES These may involve: 1 Cleaning and sterilising machine tools, pipework, etc., between charges. 2 Addition of anti-microbial chemicals to new charges of oil or emulsion. 3 Changes in procedures, such as: (a) use of clean or even de-ionised water; (b) continuous aeration or circulation to avoid malodours; (c) prevention of cross-infection; (d) re-siting tanks, pipes and ducts, eliminating dead legs; (e) frequent draining of free water from straight oils; (f) change to less vulnerable formulations 4 Continuous laboratory or on-site evaluation of infec- tion levels. Physical methods of controlling infection (heat, u.v., hard irradiation) are feasible but chemical methods are more generally practised for metal working fluids. Heat is sometimes preferred for straight oils. There is no chemical ‘cure-all’, but for any requirement the follow- ing important factors will affect choice of biocide. 1 Whether water or oil solubility is required – or both. 2 Speed of action required. Quick for a ‘clean-up’, slow for preventing re-infection. 3 pH of system – this will affect the activity of the biocide and, conversely, the biocide may affect the pH of the system (most biocides are alkaline). 4 Identity of infecting organisms. 5 Ease of addition – powders are more difficult to measure and disperse than liquids. 6 Affect of biocide on engineering process; e.g. reaction with oil formulation, corrosive to metals present. 7 Toxicity of biocide to personnel – most care needed where contact and inhalation can occur – least potential hazard in closed systems, e.g. hydraulic oils. 8 Overall costs over a period. 9 Environmental impact on disposal. Most major chemical suppliers can offer one or more industrial biocides and some may offer an advisory service. B8Component performance analysis B8.1 A useful condition monitoring technique is to check the performance of components, to check that they are performing their intended function correctly. COMPONENT PERFORMANCE The technique for selecting a method of monitoring a component is to decide what function it is required to perform and then to consider the various ways in which that function can be measured. Table 8.1 Methods of monitoring the performance of fixed components for fault detection B8 Component performance analysis B8.2 Table 8.2 Methods of monitoring the performance of moving components for fault detection B8Component performance analysis B8.3 Table 8.3 Methods of monitoring the performance of machines and systems for fault detection In addition to monitoring the performance of components, it is also useful to monitor the performance of complete machines and systems. B9 Allowable wear limits B9.1 BALL AND ROLLER BEARINGS If there is evidence of pitting on the balls, rollers or races, suspect fatigue, corrosion or the passage of electrical current. Investigate the cause and renew the bearing. If there is observable wear or scuffing on the balls, rollers or races, or on the cage or other rubbing surfaces, suspect inadequate lubrication, an unacceptable load or misalignment. Investigate the cause and renew the bearing. ALL OTHER COMPONENTS Wear weakens components and causes loss of efficiency. Wear in a bearing may also cause unexpected loads to be thrown on other members such as seals or other bearings due to misalignment. No general rules are possible because conditions vary so widely. If in doubt about strength or efficiency, consult the manufacturer. If in doubt about misalignment or loss of accuracy, experi- ence of the particular application is the only sure guide. Bearings as such are considered in more detail below. JOURNAL BEARINGS, THRUST BEARINGS, CAMS, SLIDERS, etc. Debris If wear debris is likely to remain in the clearance spaces and cause jamming, the volume of material worn away in intervals between cleaning should be limited to 1 ⁄ 5 of the available volume in the clearance spaces. Surface treatments Wear must not completely remove hardened or other wear resistant layers. Note that some bearing materials work by allowing lubricant to bleed from the bulk to the surface. No wear is normally detectable up to the moment of failure. In these cases follow the manufacturer’s maintenance recommendations strictly. Some typical figures for other treatments are: Surface condition Roughening (apart from light scoring in the direction of motion) usually indicates inadequate lubrication, over- loading or poor surfaces. Investigate the cause and renew the bearing. Pitting usually indicates fatigue, corrosion, cavitation or the passage of electrical current. Investigate the cause. If a straight line can be drawn (by eye) across the bearing area such that 10% or