The same action would also damage the internal splines of the impeller, but to a lesser extent because of its greater to have been produced by erosion. Thus, the internal splines suffered slight abrasive damage and somewhat more dama.o hardness and thus greater abrasion resistance. However, the internal splines exhibited significant damage, which appear by er Conclusions. Failure of the shaft was the result of excessive wear on the splines caused by vibration and the abrasive action of sand and metal particles Recommendations. To increase resistance to wear and abrasion, the surfaces of the spline teeth should be case hardened. Although the drive shaft exhibited reasonably high strength, its resistance to wear and abrasion was inadequate for the conditions to which it was exposed Wire Wooling. Abrasive-wear failure of shafts by wire wooling has been observed under certain circumstances where contact occurs between the shaft and a stationary part, resulting in removal, by machining, of fine wire shavings that resemble steel wool. This type of failure has been found on turbine and turbine-generator shafts made of 3Cr-05Mo steels, on 12% Cr stainless steels, on 18Cr-8Ni stainless steels, and on nonchromium steels in the presence of certain chloride-containing oils. Wire wooling has also been observed on thrust bearings and on centrifugal compressor shafts Figure 9 shows the end of a 133-cm(5.25-in. )diam shaft that was worn by wire wooling. The worn surface was contact with a labyrinth seal and was not a bearing surface. The shaft had operated for about 3- years before the damage was discovered. Although records were not available, the machine was known to have been opened at least once during that time, but damage under the seal had not been noticed. Figure 9(b) shows a micrograph of a section through the material found in the circumferential grooves in the shaft. The material was loose in the gre identical in appearance with steel wool, whereas other pieces were coarser and more like slivers doves; some was almost (b) Fig 9 Shaft that was severely damaged by wire wooling.(a) End of shaft.() Micrograph of an unetched section through loose material found in grooves worn in shaft by wire wooling 50x Although the mechanisms of wire wooling are not clearly understood, it is known that the process requires contact between a shaft(or shaft sleeve)and a labyrinth(or bearing ), either directly or through buildup of deposits. If the deposit or the stationary part contains hard particles, fine slivers can be cut or spun off the shaft surface. As fine slivers or pieces come off, additional hard particles may be formed by reaction between the steel and the oil or gas present if the resultant
The same action would also damage the internal splines of the impeller, but to a lesser extent because of its greater hardness and thus greater abrasion resistance. However, the internal splines exhibited significant damage, which appeared to have been produced by erosion. Thus, the internal splines suffered slight abrasive damage and somewhat more damage by erosion. Conclusions. Failure of the shaft was the result of excessive wear on the splines caused by vibration and the abrasive action of sand and metal particles. Recommendations. To increase resistance to wear and abrasion, the surfaces of the spline teeth should be case hardened. Although the drive shaft exhibited reasonably high strength, its resistance to wear and abrasion was inadequate for the conditions to which it was exposed. Wire Wooling. Abrasive-wear failure of shafts by wire wooling has been observed under certain circumstances where contact occurs between the shaft and a stationary part, resulting in removal, by machining, of fine wire shavings that resemble steel wool. This type of failure has been found on turbine and turbine-generator shafts made of 3Cr-0.5Mo steels, on 12% Cr stainless steels, on 18Cr-8Ni stainless steels, and on nonchromium steels in the presence of certain chloride-containing oils. Wire wooling has also been observed on thrust bearings and on centrifugal compressor shafts. Figure 9 shows the end of a 13.3-cm (5.25-in.) diam shaft that was worn by wire wooling. The worn surface was in contact with a labyrinth seal and was not a bearing surface. The shaft had operated for about 3 1 2 years before the damage was discovered. Although records were not available, the machine was known to have been opened at least once during that time, but damage under the seal had not been noticed. Figure 9(b) shows a micrograph of a section through the material found in the circumferential grooves in the shaft. The material was loose in the grooves; some was almost identical in appearance with steel wool, whereas other pieces were coarser and more like slivers. Fig. 9 Shaft that was severely damaged by wire wooling. (a) End of shaft. (b) Micrograph of an unetched section through loose material found in grooves worn in shaft by wire wooling. 50× Although the mechanisms of wire wooling are not clearly understood, it is known that the process requires contact between a shaft (or shaft sleeve) and a labyrinth (or bearing), either directly or through buildup of deposits. If the deposit or the stationary part contains hard particles, fine slivers can be cut or spun off the shaft surface. As fine slivers or pieces come off, additional hard particles may be formed by reaction between the steel and the oil or gas present if the resultant
