ME 343: Mechanical Design-3

Transcription

1 What are bearings? ME 343: Mechanical Design-3 Bearings Dr. Aly Mousaad Aly Department of Mechanical Engineering Faculty of Engineering, Alexandria University Machines with moving parts must have some way to guide their motions in turning, sliding, or rolling. Structures that enable these motions are often called bearings. Just think of anything that rotates and you are sure to find a bearing in one form or other in it. Slides 3: Bearings 2 Why bearings? The mean objective is to reduce friction i and wear. Example adding a sleeve to protect a rotating shaft. They assist in easy rotary action thereby reducing the effort or power required. Just think about pulling your luggage at the airport without any wheels or without any bearings to the wheels. They assist in carrying loads or assist in transferring the loads from something that tends to rotate to something which is stationary and has the capability to carry the loads. History The oldest instance of the bearing principle dates to the Egyptians when they used tree trunks under sleds. There are also Egyptian drawings of bearings used with hand drills. Slides 3: Bearings 3 Slides 3: Bearings 4

2 Types of bearings Journal and roller bearings Plain bearing, also known by the specific styles: bushings, journal bearings, sleeve bearings, rifle bearings. Rolling-element bearings such as ball bearings and roller bearings. Fluid bearings, in which the load is carried by a gas or liquid. Magnetic bearings, in which h the load is carried by a magnetic field. Slides 3: Bearings 5 Slides 3: Bearings 6 Windmill Dental drill Diameter range:0.5 m - 6m, Weight range::75.8kg-4000kg Pneumatic bearings can allow for very high speeds (300,000 to 400,000 rpm ). Slides 3: Bearings 7 Slides 3: Bearings 8

3 Roller bearings Roller bearings versus sleeve bearings Antifriction i i bearings! Load, speed, and the operating viscosity of the lubricant do affect the frictional characteristics of a rolling bearing. An issue with ball and roller bearings is that the balls or rollers rub against each other causing additional friction which can be prevented by enclosing the balls or rollers in a cage (separator or retainer). Slides 3: Bearings 9 Slides 3: Bearings 10 Roller bearings versus sleeve bearings Roller bearings versus sleeve bearings Roller bearings The main load is transferred through elements in rolling contact. The starting friction is about twice the running friction. Require less axial space but more diametrical space High initial cost Low maintenance cost Lower reliability Sleeve bearings The main load is transferred through sliding contact. Very high starting friction (metal to metal contact). Require more axial space but less diametrical space Low initial cost High maintenance cost High reliability Slides 3: Bearings 11 Slides 3: Bearings 12

4 Roller bearings versus sleeve bearings Characteristics Journal bearings Roller bearings Shock Oil film, excellent Direct contact between rolling element, fixed and rotating parts. Vibrations Oil is excellent for No damping effect, damping vibration due to changing rigidity during rotation. Dust < ho and embeding, no contact with Cause small pits, i.e. failure surfaces Maitenance Permenant care about lubrication. Lubrication, grease can be once or twice a year Slides 3: Bearings 13 Roller bearings versus sleeve bearings Characteristics Journal bearings Roller bearings Repair Can be repaired No repair Service life Perminant oil Fatigue life infinite life (take care of running up) Accuracy of rotation Easier to reach Depends on case excentercity, roundness and accurecey of the rolling. Slides 3: Bearings 14 Comparison of bearing frictions It is advantageous to use ball bearing and roller bearing at low speeds. Journal bearings are mostly suited for high speeds and high loads. Advantages of rolling bearings Coefficient i of ffriction is low (μ = ) 003) compared with plain bearings especially at low speeds. This results in lower power loss. Wear is negligible if lubrication is correct. They are much shorter than plain bearings and take up less axial space. Because of extremely small clearance they permit more accurate location; important for gears for example. Self-aligning types permit angular deflection of the shaft and misalignment. Slides 3: Bearings 15 Slides 3: Bearings 16

5 Disadvantages of rolling bearings The outside diameter is large. The noise is greater than for plain bearings, especially at thigh hspeeds. There is greater need of cleanliness when fitted to achieve correct life. They cannot always be fitted, e.g. on crankshafts because of there lower shock resistance. They are more expensive for small quantities but relatively cheap when produced in large quantities. Failure may be catastrophic. Main characteristics of sliding bearings High starting friction i due to metal to metal contact. This requires the use of low-friction metals like bronze alloys and babbitt metals. Under certain combination of force, speed, fluid viscosity, and bearing geometry, a fluid film (hydrodynamic y y film) forms and separates the containing surface. A fluid film can also be formed with a pumping unit (hydrostatic film). Slides 3: Bearings 17 Slides 3: Bearings 18 Classification of roller bearings Bearing nomenclature Bearings are manufactured to take pure radial loads, pure thrust loads, or a combination of the two kinds. Essential parts of a roller bearing are the outer ring, the inner ring, the balls or rolling elements, and the separator. Slides 3: Bearings 19 Slides 3: Bearings 20