more of the metal is missing due to pits, then renew the components. Scoring usually indicates abrasives either in the lubri- cant or in the general surroundings. Journal bearings with smoothly-worn surfaces The allowable increase in clearance depends very much on the application, type of loading, machine flexibilities, importance of noise, etc., but as a general guide, an increase of clearance which more than doubles the original value may be taken as a limit. Wear is generally acceptable up to these limits, subject to the preceding paragraphs and provided that more than 50% of the original thickness of the bearing material remains at all points. Thrust bearings, cams, sliders, etc. with smoothly-worn surfaces Wear is generally acceptable, subject to the preceding paragraphs, provided that no surface features (for example jacking orifices, oil grooves or load generating profiles) are significantly altered in size, and provided that more than 50% of the original thickness of the bearing material remains at all points. CHAINS AND SPROCKETS For effects of wear on efficiency consult the manu- facturers. Some components may be case-hardened in which case data on surface treatments will apply. CABLES AND WIRES For effects of wear on efficiency consult the manu- facturer. Unless there is previous experience to the contrary any visible wear on cables, wires or pulleys should be investigated further. METAL WORKING AND CUTTING TOOLS Life is normally set by loss of form which leads to unacceptable accuracy or efficiency and poor surface finish on the workpiece. B10Failure patterns and analysis B10.1 THE SIGNIFICANCE OF FAILURE Failure is only one of three ways in which engineering devices may reach the end of their useful life. In the design process an attempt is usually made to ensure that failure does not occur before a specified life has been reached, or before a life limit has been reached by obsolescence or completion. The occurrence of a failure, without loss of life, is not so much a disaster, as the ultimate result of a design compromise between perfection and economics. When a limit to operation without failure is accepted, the choice of this limit depends on the availability required from the device. Availability is the average percentage of the time that a device is available to give satisfactory performance during its required operating period. The availability of a device depends on its reliability and maintainability. Reliability is the average time that devices of a particular design will operate without failure. Maintainability is measured by the average time that devices of a particular design take to repair after a failure The availability required, is largely determined by the application and the capital cost. FAILURE ANALYSIS The techniques to be applied to the analysis of the failures of tribological components depend on whether the failures are isolated events or repetitive incidents. Both require detailed examination to determine the primary cause, but, in the case of repeated failures, establishing the temporal pattern of failure can be a powerful additional tool. Investigating failures When investigating failures it is worth remembering the following points: (a) Most failures have several causes which combine together to give the observed result. A single cause failure is a very rare occurrence. (b) In large machines tribological problems often arise because deflections increase with size, while in general oil film thicknesses do not. (c) Temperature has a very major effect on the perform- ance of tribological components both directly, and indirectly due to differential expansions and ther- mal distortions. It is therefore important to check: Temperatures Steady temperature gradients Temperature transients Causes of failure To determine the most probable causes of failure of components, which exist either in small numbers, or involve mass produced items the following procedure may be helpful: 1 Examine the failed specimens using the following sections of this Handbook as guidance, in order to determine the probable mode of failure. 2 Collect data on the actual operating conditions and double check the information wherever possible. 3 Study the design, and where possible analyse its probable performance in terms of the operating conditions to see whether it is likely that it could fail by the mode which has been observed. 