riction and heat are sufficient. Scabs or solid chunks of laminated or compacted slivers and other deposits are sometimes formed(Fig. 9b). Bearings or labyrinths of babbitt or copper alloys have been associated with this type of failure Contact-area atmospheres, in addition to bearing oils, have contained air and methy l chloride Methods for prevention of wire wooling include changing the shaft material, using a softer bearing or labyrinth material, changing to a different oil, eliminating the deposits, and providing greater clearance Adhesive Wear of Shafts. Adhesive wear, sometimes called scoring, scuffing, galling, or seizing, is the result of microscopic welding at the interface between two mutually soluble metals, such as steel on steel. It frequently occurs on shafts where there is movement between the shaft and a mating part, such as a gear, wheel, or pulley Fretting on a shaft can be a source of serious damage. Production of a reddish-brown powder is characteristic of fretting on steel. Typical locations for fretting are at splined or keyed hubs, components that are press fitted or shrink fitted to a shaft, and clamped joints. Shot peening, glass-bead peening, and surface rolling are methods of reducing the possibility of atigue fracture of shafts because of fretting of joints. Fretting is discussed in the article"Fretting Wear Failures"in this Volume Adhesive wear has a characteristic torn appearance because the surfaces actually weld together, then are torn apart by continued motion, creating a series of fractures on both surfaces. This indicates that metal-to-metal contact took place between clean, uncontaminated mating surface Because excessive frictional heat is generated, adhesive wear often can be identified by a change in the microstructure of the metal. For example, steel may be tempered or rehardened locally by the frictional heat generated. Additional information is provided in the article" Fundamentals of Wear Failures"in this volume Influence of Bearings and Seals. If acceptable wear resistance is not obtained by optimizing selection of shaft material and heat treatment, consideration should be given to the sliding(sleeve) bearing and seal material and its compatibility with the shaft. Very often, the high-wear area of a shaft may be chromium plated or may be covered with a sleeve that can methods involve risks, and steps should be taken to avoid or offset their effect. ve shaft or replacing the bearings ).These be discarded and replaced when worn(rather than throwing away an expen Failures of shafts Revised by Donald ]. Wulpi, Metallurgical Consultant Brittle fracture of shafts Brittle fractures are associated with the inability of certain materials to deform plastically in the presence of stress at the root of a sharp notch, particularly at low temperatures. Brittle fractures are characterized by sudden fracturing at extremely high rates of crack propagation, perhaps 1830 m/s(6000 ft/s)or more, with little evidence of distortion in the region of fracture initiation. This type of fracture is frequently characterized by marks known as herringbone or chevron patterns on the fracture surface. The chevrons point toward the origin of the fracture. Additional information on brittle fracture can be found in the article "Mechanisms and Appearances of Ductile and Brittle Fractures"in this Volume Failures of shafts Revised by Donald J. Ulpi, Metallurgical Consultant Ductile fracture of shafts Ductile fractures, which result from microvoid coalescence, exhibit evidence of distortion(plastic flow)at the fracture surface similar to that observed in ordinary tensile-test or torsion-test specimens. When a shaft is fractured by a single application of a load greater than the strength of the shaft, there is usually considerable plastic deformation before fracture. This deformation is often readily apparent upon visual inspection of a shaft that fractured in tension, but is often not obvious when the shaft fractured in torsion. This ability of a material to deform plastically(permanently) is a property known as ductility. The appearance of the fracture surface of Thefileisdownloadedfromwww.bzfxw.com
