Rock Products

APR 2018

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22 • ROCK products • April 2018 R olling bearings are precision machine elements found in a wide variety of applications. They are typically very reliable even under the toughest conditions. Under normal operating conditions, bearings have a substantial "service life" which is expressed as either a period of time or as the total number of rotations before the rolling elements or inner and outer ring fatigue or fail. Less than 1 percent (0.35 per- cent specifically – see Figure 1) of rolling bearings do not reach their expected life. Premature Bearing Failure When a bearing does fail prematurely, it is usually due to causes that could have been avoided. For this reason, the pos- sibility of reaching conclusions about the cause of a defect by means of studying its appearance is very useful. It's most important to correct the causes and prevent future failures and the costs that follow. Most bearing failures such as flaking and pitting, spalling unusual wear patterns, rust and corrosion, creeping, skewing and others (as shown) are usually attributed to a relatively small group of causes that are often interrelated and correct- able. These causes include lubrication, mounting, operational stress and bearing selection and environmental influence. Proper/Improper Lubrication and 'Grease Service Life' The purpose of lubricating the bearing is to cover the rolling and sliding contact surfaces with a thin oil film to avoid direct metal-to-metal contact. When done effectively it:  1. Reduces friction and abrasion.  2. Transports heat generated by friction.  3. Prolongs service life.  4. Prevents rust and corrosion.  5. Keeps foreign objects and contamination away from rolling elements. Grease is generally used for lubricating bearings because it is easy to handle and simplifies the sealing system while oil lubrication is generally suitable for high speed or high tem- perature operations. Generally, lubrication failures occur due to:  1. Using the wrong type of lubricant.  2. Too little grease/oil.  3. Too much grease/oil.  4. Mixing of grease/oil.  5. Contamination of the grease/oil by objects or water. While we have spoken about a normal bearing service life it is also important to take into consideration the normal "grease service life" as the two should be considered together to max- imize bearing life. Grease service life is the time over which proper bearing function is sustained by a particular quantity and category of grease. This is especially critical in pump, compressor, motor and super precision applications. Mounting and Installation of Bearings It is critical in the mounting and installation process to pay strict attention to:  1. Use of proper tools and ovens/induction heaters. Use a sleeve to impact the entire inner ring face of the ring being press fit.  2. Verify the shaft and housing tolerances. If the fit is too tight, you will create too much preload and if the fit is too loose, you will create too little preload which may allow the shaft to rotate or creep in the bearing. Check for proper diameters, roundness and chamfer radius.  3. Avoid misalignment or shaft deflection. This is espe- cially critical in mounting bearings that have separable components such as cylindrical roller bearings where successful load bearing and optimal life are established or diminished at installation.  4. Be aware of "Radial Internal Clearance" (See Figure 2). It is critical to maintain the proper R.I.C that was estab- Bearing Failure and Analysis The Accurate Diagnosis of a Bearing Failure Is Imperative to Prevent Repeat Failure and Additional Expense. By Steven Katz Figure 1: Causes of failure in rolling bearings (Source: antriebstechnik 18 (1979) No. 3, (71-74). Only about 0.35% of all rolling bearings do not reach expected life. Bearing Failure

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