Chapter 2 Shaft and Bearing TOPIC 1 SHAFT
Chapter 2: Shaft and Bearing TOPIC 1: SHAFT
Chapter 2: Shaft and Bearing SHAFT - Brief introduction to shaft. - Discuss reasonable geometries for shaft and reliable transmitting element. - Compute force and torque exerted on shaft.
Chapter 2: Shaft and Bearing Objectives o Propose reasonable geometries for shafts to carry a variety of types of power-transmitting elements, providing for the secure location of each element and the reliable transmission of power. o Compute the forces exerted on shafts. o Determine the torque distribution on shafts. o Prepare shearing force and bending moment diagrams for shafts in two planes.
Chapter 2: Shaft and Bearing INTRODUCTION § A shaft is a rotating member, usually of circular cross section, used to transmit power or motion. § It provides the axis of rotation, or oscillation of elements such as gears, pulleys, flywheels, cranks, sprockets, and control the geometry of their motion.
Chapter 2: Shaft and Bearing INTRODUCTION • They are mainly classified into two types. Ø Transmission shafts: - Used to transmit power between the source and the machine absorbing power. Example: - Counter shaft, Line shafts, etc. Ø Machine shafts: - These are the integral part of the machine itself. Example: - Crank shaft.
Chapter 2: Shaft and Bearing INTRODUCTION § A driveshaft or driving shaft is a mechanical device for transferring power from the engine or motor to the point where useful work is applied. Most engines or motors deliver power as torque through rotary motion: § Driveshaft are carriers of torque: they are subject to torsion and shear stress, which represents the difference between the input force and the load.
Chapter 2: Shaft and Bearing INTRODUCTION § They thus need to be strong enough to bear the stress, without imposing too great an additional inertia by virtue of the weight of the shaft. Often connected with a flexible driveshaft coupling or rag joint.
Chapter 2: Shaft and Bearing 2. 1. 1 - Shaft Design Procedures 1. Determine the rotational speed of the shaft. 2. Determine the power or the torque to be transmitted by the shaft. 3. Specify the location of bearings to support the shaft. 4. Propose the general form of the geometry for the shaft.
Chapter 2: Shaft and Bearing 2. 1. 1 - Shaft Design Procedures 5. Determine the magnitude of torque that the shaft sees at all points. 6. Determine the forces that are exerted on the shaft, both radially and axially. 7. Resolve the radial forces into components in perpendicular directions.
Chapter 2: Shaft and Bearing 2. 1. 1 - Shaft Design Procedures 8. Solve for the reactions on all support bearings in each plane. 9. Produce the complete shearing force and bending moment diagram to determine the distribution of bending moments on the shaft. 10. Select appropriate materials and specify the necessary treatment for them.
Chapter 2: Shaft and Bearing 2. 1. 1 - Shaft Design Procedures 11. Determine an appropriate design stress. 12. Analyse each critical point of the shaft to determine the minimum acceptable diameter of the shaft. 13. Specify the final dimensions for each point on the shaft.
Chapter 2: Shaft and Bearing Chapter 2: and Shaft and Bearing 2. 1. 2 - Force Stress Analysis FORCES EXERTED ON SHAFTS BY MACHINE ELEMENTS • Gears, belt sheaves, chain sprockets, and other elements typically carried by shafts exert forces on the shaft that cause bending moments. • The following is a discussion of the methods for computing these forces for some cases. • In general, you will have to use the principles of statics and dynamics to determine the forces for any particular element.
Chapter 2: Shaft and Bearing Chapter 2: and Shaft and Bearing 2. 1. 2 - Force Stress Analysis • The torque transmitted by spur gears is • Forces on spur gears, helical gears, bevel gears Ø Namely tangential forces and radial forces.