friction and heat are sufficient. Scabs or solid chunks of laminated or compacted slivers and other deposits are sometimes formed (Fig. 9b). Bearings or labyrinths of babbitt or copper alloys have been associated with this type of failure. Contact-area atmospheres, in addition to bearing oils, have contained air and methyl chloride. Methods for prevention of wire wooling include changing the shaft material, using a softer bearing or labyrinth material, changing to a different oil, eliminating the deposits, and providing greater clearance. Adhesive Wear of Shafts. Adhesive wear, sometimes called scoring, scuffing, galling, or seizing, is the result of microscopic welding at the interface between two mutually soluble metals, such as steel on steel. It frequently occurs on shafts where there is movement between the shaft and a mating part, such as a gear, wheel, or pulley. Fretting on a shaft can be a source of serious damage. Production of a reddish-brown powder is characteristic of fretting on steel. Typical locations for fretting are at splined or keyed hubs, components that are press fitted or shrink fitted to a shaft, and clamped joints. Shot peening, glass-bead peening, and surface rolling are methods of reducing the possibility of fatigue fracture of shafts because of fretting of joints. Fretting is discussed in the article “Fretting Wear Failures” in this Volume. Adhesive wear has a characteristic torn appearance because the surfaces actually weld together, then are torn apart by continued motion, creating a series of fractures on both surfaces. This indicates that metal-to-metal contact took place between clean, uncontaminated mating surfaces. Because excessive frictional heat is generated, adhesive wear often can be identified by a change in the microstructure of the metal. For example, steel may be tempered or rehardened locally by the frictional heat generated. Additional information is provided in the article “Fundamentals of Wear Failures” in this Volume. Influence of Bearings and Seals. If acceptable wear resistance is not obtained by optimizing selection of shaft material and heat treatment, consideration should be given to the sliding (sleeve) bearing and seal material and its compatibility with the shaft. Very often, the high-wear area of a shaft may be chromium plated or may be covered with a sleeve that can be discarded and replaced when worn (rather than throwing away an expensive shaft or replacing the bearings). These methods involve risks, and steps should be taken to avoid or offset their effects. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Brittle Fracture of Shafts Brittle fractures are associated with the inability of certain materials to deform plastically in the presence of stress at the root of a sharp notch, particularly at low temperatures. Brittle fractures are characterized by sudden fracturing at extremely high rates of crack propagation, perhaps 1830 m/s (6000 ft/s) or more, with little evidence of distortion in the region of fracture initiation. This type of fracture is frequently characterized by marks known as herringbone or chevron patterns on the fracture surface. The chevrons point toward the origin of the fracture. Additional information on brittle fracture can be found in the article “Mechanisms and Appearances of Ductile and Brittle Fractures” in this Volume. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Ductile Fracture of Shafts Ductile fractures, which result from microvoid coalescence, exhibit evidence of distortion (plastic flow) at the fracture surface similar to that observed in ordinary tensile-test or torsion-test specimens. When a shaft is fractured by a single application of a load greater than the strength of the shaft, there is usually considerable plastic deformation before fracture. This deformation is often readily apparent upon visual inspection of a shaft that fractured in tension, but is often not obvious when the shaft fractured in torsion. This ability of a material to deform plastically (permanently) is a property known as ductility. The appearance of the fracture surface of a The file is downloaded from www.bzfxw.com
