Bearing Design in Machinery

Engineering Tribology and Lubrication

Avraham Harnoy

• Publisher: CRC Press
• Year: 2002
• eBook ISBN: 9781135561628
• Topic: Ciencias físicas
• Subtopic: Diseño industrial

Covering the fundamental principles of bearing selection, design, and tribology, this book discusses basic physical principles of bearing selection, lubrication, design computations, advanced bearings materials, arrangement, housing, and seals, as well as recent developments in bearings for high-speed aircraft engines. The author explores unique so

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Classification and Selection of Bearings

1.1 INTRODUCTION

Moving parts in machinery involve relative sliding or rolling motion. Examples of relative motion are linear sliding motion, such as in machine tools, and rotation motion, such as in motor vehicle wheels. Most bearings are used to support rotating shafts in machines. Rubbing of two bodies that are loaded by a normal force (in the direction normal to the contact area) generates energy losses by friction and wear. Appropriate bearing design can minimize friction and wear as well as early failure of machinery. The most important objectives of bearing design are to extend bearing life in machines, reduce friction energy losses and wear, and minimize maintenance expenses and downtime of machinery due to frequent bearing failure. In manufacturing plants, unexpected bearing failure often causes expensive loss of production. Moreover, in certain cases, such as in aircraft, there are very important safety considerations, and unexpected bearing failures must be prevented at any cost.

During the past century, there has been an ever-increasing interest in the friction and wear characteristics of various bearing designs, lubricants, and materials for bearings. This scientific discipline, named Tribology, is concerned with the friction, lubrication, and wear of interacting surfaces in relative motion. Several journals are dedicated to the publication of original research results on this subject, and several books have been published that survey the vast volume of research in tribology. The objectives of the basic research in tribology are similar to those of bearing design, focusing on the reduction of friction and wear. These efforts resulted in significant advances in bearing technology during the past century. This improvement is particularly in lubrication, bearing materials, and the introduction of rolling-element bearings and bearings supported by lubrication films. The improvement in bearing technology resulted in the reduction of friction, wear, and maintenance expenses, as well as in the longer life of machinery.

The selection of a proper bearing type for each application is essential to the reliable operation of machinery, and it is an important component of machine design. Most of the maintenance work in machines is in bearing lubrication as well as in the replacement of damaged or worn bearings. The appropriate selection of a bearing type for each application is very important to minimize the risk of early failure by wear or fatigue, thereby to secure adequate bearing life. There are other considerations involved in selection, such as safety, particularly in aircraft. Also, cost is always an important consideration in bearing selection—the designer should consider not only the initial cost of the bearing but also the cost of maintenance and of the possible loss of production over the complete life cycle of the machine.

Therefore, the first step in the process of bearing design is the selection of the bearing type for each application. In most industries there is a tradition concerning the type of bearings applied in each machine. However, a designer should follow current developments in bearing technology; in many cases, selection of a new bearing type can be beneficial. Proper selection can be made from a variety of available bearing types, which include rolling-element bearings, dry and boundary lubrication bearings, as well as hydrodynamic and hydrostatic lubrication bearings. An additional type introduced lately is the electromagnetic bearing. Each bearing type can be designed in many different ways and can be made of various materials, as will be discussed in the following chapters.

It is possible to reduce the size and weight of machines by increasing their speed, such as in motor vehicle engines. Therefore, there is an increasing requirement for higher speeds in machinery, and the selection of an appropriate bearing type for this purpose is always a challenge. In many cases, it is the limitation of the bearing that limits the speed of a machine. It is important to select a bearing that has low friction in order to minimize friction-energy losses, equal to the product of friction torque and angular speed. Moreover, friction-energy losses are dissipated in the bearing as heat, and it is essential to prevent bearing overheating. If the temperature of the sliding surfaces is too close to the melting point of the bearing material, it can cause bearing failure. In the following chapters, it will be shown that an important task in the design process is the prevention of bearing overheating.

