HOW TO PREPARE ROLLING BEARING FOR MOUNTING

HOW TO PREPARE ROLLING BEARING FOR MOUNTING 2.1.1 Working Planning Know in advance what you are going to do so that the mounting work can proceed in a straightforward manner. Study the shop drawing to acquaint you with the design details of the application and the assembly sequence. Phase the individual operations and get reliable information on heating temperatures, mounting forces and the type and the amount of greases to be packed into the bearing. Whenever the installation and removal of rolling bearing necessitates special measures the fitter should be provided with comprehensive instructions on mounting details, including topics such as the means of transport for the bearing, the mounting equipment, measuring devices, heating facilities and type and quantity of lubricant. 2.1.2 Before starting mounting Before starting mounting the fitter should satisfy himself that bearing number stamped on the package agrees with the designation given on the drawing and in part list. He should therefore be familiar with the bearing numbering and identification system. 2.1.3 Handling of rolling bearings before mounting: Before being packed, the bearings are coated with preservative oil, which prevents corrosion. The oil does not need to be washed out when mounting the bearing. In service, the oil combines with the bearing lubricant. Do not perform any modifications on the bearings. Subsequent drilling of lubricating holes, machining of grooves, flats and the like will disturb the stress distributions in the ring resulting in premature bearing failure. There is also the risk of chips or grit entering the bearing. 2.1.4 Cleanliness in mounting An absolute “must” for the proper fitting of roller bearings is working in clean surroundings. The tools to be used should be free from dirt & fillings. In the room where the fittings of the bearings are carried out it is absolutely essential to avoid machining with metal cutting tools. If, despite these precautions, bearings get dirty by improper handling, they must not be rotated because even the smallest particles penetrating into the bearing will damage the races and in this way the service life of the bearing will be considerably reduced. Attention should also be given to the cleanliness of shaft, housing and any other mating parts. Castings must be free from sand. After cleaning the housing bore should receive a protective coating. Bearing seats on the shaft and in housing bore should be carefully cleaned from antirust components and residual paint. Turned parts must be free from burr and sharp edges. 2.1.5 Surrounding parts All surrounding parts should be carefully checked for dimensional and geometrical accuracy before starting mounting. Non-observance of the tolerances for shaft and housing seat diameters, out of roundness of these parts, out of square of abutment shoulders etc. impair bearing performances and lead to premature failure is not always easy to establish and much time can be lost in looking for the cause of failure. 2.2 Fits Good bearing performance is largely depending on adherence to the fits specified for the two rings in the drawing. No one can give a straight answer to the questions of the “right” fit. Indeed the selection of fits is determined by the operating conditions of the machines and the design characteristics of the bearing mounting. Basically, both rings should be well supported over their seating areas and should therefore be tight fits. This, however, can not be used in all cases, since other factors, such as axial freedom of the floating bearing or easy mounting must also be taken into consideration. The interference produced by tight fits expends the inner ring and contracts the outer ring resulting in a reduction of redial clearance. Hence the radial clearance should be adapted to the fits. The shaft and housing tolerances should be checked. A too loose fit causes the ring to creep on the shaft, which tends to damage both ring and shaft. It also affects the working accuracy of the machine or causes premature raceway fatigue from poor support. On the other hand, to tight a fit leads to a reduction in radial clearance, which might result in detrimental preload and hot running of the bearing. The seating areas must, moreover, be checked for out of round in addition to diameter. As the walls of the rolling bearings are relatively thin, possible poor geometry of shaft or housing is transmitted to the raceways. Out of round and deviation from true parallelism and taper should not exceed half the specified diameter tolerance. For Radial bearings with cylindrical bore of normal precision class. (Tapered & Cylindrical Bearings) Several factors like the type and magnitude of bearing load, temperature difference, method of bearing mounting and dismounting should be taking into consideration while selecting the proper fit. The recommended tolerances for shaft and housing for common applications are given below as general guide lines: Operating Type of rotationLoad Type of fitting Condition Inner Ring Outer RingConditions Inner Ring Outer RingRotationStationaryRotating inner Tight Fit Loose Fit ring load Stationary outer ring load StationaryRotationStationaryRotationRotating inner Tight Fit Loose Fit ring load Stationary outer ring load RotationStationaryDirect of Rotating or Rotating orDirection of Tight Fit Tight Fit load StationaryStationaryload indeterminate indeterminate due to variation of direction TABLE – RECOMMENDED SEATING FITS FOR SHAFTS* Load ConditionsExamplesShaft Diameter (mm) Shaft Tolerances Cylindrical Roller & Taper Roller BearingsRotatingOuter Ring LoadEasy axial displacement of inner ring on shaftdesirableWheels on Stationary AxlesAll Shaft Diameters‘g6 Easy axial displacement of inner ring on shaftunnecessaryTension Pulleys and RopeSheaves‘h6 Rotating inner Ring Load or Direction of Load Indeterminate