:Q SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING LESSON FOUR FOUNDATIONS OF bearing轴承 MACHINE DESIGN (PARTD conical roller Bearings With rolling Contact 圆锥滚柱 In bearings with rolling contact, the shaft is needle itt directly or indirectly supported by rolling elements, cylindrical such as balls, cylindrical or conical rollers, or roller needles(ie, cylindrical rollers with a high I/ D ratio).圆柱滚柱 The occasionally encountered name " antifrictionantifriction bearings"suggests that this type has little or no bearing friction. This is erroneous, since the friction is‖减摩轴承 merely of another nature than in journal bearings. In journal bearing bearings with rolling contact, friction losses are‖颈轴承 caused by the elastic deformation of the surfaces in滑动轴承 rolling contact,, sliding friction of rolling elements‖cage保持架 with cages, retaining rings and seals, or with one retaining ring another, and also by some shear of lubricant 档圈
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING LESSON FOUR FOUNDATIONS OF MACHINE DESIGN (PART l) Bearings With Rolling Contact In bearings with rolling contact, the shaft is directly or indirectly supported by rolling elements, such as balls, cylindrical or conical rollers, or needles (i.e., cylindrical rollers with a high L/D ratio). The occasionally encountered name " antifriction bearings" suggests that this type has little or no friction. This is erroneous, since the friction is merely of another nature than in journal bearings. In bearings with rolling contact, friction losses are caused by the elastic deformation of the surfaces in rolling contact, sliding friction of rolling elements with cages, retaining rings and seals, or with one another, and also by some shear of lubricant. conical roller 圆锥滚柱 needle 滚针 cylindrical roller 圆柱滚柱 antifriction bearing 减摩轴承 cage 保持架 retaining ring 档圈 bearing 轴承 journal bearing 颈轴承 滑动轴承
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Characteristics 1. Ball bearings have rolling elements in the form of balls which in all but the most inexpensive types are held in cages, separators, or retainers, and inner and outer grooved races 2. Roller bearings have mainly cylindrical, conical,or barrel-shaped rollers instead of balls, but are otherwise quite similar to ball bearings 3. Needle bearings usually have neither an inner race nor a cage. The needles are retained by integral flanges on the outer race. Transitional types between roller and needle bearings are found in many catalogs
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Characteristics l. Ball bearings have rolling elements in the form of balls, which in all but the most inexpensive types are held in cages, separators, or retainers, and inner and outer grooved races. 2. Roller bearings have mainly cylindrical, conical, or barrel-shaped rollers instead of balls, but are otherwise quite similar to ball bearings. 3. Needle bearings usually have neither an inner race nor a cage. The needles are retained by integral flanges on the outer race. Transitional types between roller and needle bearings are found in many catalogs
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Bearings with rolling contact have no lipstick effect, low starting torque and k Slapstick running friction, and unlike as in journal.滑动面粘附现象 bearings, the coefficient of friction varies little with load or speed. Low starting torque is of great advantage in railroad cars. and the railroad industry has given the main impetus impetus for the development of mass-produced roller剌激,促进 bearings in the past, mainly for this reason These bearings may take both radial and axial loads(depending upon the type), and need less space axially but more radiall (except needle bearings), and less lubrication and maintenance, than journal bearings. They are also noisier and more expensive, and cannot be repaired easily
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Bearings with rolling contact have no slipstick effect, low starting torque and running friction, and unlike as in journal bearings, the coefficient of friction varies little with load or speed. Low starting torque is of great advantage in railroad cars, and the railroad industry has given the main impetus for the development of mass-produced roller bearings in the past, mainly for this reason. These bearings may take both radial and axial loads (depending upon the type), and need less space axially but more radially (except needle bearings), and less lubrication and maintenance, than journal bearings. They are also noisier and more expensive, and cannot be repaired easily. slipstick 滑动面粘附现象 impetus 刺激，促进
