Gears and Gear Drives

Damir T. Jelaska

• Publisher: Wiley
• Year: 2012
• Print ISBN: 9781119941309
• eBook ISBN: 9781118392713
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Automotive Transportation & Engineering
• Index: Technology & Engineering

Understanding how gears are formed and how they interact or 'mesh' with each other is essential when designing equipment that uses gears or gear trains. The way in which gear teeth are formed and how they mesh is determined by their geometry and kinematics, which is the topic of this book. 

Gears and Gear Drives provides the reader with comprehensive coverage of gears and gear drives. Spur, helical, bevel, worm and planetary gears are all covered, with consideration given to their classification, geometry, kinematics, accuracy control, load capacity and manufacturing. Cylindrical gear geometry is the basis for dealing with any gear drives, so this is covered in detail. 

Key features:

• Contains hundreds of 2D and 3D figures to illustrate all types of gears and gear drives, including planetary and worm gears
• Includes fundamental derivations and explanations of formulae
• Enables the reader to know how to carry out accuracy control and load capacity checks for any gear drive
• Includes directions for the practical design of gears and gear drives
• Covers DIN and ISO standards in the area 

Gears and Gear Drives is a comprehensive reference for gears and gear drive professionals and graduate students in mechanical engineering departments and covers everything important to know how to design, control and manufacture gear drives.

Chapter 1

Introduction

1.1 Power Transmissions and Mechanical Drives

Mechanical power transmissions¹ consist of units which, in distinction from electrical, pneumatic and hydraulic ones, transfer power from the prime mover to the actuator (operational machine or operational member) with the assistance of rotary motion. These units are called mechanical drives and are situated between the prime mover and the actuator (Figure 1.1). The drive is connected with both the prime mover and the actuator by couplings or clutches forming an entirety whose function is defined by the purpose of the actuator.

Figure 1.1 Schematic account of a mechanical drive application

The embedding of a power transmission to link the prime mover and the machine operating member can be due to a number of reasons:

• The required speed of the machine operating member very often differs from the speeds of the standard prime movers.
• One prime mover has to drive several actuators.
• The driven side speed has to be frequently changed (regulated), whereas the prime mover cannot be used to full advantage for this purpose.
• Certain periods of the driven side operation may require torques far from those obtained on the motor shaft.
• As a rule, standard motors are designed for uniform rotary motion, while operating members have sometimes to move with varying speed or periodic halts.
• If a resonant vibration of some member in the chain of power transmission cannot be solved in any other way, the frequency of rotary motion can be changed by building-in a drive.
• Sometimes considerations of safety, convenience of maintenance or the dimensions of the machine, especially if the prime mover and operational machine shaft axes are not coaxial, do not allow the direct coupling of the prime mover shaft with operating member.

The capital task of the designer is to select such an assembly ‘prime mover – transmission (drive)’ which should optimally meet the needs of the operational machine or member. This act of choosing is a complex task, whose solution depends on: (i) accessibility of the energy source and its price, (ii) efficiency of the entirety of prime mover – transmission – operational machine, (iii) investment costs, (iv) operational machine features, primarily the (v) variability of its speed of rotation, (vi) service conditions, (vii) drive maintainability and so on. Within the framework of this task, a particularly complex problem is defining the transmission: mechanical or some other? This question is beyond the scope of this book, but generally it may be affirmed that the basic advantage of mechanical drives in relation to all the others is their very high efficiency, which is becoming more and more important day by day.

The comparative advantages offered by possible transmissions and drives are outlined in Table 1.1 which gives only a general illustration. Recently, a prominent feature in power transfer has been the extensive employment of electric, hydraulic and pneumatic transmissions. Frequently, such transmissions together with mechanical drives are simultaneously used to actuate various mechanisms. The proper choice of a drive for each specific case can be made only by comparing the technical and economical features of several designs.

Table 1.1 Advantages of transmissions and drives.

The mechanical drive driving shaft receives power P₁ at speed of rotation n₁ from the prime mover driven shaft, and the mechanical drive driven shaft supplies power P₂ < P₁ at speed of rotation n₂ to the operational machine driving shaft. The difference P₁ − P₂ = P_L is called power loss and the ratio:

is called efficiency; it takes a special place amongst power transmission characteristics because it shows unproductive power expenditure and so indirectly characterizes the wear of the drive and its warming up – the capital problems in power transmissions. Warming up causes strength and lifetime decrease of drive parts. Their corrosion resistance and the functional ability of lubricant are also imperilled. The importance of efficiency is raised to a power by the global lack of increasingly expensive energy and its value also decisively affects the price of the drive.

The power loss consists of constant losses which on the whole do not depend on load, and variable losses which on the whole are proportional to the load. The value of constant losses approximates the power of idle run, that is, the power needed to rotate the drive at P = 0 on the driven shaft. It depends on the weight of the drive parts, the speed of rotation and the friction in the bearings and on other surfaces of contact.

The second fundamental parameter of a mechanical drive is the transmission ratio i defined as the ratio of its driving n₁ and driven n₂ shaft speeds of rotation or angular speeds:

(1.1)

If i > 1 (n₁ > n₂) the mechanical drive is called an underdrive and its member is called a reducer. It reduces the speed of rotation and the transmission ratio is also called a speed reducing ratio. If i < 1 the mechanical drive is called an overdrive, its member is called a multiplicator and the transmission ratio is also called a speed increasing ratio. It multiplies the speed of rotation. An overdrive usually works less efficiently than an underdrive. This is especially true for a toothed wheel gearing.

1.2 Classification of Mechanical Drives

The basic division of mechanical drives falls into:

• Drives with a constant transmission ratio.
• Drives with a variable transmission ratio.

In constant transmission ratio drives, the constant speed of driving shaft rotation results in a constant speed of driven shaft rotation, n₂ = n₁/i. Their design should, as a rule, include at least the following data: (i) transmitted power of the driving (P₁) or driven (P₂) shaft or related torques, (ii) speed of rotation (rpm) of the driving (n₁) and driven (n₂) shaft, mutual location of the shafts and distanc...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Preface
• Acknowledgments
• Chapter 1: Introduction
• Chapter 2: Geometry of Cylindrical Gears
• Chapter 3: Integrity of Gears
• Chapter 4: Elements of Cylindrical Gear Drive Design
• Chapter 5: Bevel Gears
• Chapter 6: Planetary Gear Trains
• Chapter 7: Worm Gear Drives
• Further Reading
• Color Plates
• Index
• End User License Agreement