Chapter 2: Shaft and Bearing 2. 1. 2 - Force and Stress Analysis • For chain sprockets, if the torque, T, and pitch diameter, D are known, then the force is:
Chapter 2: Shaft and Bearing 2. 1. 2 - Force and Stress Analysis • Stress concentrations need to be considered for components that fasten machine elements. Some values used for Kt are: Ø Keyseats - 2. 0 (profile), 1. 6 (sled runner) Ø Shoulder fillets - 2. 5 (sharp fillet), 1. 5 (well-rounded fillet) Ø Retaining rings - 3. 0
Chapter 2: Shaft and Bearing 2. 1. 2 - Force and Stress Analysis • Design stresses Ø Several stress conditions can exist at the same time. § Torsional stress will exist on any part of the shaft. § Bending stress may exist on other parts of the shaft. Ø τd used to represent the design stress when shear stress is the basis for the design; σd for normal stress as basis. N represents the design factor.
Chapter 2: Shaft and Bearing 2. 1. 2 - Force and Stress Analysis
Chapter 2: Shaft and Bearing 2. 1. 2 - Force and Stress Analysis - Example Refer to diagrams below. The shaft is part of the drive for a large blower system supplying air to a furnace. Gear A receives 150 k. W from gear P. Gear C delivers the power to gear Q. The shaft rotates at 62. 8 rad/s.
Chapter 2: Shaft and Bearing 2. 1. 2 -Chapter Force and 2: Stress Analysis - Example Solution: It is known that the shaft will be machined from AISI 1144 OQT 1000 steel, thus several factors can be determined namely: sy = 572. 28 MPa, su = 813. 61 MPa Set CR = 0. 81, Cs = 0. 75 and N=2 From the data above, the modified endurance strength can be computed: S’n= sn. Cs. Cr = 175. 82 MPa
Chapter 2: Shaft and Bearing 2. 1. 2 -Chapter Force and 2: Stress Analysis - Example Torque in the shaft, T = P/ω = 2373 Nm. Forces on the gears can be computed and the values are:
Chapter 2: Shaft and Bearing 2. 1. 2 - Force and Stress Analysis - Example The load, shear and moment diagrams are drawn:
Chapter 2: Shaft and Bearing 2. 1. 2 - Force and Stress Analysis - Example After the corresponding combined moment are determined from the diagrams, the minimum required diameter for various points can be computed using the equation below:
Chapter 2: Shaft and Bearing Chapter Shaft and Bearing 2. 1. 32: - Shaft Rigidity • Rigidity of the shaft is also a major concern mainly because: Ø Excessive radial deflection of the shaft may cause active machine elements to be misaligned. Ø Dynamic behaviour of the shaft may be destructive. • Some measures to reduce problems with deflections or critical speeds: Ø Make the shaft rigid Ø Use shorter shaft length Ø Use stiff and rigid bearings
THANK YOU
Chapter 2: Shaft and Bearing BEARING - Brief introduction to bearing. Discuss type and design life of bearings. Compute equivalent load on bearing. Relate force and critical bearing selection factors.
Chapter 2: Shaft and Bearing TOPIC 2: BEARING
Chapter 2: Shaft and Bearing
Chapter 2: Shaft and Bearing INTRODUCTION • The bearing makes many of the machines we use every day possible. • Without bearings, we would be constantly replacing parts that wore out from friction.
Chapter 2: Shaft and Bearing INTRODUCTION • The concept behind a bearing is very simple: Things roll better than they slide. • The wheels on your car are like big bearings. If you had something like skis instead of wheels, your car would be a lot more difficult to push down the road. That is because when things slide, the friction between them causes a force that tends to slow them down.
Chapter 2: Shaft and Bearing INTRODUCTION • But if the two surfaces can roll over each other, the friction is greatly reduced. • Bearings reduce friction by providing smooth metal balls or rollers, and a smooth inner and outer metal surface for the balls to roll against. • These balls or rollers "bear" the load, allowing the device to spin smoothly.
Chapter 2: Shaft and Bearing 2. 2. 1 –BEARING LOADS • Bearings typically have to deal with two kinds of loading, radial and thrust. • Depending on where the bearing is being used, it may see all radial loading, all thrust loading or a combination of both.
Chapter 2: Shaft and Bearing 2. 2. 1 –BEARING LOADS • The bearings that support the shafts of motors and pulleys are subject to a radial load. • The bearings in the electric motor and the pulley pictured above face only a radial load. • In this case, most of the load comes from the tension in the belt connecting the two pulleys.