shaft that failed in a ductile manner is also a function of shaft shape, the type of stress to which the shaft was or m al, ductility is decreased by strength of the metal by cold work or heat treatment, by the presence of notches, fillets, holes, scratches, inclusions, and porosity in a notch-sensitive material, by increasing the rate of loading, and for many alloys, by decreasing the temperature Ductile fracture of shafts occurs infrequently in normal service. However, ductile fractures may occur if service requirements are underestimated, if the materials used are not as strong as had been assumed, or if the shaft is subjected to a massive single overload, such as in an accident. Fabricating errors, such as using the wrong material or using material in the wrong heat treated condition(for example, annealed instead of quenched and tempered), can result in ductile fractures Failures of shafts Revised by Donald J. ulpi, Metallurgical Consultant Distortion of shafts Distortion of a shaft can render the shaft incapable of serving its intended function Permanent distortion simply means that the applied stress has exceeded the yield strength(but not the tensile strength)of the material. If it is not feasible to modify the design of the shaft, the yield strength of the shaft material must be increased to withstand the applied stress. Yield strength may be increased either by using a stronger material or by heat treating the original material to a higher strength Creep, by definition, is time-dependent strain( distortion) occurring under stress imposed at elevated temperature, provided the operational load does not exceed the yield strength of the metal. If creep continues until fracture occurs, the part is said to have failed by stress rupture. Creep can result from any type of load (tensile, torsion, compression, bending, and so on) Some high-temperature applications, such as gas turbines and jet aircraft engines, require materials to operate under extreme conditions of temperature and stress with only a limited amount of deformation by creep. In other high-temperature applications, the permissible deformation is high and may not even be limited as long as rupture does not occur during the intended life of the part For this type of service, stress-rupture data, rather than long-term creep data, are used for design Buckling, a third type of distortion failure, results from compressive instability. It can occur if a long slender rod or shaft collapses from compressive axial forces. The load required to cause buckling can be changed only by design changes, not by metallurgical changes, such as heat treatment, in a given type of meta Failures of shafts Revised by Donald ]. Wulpi, Metallurgical Consultant Corrosion of shafts Most shafts are not subjected to severe reduction in life from general corrosion or chemical attack. Corrosion may occur as general surface pitting, may uniformly remove metal from the surface, or may uniformly cover the surface with scale or other corrosion products. Corrosion pits have a relatively minor effect on the load carrying capacity of a shaft, but they do act as points of stress concentration at which fatigue cracks can originate A corrosive environment will greatly accelerate metal fatigue; even exposure of a metal to air results in a shorter fatigue life than that obtained under vacuum. Steel shafts exposed to salt water may fail prematurely by fatigue despite periodic, thorough cleaning. Aerated salt solutions usually attack metal surfaces at the weakest
shaft that failed in a ductile manner is also a function of shaft shape, the type of stress to which the shaft was subjected, rate of loading, and, for many alloys, temperature. In general, ductility is decreased by increasing the strength of the metal by cold work or heat treatment, by the presence of notches, fillets, holes, scratches, inclusions, and porosity in a notch-sensitive material, by increasing the rate of loading, and for many alloys, by decreasing the temperature. Ductile fracture of shafts occurs infrequently in normal service. However, ductile fractures may occur if service requirements are underestimated, if the materials used are not as strong as had been assumed, or if the shaft is subjected to a massive single overload, such as in an accident. Fabricating errors, such as using the wrong material or using material in the wrong heat treated condition (for example, annealed instead of quenched and tempered), can result in ductile fractures. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Distortion of Shafts Distortion of a shaft can render the shaft incapable of serving its intended function. Permanent distortion simply means that the applied stress has exceeded the yield strength (but not the tensile strength) of the material. If it is not feasible to modify the design of the shaft, the yield strength of the shaft material must be increased to withstand the applied stress. Yield strength may be increased either by using a stronger material or by heat treating the original material to a higher strength. Creep, by definition, is time-dependent strain (distortion) occurring under stress imposed at elevated temperature, provided the operational load does not exceed the yield strength of the metal. If creep continues until fracture occurs, the part is said to have failed by stress rupture. Creep can result from any type of loading (tensile, torsion, compression, bending, and so on). Some high-temperature applications, such as gas turbines and jet