Introduction to Bearings

Transcription

1 20. Ball an Roller Objectives Recognize the ifferent types of rolling contact bearings an applications where they may be suitable. Calculate equivalent loa on Conra-type ball bearings an relationship of this loa to expecte lifetime of the bearing. Factor in effect of thrust loaing on the equivalent raial loa in calculating the life of a bearing. Unerstan principles of roller bearings, tapere roller bearings, thrust bearings, an neele bearings. Introuction to Ball Raial an Thrust loas Roller Raial an Thrust loas Neele Only raial loas Select appropriate type of bearings an their sizes August for 15, the 2007esign being unertaken. 1 August 15, Ball Single-row Ball Bearing Terminology Terminology Outer ring, which contains the outer raceway Inner ring, which contains the inner raceway Complement of balls Two-piece separator (also calle cage or retainer) August 15, August 15, Ball Bearing assembly Ball August 15, August 15,

2 Ball Bearing nomenclature Ball Corner raius With See the ifference in the curvature of the ball an the race) Bore Outsie iameter August 15, August 15, ife of Antifriction ife of Antifriction They are subjecte to repeate stress cycles an are likely to fail by fatigue. B 10 or 10 life is efine as the number of hours that 90% of the bearings teste will excee. The probability of failure is 10% at the 10 life. Average life is the number of hours that 50% of the bearings excee. Relative life August 15, August 15, Fig Typical bearing failure curve ife of The expecte life of the bearing is = T n 0 T = Hours of operation n = revolutions per minute August 15, August 15,

3 ife of The life of the bearing ecreases with an increase in the loa. c k C = P k = 3 for ball bearings = 10/3 for roller bearings = esire life c = life from the table (manufacturers catalog) C = ynamic rating from manufacturer P = esign loa ife of The equations can be rewritten as epening upon the variable to be calculate C = P c 1/k C c P = k August 15, August 15, ife of Antifriction Example Problem 20-1: ife Expectancy of Ball Table 12-1 lists the basic ynamic loa for bearing number 208 as 5050 pouns. What woul the expecte life be for the bearing if it were subjecte to a raial loa of 2400 pouns? If the shaft turns at 1750 rpm, how many hours woul bearing last base on 10 esign life? For a life of 1 million cycles August 15, August 15, Example Problem 20-1: ife Expectancy of Ball C = P k 10 rev T = 9.3 x 10 rev hr 1750 rev / min 0 min T = 88 hrs 3 = 9.3 x = (20-5) August 15, Example Problem 20-2: ife Expectancy of Ball For conitions in the previous example problem, if an 10 life of 200 hours is neee, select a bearing from Table 20-1 to meet criterion. August 15,

4 Example Problem 20-2: ife Expectancy of Ball T n 0 m = 10 hr 200 hr 1750 rev 0 m = 10 min hr = 21 x 10 revolutions l / k C = P c 1/3 21 C = 2400 lb 1 C = 22 lb (20-1) Bearing number 211 has a ynamic loa rating of 7500 pouns. This bearing woul be acceptable. If bearing bore is too large, a heavier series coul be substitute. (20-3) August 15, Equivalent combine raial loa For combine raial an thrust loas P = V X R + Y F t P = equivalent raial loa (lb) R = actual raial loa (lb) F t = actual thrust loa (lb) X = raial factor (usually 0.5) V = 1.0 for inner race rotating = 1.2 for outer race rotating Thrust factor Y is obtaine from Table 20.2 August 15, F t /C s Y Table 20.2 Thrust factor for Deep-groove ball bearings F t = Thrust force on the bearing C s = Static rating of the bearing (Manufacturer s catalog) Example Problem 20-3: Ball Bearing Selection Select a bearing from Table 20-1 to meet following criteria: R = 1200 pouns F t = 500 pouns n = 1500 rpm - shaft rotates 10 = 5000 hours August 15, August 15, Example Problem 20-3: Ball Bearing Selection To select bearing, assume thrust factor of Y = 1. (average of table): P =.5 R + Y F (20-) t P = lb lb P = 1472 lb Fin life: 1500 rpm 5000 hr 0 min = 10 hr = revolutions August 15, Example Problem 20-3: Ball Bearing Selection (cont.) l / k Ajust rating for life: C = P (20-3) c 1/3 450 C = 1472 lb 1 C = 11,280 lb From Table 20-1: Bearing Number 215 C = 11,400 C s = 9,700 Verify assumption for Y: Interpolate to fin factor: F (20-7) t 500 = =.0515 C 9700 s Y = ( ) August 15, 2007 Y =