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Since rolling elements and raceways are theoretically in point or line contact and thus highly stressed in the contact area by typically cyclical loads, all bearings with rolling contact will eventually fail by fatigue when operated at their rated load Metal failure is typically by shear, just below the surface of raceways or rolling elements--a result of the three principal stresses in the x, y and z axis, all compressive. The 薄片 phenomenon is popularly called spalling and is characterized by the presence of metal flakes in the grease. Even a small bearing 剥落润滑脂 load will induce contact stresses above the yield point (up to YA 500,000 psi; called Hertz stresses after the mathematician who HE analyzed them). As a result, the contact sur faces are under residual compressive stresses, balanced by tensile stresses Ei under the sur face. Friction between rolling elements and races yH causes tensile stresses in the sur face, believed to cause pitting The pits, in turn, are the starting point of fatigue failure crack
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Since rolling elements and raceways are theoretically in point or line contact and thus highly stressed in the contact area by typically cyclical loads, all bearings with rolling contact will eventually fail by fatigue when operated at their rated load. Metal failure is typically by shear, just below the surface of raceways or rolling elements--a result of the three principal stresses in the x, y and z axis, all compressive. The phenomenon is popularly called spalling and is characterized by the presence of metal flakes in the grease. Even a small bearing load will induce contact stresses above the yield point (up to 500,000 psi; called Hertz stresses after the mathematician who analyzed them). As a result, the contact surfaces are under residual compressive stresses, balanced by tensile stresses under the surface. Friction between rolling elements and races causes tensile stresses in the surface, believed to cause pitting. The pits, in turn, are the starting point of fatigue failure crack. 剥 薄 落 片 润 滑 脂 , 黄 油
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING ife Expectancy The life expectancy of ball bearings is not life expectancy uniform and is based upon statistics. Usually,‖(概率预计) rated load values in catalogs give the minimum Ap life(revolutions or hours at a given rpm) for 90% rated load of a group of bearings, the So-called B-10 life e.‖额定载荷 Mean or average life is about three times rated life, while median or 50% survival life is about rated life five times rated life 额定寿命 The considerable spread of the life expectancy curve is due to the many possible forms of favorable or unfavorable interaction between the different bearing elements on the basis of dimensional tolerances, sur face finish and varying structural properties, as well as conditions of service
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Life Expectancy The life expectancy of ball bearings is not uniform and is based upon statistics. Usually, rated load values in catalogs give the minimum life (revolutions or hours at a given rpm) for 90% of a group of bearings, the so-called B-10 life. Mean or average life is about three times rated life, while median or 50% survival life is about five times rated life. The considerable spread of the life expectancy curve is due to the many possible forms of favorable or unfavorable interaction between the different bearing elements on the basis of dimensional tolerances, surface finish and varying structural properties, as well as conditions of service. life expectancy （概率,预计） 寿命 rated load 额定载荷 rated life 额定寿命
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING For plain ball bearings, radial load ratings in pounds(as given in catalogs) are based upon a given rpm and a given number of hours of life with 90% survival. The product rpm X 60 X hr life is then equal to 106. Round per minute Other manufacturers give rated loads on the basis of 106 revolutions instead of hours of life(again, 90% survival). In that case, the number of hours of life can be found by dividing10° by rpm×60. It will be clear from the foregoing that, for a given load, changes in rpm(within the design limits as given by the manufacturer) affect life in hours inversely, thus n,/n,=H,/H 6.5 where n=rpm and H=life (hr ) Changes in load, on the other hand, affect life in revolutions exponentially: thus
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING For plain ball bearings, radial load ratings in pounds (as given in catalogs) are based upon a given rpm and a given number of hours of life with 90% survival. The product rpm × 60 × hr life is then equal to 106 . Other manufacturers give rated loads on the basis of 106 revolutions instead of hours of life (again, 90% survival). In that case, the number of hours of life can be found by dividing 106 by rpm × 60. It will be clear from the foregoing that, for a given load, changes in rpm (within the design limits as given by the manufacturer) affect life in hours inversely; thus, n1 /n2 =H2 /H1 6.5 where n=rpm and H=life (hr.). Changes in load, on the other hand, affect life in revolutions exponentially; thus, Round per minute
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING (F1/F2)=B2/B1 6.6 where F=load (b) and b= life(revolutions). Values of k range between 3 and 4(3 for ball bearings; 3.3 for most roller and needle bearings). Note that for k-3, doubling the load means reducing the life in revolutions by a factor of 8 The relation between B (life in revolutions) and H (life in hours), is as follows B=H×rpm×60 6.7
