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Ball and Roller Bearings - Design, Operation, and Some Failure Analysis

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Ball and Roller Bearings - Design, Operation, and Some Failure Analysis Ball and Roller Bearings - Design, Operation, and some Failure Analysis by Neville W. Sachs, P.E. © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 1 My hopes and goal for today Have you understand more about: 1. The general construction of ball and roller (rolling element) bearings. 2. How rolling element bearing lubrication works at different relative component speeds 3. The critical importance of correct shaft and housing dimensions 4. The contrast between the design failure mechanism and the most common failure mechanism. We’ll go through some of the slides very rapidly. My intent with those is to give you a reference for future use. © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 2 Some questions 1. What is the most common metallurgy for rolling element bearings? 2. Where are ceramic components used? Why? 3. With rolling element bearings, why is a VERY SLIGHT internal interference a good idea? 4. What is the mechanism that allows the typical lubricating oil to separate the rolling elements from the bearing rings? 5. How flat should a typical machine base be? © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 3 Rolling Element Bearings We’ll discuss ball, roller, and needle bearings, not plain or sleeve bearings. Earliest rolling element bearing concept was in ancient times, i.e., logs acting as rollers Modern steel bearings became practical in the late 1880’s with the ability to precisely control the element dimensions. Most bearings are through hardened and, in the industrial world, there are several standards for through hardened bearing steels. In North America, it’s AISI 52100. Some Timken tapered roller bearings and some special use bearings are case hardened with numerous alloy options. © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 4 A little history Earliest Historical Evidence – around 300 BC when Greek engineer Diades developed a roller supported battering ram, maybe earlier on pyramids . Oldest existing examples of bearings are in the Danish National Museum and date from around 200 BC. In the mid-18th century Dutch windmills had centerposts that were supported by cast iron balls. Later in the 18th century carriage wheel bearings were developed. They were filling slot bearings, used cast iron balls, and had no cages. © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 5 More History The major problem with these early applications was the lack of control of the rolling element diameters. (In discussing bearings, 0.0005” [12.7 microns] is a large overall dimension. With the rolling elements, 0.00005” [1.3 microns] is a common tolerance.) With that poor dimensional control, a few of the elements took the majority of the load, resulting in high Hertzian Fatigue Stresses on the larger elements. However, in 1883 Freidrich Fischer (FAG) developed a ball grinding machine with relatively good tolerance control. In 1898 Henry Timken produced the first tapered roller bearings and in 1907 Sven Wingquist (SKF) developed the self-aligning ball bearing. © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 6 Ball Bearing Parts Cross section of this deep-groove Outer Ring ball bearing (race) Balls (behind cage) Inner Ring (race) Cage Balls, rollers, and rings are usually made from Cages are made from a variety hardened steels of materials and really depend on the © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failureapplication Analysis 7 Some other types Typical Deep Groove Common everyday “general duty” (Conrad) Ball Bearing bearing. Good radial and some thrust capacity Angular Contact Ball Applications where heavier thrust loads are expected such as pumps Bearing (more thrust and fans . Must have some thrust capacity) loading. Roller Bearing - high Used where heavy radial loads radial but no thrust are present, i.e., large presses, capacity rolling mills, etc. Spherical Roller Bearing Very common where precise - good radial and thrust alignment is impossible, but heavy radial loads are load capacity present. Conveyor head drums, large fans, etc.. © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 8 Different Spherical