5 Example Problem 20-3: Ball Bearing Selection (cont.) Example Problem 20-3: Ball Bearing Selection (cont.) Verify: P =.5 R + Y Ft P =.5 (1200) (500) P = 1550 lb 1/ k C = P c 1/ C = C = 11,875 lb This is not acceptable. Try next larger size bearing. Try bearing number 21. Interpolate: Y = ( ) Y = 1.79 P =.5 (1200 lb) (500 lb) P = 157 lb 1/3 450 C = C = 12,005 lb Ft 500 = =.048 C 10,500 s August 15, Bearing number 21 meets esign criteria. August 15, Example Problem 20-4: ife Expectancy of Ball Example Problem 20-4: ife Expectancy of Ball Estimate life for a 200 series bearing that has an insie iameter of 30 mm on a stationary shaft where outsie race rotates. There is a raial loa of 1,000 pouns an a thrust loa of 300 pouns. Fin thrust factor as bearing is known: Ft 300 = =.129 Cs 2320 Interpolate from Table 20-2: Y = ( ) Y = 1.41 P = V X R + Y F P = 1.2 (.5) (300) P = 1095 lb t (20-7) (20-) August 15, August 15, Example Problem 20-4: ife Expectancy of Ball (cont.) Static strength capacity C = 10 P = 28. x 10 revolutions k 3350 = (20-5) It is efine as the maximum loa that can be applie without bearing amage when neither race is moving. The permanent eformation oes not excee in. per in. of iameter of the rolling element. August 15, August 15,

6 Some useful facts oa carrying capacity is roughly proportional to the iameter of the rolling elements. Roller bearings have much grater loa-carrying capacity than ball bearings. are subjecte to repeate stress cycles with max compressive stress of approx 150,000 psi. This cause fatigue failure. oa of a ball bearing is ouble, fatigue life is reuce by a factor of 8. August 15, Some useful facts If the loa of roller bearing is ouble, the fatigue life is reuce by a factor of 10. Deformation is the cause of rolling resistance. Hence haren all surfaces to about Rc 58. Wheel bearing of automobile is esigne for 10 life of 100,000 miles, 10% of the bearing may fail before 100,000 miles are reache. For properly loae bearings force require to pull a loa of 1 ton: 1. lb for ball bearing 3.0 lb for roller bearing August 15, Type of Rolling Contact Maximum capacity Ball Maximum number of balls can be inserte August 15, August 15, Maximum capacity Ball Ball Bearing with shiels Maximum number of balls can be inserte Greater raial loa carrying capacity August 15, August 15,

7 Angular contact Angular contact Balls are inserte by thermally expaning the outer ring. Can take greater thrust loa than the maximum capacity ball bearing from only one irection. August 15, August 15, Double row angular contact Double row angular contact Takes thrust in both irections August 15, August 15, piece inner-ring Ball arge number of balls higher loacarrying capacity Deep ball raceway - Higher thrust loa capacity Thrust loas from both irections 2-piece inner-ring Ball Avantages The maximum complement of balls makes for higher loa carrying capacity. The very eep ball raceway shouler makes for high thrust-loa capacity. Thrust loas can be hanle in both irections. August 15, August 15,

8 Roller Roller Because of the line contact, they can take greater loas They also have more rolling resistance ( for roller an for ball bearing) Types of roller bearings Cylinrical Tapere Spherical August 15, August 15, Tapere Roller Neele Use large number of small iameter rollers. There is no space between rollers (neeles). No cage require. Drawn cup type is very thin an compact because of the esign. Use for lighter loas. August 15, August 15, Neele Neele Some types rollers contact irectly the shaft (Fig. 4.21) without any inner race. The shaft shoul be harene (Rc 58) for proper operation (Fig. 4.22). They have large loa-capacity/size ratio. It requires very little raial space. One application is the roller type cam follower (Fig. 4.23). They have higher coefficient of friction. Typical value is compare to for roller bearings. They have lower max spee limit. August 15, August 15,

9 Neele cam follower Thrust August 15, August 15, Cylinrical roller Thrust Conical Thrust August 15, August 15, ubrication of Antifriction Provie a lubricating film between the rolling elements an the separator at the areas of contact Dissipate heat cause by eformation of the rolling elements an raceways as well as heat cause by the sliing contact between the rolling elements an the separator Prevent corrosion of bearing components Ai in preventing irt an other contamination from entering the critical areas of the bearing where sliing or rolling contact takes place August 15, Avantages Journal bearings or Plain surface bearings Requires little raial space Run quietly Have a longer life span Are less sensitive to contamination Are less costly Can better sustain shock loas Requires less precise mounting Are available in split halves August 15,

10 Avantages Antifriction bearings Have less friction Requires no wearing-in perio Requires less axial space Can run at higher spees Have fewer maintenance problems Have fewer lubrication ifficulties Allow for consierable misalignment Are easy to replace Allow for greater precision Reaily available in large varieties August 15, August 15, Summary Antifriction bearings operate with rolling contact. There are a number of ways in which the balls are loae into a ball bearing. Roller bearings take larger loas than ball bearings because of the line contact. Neele bearings requires little space. Tapere roller bearings can take axial as well as raial loas. Summary fail because of fatigue. oa capacity of a bearing can be calculate using the supplie formulae. ubrication in bearings serve a number of functions such as issipating heat, prevent corrosion an forming a film between rolling element an the raceway. Pre-mounte bearings are complete bearing assemblies. August 15, August 15,