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING (F1 /F2 ) k = B2 /B1 6.6 where F= load (lb) and B= life (revolutions). Values of k range between 3 and 4 (3 for ball bearings; 3.3 for most roller and needle bearings). Note that for k=3, doubling the load means reducing the life in revolutions by a factor of 8. The relation between B (life in revolutions) and H (life in hours), is as follows: B = H × rpm × 60 6.7
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Other Factors Affecting Bearing Life The rated life of any bearing with rolling contact is based upon proper application conditions, such as adequate lubrication, good alignment alignment, and adherence to recommended 调整 inter ference fit values for the races, if applicable Any negative variance from these conditions adherence will affect fatigue life unfavorably 附着,坚持 On the other hand, life can be extended interference fit considerably by providing superior lubrication.过盈配合 Also. for the same size and materials. the manufacturer can produce a bearing with greater life expectancy by increasing the accuracy of manufacture i.e. by reducing manufacturing tolerances
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING On the other hand, life can be extended considerably by providing superior lubrication. Also, for the same size and materials, the manufacturer can produce a bearing with greater life expectancy by increasing the accuracy of manufacture i.e. by reducing manufacturing tolerances. Other Factors AffectingBearing Life The rated life of any bearing with rolling contact is based upon proper application conditions, such as adequate lubrication, good alignment, and adherence to recommended interference fit values for the races, if applicable. Any negative variance from these conditions will affect fatigue life unfavorably. alignment 调整 adherence 附着，坚持 interference fit 过盈配合
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Static And Dynamic Load In tables giving permissible load data for ball bearings. a distinction is often made between static and dynamic loads, i. e, with the bearing stationary or rotating. The permissible static load is smaller than the permissible dynamic load due indentation he possibility of permanent indentation‖压痕 ( flattening of the contact areas when the bearing is at rest). This phenomenon is called Brinelling. Brinell Some permanent indentation is almost‖布里涅耳 unavoidable due to the high contact stresses however, it has been found that permanent deformations smaller than 10-4 times the rolling element diameter usually do not unfavorably affect the bearings operation
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Static And Dynamic Load In tables giving permissible load data for ball bearings, a distinction is often made between static and dynamic loads, i.e., with the bearing stationary or rotating. The permissible static load is smaller than the permissible dynamic load due to the possibility of permanent indentation (flattening of the contact areas when the bearing is at rest). This phenomenon is called Brinelling. Some permanent indentation is almost unavoidable due to the high contact stresses, however, it has been found that permanent deformations smaller than 10-4 times the rolling element diameter usually do not unfavorably affect the bearing's operation. indentation 压痕 Brinell 布里涅耳
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Rotation Factor When the outer race rotates and the inner race is stationary(the reversal of common practice), the rated load may be increased by a factor of 1.2, since a ball rolls further per revolution on the outer race when the outer race rotates (the ball rotates in the same sense as the race) than on the inner race(where ball rotation is opposite to that of the race) and life is limited by ball revolutions Equivalent Dynamic Load a plain ball bearing is desi gned to take radial loads However. due to the curvature of the ball tracks in an axial plane of both inner and outer races, some axial load may be imposed on a radial ball bearing as well. Note that an axial load is taken by all balls, at least in theory, while a radial load is carried by less than half their number
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Rotation Factor When the outer race rotates and the inner race is stationary (the reversal of common practice), the rated load may be increased by a factor of l.2, since a ball rolls further per revolution on the outer race when the outer race rotates (the ball rotates in the same sense as the race) than on the inner race (where ball rotation is opposite to that of the race), and life is limited by ball revolutions. Equivalent Dynamic Load A plain ball bearing is designed to take radial loads. However, due to the curvature of the ball tracks in an axial plane of both inner and outer races, some axial load may be imposed on a radial ball bearing as well. Note that an axial load is taken by all balls, at least in theory, while a radial load is carried by less than half their number