6 Types of rolling bearings Types of rolling bearings Slides 3: Bearings 21 Slides 3: Bearings 22 Types of rolling bearings Types of rolling bearings Cylinder roller bearings provide greater force because of the greater contact area. Combines the advantages of ball and roller bearings. Slides 3: Bearings 23 Slides 3: Bearings 24

7 Service Life For plain bearings some materials give much longer life than others. Some of the John Harrison ( ) clocks still operate after hundreds of years because of the lignum vitae wood employed in their construction, whereas his metal clocks are seldom run due to potential wear. Although long bearing life is often desirable, it is sometimes not necessary. For example, a bearing for a rocket motor oxygen pump is not required to have several hours life, far in excess of the several tens of minutes life needed. Service Life Fluid and magnetic bearings can have practically indefinite it service lives. In practice, there are fluid bearings supporting high loads in hydroelectric plants that have been in nearly continuous service since about 1900 and which show no signs of wear. Rolling element bearing life is determined by load, temperature, maintenance, lubrication, material defects, contamination, handling, installation and other factors. The service life of bearings in one application was extended dramatically by changing how the bearings were stored before installation and use, as vibrations during storage caused lubricant failure even when the only load on the bearing was its own weight. Bearing life varies because microscopic structure and contamination vary greatly even where macroscopically they seem identical. Slides 3: Bearings 25 Slides 3: Bearings 26 Choice of lubricant Grease is used for slower rotational speeds, lower temperatures, and low to medium loads. Grease is used in situations where maintenance is more difficult or irregularly scheduled. It can be used in dirty environments if seals are provided. Slides 3: Bearings 27 Slides 3: Bearings 28

8 Comparison between grease and oil lubrication Use grease when Temperature is not over 90 C (special types of grease may allow for higher temp.) Low speed Unusual protection is required from the enterence of forign matter. Bearing enclusures are desired Opperation for long period without attenion to lubrication. Use oil when High temperatuers High speeds Piltight seals are readily employed. Bearing type is not suitable for graese The bearing is lubricated from a supply whish is also used for other machine parts (gearbox). Bearings: maintenance Many bearings require periodic maintenance to prevent premature failure, although some such as fluid or magnetic bearings may require little maintenance. Most tbearings in high hcycle operations need periodic lubrication and cleaning, and may require adjustment to minimize the effects of wear. Bearing life is often much better when the bearing is kept clean and well-lubricated. However, many applications make good maintenance difficult. For example bearings in the conveyor of a rock crusher are exposed continually to hard abrasive particles. Cleaning is of little use because cleaning is expensive, yet the bearing is contaminated again as soon as the conveyor resumes operation. Thus, a good maintenance program might lubricate the bearings frequently but clean them never. Slides 3: Bearings 29 Slides 3: Bearings 30 Defective bearing installation (a) Misaligment (out-of-line), (b) shaft deflection, (c) crooked or tilted outer race, (d) crooked or tilted inner race. Slides 3: Bearings 31 Defective rolling bearings Regardless of fthe failure mechanism, defective rolling element bearings generate mechanical vibrations at the rotational speeds of each component. These characteristic frequencies, which are related to the raceways and the balls or rollers, can be calculated from the bearing dimensions and the rotational speed of the machine. Mechanical vibration analysis techniques are commonly used to monitor these frequencies in order to determine the condition of the bearing. Slides 3: Bearings 32

9 Reading Mechanical Engineers Reference Book, Ch. 9, Tribology. Shigley s Mechanical Engineering Design, Eighth Edition, The McGraw Hill Companies, Inc., (11. Rolling Contact Bearings) HW Solve problem 2 of sheet t1 (Sheet1 Shaft design (continue).pdf). Select two deep groove ball bearings to carry the loads at A and E. Solve problem 3 of sheet 1 (Sheet1 Shaft design (continue).pdf). Please submit the assignment to your TA during tutorials in the week starting Saturday 23, April Late assignments will not be considered. Slides 3: Bearings 33 Slides 3: Bearings 34