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  When it comes to rolling bearing life, engineers will often ask the question:

  "You said the life of the rolling bearing?"

  "How do you know when are you going to host the end of life?"

  "This is when the bearing stops running?"

  "This is when the bearings reside within the machine reaches a specific working hours?"

  Typically, the answers to these questions may include: "at the end of life, bearing orbearing no longer suitable for its intended purpose," or "when it stops turning. Unfortunately, the answer is neither specific nor adequate.

  Bearing Manufacturers Directory and most of the design book, limit is called rolling fatigue of bearing life and reliability. This phenomenon has been studied for more than 120 years began in 1890 and Richard Stribeck pioneering work in Germany, as well as Vader Palmgren o yuehangudeman in the UK and Sweden, early in the 20th century.

  Palmgren's contribution may be the most significant technology of rolling bearings. 1924-he provides a basis, rolling bearing life calculation. He said that bearing life uncertainty distribution. He meant that no two bearings in a group under the same operating conditions, will also be damaged. He suggested a L10 life or time at 90 bearing concepts will survive, of which 10 have failed. Maybe he put forward reasonablemethod to calculate the life of the mechanical components of the first one.

  Most engineers source of ball bearings and roller bearings, know-how from the bearing manufacturer directory. About 90 to 95 per cent of applications in mechanical design, equation of the bearing manufacturer's catalog and suggestions, provide safeand reliable design. Usually, the remaining 5 to 10 of the application requires expertise and analysis to avoid problems.

  Failure mode

  Ultimate failure mode limits the service life of the bearing is a bearing or roller rolling fatigue. Rolling fatigue is very variable, but statistical prediction based on steel, steel processing, heat treatment, manufacture and type of bearings, lubrication and operating conditions.

  Failure comes from peeling is a finite orbit width of maximum shear stress in the contact surface and depth. Spallation is the origin of surface or underground. Spall surface usually begins with a surface defect or elastic indentation crack formation spalling cracks propagate to the network. Cracks begin to rise in stress, such as in the orbit of maximum shear stress region following rigid inclusions, cracks also propagateto the network layer.

  Fatigue failure occurs at the contact surface is called the classic rolling fatigue. Classic rolling fatigue failure is similar to human deaths caused by old age. Most bearings, however, is from the service for other reasons.

  Other than the classic rolling bearing fatigue is caused if you don't overload and rational design, are considered to be inevitable failure, installation and lubrication. Improvement of bearing manufacturing and iron and steel processing, as well as advanced lubrication technology, potential improvements in the bearing life can be as high as 80 times, or 400 times in in the late 1950 of the 20th century, 1940 to achieve.

  Basic bearing life

  As mentioned earlier, the L10 life, revolution in the inner ring, is a bearing 90 percent of the population should be equal to or exceed their theories work load of trouble-free life. It is based on the classic rolling fatigue. "The basic bearing life" is oftenmentioned in the bearing manufacturer directory L10 life inanimate factors, depending on the bearing type, bearing steel, steel processing, heat treatment, lubrication and operating conditions.

  Most bearing selection and size based on the "basic bearing life" calculations and, occasionally, inanimate factors. Need to be aware of is that in time, bearing the loadand speed run this 10 per cent to fail. Many engineers don't realize that they've worked out before your life is not based on time will not fail in this time of 10 of the bearings can fail. This error causes the device manufacturer for warranty and product liability claims.

  Life of the bearing system

  Because it can be assumed that rational determination of any rotating machinery will have two system consisting of two or more bearings, you must also determine theindividual bearing life life of the bearing system. This can be achieved by a single system of bearing life into a life.

  System life, strictly series reliability needs to know. Remember, the bearing life is asystem will always be equal to or less than the shortest life of the bearing system. For example, say you have two run gear bearing the same bearings running at 900 RPM 3600 RPM output supported two types of gear there is a simple reduction gear box. Or torque at full load (100 per cent), and each input bearing life is 2,500 hours,and each output bearing life is 10,000 hours. System 10 percent of calculated to 1124 hours of life. This means that if you distribute 1000 gearbox and they run at maximum torque to 1124 hours, 100 transmissions will be at least a bearing failure. Question becomes can you manipulate the gearbox trouble-free under these conditions.

  If you live in the shortest possible bearing system is 2,500 hours, might reasonablybe expected the first 133 hours of operation each of the gearbox will not be bearingfailure. However, transmission can not output torque at all times in the whole operation. Assuming the gearbox in the maximum torque to 50 per cent of the time, with20 per cent of the time and half the 30 percent of torque one-fourth torque. For a calculation of total life, you need to evaluate each condition L10 life.

  L10 life equal to 2,671 hours, all gearboxes in service 10 percent theoretically allowone or more failed bearings. If, as in the previous example, you service 1000 gearbox, gearbox 100 fails, you will have 100 failures of bearings with 4000. In other words, 10 of the gearbox does not take into account the service fails bearing 2.5 per cent. Bearing the other 97.5 per cent can reasonably be considered to be unavailable.

  That is why there is no damage to the bearing removed from service, the vast majority never reach the calculated L10 life. Therefore, it is practical and cost-efficient inspection, rework and local services that did not damage the bearings before they can reach their L10 life removed.

  Reasons for elimination

  Up to now this discussion is only based on the classic rolling fatigue failure mode and the disassembly of the bearing. However, the bearings may be less than 5 per cent was removed from the service, because the rolling fatigue, whether undergroundor surface origin.

  Table 1 has a bearing removal and estimated the list of possible causes related to their careers. In addition to this list of reasons bearing disassembly and failure modes related to the category of "other" includes bearing right, hardness of true and false, excessive overload of thrust bearing, lubrication, heat, thermal load, roller edge stresses, fracture of cage, element or a ring fracture and slip injuries, and ARC discharge.

  For these reasons, the removal and destruction of the bearings can minimize and/orreduction through good design, proper bearing installation, maintenance and lubrication good practices in a timely manner. However, they cannot be completely eliminated, which makes understanding and determining bearing life is more important.


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