Chapter 2: Shaft and Bearing 2. 2. 1 –BEARING LOADS • The bearings in this stool are subject to a thrust load. • This bearing is like the one in a barstool. • It is loaded purely in thrust, and the entire load comes from the weight of the person sitting on the stool.
Chapter 2: Shaft and Bearing 2. 2. 1 –BEARING LOADS • This bearing is like the one in the hub of your car wheel. • This bearing has to support both a radial load and a thrust load. • The radial load comes from the weight of the car, the thrust load comes from the cornering forces when you go around a turn.
Chapter 2: Shaft and Bearing 2. 2. 2 – TYPES OF BEARINGS • There are many types of bearings, each used for different purposes. • These include ball bearings, roller bearings, ball thrust bearings, roller thrust bearings and tapered roller thrust bearings.
Chapter 2: Shaft and Bearing 2. 2. 2 – TYPES OF BEARINGS Ball bearings They are found in everything from inline skates to hard drives. These bearings can handle both radial and thrust loads, and are usually found in applications where the load is relatively small.
Chapter 2: Shaft and Bearing 2. 2. 2 – TYPES OF BEARINGS
Chapter 2: Shaft and Bearing 2. 2. 2 – TYPES OF BEARINGS • Roller bearings like the one illustrated is used in applications like conveyer belt rollers, where they must hold heavy radial loads. • In these bearings, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line.
Chapter 2: Shaft and Bearing 2. 2. 2 – TYPES OF BEARINGS • This spreads the load out over a larger area, allowing the bearing to handle much greater loads than a ball bearing. However, this type of bearing is not designed to handle much thrust. • A variation of this type of bearing, called a needle bearing, uses cylinders with a very small diameter. • This allows the bearing to fit into tight places.
Chapter 2: Shaft and Bearing 2. 2. 2 – TYPES OF BEARINGS Ball Thrust Bearing Ball thrust bearings like the one shown is mostly used for low-applications and cannot handle much radial load.
Chapter 2: Shaft and Bearing 2. 2. 2 – TYPES OF BEARINGS Roller Thrust Bearing Roller thrust bearings like the one illustrated can support large thrust loads. They are often found in gear sets like car transmissions between gears, and between the housing and the rotating shafts. The helical gears used in most transmissions have angled teeth -- this causes a thrust load that must be supported by a bearing
Chapter 2: Shaft and Bearing 2. 2. 2 – TYPES OF BEARINGS Tapered Roller Bearings • Tapered roller bearings can support large radial and large thrust loads. • 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.
Chapter 2: Shaft and Bearing 2. 2. 3 – LOADS ON BEARINGS All bearings will have finite life due to fatigue under high contact stresses. Relationship between load, P, and life, L:
Chapter 2: Shaft and Bearing 2. 2. 4 – DESIGN LIFE OF BEARINGS
Chapter 2: Shaft and Bearing 2. 2. 4 – DESIGN LIFE OF BEARINGS
Chapter 2: Shaft and Bearing 2. 2. 4 – DESIGN LIFE OF BEARINGS
Chapter 2: Shaft and Bearing 2. 2. 4 – DESIGN LIFE OF BEARINGS
Chapter 2: Shaft and Bearing 2. 2. 4 – DESIGN LIFE OF BEARINGS
Chapter 2: Shaft and Bearing 2. 2. 5 – MOUNTING OF BEARINGS Primary considerations in mounting: • The shaft seat diameter and tolerances • The housing internal bore and tolerances • The shaft shoulder diameter against which the inner race of the bearing will be located. • The housing shoulder diameter provided for locating the outer race. • The radius of the fillets at the base of the shaft and housing shoulders. • The means of retaining the bearing in position.
Chapter 2: Shaft and Bearing 2. 2. 6 – PRACTICAL CONSIDERATIONS Other considerations include: • Lubrication • Installation • Preloading • Stiffness of bearings • Operation under varying loads • Sealing • Limiting speed • Standards • Bearing tolerance
THANK YOU
- Slides: 51