4 If this suggests that the component should have operated satisfactorily, examine the various operating conditions to see how much each needs to be changed to produce the observed failure. Investigate each operating condition in turn to see whether there are any factors previously neglected which could produce sufficient change to cause the failure. Figure 10.1 The relationship between availability, reliability and maintainability. High availabilities can only be obtained by long lives or short repair times or both B10 Failure patterns and analysis B10.2 Repetitive failures Two statistics are commonly used:- 1 MTBF (mean time between failures) = L 1 + L 2 + +L n n where L 1 , L 2 , etc., are the times to failure and n the number of failures. 2 L 10 Life, is the running time at which the number of failures from a sample population of components reaches 10%. (Other values can also be used, e.g. L 1 Life, viz the time to 1% failures, where extreme reliability is required.) MTBF is of value in quantifying failure rates, particularly of machines involving more than one failing component. It is of most use in maintenance planning, costing and in assessing the effect of remedial measures. L 10 Life is a more rigorous statistic that can only be applied to a statistically homogeneous population, i.e. nominally identical items subject to nominally identical operating conditions. Failure patterns Repetitive failures can be divided by time to failure according to the familiar ‘bath-tub’ curve, comprising the three regions: early-life failures (infantile mortality), ‘mid-life’ (random) failures and ‘wear-out’. Early-life failures are normally caused by built-in defects, installation errors, incorrect materials, etc. Mid-life failures are caused by random effects external to the component, e.g. operating changes, (overload) lightning strikes, etc. Wear-out can be the result of mechanical wear, fatigue, corrosion, etc. The ability to identify which of these effects is dominant in the failure pattern can provide an insight into the mechanism of failure. As a guide to the general cause of failure it can be useful to plot failure rate against life to see whether the relationship is falling or rising. Figure 10.2 The failure rate with time of a group of similar components Figure 10.3 The failure rate with time used as an investigative method B10Failure patterns and analysis B10.3 Weibull analysis Weibull analysis is a more precise technique. Its power is such that it can provide useful guidance with as few as five repeat failures. The following form of the Weibull probability equation is useful in component failure analysis: F(t) = 1 – exp[␣(t – ␥) ␤ ] where F(t) is the cumulative percentage failure, t the time to failure of individual items and the three constants are the scale parameter (␣), the Weibull Index (␤) and the location parameter (␥). For components that do not have a shelf life, i.e. there is no deterioration before the component goes into service, ␥ = 0 and the expression simplifies to: F(t) = 1 – exp[␣t ␤ ]. The value of the Weibull Index depends on the temporal pattern of failure, viz: early-life failures ␤ = 0.5 random failures ␤ =1 wear out ␤ = 3.4 Weibull analysis can be carried out simply and quickly as follows: 1 Obtain the values of F(t) for the sample size from Table 10.1 2 Plot the observed times to failure against the appro- priate value of F(t) on Weibull probability paper (Figure 10.5). 3 Draw best fit straight line through points. 4 Drop normal from ‘Estimation Point’ to the best fit straight line. 5 Read off ␤ value from intersection on scale. For n > 20 – Calculate approximate values of F(t) from 100(i – 0.3) n + 0.4 where: i is the ith measurement in a sample of n arranged in increasing order. Figure 10.4 The relationship between the value of ␤ and the shape of the failure rate curve Table 10.1 Values of the cumulative per cent failure F(t) for the individual failures in a range of sample sizes B10 Failure patterns and analysis B10.4 Figure 10.5 Weibull probability graph paper B10Failure patterns and analysis B10.5 Figure 10.6 gives an example of 9 failures of spherical roller bearings in an extruder gearbox. The ␤ value of 2.7 suggests wear-out (fatigue) failure. This was confirmed by examination of the failed components. The L 10 Life corresponds to a 10% cumulative failure. L 10 Life for rolling bearings operating at constant speed is given by: L 10 Life (hours) = 10 6 C x nP Where n = speed (rev/min), C = bearing capacity, P = equivalent radial load, x = 3 for ball bearings, 10/3 for roller bearings. Determination of the L 10 Life from the Weibull analysis allows an estimate to be made of the actual load. This can be used to verify the design value. In this particular example, the exceptionally low value of L 10 Life (2500 hours) identified excessive load as the cause of failure. Figure 10.6 Thrust rolling bearing failures on extruder gearboxes [...]