Roller Bearings Bearing Tapered Roller ACBB © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering andRoller Failure Analysis Thrust Bearing 9 Speed Ratings Bearing Type Relative Safe Speed (Normal Lube) Deep Groove Ball Angular Contact Ball Cylindrical Roller Tapered Roller Spherical Roller 0 2 4 6 8 BUT, with special processing these can be doubled! © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 10 Approximate Radial Capacities Load Axial Load Bearing Type Radial Load Capacity Axial Load Capacity Deep Groove Ball Angular Contact Ball Cylindrical Roller Tapered Roller Spherical Roller 0 2 4 0 2 4 © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 11 Bearing Nomenclature 2 3 0 1 8, 9 4 6209 Bearing – Last two numbers – 09 - indicates the bore diameter, i.e., 09 x 5 = 45 mm = 1.77 in. 6 shows that it is a deep groove ball bearing 2 shows that it is a #2 diameter series with 0 as the lightest and 3 as the heaviest in common usage (Note that the bore doesn’t change.) © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 12 Bearing Nomenclature Says 6313 C3 Look carefully for a suffix on that bearing number! 1. Seals, shields, special clearances, special cage, etc. 2. With sealed and shielded bearings it will specify the lubricant. © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 13 Internal Clearances for ball bearings C3 and C4 suffix Bore Range Radial Internal Clearance Millimeters Metric (min/max in microns) Inch (min/max in 0.0001") over up to Normal C3 C4 Normal C3 C4 10 18 3/18 11/25 18/33 1/7 4/10 7/13 18 24 5/20 13/28 20/36 2/8 5/11 8/14 24 30 5/20 13/28 23/41 2/8 5/11 9/16 30 40 6/20 15/33 28/46 2/8 6/13 11/18 40 50 6/23 18/36 30/51 2.5/9 7/14 12/20 50 65 8/28 23/43 38/61 3.5/11 9/17 15/24 65 80 10/30 25/51 46/71 4/12 10/20 18/28 80 100 12/36 30/58 53/84 4.5/14 12/23 21/33 100 120 15/41 36/66 61/97 6/16 14/26 24/38 120 140 18/48 41/81 71/114 7/19 16/32 28/45 140 160 18/53 46/91 81/130 7/21 18/36 32/51 160 180 20/61 53/102 91/147 8/24 21/40 36/58 180 200 25/71 63/117 107/163 10/28 25/46 42/64 Similar values are specified by ANSI/AFBMA for roller bearings. Thermal expansion of steel is 6.3 x 10-6 in/in/0F © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 14 Internal Clearances for Ball Bearings C3 and C4 suffix Bore Radial Internal Clearances mm in Metric (microns) Inch (0.0001”) Original C3 C4 Original C3 C4 12 1/2 3/18 11/25 18/33 1/7 4/10 7/13 180 7.0 25/71 63/117 107/163 10/28 25/46 42/64 Thermal expansion of mild steel = 0.0075”/ft/1000F Thermal expansion of 300 series stainless steel = 0.011”/ft/1000F Thermal expansion of aluminum = 0.016”/ft/1000F © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 15 One reason why those dimensions are important Life Effective Clearance - inches -0.002 -0.001 0 +0.001 +0.002 +0.003 vs. 1.2 Internal Clearance 1.0 0.8 0.6 6310 Ball Bearing with 0.4 a 3,350 N (340 kgf) radial load 0.2 (data from NSK text Technical Report - 1992 Relative life compared to ideal to compared life Relative 0 -60 -40 -20 0 +20 +40 +60 +80 Effective Clearance (micrometers) © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 16 Bearing bore and housing dimensions are critical because of the internal temperatures and clearances The specified shaft and housing fits depends on the application. Both fits can’t be tight because the heat generated within the bearing will cause dimensional changes and increased stresses. If both fits are tight, enough expansion would eventually cause a seizure. There has to be good thermal conductivity through one of the rings to control expansion and lubricant temperatures. Typical fan/motor bearing – tight shaft, loose housing. Typical wheel bearing – tight housing, loose shaft Heavy loads – very tight fits © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 17 Fit examples © 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 18 Shaft and Housing Fits Allowances have to be made for thermal expansion. On a typical shaft, with over 18” between centers, one bearing will be fixed and the other will be floating to allow for thermal growth. On shorter spans the growth allowance depends on the internal clearance. The shaft and housing tolerances and fits are critical and depend on the: 1. Load direction Neville’s opinion – 2. Load magnitude After lubrication, poor 3.
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