aircraft engines, require materials to operate under extreme conditions of temperature and stress with only a limited amount of deformation by creep. In other high-temperature applications, the permissible deformation is high and may not even be limited as long as rupture does not occur during the intended life of the part. For this type of service, stress-rupture data, rather than long-term creep data, are used for design. Buckling, a third type of distortion failure, results from compressive instability. It can occur if a long slender rod or shaft collapses from compressive axial forces. The load required to cause buckling can be changed only by design changes, not by metallurgical changes, such as heat treatment, in a given type of metal. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Corrosion of Shafts Most shafts are not subjected to severe reduction in life from general corrosion or chemical attack. Corrosion may occur as general surface pitting, may uniformly remove metal from the surface, or may uniformly cover the surface with scale or other corrosion products. Corrosion pits have a relatively minor effect on the loadcarrying capacity of a shaft, but they do act as points of stress concentration at which fatigue cracks can originate. A corrosive environment will greatly accelerate metal fatigue; even exposure of a metal to air results in a shorter fatigue life than that obtained under vacuum. Steel shafts exposed to salt water may fail prematurely by fatigue despite periodic, thorough cleaning. Aerated salt solutions usually attack metal surfaces at the weakest
points, such as scratches, cut edges, and points of high strain. To minimize corrosion fatigue, it is necessary to select a material that is resistant to corrosion in the service environment or to provide the shaft with a protective Most large shafts and piston rods are not subject to corrosion attack. However, because ship-propeller shafts are exposed to salt water, they are pressure rolled, which produces residual surface-compressive stresses and nhibits origination of fatigue cracks at corrosion pits. Also, rotating parts, such as centrifugal compressor impellers and gas-turbine disks and blades, often corrode. Centrifugal compressors frequently handle gases that contain moisture and small amounts of a corrosive gas or liquid. If corrosion attack occurs, a scale is often formed that may be left intact and increased by more corrosion, eroded off by entrained liquids(or solids),or thrown off from the rotating shaft Stress-corrosion cracking occurs as a resuit of corrosion and stress at the tip of a growing crack. Stres corrosion cracking is often accompanied or preceded by surface pitting, however, general corrosion is often absent, and rapid, overall corrosion does not accompany stress-corrosion cracking The tensile-stress level necessary for stress-corrosion cracking is below the stress level required for fracture without corrosion. The critical stress may be well below the yield strength of the material, depending on the material and the corrosive conditions. Evidence of corrosion, although not al ways easy to find, should be present on the surface of a stress-corrosion-cracking fracture up to the start of final rupture All of the common materials used in shafts may undergo stress-corrosion cracking under certain specific conditions. Factors that influence stress-corrosion cracking, either directly or indirectly, include microstructure yield strength, hardness, corrodent(s), concentration of corrodent(s), amounts and nature of water, pH, and applied and residual stresses, degree of cold working, and chemical composition of the base metal. Additional information is available in the article "Stress-Corrosion Cracking"in this volume Corrosion fatigue results when corrosion and an alternating stress--neither of which is severe enough to cause failure by itself--occur simultaneously; this can cause failure. Once such a condition exists, shaft life will robably be greatly reduced. Corrosion-fatigue cracking is usually transgranular; branching of the main cracks occurs, although usually not as much as in stress-corrosion cracking. Corrosion products are generally present in the cracks, both at the tips and in regions nearer the origins The article"Corrosive Wear Failures" in this Volume contains more detailed information on the effect of combined corrosion and fluctuating stress ilures of shafts Revised by Donald J. Wulpi, Metallurgical Consultant Common Stress Raisers in Shafts Most service failures in shafts are largely attributable to some condition that intensifies stress. In local regions the stress value is raised above a value at which the material is capable of withstanding the number of loading cycles that corresponds to a satisfactory service life Only one small area needs to be