...B10 Failure patterns and analysis Figure 10.7 gives an example for 17 plain thrust bearing failures on three centrifugal air compressors The ␤ value of 0.7 suggests a combination of early-life and random failures Detailed examination of the failures showed that they were caused in part by assembly errors, in part of machine surges Figure 10.7 Plain thrust bearing... bearing surface and fatigue in extreme cases, sometimes in nominally lightly loaded areas Local areas of poor bedding on the back of the bearing shell, often around a ‘hard’ spot Causes Causes Entrapment of large particles of dirt (e.g swarf), between bearing and housing, causing distortion of the shell, impairment of heat transfer and reduction of clearance (see next column) Entrapment of dirt particles... journal Foreign matter Fatigue Characteristics Characteristics Severe scoring and erosion of bearing surface in the line of motion, or along lines of local oil flow Cracking, often in mosaic pattern, and loss of areas of lining Causes Causes Contamination of lubricant by excessive amounts of dirt particularly non-metallic particles which can roll between the surfaces Excessive dynamic loading or overheating... Foreign matter Characteristics Wiping Characteristics Fine score marks or scratches in direction of motion, often with embedded particles and haloes Surface melting and flow of bearing material, especially when of low-melting point, e.g whitemetals, overlays Causes Causes Dirt particles in lubricant exceeding the minimum oil film thickness Inadequate clearance, overheating, insufficient oil supply, excessive... During running the cracks extended and the flange collapsed A bearing must never be fitted so that the fitting load is transmitted via the rolling elements Causes Causes Insufficient interference between race and housing Particularly noticeable with heavily loaded bearings having thin outer races Too little interference, often slight clearance, between the inner race and the shaft combined with heavy... surface and of journal; may cause rapid failure in extreme cases Pitting and pick-up on bearing surface Causes Causes Electrical currents from rotor to stator through oil film, often caused by faulty earthing Vibration transmitted from external sources, causing damage while journal is stationary B11.5 B11 Plain bearing failures Overheating Thermal cycling Characteristics Characteristics Extrusion and cracking,... of the weakened material Causes Impact fatigue caused by collapse of vapour bubbles in oil film due to rapid pressure changes Softer overlay (Nos 1, 2 and 3 bearings) attacked Harder aluminium –20% tin (Nos 4 and 5 bearings) not attacked under these particular conditions Causes Formation of organic acids by oxidation of lubricating oil in service Consult oil suppliers; investigate possible coolant... film associated with interrupted flow Rapid advance and retreat of journal in clearance during cycle It is usually associated with the operation of a centrally grooved bearing at an excessive operating clearance B11.3 B11 Plain bearing failures Cavitation erosion Corrosion Characteristics Characteristics Attack of bearing material in isolated areas, in random pattern, sometimes associated with grooves... surfaces under operating loads Excessive interference Misalignment Characteristics Characteristics Distortion of bearing bore causing overheating and fatigue at the bearing joint faces Uneven wear of bearing surface, or fatigue in diagonally opposed areas in top and bottom halves Causes Causes Excessive interference fit or stagger at joint faces during assembly Misalignment of bearing housings on assembly,... Characteristics ‘Sulphur’ corrosion Characteristics Formation of hard black deposit on surface of whitemetal lining, especially in marine turbine bearings Tin attacked, no tin-antimony and copper-tin constituents Deep pitting and attack or copper-base alloys, especially phosphor-bronze, in high temperature zones such as small-end bushes Black coloration due to the formation of copper sulphide Causes Electrolyte . Wear and corrosion of production machinery, blocked pipe-lines, valve and pump failure. 8 Staff problems due to smell and possibly health. ANTI-MICROBIAL MEASURES These may involve: 1 Cleaning and. operating period. The availability of a device depends on its reliability and maintainability. Reliability is the average time that devices of a particular design will operate without failure. Maintainability. motion, often with embedded particles and haloes. Causes Dirt particles in lubricant exceeding the minimum oil film thickness. Foreign matter Characteristics Severe scoring and erosion of bearing