Design Philosophy - 1 - Design of Machine Elements - Mechanical Engineering

Introduction

Design is fundamentally a decision-making process. Given a problem, design is the systematic formulation of a plan to satisfy a need and to create an artefact with physical reality. For example, in the design of a chair, several factors must be considered before a final solution is chosen:

• Purpose - whether the chair is an easy chair, an office chair, or a dining chair.
• User characteristics - whether it is intended for an adult or a child.
• Material, strength and cost - choice of material, required strength, durability and budget constraints.
• Aesthetics and ergonomics - shape, finish and comfort for the intended user and setting.

Design activity occurs in everyday life and in engineering practice because real problems require solutions: choices must be made and consequences evaluated. A good design follows from sound choices; a poor decision at an early stage can make the entire product unsatisfactory.

Basic Concept of Machine Design

Machine design concerns the conception and specification of individual machine elements and their assembly so that they transmit forces safely and perform a required task. A machine may be defined as a combination of bodies with successfully constrained relative motions, used to convert or transmit energy for useful work.

If a machine converts heat into mechanical energy it is a heat engine. The piston-cylinder arrangement used in many engines is a typical example (see figure reference below).

Figure 1.1.2.1 - Conversion of heat to mechanical energy in a piston-cylinder arrangement.

Many machines do not generate energy but receive mechanical energy and modify it for a specific purpose. Examples include a hoist, a bicycle and a hand winch. Such modification or transmission of energy requires several machine elements, some small and some large. Machine design therefore primarily involves:

• Designing individual elements so that they can transmit the required forces or moments without failure,
• Assembling these elements in a reliable and economical manner,
• Ensuring required kinematic relationships and functional behaviour, and
• Considering manufacturability, maintainability and cost throughout the design.

Consider two simple mechanisms as illustrations:

• Hand winch
• Small press operated by a power screw

Each mechanism provides useful work through a combination of several parts. Designing them requires the sizing and selection of these parts followed by proper assembly and provision for safety and serviceability.

Types of Design

Design tasks differ according to the degree of novelty and the approach used. Common classifications are:

• Adaptive design - adoption of an existing design or standard product for a new application. Examples: adapting conveyor belts or control systems to a new machine, or using an existing haulage system with minor changes.
• Developmental design - starting from an existing product but producing a modified or improved version. Examples: successive models of an automobile where the basic concept remains but many features are changed.
• New design - an entirely new solution based on existing scientific principles; requires creative thinking and possibly some research. Examples: designing a small vehicle for personnel transport on board a ship or in a desert environment.

Types of Design Based on Methods

Design methods prescribe how engineers arrive at dimensions and configurations. Important categories are:

• Rational design - based on analysis of stresses, strains and mechanical behaviour of components using the laws of mechanics. Dimensions and materials are chosen using calculations and scientific reasoning.
• Empirical design - based on experimental results, rules of thumb and accumulated experience rather than first-principle analysis. Such relations are useful where exact analysis is difficult. Example: a commonly used empirical relation for the approximate tightening force P (in kg) when a nut is tightened on a bolt is P = 284 d, where d is the bolt diameter in millimetres. This equation is an observation-based guideline rather than a derivation from first principles.
• Industrial design - decisions guided by market considerations, production capabilities and economics. Emphasis is on external appearance, standardisation, ease of manufacture, low cost and meeting customer expectations.

Objectives and Constraints in Machine Design

The primary objectives of machine design are:

• Functionality - the machine must perform the intended task reliably.
• Safety - components should withstand loads with suitable margins to prevent failure.
• Durability and reliability - adequate life under expected service conditions.
• Economy - reasonable cost of manufacture, assembly and operation.
• Maintainability and interchangeability - ease of repair, use of standard parts and spares.
• Aesthetics and ergonomics - acceptable appearance and comfortable use where applicable.

Constraints that commonly influence design include material availability, production processes, standard sizes, weight, space limitations, environmental conditions (temperature, corrosion), and statutory or safety standards.

Design Process and Decision Making

Typical stages of the design process are:

• Problem definition and requirements specification.
• Concept generation and selection of a basic solution approach.
• Preliminary design - sizing of principal components and layout.
• Detailed design - detailed drawings, selection of materials, tolerances, surface finish and fasteners.
• Prototyping and testing.
• Final design adjustments and production release.

Decision making is present at every stage. Early critical choices - for example the selection of engine capacity, power transmission arrangement or material family - strongly constrain subsequent decisions. Designers often use trade-off studies and optimisation to balance conflicting requirements such as strength versus weight or cost versus performance.

Factors Affecting Component Design

Components are designed with attention to the following technical factors:

• Strength - static and dynamic loads, including fatigue and impact considerations.
• Stiffness - control of deflections and vibrations so performance is not impaired.
• Wear resistance - where relative motion or abrasive contact occurs.
• Manufacturability - choice of geometries and tolerances compatible with available manufacturing processes.
• Assembly and maintenance - accessibility for assembly, inspection and repair.
• Standardisation - use of standard fasteners, bearings, seals and other catalogued parts to reduce cost and simplify maintenance.

Safety, Reliability and Factors of Safety

Design must ensure that the probability of failure is acceptably low over the intended life. The usual approach is to apply a factor of safety to account for uncertainties in loading, material properties, manufacturing defects and operating conditions. The chosen factor depends on the consequences of failure, variability of loads, and reliability requirements.

Standardisation and Interchangeability

Standardisation reduces variety, lowers costs and facilitates mass manufacture. Interchangeability of parts allows rapid repair and replacement without custom fitting. Designers should prefer standard components (bolts, bearings, seals, springs) where they meet functional requirements.

Summary

Design in mechanical engineering is a structured process of making decisions to convert requirements into a physical product. Machine design concentrates on individual elements and their assembly so that machines transmit or modify energy reliably and economically. Approaches include adaptive, developmental and new design; methods include rational, empirical and industrial design. Effective design balances functionality, safety, durability and economy while observing constraints of manufacturability, standardisation and user requirements.