Chapter6 第6章

Design from a perspective of failure prevention
When selecting bearings, designers must: Routinely examine both the ability of a bearing to resist surface fatigue failure (basic dynamic load rating) and the ability of a bearing to resist brinelling failure (basic static load rating).
6.3 Potential Failure Modes
Under typical operating conditions, surface fatigue failure is the most likely failure mode. The cyclic subsurface Hertz shear stresses produced by the curved surfaces in rolling contact may initiate and propagate minute cracks that ultimately dislodge particles and generate surface pits. In some cases, static loads on a bearing during idle nonrotating segments of a duty cycle may cause brinelling of the races, resulting in subsequent generation of noise, vibration, and heat as the rolling elements pass the local discontinuities in the raceway.
6.1 Characteristics of Rolling Element Bearings
Bearing specialists must consider such matters as fatigue loading, friction, heat, corrosion resistance, kinematic problems, material properties, lubrication, machining tolerances, assembly, use, and cost. Most bearing manufacturers publish engineering manuals containing extensive descriptions of the various types and sizes available, and procedures for making appropriate selections. Designer of machines or bearing users should at least be able to choose the appropriate type and specifications of the bearing they need.
6.4 Bearing Materials
Vacuum-degassed high-carbon chrome steel, hardened and tempered for optimum strength and toughness, sometimes stabilized for precise dimensional control. Stainless-steel alloys are sometimes used when corrosion resistance is needed or moderately elevated temperatures must be accommodated. For higher-temperature applications, cobalt base alloys may be used. GCr15 steel is widely used for both races and balls, and is typically through-hardened to Rockwell C61-C64.
Chapter 6 Rolling Bearings
6.1 Characteristics of Rolling Bearings 6.2 Types of Rolling Element Bearings 6.3 Potential failure modes 6.4 Bearing Materials 6.5 Bearing Selection 6.6 Mounting and Enclosure Example
Advantages of rolling element bearings
High reliability with minimum maintenance Minimum lubrication required. Lubricant can often be sealed in for the "lifetime" of the bearing Good for low-speed operation Low starting friction and low power loss due to frictional drag Can readily support radial, thrust, or combined radial and thrust loads Small axial space required Nearly universal interchange ability among manufacturers due to industry-wide standardized sizes and closely controlled tolerances Can be preloaded to eliminate internal clearances, improve fatigue life, or increase bearing stiffness Increase in operational noise level warns of impending failure
6.2 Types of Rolling Element Bearing
Rolling element bearings may be broadly classified as either ball bearings or roller bearings. They are manufactured to take pure radial loads, pure thrust loads, or a combination of these two. Rolling element bearings are almost universally standardized. Within each of these broad categories there are a host of geometrical configurations commercially available.
(a)Straight roller bearings
Байду номын сангаас
(c)Taped roller thrust bearing
(e)Tapered roller bearings
Characteristics of taped roller bearings
Tapered roller bearings combine the advantages of ball and straight roller bearings, since they can take either radial or thrust loads or any combination of the two, and in addition, they have the high load-carrying capacity of straight roller bearings. Because of the contacts of tapered roller and the conically angled raceways, when a purely radial external load is applied to a tapered roller bearing, a thrust (axial) load is induced within the bearing because of the taper. Hence, as for the case of angular contact ball bearings, tapered roller bearings are usually used in pairs to resist the thrust reaction or to provide desired stiffness.
6.5 Bearing Selection
In selecting an appropriate bearing for any given application, design decisions must be made about bearing type, size, space allocation, mounting methods, and other details. Designer experience plays an important role in the selection process. Table 6.1 provides overall guidance in selecting an appropriate bearing type, but to use the table a designer must first summarize the specific design requirements at the bearing site in terms of radial loading and thrust loading spectrum, operating speeds, design life requirements, stiffness requirements, and operating environment.