Design 3. Bearings(1/2) Bearings. Bearings. Provides support for rotating machine elements. Contents. Hamidreza G.Darabkhani. Course Leader: A.

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1 Design 3 Bearings Hamidreza G.Darabkhani Course Leader: A. Starr Contents Bearings Bearings Classification Loads on Bearings Rolling elements bearings Ball bearings Roller bearings Needle bearings Linear bearings Thrust bearings Sliding bearing Magnetic bearings Application of bearings Bearing calculations Bearing mounting Bearings Provides support for rotating machine elements Bearings(1/2) 1

2 Bearings(2/2) Bearings Classification Bearings for Rotary Motion Magnetic Bearings Sliding Bearings (Fluid film bearings) Rolling Elements Ball Roller Needle Cylindrical Taper Barrel Allocated Loads on Bearings Rolling Elements Bearings Ball Bearings Radial and Axial loads Roller Bearings Radial and Axial loads Needle Bearings Only radial loads 2

3 Single row radial ball bearing Ball Bearings with shields Good Radial Load Moderate Axial Load Very good speed See the difference in the curvature of the ball and the race. (Point Contact) The shields are use to exclude dirt and foreign particles Angular contact Bearings Balls are inserted by thermally expanding the outer ring. Can take greater axial load Good Radial Loads Poor Axial loads Moderate Speed Double row ball Bearings Good Radial Loads Good Axial loads Good Speed 3

4 piece inner-ring ball bearings Advantages: The maximum complement of balls makes for higher load carrying capacity. The very deep ball raceway shoulder makes for high thrustload capacity. Thrust loads can be handled in both directions. Roller Bearings Because of the line contact, they can take greater loads They also have more rolling resistance Types of roller bearings Cylindrical Tapered Barrel (Spherical) Cylindrical roller bearings Are used in applications like conveyer belt rollers, where they must hold heavy radial loads. Line contact to handle much greater loads than a ball bearing. Is not designed to handle much axial loading. Tapered roller bearings Are used in car hubs, where they are usually mounted in pairs facing opposite directions so that they can handle thrust in both directions. Very good Radial Loads Poor Axial loads Moderate to very good Speed Very good Radial Loads Good Axial loads Moderate Speed 4

5 Needle bearings(1/2) Very good Radial loads No Axial loads Poor to moderate speed Linear bearings Are used primarily where something needs to be moved along a straight line with high accuracy. In other words, the object needs to only translate in one direction, and possibly move back to its starting position with high repeatability. The implications for robotic pick-and-place are obvious, as are uses for optical test fixturing and calibration. Use large number of small diameter rollers Usually there is no space between rollers (needles). No cage required. Thrust bearings Good Axial Loads No Radial Loads Poor Speeds Ball thrust bearings are mostly used for low-speed applications Thrust Ball Bearings No Radial Loads Good Axial loads Poor to Moderate Speed Cylindrical roller Spherical roller 5

6 Roller thrust bearings 1. They can support large thrust loads 2. They are often found in gear sets like car transmissions between gears, and between the housing and the rotating shafts. 3. The helical gears used in most transmissions have angled teeth in this causes a thrust load that must be supported by a bearing Very good Axial Loads No Radial Loads Poor Speeds Animations 1- The rotation of inner and outer race in ball bearings. 2- Directions of radial and thrust loads on the bearings Sliding Bearings (Fluid film bearings) Sliding Bearings (Fluid film bearings) Fluid film bearings have no rolling elements Lubricated by a film of liquid or gas which separate moving member from the stationary member The fluid (usually Oil) is circulated so that fresh, cool oil is constantly entering the space between the stationary and rotating pieces. Industrial machinery with high horsepower and high loads, such as: steam turbines, centrifugal compressors, pumps and motors 6

7 Magnetic Bearings Magnetic bearing systems represent a completely different approach to the support of rotating equipment. (NON-CONTACTING TECHNOLOGY) Magnetic bearings Are a non-contacting technology Negligible friction loss and no wear Higher reliability Enables previously unachievable surface speeds to be attained Lubrication is eliminated Magnetic Bearings are operating in: compressors, electric motors, pumps, turbo expanders, steam turbines, gas turbines, centrifuges, etc. Conical Magnetic Bearings Developed for Active Stall Control in Gas Turbine Engines Magnetic Bearings for High Temperature Smart Aero Engine Bearing applications Bearings in flight systems Pumps Air Condition systems Helicopter bearing ball bearing Gearboxes Mixers Wheels Piston engine propeller Helicopter ball bearing 7

8 AEROKOPTER Project Tapered Roller bearing Pulley cage rear side plate In AEROKOPTER Main rotor gearbox assembly See: Bearing calculations - Lifetime Dynamic load C is given by the manufacturer for a specified lifetime (e.g. L= 10 6 cycles) and a probability of survival, usually L The life (in millions of cycles) is given by k C L = P (for steady load) where C is the rated Dynamic load capacity and P is the load. k is an empirical factor depending on the bearing: 3 for ball bearings, 3.33 for roller bearings Bearing calculations - % failure Bearings are designed for a finite life, with a given probability of failure Test (fatigue) data Bearing Load/Life L x L 10 log = log ( p) x where p = 1 ( 0.9) 100 L x is the number of cycles at which x% of the bearings fail; L 10 is the number of cycles at which 10% fail Radial Load (lbs) Empirical relationship: k=3.0 (ball) k=3.33 (roller) L 10 Life (cycles) k L P 2 1 = L P 1 2 8

9 Manufacturer s Data Vendors publish the Basic Dynamic Load rating (C) of a bearing at an L 10 life of 1 million cycles. Stress Analysis Contact Stress σ c =300,000 psi is not unusual Balls, rollers and races are made from extremely high strength steel e.g. AISI s y = 260,000 psi s u =322,000 psi Bearing Selection Determine the design life (in cycles) Determine the design load P d = V R V=1 for inner race rotation V=1.2 for outer race rotation Calculate the required basic dynamic load 1 L k d Creq' d Pd 6 10 = Select a bearing with (C > C req d ) and a bore that closely matches the shaft diameter. Equivalent combined radial load For combined radial and thrust loads P = V X R + Y Ft where: P = equivalent radial load (lb) R = actual radial load (lb) Ft = actual thrust load (lb) X = radial factor (usually 0.56) V = 1.0 for inner race rotating = 1.2 for outer race rotating Y = Thrust factor( from chart) 9

10 Thrust factors, Y Bearing Mounting Deep -groove, ball bearings X = 0.56 for all values of Y Bearing Mounting Shaft/bearing bore has a light interference fit. Housing/outer race has a slight clearance fit. (Check manufacturers catalog) Match maximum permissible fillet radius. Shaft or housing shoulders not to exceed 20% of diameter. Outer ring of one bearing clamped e.g. cover plate Ball Bearing Typical bearing mounting bearing free to float Other ways to locate bearings: retaining rings, washers, screws, cotter pins, taper pins Shoulders locate bearings Nuts clamp inner ring to shaft 10

11 Sample schematic of bearing mounting and gears Some useful references 1. Mechanical Engineering Design 7th Edition, J E.Shigley C R.Mischke and R G.Budynas 2. Rolling Bearing Analysis, 4 th edition, Tedric A. Haris 3. Rolling bearings, T S Nisbet X X Ball Bearing C Ball Bearing D _Bearings2_2005_3_29.ppt 9. gn_files/1%20posters/ppt%20poster/bearing.ppt 11