repeatedly stressed above the fatigue strength of the material for a crack to be initiated An apparently insignificant imperfection, such as a small surface irregularity, may severely reduce the fatigue strength of a shaft if the stress level at the imperfection is high. The most vulnerable zone in torsional and bending fatigue is the shaft surface; an abrupt change in surface configuration may have a damaging effect, depending on the orientation of the discontinuity to the direction of stress ll but the simplest shafts contain oil holes, keyways, or changes in shaft diameter(threads, fillets, annular grooves, and so on). The transition from one diameter to another, the location and finish of an oil hole, and the type and shape of a key way exert a marked influence on the magnitude of the resulting stress-concentration and fatigue-notch factors, which often range in numerical value from 1 to 5 and sometimes attain values of 10 or Types of Stress Raisers. The majority of stress raisers can be placed into one of the following general groups Thefileisdownloadedfromwww.bzfxw.com
points, such as scratches, cut edges, and points of high strain. To minimize corrosion fatigue, it is necessary to select a material that is resistant to corrosion in the service environment or to provide the shaft with a protective coating. Most large shafts and piston rods are not subject to corrosion attack. However, because ship-propeller shafts are exposed to salt water, they are pressure rolled, which produces residual surface-compressive stresses and inhibits origination of fatigue cracks at corrosion pits. Also, rotating parts, such as centrifugal compressor impellers and gas-turbine disks and blades, often corrode. Centrifugal compressors frequently handle gases that contain moisture and small amounts of a corrosive gas or liquid. If corrosion attack occurs, a scale is often formed that may be left intact and increased by more corrosion, eroded off by entrained liquids (or solids), or thrown off from the rotating shaft. Stress-corrosion cracking occurs as a resuit of corrosion and stress at the tip of a growing crack. Stresscorrosion cracking is often accompanied or preceded by surface pitting; however, general corrosion is often absent, and rapid, overall corrosion does not accompany stress-corrosion cracking. The tensile-stress level necessary for stress-corrosion cracking is below the stress level required for fracture without corrosion. The critical stress may be well below the yield strength of the material, depending on the material and the corrosive conditions. Evidence of corrosion, although not always easy to find, should be present on the surface of a stress-corrosion-cracking fracture up to the start of final rupture. All of the common materials used in shafts may undergo stress-corrosion cracking under certain specific conditions. Factors that influence stress-corrosion cracking, either directly or indirectly, include microstructure, yield strength, hardness, corrodent(s), concentration of corrodent(s), amounts and nature of water, pH, and applied and residual stresses, degree of cold working, and chemical composition of the base metal. Additional information is available in the article “Stress-Corrosion Cracking” in this Volume. Corrosion fatigue results when corrosion and an alternating stress—neither of which is severe enough to cause failure by itself—occur simultaneously; this can cause failure. Once such a condition exists, shaft life will probably be greatly reduced. Corrosion-fatigue cracking is usually transgranular; branching of the main cracks occurs, although usually not as much as in stress-corrosion cracking. Corrosion products are generally present in the cracks, both at the tips and in regions nearer the origins. The article “Corrosive Wear Failures” in this Volume contains more detailed information on the effect of combined corrosion and fluctuating stress. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Common Stress Raisers in Shafts Most service failures in shafts are largely attributable to some condition that intensifies stress. In local regions, the stress value is raised above a value at which the material is capable of withstanding the number of loading cycles that corresponds to a satisfactory service life. Only one small area needs to be repeatedly stressed above the fatigue strength of the material for a crack to be initiated. An apparently insignificant imperfection, such as a small surface irregularity, may severely reduce the fatigue strength of a shaft if the stress level at the imperfection is high. The most vulnerable zone in torsional and bending fatigue is the shaft surface; an abrupt change in surface configuration may have a damaging effect, depending on the orientation of the discontinuity to the direction of stress. All but the simplest shafts contain oil holes, keyways, or changes in shaft diameter (threads, fillets, annular grooves, and so on). The transition from one diameter to another, the location and finish of an oil hole, and the type and shape of a keyway exert a marked influence on the magnitude of the resulting stress-concentration and fatigue-notch factors, which often range in numerical value from 1 to 5 and sometimes attain values of 10 or higher. Types of Stress Raisers. The majority of stress raisers can be placed into one of the following general groups: The file is downloaded from www.bzfxw.com