1.1.1 Radial and Thrust Bearings

Bearings can also be classified according to their geometry related to the relative motion of elements in machinery. Examples are journal, plane-slider, and spherical bearings. A journal bearing, also referred to as a sleeve bearing, is widely used in machinery for rotating shafts. It consists of a bushing (sleeve) supported by a housing, which can be part of the frame of a machine. The shaft (journal) rotates inside the bore of the sleeve. There is a small clearance between the inner diameter of the sleeve and the journal, to allow for free rotation. In contrast, a plane-slider bearing is used mostly for linear motion, such as the slides in machine tools.

A bearing can also be classified as a radial bearing or a thrust bearing, depending on whether the bearing load is in the radial or axial direction, respectively, of the shaft. The shafts in machines are loaded by such forces as reactions between gears and tension in belts, gravity, and centrifugal forces. All the forces on the shaft must be supported by the bearings, and the force on the bearing is referred to as a bearing load. The load on the shaft can be divided into radial and axial components. The axial component (also referred to as thrust load) is in the direction of the shaft axis (see Fig. 1-1), while the radial load component is in the direction normal to the shaft axis. In Fig. 1-1, an example of a loaded shaft is presented. The reaction forces in helical gears have radial and axial components. The component F_a is in the axial direction, while all the other components are radial loads. Examples of solved problems are included at the end of this chapter. Certain bearings, such as conical roller bearings, shown in Fig. 1-1, or angular ball bearings, can support radial as well as thrust forces. Certain other bearings, however, such as hydrodynamic journal bearings, are applied only for radial loads, while the hydrodynamic thrust bearing supports only axial loads. A combination of radial and thrust bearings is often applied to support the shaft in machinery.

FIG. 1-1 Load components on a shaft with helical gears.

1.1.2 Bearing Classification

Machines could not operate at high speed in their familiar way without some means of reducing friction and the wear of moving parts. Several important engineering inventions made it possible to successfully operate heavily loaded shafts at high speed, including the rolling-element bearing and hydrodynamic, hydrostatic, and magnetic bearings.

1. Rolling-element bearings are characterized by rolling motion, such as in ball bearings or cylindrical rolling-element bearings. The advantage of rolling motion is that it involves much less friction and wear, in comparison to the sliding motion of regular sleeve bearings.

2. The term hydrodynamic bearing refers to a sleeve bearing or an inclined plane-slider where the sliding plane floats on a thin film of lubrication. The fluid film is maintained at a high pressure that supports the bearing load and completely separates the sliding surfaces. The lubricant can be fed into the bearing at atmospheric or higher pressure. The pressure wave in the lubrication film is generated by hydrodynamic action due to the rapid rotation of the journal. The fluid film acts like a viscous wedge and generates high pressure and load-carrying capacity. The sliding surface floats on the fluid film, and wear is prevented.

3. In contrast to hydrodynamic bearing, hydrostatic bearing re...

Table of contents

• Cover
• Title Page
• Copyright Page
• Dedication
• Preface
• Table of Contents
• Symbols
• Chapter 1 Classification and Selection of Bearings
• Chapter 2 Lubricant Viscosity
• Chapter 3 Fundamental Properties of Lubricants
• Chapter 4 Principles of Hydrodynamic Lubrication
• Chapter 5 Basic Hydrodynamic Equations
• Chapter 6 Long Hydrodynamic Journal Bearing
• Chapter 7 Short Journal Bearings
• Chapter 8 Design Charts for Finite-Length Journal Bearings
• Chapter 9 Practical Applications of Journal Bearings
• Chapter 10 Hydrostatic Bearings
• Chapter 11 Bearing Materials
• Chapter 12 Rolling Element Bearings
• Chapter 13 Selection and Design of Rolling Bearings
• Chapter 14 Testing of Friction and Wear
• Chapter 15 Hydrodynamic Bearings Under Dynamic Conditions
• Chapter 16 Friction Characteristics
• Chapter 17 Modeling Dynamic Friction
• Chapter 18 Case Study: Composite Bearing—Rolling Element and Fluid Film in Series
• Chapter 19 Non-Newtonian Viscoelastic Effects
• Chapter 20 Orthopedic Joint Implants
• Appendix A Units and Definitions of Material Properties
• Appendix B Numerical Integration
• Bibliography