Group 1: Nonuniformities in the shape of the shaft, such as steps at changes in diameter, broad integral collars, holes, abrupt corners, keyways, grooves, threads, splines, and press-fitted or shrink-fitted attachments Group 2: Surface discontinuities arising from fabrication practices or service damage, such as seams, nicks, notches, machining marks, identification marks, forging laps and seams, pitting, and corrosion Group 3: Internal discontinuities, such as porosity, shrinkage, gross nonmetallic inclusions, cracks, and voIds Most shaft failures are initiated at primary(group 1)stress raisers, but secondary(group 2 or 3)stress raisers may contribute to a failure. For example, a change in shaft diameter can result in stress intensification at the transition zone, if there is a surface irregularity or other discontinuity in this zone, the stress is sharply increased around the discontinuity Failures of shafts Revised by Donald ]. Wulpi, Metallurgical Consultant Influence of changes in Shaft diameter A change in shaft diameter concentrates the stresses at the change in diameter and in the smaller-diameter portion. The effects of an abrupt change and three gradual changes in section on stress concentration are shown schematically in Fig 10. The sharp corner at the intersection of the shoulder and shaft in Fig. 10(a)concentrates the stresses at the corner as they pass from the large to the small diameter. The large-radius fillet shown in Fig. 10(d) permits the stresses to flow with a minimum of restriction. However, the fillet must be tangent with the smaller-diameter section, or a sharp intersection will result, overcoming the beneficial effect of the large-radius fillet (b) (d) Fig. 10 Effect of size of fillet radius on stress concentration at a change in shaft diameter. See text for discussion xample 4: Fatigue Fracture of a 6150 Steel Main Shaft in a Coal Pulverizer Unusual noises were noted by the operator of a ball-and-race coal pulverizer(shown schematically in Fig. 1la), and the unit was taken out of service to investigate the cause. The pulverizer had been in service for ten years
· Group 1: Nonuniformities in the shape of the shaft, such as steps at changes in diameter, broad integral collars, holes, abrupt corners, keyways, grooves, threads, splines, and press-fitted or shrink-fitted attachments · Group 2: Surface discontinuities arising from fabrication practices or service damage, such as seams, nicks, notches, machining marks, identification marks, forging laps and seams, pitting, and corrosion · Group 3: Internal discontinuities, such as porosity, shrinkage, gross nonmetallic inclusions, cracks, and voids Most shaft failures are initiated at primary (group 1) stress raisers, but secondary (group 2 or 3) stress raisers may contribute to a failure. For example, a change in shaft diameter can result in stress intensification at the transition zone; if there is a surface irregularity or other discontinuity in this zone, the stress is sharply increased around the discontinuity. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Influence of Changes in Shaft Diameter A change in shaft diameter concentrates the stresses at the change in diameter and in the smaller-diameter portion. The effects of an abrupt change and three gradual changes in section on stress concentration are shown schematically in Fig. 10. The sharp corner at the intersection of the shoulder and shaft in Fig. 10(a) concentrates the stresses at the corner as they pass from the large to the small diameter. The large-radius fillet shown in Fig. 10(d) permits the stresses to flow with a minimum of restriction. However, the fillet must be tangent with the smaller-diameter section, or a sharp intersection will result, overcoming the beneficial effect of the large-radius fillet. Fig. 10 Effect of size of fillet radius on stress concentration at a change in shaft diameter. See text for discussion. Example 4: Fatigue Fracture of a 6150 Steel Main Shaft in a Coal Pulverizer. Unusual noises were noted by the operator of a ball-and-race coal pulverizer (shown schematically in Fig. 11a), and the unit was taken out of service to investigate the cause. The pulverizer had been in service for ten years